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Full text of "The study of plant communities : an introduction to plant ecology"

r 







THE STUDY OF 



a 



PLANT COMMUNITIES 

An Introduction to Plant Ecology 



BY 



HENRY J. OOSTING 

Associate Professor of Botany 
Duke University 




1948 
W. H. FREEMAN AND COMPANY 

San Francisco, California 



Copyright, IQ48, by Henry J . Oosting 



All rights to reproduce this book in whole or in 
part are reserved, with the exception of the right 
to use short quotations for review of the book. 



PRINTED IN THE UNITED STATES OF AMERICA 
TYPOGRAPHY BY MACKENZIE & HARRIS, INC., SAN FRANCISCO 



TO 
MY STUDENTS 

from whom 

I have learned much more 

than they realize 



Preface 



This book grew out of several successive reorganizations of an 
introductory course in plant ecology. Since it is intended as an 
introduction to plant ecology, effort has been made to make it as 
stimulating as possible while presenting basic information. From 
experience we know that this ideal is best achieved through study 
of plant communities with emphasis on field work. The plant 
community, therefore, is made the basis of this book. 

The plan, in brief, proceeds from a consideration of the nature 
and variation of communities to methods of distinguishing and 
describing them. This is followed by a discussion of the factors 
which limit, maintain, and modify communities both locally and 
regionally. Thus the interrelationships between organisms and 
environment are emphasized and a foundation is laid for presenta- 
tion of the concepts of succession and climax. Then the climax 
regions of North America become a logical consideration since 
they are illustrative of all that comes before. To answer the ques- 
tions which must arise regarding the permanence of climax, a sec- 
tion is devoted to past climaxes and their study and reconstruction. 
Finally, the potentialities of the ecological point of view in prac- 
tical considerations are emphasized by a survey of its possible and 
desirable applications in range management, agriculture, conserva- 
tion, landscaping, forestry, and even human relations. 

The intent has been to write a textbook with a wide usefulness. 
It was assumed that, in some instances, the text material might 
serve as the complete subject-matter of a course. To this end, the 
presentation aims at a fairly broad but solid foundation for eco- 
logical thinking and appreciation. At the same time there is no 
attempt at completeness, either in subject matter or bibliography, 
such as might be expected in a reference volume. Although con- 
troversial issues are not deliberately obscured, they are not em- 

6 



PREFACE 



phasized. The assumption has been that a beginning student should 
acquire a working knowledge and appreciation of the field before 
he is introduced to matters that might confuse him. 

A reasonable background of botanical and scientific experience 
is assumed so that, in general, college juniors and seniors might be 
expected to have the greatest appreciation of a course of this kind. 
A reasonable knowledge of plant physiology is expected, at least 
enough for comprehension of ordinary physiological processes. 
Although a student without some taxonomic training could hardly 
fully appreciate or enjoy an ecology course dealing with com- 
munities, he could use this book if he had some knowledge of 
plants. Both common and scientific names have been given reg- 
ularly or at least the first time a species is mentioned. The plants 
which are named are almost without exception rather generally 
known species of long standing. It is not considered necessary, 
therefore, to include authorities with scientific names since they 
may invariably be found in standard manuals. 

Suggestions for collateral reading may be found in the selected 
general references at the ends of chapters. Cited references are in- 
dicated in the text by number only and are listed in the bibliog- 
raphy at the end of the book. Citations are made where it seemed 
desirable to indicate the authority for or give credit for state- 
ments used in the text. Again, for those who may wish to go to 
original sources, references to survey and review papers are in- 
cluded. The bibliographies of these references are usually so ex- 
tensive that the advanced student who uses them may quickly 
accumulate all the source material he needs. 

Those who contributed directly or indirectly to the develop- 
ment of this book are too numerous to mention specifically, but I 
am deeply aware of my debt to former instructors, my colleagues, 
and my students. Many have given invaluable aid in the actual 
preparation of the book. A very special acknowledgment of as- 
sistance is due Miss Ruby Williams who, through a careful reading 
of the manuscript, did much to improve the mechanics of organ- 
ization and to clarify and simplify the presentation. 

The use of the book in mimeographed form provided a test of 
its value under a variety of conditions in different sections of the 
country. It was used in classes by Dr. W D. Billings at the Uni- 



8 THE STUDY OF PLANT COMMUNITIES 



versity of Nevada, Dr. M. F. Buell at Rutgers University, Dr. R. B. 
Livingston at the University of Missouri, and by Dr. J. F. Reed at 
the University of Wyoming, as well as at Duke University. The 
comments and suggestions derived from both students and instruc- 
tors led to revisions and additions which are invaluable, particular- 
ly in their contribution to wider utility. It is truly with deep 
appreciation that the cooperation and assistance from all these 
sources is acknowledged. 

Finally, although credit lines indicate the sources of illustrations, 
it is a real pleasure to acknowledge the courtesies and helpfulness 
of the numerous individuals and organizations involved. The ex- 
cellent material they made available, sometimes with considerable 
trouble to themselves, often made it necessary to choose from sev- 
eral possibilities for a single illustration. It is regretted that not all 
the pictures could be used. The line-drawings were done by 
George A. Thompson and Robert Zahner whose assistance is 
gratefully acknowledged. 

HENRY J. OOSTING 

Durham, North Carolina 
February, 1948. 



Table of Contents 



PART 1-INTRODUCTION Page 

Chapter I. The Subject Matter of Ecology . ... 11 

PART 2-THE PLANT COMMUNITY 

CHAPTER II. Nature of the Community 21 

CHAPTER III. Yegetational Analysis : 
Quantitative Methods % 33 

CHAPTER IV. Yegetational Analysis : 
Phytosociological Objectives 55 

PART 3-FACTORS CONTROLLING THE 

COMMUNITY : THE ENVIRONMENT 

CHAPTER V. Climatic Factors : The Air 75 

CHAPTER VI. Climatic Factors : Radiant Energy. 
Temperature and Light 11,5 

CHAPTER VII. Physiographic Factors 144 

Chapter VIII. Biological Factors 1 88 

PART 4-COMMUNITY DYNAMICS 

Chapter IX. Plant Succession 211 

CHAPTER X. The Distribution of Climax Communities : 
Present Distribution of Climaxes 234 

Chapter XI. The Distribution of Climax Communities : 
Shifts of Climaxes with Time 3 00 

PART 5-PRACTICAL CONSIDERATIONS 

Chapter XII. Applied Ecology 315 

References Cited 3^7 

Index ?71 

63307 



Part 1 • Introduction 



CHAPTER I 




THE SUBJECT MATTER OF ECOLOGY 

"What is Ecology and What Good Is It?" 250 - the title of an 
address made before the Ecological Society of America some 
years ago, is a compact, perhaps oversimplified, statement of the 
questions this textbook aims to answer. Its intention is to present 
an adequate introduction to the various phases of the subject, to 
show its position in relation to other sciences, and to indicate the 
possibilities and advantages of applying the methods and point of 
view of ecology in solving biological problems. 

THE TERM AND BASIC CONCEPTS 
The term, ecology, carries a more familiar ring than it did a 
relatively few years ago. Although it was used commonly in many 
fields of science, it did not, until recently, appear elsewhere. Now, 
it is occasionally seen in magazines and sometimes even in news- 
papers. This is partly the outgrowth of a gradual maturing of the 
science and partly the result of a growing appreciation of its mean- 
ing and potentialities. 

Although the subject matter of ecology is as old as that of any 
other science and although much of it has long been a part of sci- 
entific knowledge, ecology as a field of science is relatively new. 
The name first appeared, in 1869, as "oecology;' 112 but the great- 
est advancement has come during the past fifty years, following 
the impetus supplied by the writing and thinking of a few men in 
the late 1890's. The term "ecology" is particularly appropriate. Its 
Greek root, oikos, means home and thus indicates a dwelling place; 
this, of course, implies that organisms are present and that certain 
conditions link the two. Ecology is, therefore, the study of organ- 
isms, their environment, and all the interrelationships between the 
two. It is commonly defined as the study of organisms in relation 
to their environment. 

11 



12 THE STUDY OF PLANT COMMUNITIES • Chapter I 



ALL LIFE BOUND TO ENVIRONMENT 

An organism without environment is inconceivable, 122 for living 
things have certain requirements that must be satisfied by their sur- 
roundings if life is to continue. Their physiological processes, 
which, to sustain life, must all continue at rates above definite min- 
ima, are largely controlled by environmental conditions or sub- 
stances. Most of the processes use water or require its presence; 
food manufacture is dependent upon carbon dioxide and light con- 
ditions; the universal process of respiration requires oxygen; and 
all processes are limited by, or vary with, temperature. 

Since organisms must grow and reproduce to survive, they re- 
quire energy, which they derive from food by respiration. Food, 
therefore, becomes a major consideration in explaining the activi- 
ties of organisms. Green plants must be able to manufacture 
enough food to grow and reproduce and still leave a surplus for 
dependent organisms. Among the latter, there are usually several 
dependent upon each other for food in a relationship called a food- 
chain. For example, in aquatic environments the food-producing 
algae are eaten by miscroscopic animals that may in turn be eaten 
by larger animals upon which small fish feed. Small fish are often 
eaten by larger fish, and many of these are eaten by man. Any 
number of things may disrupt such a food-chain, but, under nor- 
mal conditions, all the organisms are interrelated by their mutual 
requirement of food, whose ultimate production is dependent 
upon algal activity in the presence of light. 

Regardless of the environment and the group of organisms 
adapted to survival in it, similar food-chains and dependencies can 
be found everywhere. Thus we see that the basic relationship 
binding all organisms to each other and to the environment is, in- 
variably, one traceable to energy needs and uses; and, because the 
ultimate source of energy for both plants and animals is the sun, 
all organisms are mutually related to each other and to their en- 
vironment. 

If groups of organisms live together successfully, their demands 
and effects upon the energy cycle will not disrupt it. All the proc- 
esses and activities taking place within the group will be in balance 
with the available supply of energy. A major concern of ecology, 
therefore, is to learn what that balance is and what controls it. 



THE SUBJECT MATTER OF ECOLOGY 13 

ENVIRONMENT A COMPLEX OF FACTORS 

Environment includes everything that may affect an organism 
in any way. It is, therefore, a complex of factors, which may be : 
substances, such as soil and water; forces, such as wind and grav- 
ity; conditions, such as temperature and light; or other organisms. 
These factors may be studied or measured individually, but they 
must always be considered in terms of their interacting effects 
upon organisms and each other. The resulting complexity of en- 
vironment and the array of subject matter encompassed suggest 
the necessity for drawing upon the knowledge of all fields of sci- 
ence for its understanding. Therein lie a complete justification of 
and explanation for ecology. Its special function is to consider 
such subject matter in terms of organisms. Any one field of science 
is relatively restricted to its own subject matter, whereas ecology 
brings together the knowledge of various sciences with the object 
of interpreting the responses of organisms to their environment. 

Since all plants and animals, including man, are organisms, and 
since environment can at times include almost anything in the uni- 
verse, the subject matter of ecology is almost unlimited. As a re- 
sult, it is dependent upon the specialized fields of science for much 
of the knowledge it uses. It requires an understanding of the funda- 
mentals of other sciences, an alertness to changes and new discov- 
eries in various fields, and a constant consideration of the possibili- 
ties of using such information for interpreting or explaining the 
peculiarities, responses, and nature of organisms under the con- 
ditions in which they live. 

SCOPE OF THE FIELD OF ECOLOGY 

Since the subject is concerned with organisms, it must include 
both plants and animals. Such a broad biological basis presupposes 
a solid foundation in both botany and zoology, and, if man is to be 
considered, an additional need for understanding of sociological, 
psvchological, and economic problems. Although the latter are 
not ordinarily considered biological subjects, they may become 
more so in the future. Sociologists are more and more concerned 
with "human ecology'' and some phases of ecology have come to 
be known as "plant sociology!' 

It is, unfortunately, unusual to find students, teachers, or inves- 



14 THE STUDY OF PLANT COMMUNITIES * Chapter I 

tigators today with sufficient training or experience to deal ade- 
quately with the entire field of biology. This explains why special- 
ists usually concentrate on either plant ecology or animal ecology, 
and why textbooks emphasize either plants or animals, even though 
all organisms should be considered. In an introduction to the sub- 
ject, however, it is probably advantageous to restrict the subject 
matter for effective discussion. We shall, therefore, be concerned 
primarily with the ecology of plants, although their relationships 
to animals will not be ignored. Furthermore, the major emphasis 
will be upon natural groupings or communities of plants and the 
reasons for finding them as we do. 

BROAD TRAINING DESIRABLE 

At first thought, the diversity of subject matter included in the 
scope of ecological application is discouraging. It ranges through 
all the sciences, but obviously one person can hardly become mas- 
ter of all scientific knowledge. Specialists, however, working on 
different phases of a problem, can contribute to its solution, pro- 
vided they all have the same objectives and points of view. Most 
ecologists are specialists in some phase of the subject, but the 
ecological approach provides the necessary unity for holding their 
interests together. A truly complete ecological training is impos- 
sible; yet it is possible to acquire a broad enough training to appre- 
ciate the importance of subject matter in fields with which one 
may not be entirely familiar. 

An appreciation of ecology necessitates certain fundamentals of 
training for a background. The specialist then expands his knowl- 
edge along lines of interest. A basic biological foundation is, of 
course, a necessity, with taxonomy and physiology as absolute 
prerequisites because of their constant use. Because ecological 
problems frequently range through any of the biological fields 
from morphology to pathology to genetics, the advantages of an 
extensive preparation should be evident. 

The desirability of a basic understanding of physics and chem- 
istry need hardly be emphasized since both have their obvious 
uses in the interpretation of environmental conditions as well as in 
applications to physical and physiological problems. Some knowl- 
edge of geology is very useful, and, for certain types of work, a 



THE SUBJECT MATTER OF ECOLOGY 15 

broad training in this field is a necessity. Soils are a constant con- 
cern of the ecologist both as to their origin and development and 
as to the paralleling vegetational characteristics as modified by 
water, aeration, and nutrition. The frequent recurrence of prob- 
lems related to climatology suggests its desirability, and the in- 
creasing use of quantitative methods requires an appreciation of, if 
not actual facility in, the use of statistical methods and experi- 
mental design. Also, ecological problems frequently overlap those 
of applied fields such as agriculture, forestry, and range manage- 
ment. In addition to the terrestrial ecology with which we shall 
primarily concern ourselves in this text, there are the special fields 
of limnology, dealing with fresh-water environments, and marine 
ecology and oceanography with all their particular problems. 

These suggestions are indicative of the diversity of subject mat- 
ter included in ecology. Specialization is a natural and desirable 
result so long as it contributes to the ultimate goal of understand- 
ing the interrelationships of organisms and environment and to 
clarifying the natural laws under which the complex operates. 

HISTORICAL DEVELOPMENT OF PLANT ECOLOGY 

The origins of modern plant ecology are, of necessity, diverse. 
Designation of the limits and ranges of species by Linnaeus and 
other early systematic botanists led to the development of floris- 
tic plant geography, which considers the origin and spread of 
species. The next step was in the direction of explaining distribution 
of species. Humboldt, a taxonomist who was a great traveler, was 
impressed by the correlations with climate that he observed. As a 
result, he developed his ideas so effectively at the beginning of 
the nineteenth century 126 that the influence of his thinking is 
still apparent in the interpretations of climatic plant geography. 
Schouw, 214 one of Humboldt's students, was the first to attempt 
the formulation of laws regarding the effectiveness of light, mois- 
ture, and temperature in species distribution. Somewhat later 
(1855), still another taxonomist, A. de Candolle, published studies 
along this line but with major emphasis upon temperature as a 
controlling factor. Attempts to correlate vegetational distribution 
with single factors continued for several years and culminated in 
Merriam's 173 study of temperature zones for all of North America. 



1 6 THE STUDY OF PLANT COMMUNITIES • Chapter I 



The geographer's preoccupation with climatic causes for the 
distribution of species was paralleled by another significant trend 
of interest initiated by the writings of Grisebach in the nineteenth 
century. He recognized groups of plants, or communities, as units 
of study and described the vegetation of the earth on this basis. 111 
This was the first step in the direction of modern studies of plant 
communities. Although further expanded by the publications of 
Drude, 94 the trend received its greatest impetus from the writings 
of Warming, particularly his Oecology of Plants, 266 originally 
published in Danish in 1895. This publication marks the beginning 
of modern ecology as it is concerned with communities and the 
interrelationships of organisms and environment. Although Warm- 
ing must be credited with recognizing the complexity of these re- 
lationships, he tended to place too much stress on water as a con- 
trolling factor. In 1898, Schimper published his monumental Plant 
Geography upon a Physiological Basis, which was later (1903) 
translated into English from the German. Its author followed the 
general plan of presentation begun by Warming but contributed 
substantially from his broad experience and travels. He came near- 
er to the modern interpretation of causes of distribution of vege- 
tation by emphasizing the complexity of environment and the 
interraction of factors. 

These, briefly, are the foundations of modern community studies 
and the philosophy of modern ecology. From them stem studies 
of the structure and classification of communities as emphasized 
by continental European ecologists particularly, intensive studies 
of habitat in the search for causes, and analysis and interpretation 
of vegetational change as developed by American and English 
workers. The history of modern ecology is so brief that the last 
of these developments can hardly be treated historically. They are 
the fundamentals of ecology today and, therefore, will be consid- 
ered as part of the text material of this book. 

APPROACHES TO THE SUBJECT 

Considering the diversity of subject matter in ecology and the 

variety of possible interests, it is not surprising that problems have 

been studied in many different ways. Certain investigations must 

be made in the laboratory and others in the field. Some ecologists 



THE SUBJECT MATTER OF ECOLOGY 17 

have focused all their attention upon single factors; others have at- 
tempted to analyze the joint effect of several factors. 

Autecology and Synecology.— Certain problems can best be 
solved by working with individual organisms or species in the lab- 
oratory or in the field. Others can be solved only when the group- 
ings of organisms are investigated as they occur naturally. Similar- 
ly, the environment may be analyzed one factor at a time or 
considered in its entirety as a complex of several factors. Each 
approach has its merits under conditions that should become ap- 
parent later. The two are distinguished as autecology— from the 
Greek root autos meaning self— which deals with individual or- 
ganisms or factors, and synecology— from the Greek prefix syn 
meaning together— applied to studies of groups of organisms or to 
complexes of factors. 

Autecology is not always distinguishable from some kinds of 
physiology; in fact, there is probably no point in doing so. The 
very nature of autecology brings about overlapping with other 
fields. Autecology is, nevertheless, justifiable because of the con- 
tributions it can make to synecology. The latter is clearly a field in 
itself whose objectives make it distinct from all other fields of 
science. This is a partial reason for giving major consideration to 
synecology in this text and for bringing in autecology only when 
it contributes to the understanding of discussions of community 
problems. 

Static and Dynamic Viewpoints.— Plant communities may be 
studied as they are, without regard to what mav have preceded 
them or to what their natural future may be. This leads to con- 
sideration of the abundance and significance of the species making 
up the community and permits detailed descriptions and precise 
classification of communities according to one system or another. 
It is typical of the work of several early continental Europeans, 
who, as a result, developed systems of classifying and describing 
communities and their structure. In America and England, the 
view was early adopted that a community is a changing thing 
whose origin, development, and probable future can be recon- 
structed or predicted. These two approaches have come to repre- 
sent what are known as the static and dynamic points of view in 
community studies. The static approach is undoubtedly a product 



18 



THE STUDY OF PLANT COMMUNITIES " Chapter I 





, 




FIG 1. Communities of contrasting life form as illustrated by vegetation 
on Roan Mountain in the southern Appalachians. (1) Deciduous forest of 
beech and maple. (2) Portion of a grassy bald in which grasses and sedges 

of the restricted areas of study in Europe where civilization has 
long since destroyed or modified most natural communities. In the 
same way, the vast areas of virgin forest and grassland in America, 



THE SUBJECT MATTER OF ECOLOGY 



19 





i *V- ' 









predominate. (3) Portion of a shrub community made up largely of rhodo- 
dendron (open coniferous forest in background). (4) Moss community in 
which young conifers are becoming established.— Photos by D. M. Brown. 

with opportunities to observe natural variation on a large scale and 
under a variety of circumstances, must have contributed to de- 



20 THE STUDY OF PLANT COMMUNITIES • Chapter I 

velopment of the dynamic point of view. Undoubtedly each 
method has its place and usefulness. In fact, each has profited from 
the other, but, since the dynamic point of view has the broadest 
usefulness in both pure and applied ecology, it will be emphasized 
here. 

BACKGROUND FOR COMMUNITY STUDY 

Systems of description of vegetation that are based upon appear- 
ance or general nature of the plants have been used with some suc- 
cess, particularly by plant geographers. Such systems indicate size 
and form of plants; whether they are evergreen or deciduous, 
herbaceous or woody; 210 position of buds in the dormant season, 202 
and various other characters classified under the general headings 
of growth forms or life forms. This makes possible the visualization 
and superficial comparison of otherwise unfamiliar vegetation and 
likewise may serve to bring out certain characteristics of com- 
munities that otherwise might not be apparent. Such systems are 
either based upon previous detailed studies of the species, or they 
may be a means of superficially characterizing vegetation of which 
the taxonomy is still inadequately known. They can only supple- 
ment studies based upon taxonomy since description of a commu- 
nity, to be adequate for all purposes, must be based upon species. 
The field ecologist must, therefore, have a thorough working 
knowledge of taxonomy and, preferably, some experience with 
the flora of the region of his studies 

Just as the study of vegetation must remain more or less super- 
ficial without a solid taxonomic foundation, so will interpretations 
and explanations be limited by the amount of autecological infor- 
mation available about the species and their environments. Physi- 
ological-ecological investigations, in the field and under natural 
conditions, constantly modify synecological conclusions that have 
been made deductively, or they suggest new interpretations and 
investigations. The quality of community studies, therefore, de- 
pends upon certain fundamentals, which include a knowledge of 
the individual species and their requirements and responses. 



Part 2 • The Plant Community 



CHAPTER II 
NATURE OF THE COMMUNITY 

Recognition of a plant community or distinguishing one com- 
munity from another is probably simpler than recording the char- 
acteristics by which the community is recognizable. To refer to a 
stand of pine, a grassy field, or a lowland forest is, in a sense, rec- 
ognizing communities, and most of us have done this from child- 
hood. Such communities are the basic vegetational units of the 
ecologist, and, therefore, their specific and general characters 
should be stated to insure agreement as to concepts. 

DEFINITION 

A good working definition is as follows: A community is an 
aggregation of living organisms having mutual relationships among 
themselves and to their environment. This applies to the specific 
example which one has in mind or which one is observing— that is, 
the concrete community or stand. At the same time, it does not 
exclude the possibility of visualizing an abstract community syn- 
thesized from several or many concrete examples or stands. Thus 
a particular stand of pine would be a concrete community and the 
community in the abstract would include all the stands of that 
species. 

A stand need not be limited to trees. Any group of plants satis- 
fying the definition of a community may be so termed— a mat of 
lichens on a rock, covering only a few square inches, an algal mat 
on a pond, or a forest of fairly homogeneous composition extend- 
ing- over a thousand acres or more. 

MUTUAL RELATIONSHIPS AMONG ORGANISMS 

These include all the direct or indirect effects that organisms 
have upon each other. Foremost among these is competition, 

21 



22 THE STUDY OF PLANT COMMUNITIES • Chapter II 




FIG. 2. A stand of mixed conifers in Idaho— U. S. Forest Service. 



which results whenever several organisms require the same things 
in the same environment. The intensity of competition is deter- 
mined by the amount by which the demands exceed the supply. 
Competition may occur between individuals of the same species. 
Because they are alike, their demands are identical, and, if the sup- 
ply of water or nutrients or light is insufficient to satisfy the needs 
of all, then some will be eliminated. This is particularly notice- 
able in young, crowded forest stands but is equally true among 
roadside w r eeds or in a vegetable garden. All plants may survive 
for a time in a stunted condition; then some individuals are gradu- 
ally eliminated. Whether in the forest or in the garden, thinning 
to reduce competition between species usually pays with more 
lumber or better vegetables. 

Stratification.— Usually there are several species involved in 
competition within a stand. If plants of several species that start 
simultaneously make the same demands upon the habitat, they may 
survive in about equal numbers and occupy the same position in 
the community. Those whose requirements differ will affect each 



NATURE OF THE COMMUNITY 



23 



other less but will most certainly not be of equal importance in the 
community. A tall-growing species outgrows a potentially short 
one under the same conditions. If the latter then survives, it does 
so because its light requirements are not great. Thus the one tends 
to occupy a higher level than the other and to form an overstory. 
In this way stratification may develop in a stand in which the 
upper stratum of plants usually includes the controlling and char- 
acteristic species for the community. These are termed the dom- 
inant individuals. If they are removed for any reason, as by selec- 
tive cutting or disease, dominance is usually assumed by other 
species, and the character of the community is changed completely. 
This is not true when lesser species in subordinate strata are re- 
moved, for, with the dominants intact, the same type of commu- 
nity can regenerate itself. 

Stratification may likewise be seen among the shrubs and herbs 
beneath the trees, since some may be tall and some low. The lowest 




Fig. 3. A stand of moss (Hypnum crista-castrensis) on the forest floor in 
northern Wisconsin. Although this species is a dependent within the forest 
community, it forms a stand nevertheless— Photo by L. E. Anderson. 



24 THE STUDY OF PLANT COMMUNITIES ■ Chapter II 







FlG. 4. Very much overstocked stand of naturally seeded, eight-year-old, 
loblolly pine. Although many individuals will die in the next few years and 
thus produce natural thinning, the remaining trees will remain spindly and 
growth will not be satisfactory. Artificial thinning to reduce competition is 
apt to pay dividends in such stands.— Photo by C. F. Korstian. 




Fig. 5. Young loblolly pine stand, which was overstocked (left) for best 
growth. The stand was thinned experimentally soon after it was photo- 
graphed. Same stand (right) onlv two years after thinning, shows marked 
increase in size in the reduced number of trunks. The increase in rate of 
growth will be apparent for a number of years.— Photo by C. E Korstian. 



exposed stratum is made up of mosses, lichens, and sometimes 
algae, which may form a mat or ground cover on the forest floor, 



NATURE OF THE COMMUNITY 



25 



and a final stratum of fungi, bacteria, and algae in the upper layers 
of the soil can also be recognized. The species making up these 
lesser strata probably offer little direct competition to the trees 
above them. Most of these plants have appeared, and are able to 
survive, here because of conditions provided by the tree strata. 




FlG. 6. Stratification in an oak-hickory forest community as seen in spring 
when the subordinate tree stratum is especially marked by flowering of dog- 
wood and redbud.— Photo by H. L. Blomquist. 

Indirectly, however, they may offer serious competition to the 
continued dominance of the trees because, if the trees are to main- 
tain themselves in the community, they must be able to reproduce 
themselves. If the seedlings of tree species cannot meet the compe- 
tition of lesser species, whether it be in the herb or shrub stratum, 
such trees must eventually disappear from the community. Thus 
permanent or true dominance involves the ability to compete suc- 
cessfully in all strata of the community. The effects of competition 
are most apparent in the lesser strata, and undoubtedly competi- 
tion is greatest between the seedlings of species of all strata since 
all must start small and in the same restricted environment of the 
forest floor. 

Some ecologists maintain that each of these strata is itself a com- 
munity (synusia), which should be considered as a distinct unit of 
vegetation. Whether or not the strata are so recognized, they can- 
not be neglected in any study of communities. Often an under- 



26 THE STUDY OF PLANT COMMUNITIES * Chapter II 

standing of the community as a whole is possible only after infor- 
mation is complete on the individual strata. 

Dependence.— Within any community some species, although 
a part of the community, are at the same time dependent upon the 
whole for their survival. To a great extent, these are inconspicuous 
organisms, which, at first glance, might well be overlooked or 
ignored. Most of the bryophytes and thallophytes, as well as a few 
vascular plants, require the special conditions provided by larger 
seed plants; shade and moisture are usually of greatest importance 
to their survival. Such dependent organisms would soon disappear 
if the dominant vegetation were removed. 

Epiphytes grow on the trunks, the branches, and even on the 
leaves of the larger plants. In subtropical and tropical forests they 
may be conspicuous because of both size and abundance. In for- 
ests of temperate zones they may be easily overlooked, for they 
are usually mosses, liverworts, or lichens. These may be restricted 
to certain communities, and sometimes individual species will grow 
only on specific trees. Fungi, including bacteria, make up an im- 
portant part of many communities, especially forests. Here they 
may be parasitic and cause diseases that may at times become so 
serious as to destroy a stand or even to eliminate a community. 
Other saprophytic fungi, living in the soil or litter of the forest 
floor, although dependent upon the community, likewise contrib- 
ute to its perpetuation through their activities in decomposition of 
organic matter. Still others, again often host specific, live in an as- 
sociation with the roots of vascular plants in a relationship termed 
mycorhiza (see Fig. 91). 

Finally, animals, largely as dependents but also as influents, are 
likewise a part of the biotic community. Large species such as deer, 
which move about freely, are not necessarily associated with a 
single community. However, many smaller, less widely ranging 
species are definitely restricted to single communities, and even 
some birds and flying insects may be constantly associated with 
certain types of vegetation. Many beetles, borers, moths, etc. are 
extremely destructive parasites, while other similar small animals 
live on the remains of dead plants. The animals are apt to be re- 
lated to the community through food requirements and, if present 
in large numbers, may have extremely destructive effects. 



NATURE OF THE COMMUNITY 



27 




MUTUAL RELATIONSHIPS TO ENVIRONMENT 

Plants must be adapted to the environment in which they live if 
they are to survive for long. Some can withstand heat, some cold; 



28 THE STUDY OF PLANT COMMUNITIES ' Chapter II 

some require a large continuous supply of moisture, others require 
only a small amount which need be available only periodically. 
Thus the climate of a region definitely controls the kinds of plants 
that may grow there. The general vegetation type or growth 




FIG. 8. Spanish "moss" (Tillandsia usneoides), an epiphytic flowering 
plant, growing on live oak, North Carolina coast— Photo by H. L. Blomquist. 

form, such as grassland, desert, or forest, is a product of the com- 
plex of climatic factors effective in a region and can be used as a 
generalized basis for evaluating the climate. For example, knowing 
something of the growth forms able to survive under the extreme 
conditions of moisture and temperature associated with a desert, a 
repetition of these growth forms anywhere else in the world auto- 
matically may be accepted as indicative of desert conditions. The 
scrubby broad-leaved evergreens (chaparral) that cover much of 
southern California are a product of the climatic conditions pe- 



NATURE OF THE COMMUNITY 



29 



culiar to the area. The same growth form is repeated in a few 
widely separated regions of the world where, although made up 
of quite different species, it is a product of a similar complex of cli- 
matic conditions. In the same way the vast expanses of deciduous 
or coniferous forests in the temperate regions of the world are 
each found where climatic characteristics fall within definite 
limits, similar throughout. 

General Climate and Vegetation Type.— Within the general 







FlG. 9. Transition zones between stands of two life forms. The forest at 
right (mostly buckeye) shows the usual gradual transition from a closed 
stand to scattered, widely spaced individuals over a wide band— such as is 
typical of most transitions from one community to another. The abrupt 
transition from beech forest to grassland (at left) is unusual— Photo by D. 
M. Brown. 



vegetation type, certain variations may be expected. Species dif- 
ferences are not uncommon although the growth form may be 
uniform for all. Such differences are most pronounced when a 
type of growth form extends over a wide latitudinal range. In the 
arctic flora, which has an otherwise uniform physiognomy, the 
number of species declines steadily northward. Within the grass- 
land areas of the Middle West, there is obvious uniformity of 
growth form from Canada to Texas, yet some species found in the 
south are not found in the north and other species may be found 
only in the north. Even those species that seem to range from one 



30 TH E STUDY OF PLANT COMMUNITIES * Chapter II 

limit of a growth form to another may likewise have certain char- 
acteristics, probably physiological, which limit the extent of their 
area of favorable growth. Recently it has been shown that certain 
grasses that seem to range throughout the latitudinal extent of the 
prairie cannot be satisfactorily used to reseed northern areas when 
the seed has been obtained in the south. Foresters, too, recognize 
that it is advisable to replant with seedlings grown from locally 
produced seed. 

The more extreme (less favorable) the climatic conditions, the 
less diversity can there be in the species and the fewer the species 
will be because not many will have the adaptations necessary for 
their survival. The numbers of species in a general vegetation type 
are by no means constant throughout, especially nearing the limits 
of the type. Here it might be expected that conditions would be 
something less than optimum and that some species would be less 
well adapted to the extremes than others. The same can be said for 
numbers of individuals of a species. As conditions favoring a spe- 
cies vary from their maximum, the number of individuals may be 
expected likewise to fluctuate, and, near the limits of the range of 
a growth form, the numbers of individuals of that growth form 
would also decline. In the same sense, but in the opposite direction, 
this marginal area would support a few species and individuals of 
the contiguous growth form; thus transition zones between com- 
munities are characteristic. Sometimes these transitions are wide, 
sometimes relatively narrow, but rarely does one community, 
large or small, have a sharp line of demarcation between itself and 
its neighbor. 

Local Habitats and Species Differences.— Climatic areas are of 
considerable extent and usually include local diverse conditions of 
soil or topography. Often these variations are so great as to result 
in localized environments (habitats) quite unfavorable to the spe- 
cies and even to the growth form of the region as a whole. Often 
the conditions may be so much more favorable than those of the 
general climate that a growth form from a neighboring region can 
compete successfully. This is well illustrated by the trees and 
shrubs extending far into the prairie along the streams, where the 
favorable soil moisture is sufficient for them to compete success- 
fully in a grassland climate. 



NATURE OF THE COMMUNITY 



31 



A south-facing bluff forms a habitat that is almost always warm- 
er and drier than the average for the region, while a north-facing 
bluff is cooler and wetter. Barren exposures of rock or high, rocky 
ridges represent one extreme in local habitats, while flood plains 
of streams, lakes, and lake margins represent the other. Such habi- 




FlG. 10. Aerial view of the forest that extends along the meandering 
Sauris River far into the grassland of Nebraska —U. S. Forest Service. 



tats are bound to support numerous species that are not character- 
istic of the general climate and may even differ in their growth 
forms. These local variations may be extremely restricted in area, 
scarcely affecting the general physiognomic picture, as would be 
true of the vegetation around a spring, or on a boulder in the 
woods; but they may also be so extensive as to be misleading. 
Cypress or cypress-gum swamps in some sections of the southern 
states are so large that they might be viewed solely as a product 
of climate, especially where little drained land is near supporting 
upland vegetation. Many of the pine forests of New England and 
the other northern states lie in a climatic region where spruce and 
fir should eventually predominate. They are so extensively distrib- 
uted that some ecologists recognize them as the ultimate growth 
form for the region and as strictly controlled by climate, whereas, 
their occurrence within the climatic area is closely associated with 
light, sandv soils. 



32 THE STUDY OF PLANT COMMUNITIES ' Chapter II 



The kinds of plants, as to form and appearance, that can grow in 
a climatic region are, therefore, determined by the overall climate. 
The species within the general growth form may vary from place 
to place or from one limit of the climatic area to another as de- 
termined by local variations in some factors. Local habitats may 
have such marked differences in growing conditions that not only 
will species differ but even the growth form may not be that of 
the climate of the region. 



GENERAL REFERENCES* 

J. Braun-Blanquet. Plant Sociology : The Study of Plant Communities. 

A. E W SCHIMPER. Plant Geography upon a Physiological Basis. 

E. Warming. O ecology of Plants. 

J. E. Weaver and E E. Clements. Plant Ecology. 

*See References Cited on page 362 and following for complete listings. 



CHAPTER III 




VEGETATIONAL ANALYSIS 
QUANTITATIVE METHODS 

FIXING THE CONCEPT OF A COMMUNITY 
The fallacy of doing detailed physiological studies with an un- 
named plant is obvious. If the physiologist does not know the 
species with which he is working, his conclusions will be limited 
to the particular group of plants he is using in his experiments. 
The studies of taxonomists, floristic geographers, and geneticists 
represent an accumulation of information and data upon which 
the physiologist can draw and which he can use to make general- 
izations and comparisons. All this information is connoted by the 
scientific name of the plant being studied. 

The ecologist, although working with communities, deals with 
problems similar to those of the physiologist when he sets up the- 
ories, attempts to find causes, to draw conclusions, or to formulate 
laws. But the ecologist is faced with the necessity of determining 
the make-up of the community with which he works before he 
can proceed to an investigation of causes or to experimental con- 
siderations. At present, most of the larger, regional, climatic vege- 
tation types are so well known that their concepts are probably as 
distinct to the ecologist as are those of most common species to 
the taxonomist. For lesser communities, however, this is not true. 
Furthermore, identification of such a community in terms of a 
specific concept requires more than a superficial examination. Per- 
haps an ecological classification of plant communities will never be 
achieved with the same degree of perfection found in taxonomic 
classification; perhaps such perfection is not necessary. It is neces- 
sary, however, that there be means of characterizing a community 
with sufficient accuracy to permit identification at any time, to 
compare it with other similar communities, and to have an ade- 
quate permanent record of its nature and occurrence. Undoubted- 
ly, if such work is well done, it is justified on its own merits as a 
phase of ecological investigation. 

33 



34 the stud y of plant communities * Chapter HI 

If the major interest in a community is an experimental one and 
the preliminary analysis and description of the vegetation have not 
previously been made, the experimenter must first learn and re- 
cord the characteristics of the community with which he intends 
to work. Again, after experimentation or treatment, whether it be 
of the community as a whole or of individual species, it often be- 
comes necessary to evaluate the results in terms of the community 
as a whole. There must also be a means of comparing the original 
and the resulting communities at the beginning and at the end of 
each experiment or treatment. The relationship of the individual 
species to the community and the responses of the individual spe- 
cies can best be interpreted when the constitution of the entire 
community is positively established. 

It is illogical to proceed with explanations when the subject it- 
self is indefinite or unknown. Therefore, the first objective in 
ecological work is to learn the composition and structure of the 
community under consideration. Then, and only then, logically 
follow a search for causes, experimentation, and interpretations 
based upon a firm foundation. 

QUANTITATIVE DATA A NECESSITY 

In the early days of ecology, observation and description were 
considered adequate for recording the characteristics of a com- 
munity, but few observers see the same thing in the same way, and 
few writers have the ability to translate exactly into words the 
things they have seen. Thus, as in other sciences, ecology has be- 
come more precise as it has developed and, with its concern for 
greater detail, has demanded accurate measurement and precise 
records of vegetation. This has led naturally to quantitative meth- 
ods and terminology, which are becoming more uniform and, 
therefore, more useful. Their use permits positive statements con- 
cerning the numbers and sizes of individuals as well as the space 
they occupy within a stand. With such data in hand, it is possible 
to make comparisons of species or groups of species within a stand 
or between stands. Likewise, the data constitute a permanent rec- 
ord, which can be referred to again if the same stand or similar 
stands are studied later. Also, as a permanent record, they are sub- 



VEGETATIOXAL ANALYSIS 35 



ject to reconsideration by other investigators, who may reinter- 
pret them in the light of additional experience or information. 

SAMPLING 

The need for quantitative records has made it necessary to give 
serious consideration to methods of sampling. Usually the mem- 
bers of an entire community cannot be counted or measured, and 
even if this were done, the information would be no more useful 
or significant than an adequate set of data acquired by proper sam- 
pling. Since this is true, it becomes of prime importance to deter- 
mine what constitutes an adequate sample in terms of the commu- 
nity as a whole and how to obtain such a sample with a minimum of 
effort. At best, sampling for vegetational data is tedious and time- 
consuming; often it may be extremely hard work. Nevertheless, 
sampling conserves both time and labor as compared with an at- 
tempt to analyze a whole community, and its results are much 
more significant than those obtained by mere observation. 

In this connection it should be emphasized that the early pro- 
cedures of observation and reconnaissance are still of extreme im- 
portance in determining where, how, and what to sample. These 
activities are still a necessary part of community study although 
they cannot be substituted for detailed analysis. They serve to 
form a basis for theories or ideas that may in turn be substantiated 
by quantitative evidence obtained by sampling. Preliminary recon- 
naissance may likewise help to reduce the effort expended in sam- 
pling. No sampling should be done without a thorough knowledge 
of the history, physiography, and vegetation of the region as a 
whole. Prior to sampling, the community should have been ob- 
served repeatedly in different parts of its range and more particu- 
larly under the varying local conditions where it exists. Finally, 
the specific stand should be observed thoroughly to determine its 
obvious variations, its extent, limits, and transitions to contiguous 
communities. Then, knowing all this, together with the size of in- 
dividual plants, the strata present, and the purposes for which the 
sampling is to be done, one may plan his procedure in terms of the 
desired results, the necessary degree of accuracy, and the time 
available for doing the work. 

Ecologists call a sample area or plot a quadrat, and the method 



36 THE STUDY OF PLANT COMMUNITIES * Chapter 111 

of sampling by the use of plots is commonly called the quadrat 
method. The use of the sample plot is by no means restricted to 
ecology, but its application in the sampling of natural vegetation 
has led to methods peculiarly adapted to the ecologist's needs. The 
quadrat has almost unlimited applications and has been used in a 
great variety of ways. 

Kinds of Quadrats.— The list-count quadrat is probably most 
commonly used. With this the species are recorded and their num- 




FlG. 11. A small quadrat laid out with meter sticks, which are pinned at 
corners. Ready for list-count. This is a permanent quadrat that can be relo- 
cated by paint markings on boulders. At Glacier Bay, Alaska, for the study 
of early development of vegetation on raw morainic soil. 75 Ice covered this 
area thirty-seven years before picture was taken.— Photo by W. S. Cooper. 

bers determined by count. This method is subject to many mod- 
ifications depending upon circumstances. For trees, the individual 
diameters might be recorded and later used for segregating size 
classes, or perhaps for computing basal area (indicative of dom- 
inance) for species. Bunch grasses, too, are often measured across 
the base to obtain a basal area figure, which, combined with the 
count, will give a better expression of the relative importance of 
species. With herbs it is sometimes desirable to have additional in- 
formation on the weight of tops, which must, therefore, be re- 
moved for each species. In any event, the species are listed and 
tabulated by number, weight, or size. 
A chart quadrat is a more detailed record of the individuals 



VEGETATIOXAL ANALYSIS 



37 




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38 THE STUDY OF PLANT COMMUNITIES • Chapter HI 



TRENCHING STUOY "2. SHORTLEAF PINE TYPE 
CONTROL PLOT 9-12-34 




A = Andropogon ternanuS 
Ad = Aster dumosus 
Ca = Catnenna angustifoiia 
Cp = Ciadonia pyxidata 
C = Ciador..a sylvatica 
= Oicranum scoparium 
Ds = Oicranum spunum 
•Ov= Diospyros virginiana 



0.35 «Pe 
E = Eupatonum hyssopifolium 
G = Grass seeding 
H = Herbaceous seedling 
L = Lespedeza repens 
Oe = Oenothera longipedicellata 
Ox = Oxahs stncta 
P = Pamcum sphaerocarpon 
Scale 



»Pe = Pmus echinata 

Pq = Psedera qumquefoiia 

•Q = Quercus velutina 
SB = Smilax bona-nox 
S = Sohdago nemorahs 
T b Thuidium dehcatulum 

•U = Uimus alata 
V = Viburnum afhne 



0.66 leet 



FIG. 13. The system of mapping used in the study illustrated in Figure 12. 
Such a procedure is adaptable to many situations/" 



146 



present, giving their size and distribution within the area. This is 
usually time-consuming, even on small quadrats with a relatively- 
simple arrangement and few species. It does, however, permit 
study at a later date— an advantage not to be ignored under many 
circumstances. Small quadrats may be photographed with consid- 
erable success if proper equipment can be brought to them con- 
veniently. Camera stands of various sorts have been designed that 
permit vertical views, and the photographs can be studied at leis- 



VEGETATIONAL ANALYSIS 



39 



ure. Fairly accurate coverage for individual species can be deter- 
mined from the prints with a planimeter (a mechanical device for 
determining the area of a surface with irregular boundaries). Such 
records are particularly useful when the areas are to be studied 
over a period of time and when they are subject to treatment. 




FlG. 14. A4apping a quadrat by the use of a pantograph, which reduces all 
details to scale.— U. S. Forest Service. 

When a high degree of accuracy is desired for small plots, a panto- 
graph 193 can be used with a drawing board, or sketching on co- 
ordinate paper may be quite satisfactory, especially if the quadrat 
itself is marked off into a grid pattern, as with strings. For small 
quadrats of low or matted vegetation, a rigid frame permanently 
rigged with fine cross wires to form a grid (see Fig. 12) can be 
used to advantage since it can be moved from place to place, thus 
saving the time of marking off" each new quadrat. 25 Small quadrats 
in relatively tall herbaceous vegetation or among shrubs and sap- 
lings can be laid out more easily with rods or wooden strips cut to 
proper length than with tapes (see Fig. 11). There are times when 
the accurate measurement or recording of cover is too time-con- 
suming or is not actually necessary. Estimation of cover merely 
by inspection of each plot can be done with considerable accuracy 
after only a little experience, and such an estimate may be suffi- 
cient for the objectives. 



40 



THE STUDY OF PLANT COMMUNITIES ■ Chapter HI 



The use of permanent quadrats has been advocated by many 
ecologists, but few have followed their own excellent advice. 
Whenever there is a remote possibility that a sampling area may 
again be visited for further study, the quadrats should be marked 
with permanent markers, for surprisingly worth-while results may 




FlG. 15. Paired pictures illustrating slow development of vegetation on 
»cks on Isle Royale. Lower picture taken seventeen years after upper.— 



rocks on Isle Royale. Lower picture 
Photos by W. S. Cooper™ 

be obtained by restudying identical areas after a period of years. 
Such results are often valuable out of all proportion to the effort 
required, especially when compared to the initial study. Most 
quadrat studies are planned for immediate results and to help solve 
problems of the moment, but with little extra effort they could be 
used to yield returns over a period of years. Actually it would be 
well to consider the possibility of making every quadrat permanent. 
When Dr. W S. Cooper made his now widely known study 
of vegetation on Isle Royale in Lake Superior, he photographed 
his sampling areas and carefullv marked the spots even though he 
had no definite plan for restudying the area. Seventeen years later 



VEGETATIONAL ANALYSIS 44 

he was able to relocate these points exactly, and he obtained a 
striking series of matched pictures illustrating the development of 
each of the vegetation types on the island. 74 A number of similar 
illustrations could be mentioned, but they are far too few. 

Marking such plots when far afield may be something of a 
problem, but by forehanded thoughtfulness combined with in- 
genuity an adequate plan can usually be devised. A small can of 
paint is no great burden when added to regular field equipment, 
and its judicious use in conjunction with blazed trees, rock cairns, 
or the like will usually suffice (see Fig. 11). It should be added 
that experience indicates the advisability of recording in one's 
notes a careful description of the markers and their exact posi- 
tion with reference to landmarks of a permanent nature. 

Quadrats originally set up for permanent study are usually of 
an experimental nature. Perhaps they are to be subject to a treat- 
ment of some sort, as, for example, different degrees of grazing, 
watering, or thinning. For acceptable results these must always be 
laid out in pairs so that an untreated plot can be used as a check 
or control on the treated area. Usually it is desirable to replicate 
the pairs one or more times, and this must be given serious thought 
in terms of the extent of the stand and uniformity of conditions. 
Such experimental areas are often established near at hand and in 
easily accessible places, for they are to be visited regularly. With 
plans made in advance, materials for permanent marking are among 
the first equipment to be assembled. Substantial lengths of old pipe 
or scrap metal, when driven into the ground leaving a few inches 
protruding, are permanent and very satisfactory markers. If they 
are painted conspicuously and marked with numbers, there can be 
no confusion. 

Experimental quadrats are of many types. Studies of competi- 
tion and survival may involve thinning of stands, eliminating unde- 
sirable species, or introducing other species, either by seeding or 
planting seedlings, the object being to observe effects on the com- 
munity or the introduced species. Newly exposed bare areas may 
be studied to follow the natural development of vegetation, or 
areas may be denuded and attempts made to produce artificial 
communities. Perhaps the quadrats are used to evaluate the effects 
of some controlled factor such as artificial watering or shading or 



42 THE STUDY OF PLANT COMMUNITIES ■ Chapter HI 




the application of a fertilizer. Again, animals may be the factor 
under consideration, and then exclosures of the vegetation or the 
animals will be necessary, depending upon objectives. 84 Exclosures 
should not be considered lightly, for their installation may require 



VEGETATIONAL ANALYSIS 43 



considerable time and labor. Also certain types of materials may 
be surprisingly expensive, especially if plots are replicated. If the 
effects of grazing are to be studied, a barbed-wire fence will keep 
out cattle, but rabbits must also be considered. They may be at- 
tracted by the very things that nourish within the exclosure after 
the cattle are kept out. Again, small plots may be fenced for rab- 
bits and yet permit squirrels or birds to come in over the top. 
Then the entire plot must be covered. Lesser rodents may go 
through or tunnel under the wire, and suitable precautions must 
be taken to check them. 

The effects of the exclosure itself upon the vegetation should 
not be ignored since it may serve as a windbreak, which may re- 
duce transpiration and intercept snow, soil, and seeds. Small plots 
completely screened over will have quite a different micro-climate 
from unscreened areas. To hold constant a single variable within 
an exclosure is difficult, but it can be approached by having ex- 
closures as large as possible, by insuring a liberal transition or isola- 
tion strip around the margin, which will not be used in sampling, 
and by having the barriers as low and as open as possible within 
the limitations of the experiment. 

Quadrat Methods —Actually the unit sampling area can be any 
shape or size, and any number can be used in a variety of ways, 
depending upon circumstances and objectives. As one soon learns, 
the major concern is to get adequate data with a minimum of 
effort. Because vegetation is so variable, generalizations cannot be 
made to fit all situations. Because objectives are rarely the same, 
methods quite satisfactory in one instance may not be so in another. 
Set rules are not advisable for sampling, but certain generalizations 
may well be considered in the light of experience. 

Shape of Quadrat.— The term, quadrat, implies a square, and 
this shape is undoubtedly more commonly used by ecoloo-ists than 
any other. This is probably a matter of habit, for other shapes are 
just as usable and sometimes more efficient. When Raunkiaer 202 
was making his pioneer studies of frequency, he at first used a 
square frame for marking his sample areas but later used a circle 
exclusively because of its convenience. He wished to have data 
from many small quadrats that were randomly distributed. For 
marking, he used a rod to which a stick was attached at right angles 



44 TH E STUDY OF PLANT COMMUNITIES * Chapter III 

to form the desired radius. The rod was thrust into the ground at 
the sampling point and rotated so that the stick marked the limits 
of the circle to be sampled. He said, "The most convenient forms 
and sizes of the unit areas are the best!' With low vegetation, 
circular plots are sometimes a distinct advantage. An efficient 
means of laying out circular plots is to use a set of hoops or rings 
of proper size tossed in all directions from a central point. These 
cannot be used in tall vegetation of any kind since they may be 
obstructed when thrown or may be suspended above the ground. 
Larger circular plots can be quickly and accurately marked with a 
string attached to a free-turning ring on a central axis. Again, this 
method will not be found satisfactory where vegetation is more 
than waist high. 

It has been demonstrated that a rectangular plot is significantly 
more efficient in sampling than a square one of equal area since it 
will tend to include a better representation of the variation in the 
stand. Clapham, 55 who worked with low herbaceous vegetation in 
his study of this problem, concluded that plots l A x 4m. were the 
most efficient in size and that to secure the same amount of infor- 
mation with squares as with strips nearly twice as large an area 
would have to be observed. Short strips (1:4) gave less variable 
data than squares but more variable than long strips (1:16). The 
same general conclusions were reached after studies of certain 
types of sagebrush-grass range sampling. 194 

Size and Number of Quadrats —A. community is rarely homo* 
geneous throughout as to species and their distribution. Newly 
formed habitats, such as sandbars or tidal flats where often only a 
single species is a pioneer, may support a nearly homogeneous 
stand, but the usual community will have some variation. If there 
were no variation, a single relatively small sample would always be 
sufficient. Since variation is the rule, it becomes necessary to have 
samples large enough or numerous enough to include the variation 
and to have it fairly represented in the data. There is thus always 
a question of how large and how numerous the quadrats should be 
for adequate sampling. 

The literature dealing with this problem is far too extensive to 
review here. Agreement has not been reached on all phases of sam- 
pling methods, and probably different methods will be advocated 



VEGETATIONAL ANALYSIS 45 

for some time to come. Several recent papers summarize ideas and 
analyze the problem in detail. Their extensive bibliographies will 
soon lead one to the conclusions of a variety of workers. Cain's 
publications 43 > 49 have done much to clarify methods of determin- 
ing sample sizes and numbers both through his own contributions 
of methods and their applications as well as through his summaries 
of the literature. Penfound 196 has brought together and analyzed 
the usefulness of several currently favored procedures. 

Species : area curves have been used in a variety of ways. Orig- 
inally used by European ecologists to determine the "minimal 
area" to be recognized as an "association individual" (^ stand), they 
have been equally useful in arriving at numbers or sizes of plots to 
be used in sampling individual stands. A characteristic curve will 
result from plotting the number of species obtained against the 
area sampled. The accumulated number of species found may be 
expressed as a percentage of the total or as an absolute number and 
plotted on the y axis. When the corresponding numbers of plots, 
or sizes of area sampled, are plotted on the x axis, the curve formed 
by the joined points will rise abruptly with first increases in area, 
but will soon level off, and tend to rise only slightly thereafter 
with increase of sampling area. It is assumed that the added infor- 
mation represented in the slight rise of the curve is not sufficient 
to justify the time and effort needed for the extra sampling. There- 
fore, for this same type of vegetation, the sampling is assumed to 
be adequate when the size of the sample somewhat exceeds the 
area plotted against the point at which the curve flattens strongly. 

It is of interest that, when the ratio of the x to the y axis is 
shifted, it will result in a change in the form of the curve and a 
consequent shift in the position where the curve tends to flatten. 
This suggested the desirability of some means other than inspec- 
tion for determining this point. Cain 46 suggests that, in terms of 
his experience, sampling is adequate when a 10 percent increase 
in sample area results in an increase of species equaling 10 percent 
of the total present. He suggests a mechanical means of determin- 
ing this point on the curve regardless of the ratio of the x and y 
axes. When a triangle is placed so that one edge passes through the 
zero point and the point representing 10 percent of the area and 
10 percent of the species, the triangle can be pushed upward along 



46 THE STUDY OF PLANT COMMUNITIES * Chapter 111 



a ruler placed at the right until its lower edge describes a tangent 
to the curve. The point of the tangent is the center of the region 
where the 10 percent relationship holds. If greater accuracy is de- 
sired, the minimal area could be placed at the point of 5 percent 
rise for a 10 percent increase in sampling. 

By another procedure, 49 the ratio of the x-y axes can be ignored, 
a sample size or number can be selected, and a value set upon the 
sampling. If the total number of species obtained in the sampling 
is divided by the total number of sample units, the average incre- 
ment of new species per additional sample unit is obtained. The 
point on the curve is located (point A), in the region of which ad- 
dition of a unit sample produces an increment of species equal to 




Oak- Hickory Forest 
piedmont of n.c. 

MINIMUM NUMBER OF 10 X I0M. 
FOR SAMPLING TREE STRATA 



8 10 12 14 16 18 20 22 

NUMBER OF 10 X 10 M. QUADRATS 



24 26 28 30 




C 



Oak-Hickory Forest 
piedmont of n.c. 

MINIMUM NUMBER 0F4X4M. SHRUB 
AND TRANSGRESSIVE STRATA 



8 10 12 14 16 18 20 22 

NUMBER OF 4X4M. QUADRATS 



24 26 



28 



30 



FlG. 17. Species : area curves for an oak-hickory forest, (A) indicating a 
minimum of six 10 by 10 m. quadrats for sampling the arborescent strata, and 
(B) a minimum of ten 4 by 4 m. quadrats for sampling the transgressive and 
shrub strata. (C) A dune grassland community required a quadrat of hot less 



VEGETATIONAL ANALYSIS 



47 



the average increment. Beyond this point addition of samples will 
yield progressively less than the average. In the region of point B 
a sample yields only one-half the information and at point C only 
one-quarter the information obtained by a sample at point A. 
Used in combination with the tangent procedure, this should be 
helpful in interpretation of the species : area curve and the selec- 
tion of numbers or sizes of plots most suited to a vegetation type. 
If a series of quadrats of an arbitrarily set size is run in a stand, 
a species : area curve constructed from the data will indicate how 
many such quadrats would have been necessary for sampling to 
achieve a desired accuracy. The most efficient size of plot to be 
used can likewise be determined from the same preliminary series 




Dune Grassland. N.C 

MINIMUM QUADRAT SIZE IF 25 ARE USED 



SIZE OF QUADRATS SQ.M. 




Dune Grassland, N.C. 

MINIMUM NUMBER OF ^ SQ.M. 
QUADRATS 



10 15 

NUMBER OF i SQ.M. QUADRATS 



20 



25 



than % sq. m. and (D) a minimum of six such samples. The lines tangent to 
the curves were put in using Cain's triangle method described on page 000. 
In (B), point a is equivalent to the average increment per sample, at point b 
the yield is only one-half this increment, and at point c only one-quarter the 
increment. 



48 THE STUDY OF PLANT COMMUNITIES ■ Chapter III 



of data if each quadrat is subdivided into successively smaller plots 
(e.g. : 1, /z, 54, Vs sq. m.) for which the records are kept sep- 
arately. The data obtained from the smallest area then become a 
part of those for the next larger area, and so on. When the number 
of species is plotted against increase in area sampled, the usual 
curve is formed. The information regarding numbers and sizes of 
10m 4m 



4 

2 



2 





1 







\ 


1 


2 


\ 


k 



a b e 

FIG. 18. Nested quadrats. (A) shows a plan used successfully for sampling 
the several strata in forest stands. (B) and (C) show systems of dividing plots 
of any size for accumulating data to be used in determining the desirable size 
of plot by means of species : area curves. 

plots is then applied to sampling of similar or closely related com- 
munities. The procedure for determination of numbers and sizes 
of plots is well illustrated by Cain's study of sample-plot tech- 
niques applied to alpine vegetation in Wyoming. 49 

When vegetation is stratified, a series of sample plots large 
enough to include the trees will certainly be large enough for all 
plants and strata. The work involved in measuring or counting the 
lesser vegetation in such plots, however, would be unnecessarily 
great. It, therefore, becomes advisable to sample each stratum sep- 
arately with an appropriate size of plot for each. These plots can 
be "nested" one within the other and the work thus materially 
reduced. Sampling forest vegetation in the Piedmont area of North 
Carolina has been done satisfactorily by using 10 x 10 m. plots for 
trees, 4 x 4 m. plots for all other woody vegetation up to ten feet 
tall, and 1 x 1 m. plots for herbs. 183 By separating the data for 
trees into overstory and understory individuals and by recording 
separately those woody plants less than one foot tall and those 
from one to ten feet tall, five strata were distinguished. More 
might be necessary or advisable under other conditions. 



VEGETATIONAL ANALYSIS 



49 



In general, it may be said that small plots require less work than 
large plots, both in the laying out and in the obtaining of data, 
even though more small plots than large ones are needed for com- 
plete sampling. At the same time, there is a further saving of effort 
in that the total area sampled by small plots may usually be less 
than that sampled by large plots and yet give comparably valuable 
information. 

Distribution of Quadrats .—When the size, shape, and numbers 
of quadrats have been determined, there still remains the question 
of how they are to be placed efficiently and in such a fashion that 
they will give representative data for the stand as a whole. If a 
stand had a perfectly homogeneous composition, it would make no 
difference where the sampling was done, but this is rarely, if ever, 
true. Differences in the soil, drainage, and topography are usually 
present and are reflected in the vegetation. These variations must 
be fairly represented in the sample. It becomes necessary, there- 






FlG. 19. The distribution of quadrats in a stand according to three differ- 
ent systems. (A) Random distribution as determined by Tippett's numbers. 252 * 
(B) Spaced as widely and evenly as possible by survey and measurement. (C) 
Distributed evenly along lines run by compass or sighting; spacino- deter- 
mined by pacing. 

fore, to distribute the quadrats throughout the stand, and a plan 
that will eliminate the human factor in placing the individual plots 
is desirable. 

The statistician prefers a sampling system that gives him data 
obtained at random. 216 This demands a division of the entire stand 
into possible sampling areas and then a selection of actual sampling 
areas determined strictly by chance. Under such conditions, the 



50 THE STUDY OF PLANT COMMUNITIES * Chapter III 

statistician is able to express mathematically how good his sam- 
pling may be. Such a method frequently brings several sampling 
areas into close proximity at the same time that wide areas are left 
unsampled. Within these wide areas, there are very likely to occur 
a number of infrequent or unusual species in small numbers, which 
would be of little concern in a statistical treatment but whose 
presence could be of great interest to the ecologist. For him, it is 
usually desirable to have as many of the variations as possible rep- 
resented in his data because they are subject to interpretation in 
terms of experience and the nature of related communities. For 
such purposes, statistical methods are often of little help. It is, 
therefore, probable that quadrats distributed systematically 
throughout the stand as evenly and widely as possible are quite 
satisfactory for most ecological sampling. In fact, systematic sam- 
pling is likely to be better than random sampling for certain eco- 
logical purposes. 

Any method that will insure wide and even distribution of sam- 
ples should be satisfactory. The limits and extent of the stand must 
first be ascertained, and sampling plans made accordingly. Once 
the plan is made, it should be followed rigidly unless some previ- 
ously unknown irregularity, like a swamp or an outcrop of rock, 
should fall within a sample. 

In small stands it is possible to plan a grid pattern and to sample 
at regular intervals in this pattern. When stands are large but of 
reasonable uniformity, it is common practice to run one or more 
lines across the greatest extent and to space the quadrats evenly 
along these lines. It would appear that the more widely the plots 
are spaced in an area to be sampled the greater the efficiency of the 
sampling unit, provided the spacing is not so great as to make 
correlation negligible between adjacent plots. 194 Under some con- 
ditions, it may be desirable to run the lines with a surveyor's 
transit, although a compass line will usually suffice, and in open 
country it is possible to run them by sighting on some landmark. 
The spacing may sometimes require accurate measurement, but 
pacing may serve quite satisfactorily. The important thing is to 
avoid any method bordering on personal judgment in placing the 
plots once the sampling is under way. This should be remembered 
particularly when the sampling is being done to prove or disprove 



VEGETATIONAL ANALYSIS 



51 



a point. Under such conditions, there is often a strong temptation 
to shift a plot a few feet or more to include or exclude a desired or 
undesired species or condition. 




FlG. 20. Diagrammatic profile along a transect on the dunes at Ft. Macon, 
N. C. Physiographic-vegetational zones are indicated. Transect was 110 
meters long and horizontal scale is one-half the vertical. 



188 




102 t 103 

PEAK OF REAR DUNE 



. FlG. 21. Portion of field-mapped transect along profile shown in Figure 
20 from 97 m. through 104 m. across the transition from Zone 4 to Zone 5, 
where dominance changes from Andropogon to Uniola. The symbols indi- 
cate A—Andropogon, U— Uniola, H—Heterotheca, C—Cenchrus, Oe— Oeno- 
thera, L—Leptilon. Such a map gives accurate quantitative data for each spe- 
cies as well as a visual record of changes ki vegetation associated with habitat. 
See Table 1. 

Transects— A transect is a sampling strip extending across a 
stand or several stands. It is most often used when differences in 
vegetation are apparent and are to be correlated with one or more 
factors that differ between two points. From a flood plain of a 
river to the adjacent upland there would be marked changes in 
moisture conditions, and in such a place a transect can be useful 
for determining the range of moisture requirements of individual 



52 THE STUDY O F PLANT COMMUNITIES * Chapter III 

TABLE 1. Average density (D) and cover (C), by zones, of principal 
species mapped on a transect from high tide to the crest of the rear dune at 
Ft. Macon, N. C. (see Fig. 20). Both cover and density values show the pre- 
dominance of Uniola in exposed zones and of Andropogon in protected ones. 
This is correlated with salt spray. 188 





ones 






Transect I 






z 


I 


// 


III 


Ilia 


IV 


V 


Uniola paniculata L. 


D 
C 


11.8 

2.7 


7.5 
6.9 


3.3 
0.3 


0.4 
0.03 


4.0 
9.1 


10.6 
18.9 


Andropogon littoralis Nash 


D 
C 


2.1 

2.2 


2.5 
7.1 


5.7 
7.3 


6.6 
15.4 


4.1 

5.5 


0.9 

2.3 


Oenothera humifusa Nutt. 


D 
C 


1.1 
0.06 


1.8 


7.2 


1.8 


6.5 
0.9 


4.5 


Heterotheca subaxillaris (Lam.) 
Britt. and Rose 


D 
C 


4.3 
0.5 


0.5 
0.1 


0.5 


0.2 


1.1 


0.8 


Leptilon canadense (L.) Britton 


D 


0.4 


1.2 


15.1 


0.06 


6.2 


5.1 


Euphorbia polygonifolia L. 


D 


0.1 


0.3 


0.3 




0.2 




Fimbristylis castanea (Michx.) 
Vahl ' 


D 
C 






1.4 
0.2 


11.6 
16.6 


1.9 
0.1 




Myrica cerifera L. 


D 
C 








0.5 
14.7 







species. Transects are also useful in altitudinal studies and in any 
situation where transitions between communities occur. 

Sizes of transects, just as sizes of quadrats, will be determined 
by conditions. A transect reaching from one small community to 
another, across a transition zone, might need to be only a few 
meters long and perhaps a meter or less in width. Transects from 
lake margins across the several marginal girdles of vegetation that 
are usually present might be much longer. One reaching from 
high-tide mark across seaside dunes might be several hundred 
meters long. A study of the zonation of vegetation on the Sierra 
Nevada was made by mapping a transect seven miles wide and ex- 
tending across the mountain range for a distance of eighty miles. 144 

When it seems desirable to map an entire transect in detail, it is 



VEGETATIONAL ANALYSIS 



53 



advisable to do so by blocks. Values for each block may then be 
conveniently used as quadrat data, an additional means of analysis 
and expression of results. A variation of the transect is the method 
of sampling a unit area at regular intervals along a line. These inter- 
vals may be determined by distance or altitude. Such records 
taken on several lines are particularly helpful in mapping several 
vegetation types that intergrade irregularly over an extensive area. 
In the early land surveys of the northern and midwestern states, it 
was required that the characteristic trees be listed in the records 
for definite intervals along the lines run by the surveyors. Since 
the county and township lines they established still stand, it has 
been possible to reconstruct with considerable accuracy the com- 
position of the forests as they then existed as well as the limits of 




FlG. 22. Forest associations of southwestern Michigan as reconstructed from 
the field notes of the old land survey. Unshaded areas, marked B, beech- 
maple forest; X = hemlocks, constituting, along lake shore, a codominant 
with beech and maple; O = white pines (a mark for each locality of occur- 
rence noted in the survey); horizontally shaded areas, oak-hickory forest; 
obliquely shaded areas, oak-pine forest; stippled areas, dry prairies; and ver- 
tically shaded areas, swamp associations.— From Ke?Joyer. i3i) 



54 the study OF plant communities ■ Chapter HI 



forest and grassland. 139 These surveyors' "transects" were some of 
the first and longest ever run. 

Sometimes there is an advantage in the use of "line transects" in 
which the species are tabulated as they occur along a line. The 
method is adaptable to the determination of numerical abundance, 
frequency, coverage, and other characteristics. It has the advan- 
tage of speed and apparently gives accurate information, consider- 
ing the time it requires. It is particularly useful in dense stands of 
scrubby vegetation, which would be very difficult to sample with 
quadrats. Determinations of cover in dense chaparral using line 
transects gave results that compared very favorably with those ob- 
tained by complete charting, although the transects were made in 
a small fraction of the time required for the detailed procedure. 13 



r-130 



-loo 




Rain Forest Swamp Forest 

Fig. 23. Profile diagrams (bisects) of two types of tropical forest. Note 
that difference in height of trees and in form of trunk is well shown and that 
rain forest has three distinct strata of trees but swamp forest has essentially 
one.— After Beard. 15 

Bisects.— These are variations of transects in that they are sam- 
ple strips aiming to show the vertical distribution of vegetation. 
Thus they may include stratification and layer communities from 
dominant trees to seedlings on the forest floor and, in addition, 
show the stratification and root distribution of these same plants 
below ground. 

GENERAL REFERENCES 

S. A. CAIN. The Species- Area Curve. 

S. A. CAIN. Sample-Plot Technique Applied to Alpine Vegetation in Wyo. 
F. X. Schumacher and R. A. Chapman. Sampling Methods in Forestry 
and Range Management. 



CHAPTER IV 

VEGETATIONAL ANALYSIS 
PHYTOSOCIOLOGICAL OBJECTIVES 

The interest of European workers in community structure, 
their desire to describe communities precisely, and their concern 
with systems of classifying communities resulted in the develop- 
ment of a phase of ecology known as phytosociology. Its develop- 
ment was paralleled by (1), the growth of systems of terminology 
with which the characteristics of a community could be adequate- 
ly expressed, and (2), the testing and refinement of methods for 
obtaining quantitative data on the structure and composition of a 
community to support the systems of description. 

Phytosociological methods and terminology have become pro- 
gressively more standardized, but, as yet, there is not complete 
agreement among workers. The problems to be resolved are still 
of the same nature as those of earlier days as is illustrated by a re- 
cent characterization, 196 which groups them into two categories : 
(a) the size and number of quadrats to be utilized and (b) the 
conditions to be investigated. The first we have discussed at some 
length as a part of quantitative methods in community analysis. It 
should be remembered that the development of these methods has 
been strongly influenced by phytosociological interests. Although 
the quadrat method in ecology had its origins in America, its 
adaptation and refinement for complete analysis and description 
of communities must be largely credited to European workers. 

What phytosociological values are necessary for an adequate 
characterization of a community would hardly be agreed upon by 
all workers even today. Through the years this has been the subject 
of much debate. Some early workers attempted to describe com- 
munities on the basis of a single value (e.g., frequency) for each 
species. Today such a simple system would not be recommended 
by anyone, and, regardless of objectives, several values are now 
used in all phytosociological analyses. Adethods of sampling and 
objectives have always influenced each other, and, therefore, it is 
not surprising that early European workers had widely different 

55 



56 THE STUDY OF PLANT COMMUNITIES * Chapter I V 

approaches, which led to somewhat different conclusions. Several 
centers of thought and research naturally grew up, which still in- 
fluence our thinking and procedure. The ideas of the so-called 
Zurich-Montpellier school have gained rather wide favor, largely 
through the influence of Dr. J. Braun-Blanquet, 34 and they will be 
summarized in the remaining section of the chapter. 42 

Before proceeding with this summary, it seems entirely appro- 
priate to point out the unfortunate fact that Americans have been 
slow to adopt the phytosociological approach, probably because 
of a lack of appreciation of the usefulness of sociological data. Al- 
though phytosociology is, in itself, only a phase of ecology, its 
methods are useful far beyond the field for which they were de- 
veloped. Whenever communities must be described or the sig- 
nificance of individual species in a community must be evaluated, 
phytosociological concepts and methods are applicable and usu- 
ally with distinct advantages. This means that the methods are 
useful in experimental studies of communities, for comparing one 
community with another, for showing changes in a community 
from year to year, and, in fact, whenever precise information is 
needed about community structure and the part contributed by 
various species. Its possible applications are almost unlimited. To 
illustrate, various of its methods have been used to advantage in 
such diverse problems as correlating the progressive changes of 
vegetation and soil on abandoned fields, 20 showing the effects of 
different intensities of fire on the structure of pine stands, 1S4 and 
for demonstrating differences in virgin forest with changes of 

topography. 187 

STRUCTURAL CHARACTERISTICS 

The sociological characters of an individual stand or concrete 

o 

community may be conveniently grouped in two categories : 
quantitative and qualitative. Quantitative characters, obtained by 
quadrat methods, indicate numbers of individuals, their sizes, and 
the space they occupy. Qualitative characters indicate how species 
are grouped or distributed, or describe stratification, periodicity, 
and similar conditions, and are based upon the knowledge derived 
from long familiarity and observation of the community. 

Quantitative Characters. — Numbers of Individuals. — Under 
some circumstances, it mav not be practicable to make actual 



VEGETATIONAL ANALYSIS 



57 



Table 2. Portion of a list of species occurring on the east coast of Greenland at 
fourteen localities ranging from (A) 70° N latitude, southward to (N) 65° N latitude. 
Both presence and degree of importance of the species in each locality is indicated 
by the field-assigned numbers according to the following scale: 

5 — very common (important constituent of several closed communities); 4 — 

common (more scattered occurrence); 3 — here and there; 2 — uncommon; 1 — rare; 

-\ present. 

The listed species were selected to show how the system of values indicates range 
limits and progressive changes of importance with latitude. From Bocher. 24 * 





Localities 




A 


B 


C 


D 


E 
4 


F 


G 


H 
4 
4 


/ 
4 

5 


/ 

4 

4 


K 
4 


L 


M 


iV 


Cystopteris fragilis 


4 


+ 


3 


+ 


3 
4 
4 


} 


4 


4 


Cerastium alpinum 


4 


4 


5 


4 


5 


+ 


4 


4 


4 


5 


Minuartia biflora 


4 


4 


4 


4 


4 


+ 


4 


4 


4 


4 


4 


4 


4 


Silene acaulis 


5 


4 


4 


4 


4 


+ 


4 


5 


4 


4 


5 


4 


5 
5 


5 


Sedum roseum 


4 

5 


+ 


4 


2 


4 


+ 


1 


5 


5 


4 


3 
5 


4 


r 

:> 


Oxyria digyna 


5 


5 


4 


4 


4 


4 


5 


5 


5 


5 


5 


5 


Polygonum viviparum 


5 


5 


5 


5 


5 


5 


5 


5 


5 


5 


5 


5 


5 


r 


Salix herbacea 


5 


5 


5 


5 


5 


5 


5 


5 


5 


5 


r* 

5 


5 


5 


5 
































Potentilla tridentata 










1 




3 


? 

2 


3 


Polystichum lonchites 
















1 






3 


Alchemila filicaulis 








1 








3 


5 


4 




? 


5 


5 


Sagina intermedia 




+ 






+ 


3 


3 


4 




4 


+ 


3 


3 


Draba rupestris 


? 




3 








3 


3 


4 


4 


4 


4 


4 


4 


Empetrum nigrum 


3 


2 


2 


1 


2 


+ 


5 


5 


5 


5 


5 


5 


5 


5 


Salix arctophila 


+ 


+ 


+ 


+ 


+ 


+ 


4 


4 


5 


5 


5 


5 


5 


5 
































Epilobium arcticum 


3 
3 


+ 


























Potentilla pulchella 


























Ranunculus sulphur eus 


3 




2 
























Draba lactea 


4 


+ 


+ 


+ 


+ 




















Dryas octopetala 


5 


+ 


2 




2 




















Draba alpina 


4 


4 


4 


.4 


3 




5 
















Cassiope tetragona 


5 


5 


5 


5 


4 








1 


1 


4 






1 



58 TH E STUDY OF PLANT COMMUNITIES • Chapter IV 

counts, but plentifulness may rapidly be estimated according to 
some scale of abundance similar to the following : 

1. very rare 

2. rare 

3. infrequent 

4. abundant 

5. very abundant 

Such estimates are particularly useful when several similar stands 
of uniform composition are to be surveyed within a limited time. 
Assuming the sampling is adequate, the determination of actual 
numbers by counting is of greater value because it permits the ex- 
pression of density, which is abundance on a unit-area basis. 

Density is the average number of individuals per area sampled. 
Since it is an absolute expression, the significance of density in in- 
terpretation may be overemphasized unless one remembers that it 
is an average value. Not all species with equal densities are of equal 
importance in a community, or need they be similarly distributed. 
If ten individuals of a species are counted on a series of ten plots, 
the density is "one" regardless of whether they are all found in 
one plot or one in each of the ten plots. It becomes necessary, 
therefore, to interpret density values or to specify other charac- 
ters that, combined with density, serve to complete the picture. 
One such value is frequency. 

Frequency — This value is an expression of the percentage of 
sample plots in which a species occurs. In the example above, the 
plants that were all found on a single plot would have a frequency 
value of 10 percent, whereas, if they had occurred in every plot, 
the value would be 100 percent. Thus frequency becomes a very 
useful value, when used in combination with density, for then not 
onlv the number of individuals is known but also how they are 
distributed in the stand. These two characters are of prime impor- 
tance in determining community structure and, taken together, 
have a variety of uses far beyond those of other quantitative values. 
The use of frequency as a single determination in analytic pro- 
cedure has proven unsatisfactory, although numerous attempts 
have been made to show its adequacy. 

It should be emphasized that frequency values cannot be com- 



VEGETATIONAL ANALYSIS 



59 



PINE 



HARDWOODS 



39.6- 



Density-Frequency 



20.0- 



10.0- 




Overstory 



Understory 



Fig. 24. Bar diagrams of density, frequency, and basal area to compare 
pine and hardwood development in an unburned pine stand (A), with por- 
tions previously subjected to surface fire (B), and crown fire (C). Densities 
are indicated by the height of the columns above the zero line and frequen- 
cies by the width of the columns. Basal areas in square feet are indicated by 
the length of the columns below the zero line, and the width of these 
columns indicates percent of total basal area in the stand. Values for density 
and absolute basal area were modified by the factor 2 ^j~y because of their 
wide range. 



184 



]*\red unless determined with plots of equal size. The larger the 
pots, the higher the frequency. 

'*§ Frequencies may conveniently be grouped into classes, for ex- 
ample, A 1-20%, B 21-40%, C 41-60%, D 61-80%, E 81-100%. 



60 THE STUDY OF PLANT COMMUNITIES • Chapter IV 



Raunkiaer 202 used these five classes and, on the basis of more than 
eight thousand frequency percentages, found that Class A included 
53 percent of the species; B, 14 percent; C, 9 percent; D, 8 percent; 
and E, 16 percent. From these data he drew his "Law of Fre- 
quency" which states that Class A>B>C|D<E. This led 
to numerous investigations to check on the validity and univer- 



TABLE 3. The effect on frequency of increasing size of quadrat as illus- 
trated by data on Alpine fell-field vegetation in the Rockies. Quadrat sizes 
in sq. m. From( 4y ). 



Arenaria sajanensis . . . 

Selaginella densa 

Trifolium dasyphyllum 
Eritrichium argenteum . 
Sieversia turbinata .... 
Polemonium conjertum . 

Phlox caespitosa 

Sedum stenopetalum . . . 
Paronychia pulvinata . . 

Silene acaulis 

Potentilla nelsoniana . . 
Potentilla quinquefolia. 

Potentilla sp 

Polygonum bistortoides . 
Artemisia scopulorum . 

Sieversia ciliata 

Arenaria macrantha. . . 
Erigeron compositus . . . 

Total species 

Average frequencies . . 



1 1 10 



100 
100 
80 
80 
50 
40 
30 
30 
30 
20 
20 
20 
10 
10 
10 



15 

42 



i/4 



100 
100 
100 
80 
50 
40 
50 
50 
30 
30 
20 
20 
20 
20 
10 
10 



16 
45.6 



■12 



100 
100 
100 
90 
60 
40 
50 
50 
50 
30 
30 
20 
30 
20 
20 
20 



16 

48.7 



i/i 



100 
100 
100 
90 
80 
50 
60 
60 
50 
70 
30 
30 
30 
20 
20 
30 
10 
10 
18 
52.2 



sality of the principle of frequency distributions in plant com- 
munities. 137 The results have been in essential agreement regardless 
of the vegetation type. Class A will normally be very high because 
of the numerous sporadic species to be found with low frequency 
in most stands. Class E, and to a lesser extent D, must always bi 
relatively high because of the species that dominate the commu- 



VEGETATIONAL ANALYSIS 



61 



nity. If quadrats are enlarged, classes A and E will enlarge and the 
lesser classes will decrease accordingly. Frequency classes, there- 
fore, are comparable only when based upon samples of the same 
size. 

A frequency diagram is useful in indicating the homogeneity of 
a stand since floristic uniformity varies directly with the values for 



80 



50 



% 



ABODE 

Raunkiaer's 

NORMAL 



ABODE 

Kenoyer's 

NORMAL 



15 YR. 







34 YR 







90 YR 

T 



abode abode abode 

Loblolly Pine Stands-3ages 



eo 



50 



25 



% 



_n 



_□ 



ABODE abode abode abode abode 

Five Stands Virgin Red Fir- Sierra Nevada 
FlG. 25. Frequency diagrams of pine stands of different ages and of virgin 
red fir stands compared with Raunkiaer's and Kenoyer's normals. The pine 
stands were all relatively homogeneous but became slightly less so with age 
as the total number of species increased by 25 percent and the accidentals de- 
clined. Class E, the dominants, remained essentially constant throughout the 
series. All the virgin red fir stands were extremely homogeneous in spite of 
a high proportion of incidentals occurring sporadically. The stands were also 
similar to each other although widely distributed along the Sierra. 

classes A and E. When classes B, C, and D are relatively high, the 
stand is not homogeneous. In general, the higher Class E may be, 
the greater the homogeneity. 

Cover and Space.— Although density and frequency indicate 
numbers and distribution, they do not indicate size, volume of 
space occupied, or amount of ground covered or shaded. These 
characteristics are desirable additional values that contribute ma- 
terially to an understanding of the importance of a species in a 
stand, since they are closely related to dominance. 

As suggested under Quadrat Methods (Chap. 3), cover can be 



62 THE STUDY OF PLANT COMMUNITIES • Chapter IV 

estimated with some success or may be accurately determined by 
various devices for measurement and recording. When vegetation 
is stratified, the cover must be considered in terms of the stratum 
to which the species belongs. For rapid estimation, as well as for 
analysis of results, there is a distinct advantage at times in using 
several cover classes. Braun-Blanquet recommends five : 

1. covering less than 5% of the ground surface 

2. covering 5% to 25% 

3. covering 25% to 50% 

4. covering 50% to 75% 

5. covering 75% to 100% 

In studies of grassland, estimates and measurements of cover are 
extremely useful because the variations in size and form of grasses 
make counts difficult 'and of little value. For expressing cover, 
sometimes as area of coverage, sometimes as basal area of clumps, 
range ecologists frequently use the term, density . This usage is, 
of course, at variance with the phytosociological application and, 
consequently, leads to confusion of interpretation unless it is 
known, for example, that a "density list" 96 applied to grassland, 
refers to area or cover for each species, and that "square foot 
density" 247 also indicates coverage evaluated by a different method. 

Determination of the volume of space occupied by species is 
difficult and has not been widely done. When all plants are small, 
cover alone serves very well, especially when strata are distin- 
guished. With grasses, as in pasture studies, clipping and weighing 
the tops is sometimes necessary for accurate comparisons. In for- 
est studies, the estimate of volume of standing timber as used by 
foresters can be used to advantage, but a more useful value is basal 
area. Diameters can be determined accurately and quickly with a 
diameter tape, and basal area, easily obtained from standard tables, 
can add much to an evaluation in terms of size and bulk that can- 
not be visualized through the other quantitative characters. This 
provides a particularly useful means of comparing the relative im- 
portance of species of trees and, in addition, permits analysis in 
terms of size or diameter classes among the sapling and understory 
individuals. Several quantitative characters can be advantageously 
combined in the form of phytographs (Fig. 26) for evaluation. 



VEGETATIONAL ANALYSIS 



63 



SPRUCE 
FLAT 



SPRUCE 
HARDWOODS 



OLD FIELD 
SPRUCE 



SPRUCE 
SLOPE 



Picea 
rubens 



Abies 
balsamea 



Betula 
lutea 



Betula 
papyrif era 



Acer 

rubrum 




Fraxinus 
americana 



Acer 

saccharum 




Sorbus 
americana 




FIG. 26. Phytographs showing the relative importance of the dominant 
species of trees in four types of pulpwood forest in northwestern Maine. 
Radius 1, percentage of total dominant abundance; Radius 2, percentage fre- 
quency; Radius 3, percentage of total size classes represented; Radius 4, per- 
centage of total dominant basal area. The inner end of each radius represents 
the absence of its assigned sociological value. 191 



64 THE STUDY OF PLANT COMMUNITIES ■ Chapter IV 

Qualitative Characters.— These characters, which include socia- 
bility, vitality, stratification, and periodicity, are mostly not de- 
rived from quadrat studies but from observation of, and wide ex- 
perience with, the community. They describe the plan and organ- 
ization of its components, which have been evaluated previously 
in terms of measurements and counts. When the quantitative an- 
alysis has been fairly complete, especially including density or 
cover in conjunction with frequency, and when strata have been 
analyzed separately, the qualitative characters are already largely 
included in the quantitative picture. 

Sociability .—This character evaluates the degree that individuals 
of a species are grouped or how they are distributed in a stand. It 
has also been expressed as gregariousness or dispersion. Each of the 
various scales used to indicate degree of sociability include expres- 
sions which range from plants occurring singly, as one extreme, 
through intermediate conditions (patches, colonies, or groups), to 
large colonies, mats, or pure stands at the opposite extreme. 

The sociability of a species is not a constant, for it is determined 
by the habitat and the resulting competition of the species with 
which it is associated. Since habitat conditions are not constant and 
since communities change, especially in plant succession, the so- 
ciability of a species, even in the same locality, may change con- 
siderably. 

Dispersion is a statistical expression that has been applied to 
sociability. If dispersion is normal, it implies a randomized distri- 
bution such as might be expected by chance. In hyperdispersion 
there is irregular distribution, which results in crowded individ- 
uals in some areas and their complete absence from others. Hypo- 
dispersion means that the arrangement is more regular than would 
be expected by chance, as, for instance, the plants in a cornfield. 
All of these conditions are recognizable in natural communities 
and, when density-frequency values have been determined, are 
noticeable in the data. 

Vitality.— Not all species found in a given stand need belong to 
the community. Unless the plants are reproducing, they are not 
completely adapted to the conditions and may disappear entirely. 
Even species constantly present in a community mav be derived 
from seeds produced elsewhere and transported by wind or some 



VEGETATIONAL ANALYSIS 6S_ 

other agency. It becomes necessary, therefore, to know something 
of the vigor and prosperity of the species before classifying it as 
a true community member. 

Vitality need not always be listed for all species, but it must be 
considered in evaluating their importance, whether it is done sys- 
tematically or not. Vitality classes or degrees of vitality include : 
( 1 ) ephemeral adventives, which germinate occasionally but can- 
not increase, (2) plants maintaining themselves by vegetative 
means but not completing the life cycle, (3) well-developed plants, 
which regularly complete the life cycle. 

Changes in the vitality of species are often indicators of com- 
munity change or plant succession. Dominants decreasing in num- 
bers and reproducing feebly indicate future radical changes. Rap- 
idly increasing numbers of a species previously of little importance 
may suggest the new dominants to come. 

Stratification— The necessity for recognizing the strata of a 
community becomes obvious when sampling is attempted. The 
several strata that may occur were described under sampling pro- 
cedure. Diagrams of stratification combined with cover are often 
used effectively to show the relative significance of the several lay- 
ers in a stand. The physical and physiological requirements of spe- 
cies in different strata can be appreciated fully only when the 
stratification both above and below ground is clearly worked out. 
Then the micro-environments of these strata may be considered in 
terms of cause and effect. 

Periodicity .—The conspicuous rhythmic phenomena in plant 
communities are those related to seasonal climatic change, and, of 
these phenological changes, the obvious ones have been given most 
attention. Flowering and fruiting^ periods have been noted for so 
long that they are fairly well known; in fact, phenology is often 
thought of as referring only to these phenomena. In community 
studies the terms aspect dominance and seasonal dominance have 
been used to describe situations in which a species or group of 
species appears to be dominant for a portion of the year, usually 
because of conspicuous floral characters. 

Of equal importance to the community is the seasonal develop- 
ment of vegetative parts. The seasonal aspect of the individual may 
proceed through several phases, including a leafy period, a leafless 



66 THE STUDY OF PLANT COMMUNITIES « Chapter IV 



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VEGETATIONAL ANALYSIS 



67 



period, a flowering period, a fruiting period, an embryo period, 
and perhaps others. Rarely will all the species of a community 
have these periods strictly coinciding. Consequently, in temperate 
climates, the community as a whole usually has seasonal aspects, 
which are termed vernal, estival, autumnal, and hibernal. The 
structure and species of a community are strongly influenced by 




FlG. 28. Aspect dominance as illustrated by chandelier cactus (Opimtia 
arbor -esc ens) in a mixed prairie community (Bouteloua-Hilaria). El Paso 
County, Colo. The cactus makes up only 8.9 percent of the total cover.— 
Photo by R.B. Livingston. 

the extent to which periodic phenomena in the individuals are 
adjusted to each other. 

Light and moisture conditions on the floor of a deciduous forest 
during the vernal period permit the growth and maturation of nu- 
merous herbs before the estival period. When the trees and shrubs 
are in full leaf, these herbs are already declining to a fruiting or 
resting condition and are unaffected by the reduced light and 
moisture available to them. These vernal herbs are a part of the 
community and must be so considered. 

Another illustration of a periodic phenomenon that may be im- 
portant in sociological relations is the time of growth. Height 
growth has been systematically studied for numerous woody spe- 



68 THE STUDY OF PLANT COMMUNITIES * Chapter IV 

cies, but the periods of root elongation are rarely known. Studies 
of loblolly pine 205 showed, surprisingly, that it makes some root 
growth in every month of the year. Even in the winter months, 
its roots are constantly coming in contact with new supplies of 
soil water, which fact may partially explain its ability to thrive in 




FlG. 29. Vernal aspect dominance of atamasco lily (Zephyranthes atamas- 
co) in a low North Carolina meadow where only grasses, rushes, and sedges 
are visible a few weeks later.— Photo by H. L. Bloniquist. 

the southeastern states where transpiration may at times be fairly 
high during the winter. 

Periodicity may be controlled by a variety of factors. Length 
of day affects the time of flowering, some species requiring long 
days, some short. The fall of leaves in autumn is a response not to 
temperature but to length of day. Desert vegetation may flower 
or not depending upon precipitation, and semidesert plants reg- 
ularly flourish during the brief seasonal rains and exist in an almost 
dormant condition for the remainder of the year. Arctic and alpine 
areas usually receive little rain. The melting snow provides the 
moisture for vegetation. In situations where little snow accumu- 
lates or where it melts and disappears quickly, the vegetation is 



VEGETATIOXAL ANALYSIS 69 

sparse and takes on a hibernal aspect very quickly. Where snow 
patches remain well into the summer and provide a water supply 
by melting gradually, the estival aspect may carry on for several 
weeks after plants in less favorable sites near by have gone to seed. 
Plants deeply buried under snow may not be exposed until so late 
in the season that conditions are unfavorable for flowering, and, as 
a result, they produce no fruit or seed. 

SYNTHETIC CHARACTERISTICS 

It has previously been pointed out that it is often desirable as 
well as practical to consider a community in the abstract as well as 
in the concrete sense. When a community is studied on this basis, 
it becomes necessary to observe numerous stands and to determine 
whether they actually do belong to the same community and to 
what extent they vary from each other. It is desirable also to know 
which species, singly or in combination, may be taken as indica- 
tors, which species are only incidental, which ones are always 
present, and which ones occur only when a stand develops under 
a given set of conditions. 

Thus, for a complete synthetic analysis, it is desirable to have in- 
formation on as many stands as possible, or at least enough stands 
to be representative of the whole. These should be distributed 
throughout the range of the community and under all the variety 
of conditions in which they develop. Again, to make a proper an- 
alysis, only those stands should be employed that are in a com- 
parable stage of development or maturity and that are extensive 
enough to include all the important species and most of the antici- 
pated variations. 

Presence.— A most useful synthetic character involves merely 
the degree of regularity with which a species occurs in the stands 
observed. When the species present in each of the stands have 
been tabulated, the presence of each is expressed by the percentage 
of stands in which it occurred or by a five-degree scale of presence 
classes. 

1.— rare (1-20% of the stands) 

2— seldom present (21-40%) 

3. -of ten present (41-60%) 

4— mostly present (61-80%) 

5— constantly present (81-100%) 



70 THE STUDY OF PLANT COMMUNITIES • Chapter IV 

The number of stands necessary for a study of presence as well 
as the necessary extent of stands cannot be arbitrarily stated. Ma- 

TABLE 4 



Species 


/ 


2 


3 


4 


5 


6 


7 

X 
X 
X 

X 


8 

X 
X 
X 
X 
X 


9 

X 
X 
X 

X 


10 

X 
X 

X 
X 


II 

X 
X 
X 
X 


12 

X 
X 
X 
X 


13 

X 
X 
X 
X 


H 

X 
X 
X 
X 
X 
X 


15 

X 
X 
X 
X 


16 


Trees 
Abies magnified 


x 


X 
X 


X 
X 
X 

X 

\ 


X 
X 

X 
X 


X 
X 


X 
X 
X 

X 


x 


Pinus monticola 






Pinus contorta 




x 


Tsuga mertensiana 




X 




Abies concolor 




y 


Acer glabrum 








Shrubs 
Ribes viscosissimum 






X 


X 


X 


X 


X 


X 


X 


X 
X 

X 
X 

X 
X 
X 

X 
X 

X 

X 

X 
X 
X 


X 
X 

X 

X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 

X 

X 
X 


X 

X 
X 

X 
X 
X 
X 

X 

X 
X 

X 
X 


X 
X 

X 

X 
X 
X 
X 
X 

X 
X 

X 
X 

X 
X 


X 
X 
X 
X 

X 
X 
X 
X 
X 
X 
X 
X 

X 

X 

X 

x 


X 

X 
X 
X 
X 

X 

X 
X 
X 

X 
X 
X 
X 


Y 


Symphoricarpos rotundifolius 

Ribes montigenum 






X 


X 

x 


Sambucus racemosa 










X 




X 

X 

X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 

X 
X 

X 
X 
X 
X 


X 
X 

X 
X 
X 
X 

X 
X 
X 
X 
X 
X 

X 
X 


X 
X 

X 
X 
X 
X 

X 

X 

X 
X 

X 

X 
X 




Ribes cereum 


X 










Spiraea densiflora 






X 
X 
X 






X 
X 




Arctostaphylos nevadensis 

Symphoricarpos mollis 


X 




Y 


Lonicera conjugialis 








Quercus vaccinifolia 

Amelanchier alnifolia 






X 


X 


X 

X 
X 
X 
X 
X 
X 

X 
X 

X 
X 

X 
X 

X 
X 


X 

X 
X 
X 

X 

X 
X 

X 
X 

X 
X 

X 




Rubus parviflorus 










Herbs 

Chrysopsis breweri 


X 

X 


X 
X 
X 


X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 


X 
X 

X 
X 
X 

X 
X 

X 

X 
X 
X 

X 
X 


Y 


Monardella odoratissima 


Y 


Gayophytum ramosissimum 

Pedicularis semibarbata 




X 
Y 


Pirola picta 


X 


X 


Y 


Phacelia hydrophylloides ■. 


y 


Poa bolanderi 


X 
X 


X 

X 
X 


x 


Arabis platysperma 




Corallorrhiza maculata 


X 


Thalictrum fendleri 






Kelloggia galioides 




V 


Erigeron salsuginosus var. 

angustifolius 


X 


X 
X 


Y 


Uxeracium albifiorum 


X 


Lupinus andersoni var. fulcratus . . . . 

Viola purpurea 

Chimaphila umbellata 


X 
\ 


X 
X 


X 

X 
X 


X 
X 


Pentstemon zracilentus 






Pla^iobothrys hispidus 




X 


X 



VEGETATIONAL ANALYSIS 



71 



ture, homogeneous, undisturbed stands of virgin forest would re- 
quire the observation of only small portions of individual stands 
and a relatively small number of stands to give dependable infor- 
mation. In younger, less stable vegetation, more stands and a wider 
observation would be necessary so that variation would be repre- 



€0 



50 
















Presence 


30 






10 








% 






i 



Frequency 



Constance 



12345 ABCDE 12345 

FlG. 30. Presence, frequency, and Constance diagrams for Sierran red fir 
forest, based on sixteen stands. The presence diagram is normal, especially in 
the absence of a second maximum. The Constance diagram is constructed 
from regular quadrat data rather than a Constance sample. Compared to a 
frequency diagram it should show a material decrease in Class 1 because of 
the greater odds on discovery of a single plant of an accidental species in a 
restricted area. Surprisingly, with only forty species, it retains the same form 
as the presence diagram (ninety-seven species) although the high Constance 
classes are reduced. The frequency graph is normal, and indicates stands of 
relatively great homogeneity. 

sented and so that those species seldom present or rare would fall 
into their proper classes. What this minimal area should be and 
what the minimum number of species might be for the community 
must largely be determined by experience and familiarity with the 
community. 

Constance.— When a unit area in each stand instead of the en- 
tire stand is used for listing species, as for presence, the values are 
termed Constance. There is thus no fundamental difference be- 



TABLE 4. Portion of a presence table compiled from sixteen stands of vir- 
gin red fir (Abies magnified) forest in the Sierra Nevada. Only Abies mag- 
nified and Pinus monticola, of the trees, are constantly present (Class 5). 
Only one shrub, Kibes viscosissimum, is a constant, others falling in Class 3 
or lower. Five herbs are constants, eight are mostlv present (Class 4), and 
five are often present (Class 3). Eleven herbs of Class 2 (seldom present) and 
46 of Class 1 (rare) are not listed. 189 



72 THE STUDY OF PLANT COMMUNITIES • Chapter IV 

tween presence and Constance. The latter has the advantage of 
eliminating discrepancies resulting from sampling stands of un- 
equal size. The lower classes of Constance are more uniform than 
those of presence, for the larger the area examined the greater the 
number of incidental species encountered. 

Constancy bears a relationship to the abstract community very 
similar to that of frequency in the concrete community. The prob- 
lems of minimal area are similar and can to some extent be reduced 
by the use of species : area curves as used in frequency determina- 
tions. Both concepts are concerned with homogeneity, the one 
with that of the stand, the other with that of the abstract com- 
munity. If Constance values are divided into five classes and these 
are diagrammed as for frequency, the results are quite different. 
Instead of two maxima as in frequency, only the classes represent- 
ing irregular occurrences are high, and each succeeding higher 
class is apt to include fewer species. 

Fidelity. — This character is indicative of the degree with which 
a species is restricted to a particular kind of community. Species 
may be grouped into five fidelity classes. 

Fid. 1 .—Strangers, appearing accidentally 

Fid. 2.— Indifferents, without pronounced affinity for any 
community 

Fid. 3.— Preferents, present in several communities but pre- 
dominantly in one of them 

Fid. 4.— Selectives, found especially in one community but 
met with occasionally in others 

Fid. 5.— Exclusives, found completely, or almost so, in only 
one community 

Species with fidelities 3-5 are termed characteristic species in a 
community. Positive establishment of which species are character- 
istic is possible only after all communities of a region have been 
studied sociologically. Approximations can, of course, be made by 
those of wide experience, but even then the assigned values must 
be considered with skepticism. When fidelity values are accurately 
determined, they contribute strongly to the recognition and classi- 
fication of a community. However, studies of this sort have been 
so few in the United States that it will be a long time before suf- 



VEGETATIONAL ANALYSIS 



73 



TABLE 5. A summary of sociological concepts that permits presentation 
of the important data for a community in a single tabulation. The quantita- 
tive data (1) are derived from quadrats; the analytic data (A) from the study 
of some one community; the synthetic data (B) from the study of several 
different examples (stands) of the same community.— After Cain.* 2 





SOCIOLOGICAL SUMMARY 




I 

Organization 


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ficient data have accumulated to permit accurate statements of fi- 
delity for species of most communities. Under such conditions, it 
seems advisable to use Constance, an absolute value determined 
within the community in question, as a means of fixing upon the 
sociologically important species. Some ecologists consider con- 
stance of greater significance than fidelity for this purpose. It 
should be noted that characteristic species are more responsive to 
habitat variations and are consequently of greater indicator sig- 
nificance than are, in general, the species of high Constance. It 



74 THE STUDY OF PLANT COMMUNITIES * Chapter IV 

would be desirable if we had both values available for all com- 
munities. 

Coefficient of Community.— When comparing two communi- 
ties or the vegetation of two regions, a mathematical expression of 
the similarity of lists of species may be useful. If community X 
is compared to Y, the number of species common to both, ex- 
pressed as a percentage of the number for Y has been termed the 
coefficient of community. The same principle can be used for 
evaluating variation or similarity among several stands of an ab- 
stract community. Then, however, each must be compared with a 
standard or list of the characteristic species of the community as a 

whole. 130 

OBJECTIVES DETERMINE PROCEDURE 

If these several sociological concepts are grouped systematically 
in tabular form, their relationships become clearer (Table 5). Such 
a grouping has the further usefulness of presenting tabulation of 
values obtained in the field in compact and logical order for in- 
terpretation. 

When the objective is merely to describe a community as com- 
pletely as possible, it might well be desirable to have such a table 
completely filled out. In studies involving the application of phy- 
tosociological methods to special problems it is frequently onlv 
necessary to use a few of the values. This does not mean that not 
all are of significance, or that some can be ignored entirely. Rather, 
it suggests that each has its uses and that some are applicable where 
others are not. 

The limitations and possibilities of usefulness of the several con- 
cepts become increasingly understandable after one has had some 
experience with them. Nevertheless, selection of the most useful 
values for study and application to a particular problem always 
remains a matter for serious consideration. The concepts to be 
used must be selected in terms of their contribution to the object 
of the study, the time available, and the labor involved. 

GENERAL REFERENCES 

J. Braun-Blanquet. Plant Sociology : The Study of Plant Communities. 
S. A. CAIN. Concerning Certain Phytosociological Concepts. 
C. RAUNKIAER. The Life Forms of Plants and Statistical Plant Geography; 
Being the collected papers of C. Raunkiaer. 



Part 3 • Factors Controlling the 
Community: the Environment 



Vegetational analysis gives the information necessary to de- 
scribe and name a community and provides data that can be used 
to compare it with other communities or with itself after a lapse 
of time or an experimental treatment. This in itself is worth while, 
but the ecologist has the added objective of correlating the vege- 
tational record so obtained with the environment. To interpret the 
vegetational statistics, and to explain them in terms of cause and 
effect, leads to an analysis of the environment and its relationships 
to the community. 

Since the environment consists of many factors interacting upon 
each other and upon the vegetation, its complexity prohibits con- 
sideration of it as a whole. The interactions are by no means all 
clearly understood and the effects of a single factor upon an or- 
ganism may be inadequately known; therefore, it is logical to ap- 
proach the subject of environment through individual factors and 
their effects. With information as complete as possible on the 
operation of individual factors, explanations may often be found 
for plant responses among the interactions and effects of a few of 
the variable factors. The chapters of this section deal successively 
with climatic, physiographic, and biological factors as each may 
operate in the complex of factors termed environment. 

CHAPTER V 



CLIMATIC FACTORS: THE AIR 

GASES OF THE ATMOSPHERE 

The air surrounding the earth is made up of only a few gases in 

proportions that remain remarkably constant. The average volume 

percentages of dry air are : nitrogen, 78.09; oxygen, 20.95; carbon 

dioxide, 0.03; and argon, 0.93. In addition, there are minute but 

75 



76 THE STUDY OF PLANT COMMUNITIES ■ Chapter V 

measurable quantities of several rare gases, which have no part in 
our discussion. Within the limits of the atmosphere that can affect 
plants directly, there is but slight variation in the proportions of 
these gases whether over the ocean or land, at sea level or on high 
mountains. Minor but rather consistent variations have been found 
over large industrial cities where quantities of carbon dioxide are 
constantly being produced. 

Whenever an organism respires or a fire burns, oxygen is re- 
moved from the atmosphere and carbon dioxide is added to the 
air. Decomposition of organic matter also liberates carbon dioxide, 
and photosynthetic activity of plants removes carbon dioxide 
and liberates oxygen. When these processes are not in balance, 
there may be local variations in the composition of the air, but so 
long as the air is not strictly quiet, the least motion, combined with 
diffusion, is sufficient to eliminate gaseous differences almost at 
once. 

Thus, regardless of its terrestrial environment, the organism is 
almost certain to be plentifully supplied with these gases that form 
a relatively constant part of the atmosphere; therefore, these need 
not be considered as variable factors in the environment. 

GASES OF THE SOIL ATMOSPHERE 

Although normally there is never a shortage of oxygen in the 
air above ground, such a shortage sometimes occurs in the soil. 
Air space in the soil is limited and is partially, or sometimes wholly, 
occupied by water. Any change in the composition of the soil at- 
mosphere is only slowly readjusted from the atmosphere above, 
for here air movement and diffusion are relatively slow. 

Since all living structures in the soil respire, and this includes 
small animals and other microorganisms as well as roots of large 
plants, the supply of oxygen is constantly reduced and carbon 
dioxide is released. As a result, the soil atmosphere always contains 
less oxygen and more carbon dioxide than the air above. Oxygen 
decreases with depth, and carbon dioxide increases. In the soil un- 
der closed stands of vegetation, carbon dioxide often equals 5 
percent and has been found in much higher concentrations. The 
constant use of oxygen and its extremely slow rate of diffusion 
when soils are saturated soon result in oxygen deficiency. Tern- 



CLIMATIC FACTORS : THE AIR 77 



porary saturation may not be serious, but, when prolonged, it re- 
sults in death of the vegetation through inhibition of root growth 
and absorption. Under these conditions, several soil organisms may 
carry on anaerobic respiration for a time, but such activity results 
in chemical changes of several kinds, which may affect fertility of 
the soil or actually inhibit plant growth. 

Available oxygen in an aquatic habitat probably is somewhat 
higher than in a saturated soil because of the movement of the 
water and because the oxygen is more readily replaced by solution 
from the atmosphere. If, however, the water is solidly frozen over, 
it is not uncommon for the oxygen supply to fall so low that many 
of the fish die. When such conditions develop in well-stocked 
fishing lakes, it is now common practice to cut several holes 
through the ice and to pump air through the water until the de- 
pleted oxygen supply has been replaced. The mud at the bottom 
of a shallow pond is probably the least favorable habitat for plant 
roots. Most plants growing well in such places are of the emergent 
type, having at least part of their structure in the air and charac- 
terized by lacunar tissue, which permits gases to accumulate in, 
and move freely through, the plant. 

WATER CONTENT OF THE ATMOSPHERE 

In addition to the gases constituting the atmosphere, water is 
always present as vapor but in widely varying amounts. Since at- 
mospheric moisture represents the indirect source of the plant's 
water and likewise controls the amount and rate at which water 
is lost by the plant, it is an environmental factor deserving some 
attention. 

The capacity of air to hold water vapor increases as tempera- 
tures rise or pressure is reduced. The air is said to be saturated 
when it contains as much moisture as it can hold at a given tem- 
perature and pressure. If for any reason the temperature is raised 
or the pressure is decreased, the amount of water remaining con- 
stant, the air is no longer saturated. On the other hand, if the tem- 
perature of saturated air decreases, the capacity is reduced, and 
some of the vapor precipitates as a liquid. Thus air that is not sat- 
urated will become so without change of vapor content if its tem- 
perature is lowered, and, when saturation is reached, the air is said 



78 THE STUDY OF PLANT COMMUNITIES • Chapter V 

to be at the "dew point!' If the cooling continues, the vapor be- 
comes a liquid, which may condense on objects near the surface 
of the earth as dew or frost or, if condensation takes place in the 
air, may result in precipitation. 

Terminology of Atmospheric Moisture.— Several expressions 
are used to describe the moisture content of the air. Absolute hu- 
midity is commonly interpreted as the amount of water vapor per 
unit volume of air and can be expressed as grams per cubic meter 
or any other units of mass and volume. In itself the absolute hu- 
midity has little bearing on the life of a plant, for it is not the total 
atmospheric moisture that determines evaporation and transpira- 
tion, but rather the difference between the weight of vapor pres- 
ent and the maximum weight the air could hold at the time. Thus 
the relative humidity, which is an expression of percentage of sat- 
uration, is more nearly related to the rate of water loss from a free 
water surface or from a plant. Relative humidity depends upon 
temperature as well as the amount of moisture present, and, as a 
consequence, identical relative humidities do not indicate identical 
moisture conditions unless the temperatures are also the same. This 
means that every shift in temperature results in a change in rela- 
tive humidity, regardless of moisture present, and a consequent 
change in rate of evaporation or transpiration. 

Several authors have emphasized that, when considered inde- 
pendently of other factors, the actual amount of water vapor in 
the air has little if any influence upon evaporation. One illustra- 
tion 7 especially serves to emphasize the ecological significance of 
this fact. Death Valley, California, is probably the most arid region 
in the United States, yet its "dry" atmosphere contains on the aver- 
age in July almost exactly the same amount of water vapor per 
unit volume as does the "moist" atmosphere of Duluth, Minnesota, 
at the same time of the year. 

An atmosphere 70 percent saturated at 60° F. will contain much 
less water vapor than an atmosphere 70 percent saturated at 80° E, 
and the capacity to hold more water will be less in the first than 
the second case. Evaporation will, therefore, normally be more 
rapid at 80° E even though the relative humidities are the same. It 
can be seen that a statement of relative humidity alone gives little 
indication of atmospheric moisture conditions since a relative hu- 



CLIMATIC FACTORS : THE AIR 79 



midity of 80 percent may mean "dryness" if the temperature is 
high or "wetness" if the temperature is low. 

It is desirable then to have a term indicating the amount of water 
that air can take up before it becomes saturated. Vapor pressure 
is a measure of the quantity of water vapor present, the tempera- 
ture being constant, and is usually expressed in units of pressure 
(inches or mm. of Hg). Therefore, vapor pressure deficit is the 
difference between the amount of water vapor actually present 
and the amount that could exist without condensation at the same 
temperature. It is a direct indication of atmospheric moisture, 
quite independent of temperature and, therefore, compared to 
relative humidity, its values are much more indicative of the po- 
tential rate of evaporation. 

When the relative humidity is 100 percent at 68° E, the vapor 
pressure is 17.54 mm. of mercury. If the relative humidity were 70 
percent, the vapor pressure would equal 12.28 mm. (0.70 x 17.54), 
and the deficit would be 5.26 mm. (17.54-12.28). If the relative 
humidity were the same (70%) at 59° E, the vapor pressure would 
be 8.95 mm. (0.70 x 12.79) and the deficit would be only 3.84 mm. 
(12.79—8.95). Tables of saturation pressures (vapor pressures) are 
usually available in handbooks of chemistry, and it is possible to 
transform relative humidities to vapor pressure deficits quickly 
when the temperature is also known. The relationships are shown 
in Table 6. 

Greater general use of the vapor pressure deficit in ecological 
work seems desirable, for in spite of certain limitations, its ac- 
curacy is much greater than that of relative humidity. The poten- 
tial rate of evaporation cannot be indicated with a single simple 
expression of atmospheric moisture since the rate depends upon 
the vapor pressure gradient between evaporating surface and at- 
mosphere. The gradient can be determined only when the tem- 
perature and vapor pressure of the liquid are known as well as 
those of the atmosphere. Vapor pressure deficit is directly related 
to evaporation only when the temperatures of the air and of the 
evaporating surface are equal. 252 Ecologists more often than not 
measure evaporation directly, but when evaporation is not known, 
in spite of the above, vapor pressure deficits could well be used in- 
stead of relative humidities. 



80 



THE STUDY OF PLANT COMMUNITIES ■ Chapter V 





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CLIMATIC FACTORS : THE AIR 8 1 



Measurement of Atmospheric Moisture.— The psychrometer 
and the hygrometer are the two instruments most useful to ecol- 
ogists for this purpose. The former consists of two thermometers, 
one of which has the bulb wrapped with a wet piece of cloth, and 
both of which are aerated in some fashion, usually by whirling. 
Evaporation from the wet cloth is controlled by the moisture in 
the atmosphere, and the bulb is cooled in proportion to the evap- 
oration. The dry bulb gives the temperature of the atmosphere, 




FlG. 31. A sling type of psychrometer for determining relative humidity 
by the difference in temperature of the wet and dry bulb after whirling.— 
Courtesy Friez Instrument Division, Bendix Aviation Corporation. 

and the difference between the dry and wet bulb readings gives 
the wet bulb depression. Knowing the barometric pressure and 
these two values, the relative humidity can be quickly determined 
from standard tables or from nomograms. 108 The necessity for 
aeration of the thermometers, usually accomplished by rapid ro- 
tation, has led to the design of several "sling" type psychrometers. 
Because these must be whirled, they require considerable space for 
operation. The "cog" psychrometer, functioning like an egg beat- 
er, can be used in much smaller spaces. 

The hygrometer is usually a continuously recording instrument 
in which an arm marks on a rotating drum the stretching and con- 
tracting of a strand of hairs, which respond to relative humidity. 
The drum is so calibrated that relative humidity is recorded di- 
rectly. Often the device is equipped to record the temperature 
simultaneously and is then called a hygrothermograph. Naturally, 



82 



THE STUDY OF PLANT COMMUNITIES • Chapter V 



this automatic device is very convenient, particularly since it nor- 
mally needs to be serviced but once a week. If, however, several 
stations are to be maintained, the necessary instruments may not 
be available, and the psychrometer is then the only solution. 

With readings of the psychrometer and the hygrothermograph, 
the air temperature is also obtained, providing the means of calcu- 
lating vapor pressure deficits with no extra determinations. A 




FlG. 32. A hygrothermograph, which automatically gives a continuous 
record of relative humidity and temperature of the air.— Courtesy Friez In- 
strument Division, Bendix Aviation Corporation. 



simple nomogram (Fig. 33) permits direct conversion from wet 
and dry bulb temperatures to vapor pressure deficit. 

Evaporation and transpiration.— Measurement of transpiration 
under natural conditions is often practically out of the question. 
Although small plants may be potted or grown in cans and these 
may be weighed at regular intervals to determine water loss, only 
a limited number of plants can be used, and the labor involved can 



CLIMATIC FACTORS : THE AIR 83 

soon become prohibitive if a comprehensive study is to be made. 

Relative rates of transpiration can be determined by the cobalt- 
chloride method, which is rapid and permits numerous determina- 
tions in a short time. Paper treated with cobalt-chloride is blue 
when dry and turns pink as it takes up moisture. Small squares of 
the dry paper can be attached to leaves between small glass plates 
by means of a wire clip. The time required for the paper to turn 
pink is taken as a basis of comparison. To get comparable values, 
the paper must be absolutely dry and care must be taken that the 
clip fits snuggly to the leaf. For increased accuracy, standard color 
strips are usually attached to the glass to be used for comparison 
with the sensitized strip. In spite of its simplicity, the method has 
definite limitations. The close-fitting clips exclude all outside air 
and thus eliminate air movement as a factor, while at the same 
time diffusion into the air is practically stopped by the glass. Thus 
the measurement is perhaps an indicator of the moisture in the in- 
ternal atmosphere of the leaf. Rarely will two leaves on a plant 
give identical readings, for their water loss varies with their posi- 
tions on the plant and their ages. Thus several determinations must 
be made simultaneously to evaluate a single plant, while to compare 
this plant with others necessitates a considerable number of read- 
ings. In spite of these limitations, it should not be assumed that the 
method is ineffectual, for under certain conditions, it has been used 
to great advantage. 

These methods have their greatest utility in intensive studies of 
a few or of individual plants under experimental conditions in the 
laboratory or field. In studies of communities, it is often desirable 
to have a more generalized picture of transpiration conditions. 
Under those conditions, the rate of evaporation may be more use- 
ful than a limited number of measurements of transpiration. Per- 
haps the most desirable information is obtained by using plants as 
instruments (phytometers). Two or more habitats may be com- 
pared by setting up potted plants of the same species in each of 
these habitats and comparing their transpiration rates as indicated 
by loss of weight over the same period of time. Again the work 
involved is often prohibitive. As a result, ecologists have largely 
come to depend upon mechanical devices that measure evaporation 
over unit periods of time, and, since evaporation and transpiration 



84 THE STUDY OF PLANT COMMUNITIES • Chapter V 

DRY BULB TEMP. V. R D. WET BULB TEMP. 

(C°3 (mm. Hg.) ( C °) 



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COPYRIGHT 194 
W. E. Gordon 



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Bo romet e r So romerer 

FlG. 33. Nomogram for the direct conversion of wet and dry bulb tem- 
peratures to vapor pressure deficit, at barometric pressures of 30, 29, 27, and 
25 inches. To use, lay a straight edge across the appropriate temperature 
values on the wet and dry bulb scales and read off VP.D. directly. Because of 
the reduction necessary for this reproduction, extreme accuracy is not pos- 
sible in its use— By permission W. E. Gordon. 108 



CLIMATIC FACTORS : THE AIR 85 

respond similarly to the external factors affecting the latter, evap- 
oration is taken as indicative of potential transpiration. 

Evaporation is measured by the United States Weather Bureau 
by means of large open tanks of uniform size and depth, but this 
method is quite unsatisfactory for most ecological purposes. The 
bulkiness of the equipment, the necessity for frequent checking, 
and the probability of disturbance and of contamination are all 
against it. 

Various compact evaporimeters have been devised primarily for 
ecological use. Of these the now well-known Livingston atmom- 
eter has been most widely used. It consists of a porous clay sphere 
or cup connected to a reservoir by means of a tube. Water evap- 
orates from the clay surface and is constantly replaced from with- 
in. If the sphere and tube have been filled with distilled water so 
that no air bubbles are present (most easily done under water), 
the water will be drawn from the reservoir through the tube. An 
additional small-bored tube passed through the stopper of the res- 
ervoir will permit equalizing of pressure but negligible loss of 
water by evaporation. The reservoir is marked near the top and 
filled to this mark by lifting the stopper. Subsequent fillings made 
at regular intervals indicate water lost to the air by evaporation 
over the period of time involved. 

The simplicity of this device has been in its favor, and it has 
other advantages. Before they are sold, all atmometer cups are 
checked against a standard and marked with a correction factor 
which, when applied, permits direct comparison of results obtained 
with every instrument wherever it is used. If, as is frequently true, 
the cups become dirty or accumulate a film of algae, they must be 
restandardized. If algae and fungi tend to accumulate in the reser- 
voir or on the cup, they can usually be controlled by a small piece 
of copper sulfate in the water. The error produced by the solute 
is negligible as compared to that caused by a film of algae. 

The spherical form of the atmometer cup gives it the advantage 
of exposing half its surface to the sun regardless of the sun's posi- 
tion. Other evaporimeters with different shapes have been less 
useful for this reason alone. Black cups can be used in combination 
with white and the increased evaporation resulting from their 
greater heat absorption may be used as a measure of relative light 
intensity in different habitats. 



86 



THE STUDY OF PLANT COMMUNITIES ' Chapter V 



Since the cup permits evaporation, it also will absorb water dur- 
ing rainy spells. For field work, therefore, it is necessary to install 
one of the various mercury traps designed to permit water to rise 
in the tube but not to let it return to the reservoir. A simple but 
effective trap consists of a drop of mercury in the lower end of 
the tube, held in position between two plugs of pyrex glass wool. 







t r? 







FlG. 34. Two atmometers set up and in use in a study of grassland en- 
vironment. The improvised shelter was used for max-min thermometers.— 
Photo by R. B. Livingston. 

When temperatures fall below freezing, atmometers cannot be 
used because of the danger of breakage. A summary of the devel- 
opment, uses and limitations of atmometers is given by Living- 
ston. 157 

Condensation of Atmospheric Moisture.— If air is sufficiently 
cooled, its relative humidity increases to 100 percent, or saturation. 
The slightest cooling beyond this point will result in condensation 
of the vapor to form a liquid. The temperature at which condensa- 
tion occurs (dew point) will, of course, vary with the moisture 
content of the air. 



CLIMATIC FACTORS : THE AIR 87 

Cooling of air masses is commonly caused by their expansion 
when air rises in convection currents or when moving air is forced 
to rise, as when it strikes a mountain slope. Cooling also occurs 
cyclonically, for then masses of warm and cool air may meet, and, 
depending upon which is least stable, warm air moves up over the 
cool (a warm front) or the cold air underruns the warm (a cold 
front). Of considerable local ecological significance is the contact 
cooling resulting when relatively warm air moves over a cooler 
surface or when cold air moves in over a body of warm water. 
Under these conditions, fogs or clouds may form, which not only 
may result in precipitation but may also modify the effects of 
solar radiation. 

Fog.— Any minute particles of matter in the atmosphere with 
hygroscopic properties may serve as condensation nuclei (there is 
disagreement as to their necessity) about which droplets of water 
form, the size of the droplets depending upon the speed of con- 
densation. Contact cooling usually produces only small droplets, 
which remain in the air and are visible as fog. Coastal fogs are of 
this type when they are the result of prevailing winds coming off 
the warm ocean and striking a cooler land mass. Such fogs are 
usually dissipated as the day progresses, evaporating as the tem- 
perature rises. Coastal fogs may also be caused by winds blowing 
from areas of warm water across cool currents. In summer, along 
the Pacific coast, warm air moves in from far offshore across the 
cool California current flowing from the north. Fog forms over 
the cold current and is blown inland, where it disappears if the 
land is warm but persists at night when the land is cooler. Because 
they affect light, temperature, and moisture conditions, fogs may 
be of extreme importance in determining types of coastal vegeta- 
tion and the agricultural possibilities of an area. The distribution 
of coastal redwoods of our Pacific coast forms a striking example 
of the effects of fog. In a region almost without summer rainfall, 
the coastal redwood and several associated species are almost pre- 
cisely limited to the humid fog belt along the coast. Fogs inland 
are usually over low ground, swamps, or small bodies of water, 
and are common in valleys where air movement is reduced and 
radiation cooling is effective. 

Clouds.— Clouds differ from fog only in their position. Both are 



88 THE STUDY OF PLANT COMMUNITIES • Chapter V 

made up of droplets of water suspended in the air because they are 
so minute that they do not settle out. Clouds are frequently formed 
when air is carried upward by convectional currents and is cooled 
to the dew point as it rises. Cooling and condensation with con- 




FlG. 35. Ocean fog pouring over crest of Coast Range, Oregon.— Photo 
by IV. S. Cooper. 

sequent cloud formation also result when air is forced upward 
over a mountain range and from cyclonic disturbances. 

Clouds are classified on the basis of form and position, the termi- 
nology being derived from an early simple classification in which 
four types were recognized : cirrus (curly), cumulus (piled up), 
stratus (flat), and nimbus (rain or storm). Modern systems divide 
clouds into families, each with its own type of clouds distinguished 
by descriptive names that are combinations of the old terminol- 
ogy. 265 For details about clouds and cloud forms, an illustrated 
manual should be consulted. 261 ' 128 

Precipitatio72.—Fogs and clouds reduce intensity of solar radia- 
tion that reaches the earth and may thus be of constant, though 
minor, ecological significance in certain areas. But, of more gen- 
eral importance, they are the source of precipitation when, be- 
cause of rapid condensation, their tinv droplets increase in size 
sufficiently to respond to gravity and fall to the earth. Not all 
clouds produce rain because convection may not be rapid enough 
or persistent enough to produce drops of sufficient size. Summer 
rains are frequently short and heavy because of local, vertically 
ascending air currents of high velocity. During cooler seasons, rain 



CLIMATIC FACTORS : THE AIR 



89 



is more apt to result from the slow ascent of warm air currents 
along atmospheric fronts or great shifting air masses. In the vicinity 
of mountains the same effect is obtained by moist air being forced 
upward to altitudes of lower temperature and density. The high- 
est precipitation records are usually found on windward slopes of 
mountains and are produced by such forced ascents of air. Tropi- 
cal rains, although often very heavy, are usually convectional in 
origin. 




FlG. 36. Coastal redwood forest in California, showing the characteristic 
morning fog that is a factor in its survival.— U. S. Forest Service. 



90 



THE STUDY OF PLANT COMMUNITIES ■ Chapter V 



Other forms of precipitation include snow, which is formed 
like rain but at temperatures below freezing and under conditions 
that permit the crystals to fall before they melt. Sleet is rain that 
falls through air strata of low temperature and then reaches the 
earth as clear pellets of ice. If rain falls on a cold surface and 
freezes, it is called glaze. Hail, which falls almost exclusively in 




FlG. 37. Northern hardwood stand of birch, hard maple, elm, and ash 
after a glaze storm in New York. Scarcely a tree escaped damage.— U. S. For- 
est Service. 



summer because of its dependence on convectional storms, starts 
with a snow or ice nucleus, which falls to a stratum of sufficiently 
high temperature to be partially melted. When carried upward 
again, the moisture on the surface freezes, and condensation adds 
to the size. If the process of falling and being carried up again 
is repeated several times, the successive thawing, freezing, and con- 
densation will form a concentrically layered mass of snow and ice 
of sufficient size to fall to the earth as a hailstone. 

Since hail is primarily a summer phenomenon occurring only 
under exceptional conditions, it is of little consequence to plants 
as a source of water. It may, however, do serious phvsical damage, 
often stripping foliage completely from woody plants and damag- 
ing herbaceous structures beyond recovery. Sleet and glaze are in 



CLIMATIC FACTORS : THE AIR 



91 



the same category. Glaze may be so heavy as to cause great dam- 
age to forest trees through breakage. Conifers are particularly sus- 
ceptible to such damage because of the load of ice that can accu- 
mulate on their many needles. In young stands, the trees may be 
broken down so that they die, or they may be so bent and twisted 
that, should they grow to maturity, they form badly distorted 
trees. 





FlG. 38. Average snow pack as it appears in March in the Sierra Nevada. 
Echo Summit, Calif —Courtesy of W. D. Billings. 

Snow is an important source of soil moisture and, in addition, 
may serve to modify the effects of low temperatures. Roughly 
ten inches of snow are equivalent to an inch of rain although the 
moisture content of snow is highly variable. Under average tem- 
perature conditions, water derived from melting snow might 
make up from 5 to 25 percent or more of the total precipitation, 
but its importance is not determined entirely by amount. Since 
conditions in the spring may be such that a heavy blanket of 
snow disappears in a few hours, the water may run off rapidly, 



92 THE STUDY OF PLANT COMMUNITIES • Chapter V 

especially if the soil is frozen, and be of no more significance 
than that of an extremely heavy rain of short duration. That 
same amount of snow, if it melts over a period of weeks, can re- 
lease water so slowly that practically all of it will soak into the 
soil, to become a part of a reservoir to be drawn upon during dry 
periods weeks or months later. Again, under semidesert condi- 
tions where the vapor pressure deficit is high, this may not be 
true because, if the snow remains for long periods, much of it may 
be lost by evaporation or sublimation. 

2.00 



1.60 



K 

O 1.20 

I 



i 

H 0.80 



0.40 





'rainfall 


~r 


(FIELD) ' 


1 1 

C FOREST) 


1 1 


-t — p 
1 \ 

1 ' 
1 - 

T 

t 


\ RUNOFF 


CFIELD) 


1 

RUNOFF f FOREST) 












-i 










1 


i 
i 
t 
n 
ii 
ii 










_ 


ii 
ii 
u 
• i 










\ 


1 1 

-i i 








. 1 INFILTRATION CFOREST) 


\ — 


!\ 










\ 


i / 
i / 


X -„INF_ILT RATION 


_CFIELD)_ 


\ 


N. 


i / 


1 


1 


_[ 


* 1 1 


1 ^s 



20 



40 



60 80 100 

MINUTES FROM START 



120 



140 



FlG. 39. A comparison of surface runoff and infiltration on forested pine- 
land (55 yr.) and on bare, abandoned land in Mississippi when precipitation 
was at essentially the same rate for both areas.— Adapted from Sherman and 
Musgrave. 232 



This reserve of ground water derived partly from snow be- 
comes of greatest importance where the total precipitation is 
relatively low. The grasslands of our Middle West are much 
more dependent upon the reserve of ground water than are 
forested regions where the total precipitation is greater and is 
more evenly distributed throughout the year. The success of 
agriculture, especially wheat production in the mixed prairie 
region of the Dakotas, Nebraska, Colorado, and Wyoming, is, to 
a great extent, dependent upon the reserve of soil water derived 



CLIMATIC FACTORS : THE AIR 



93 



from snow. To be sure, where snowfall comprises a high per- 
centage of the total precipitation, it must be of relatively greater 
importance than elsewhere. Subalpine forests are often almost 
completely dependent upon soil water derived from snow. The 
red fir forest along the crest of the Sierra Nevada receives prac- 
tically no rain throughout the growing season. However, the 
cool summer days at these high altitudes do not create high water 
losses, and since snow falls of sixty feet have been recorded here, 
the resulting water is adequate to maintain the forest and to pro- 
vide, as it runs off, an excess usable for agriculture at lower alti- 
tudes. 




FlG. 40. A standard rain gauge and measuring stick. Cutaway view to 
show funnel and inner tube.— Courtesy Friez Instrument Division, Bendix 
Aviation Corporation 



94 THE STUDY OF PLANT COMMUNITIES ■ Chapter V 



Snow water is of prime importance in those arctic and alpine 
regions where there is practically no rain. Here the plants are 
shallowly rooted, not uncommonly limited to the surface soil by 
perpetual frost a few inches below. Surface water must then be 
supplied continuously to maintain plant life. This is provided by 
the melting snow, some of which, in depressions or other pro- 



AVERAGE ANNUAL PRECIPITATION FOR THE UNITED STATES 

EXPRESSED IN INCHES 





75 10 100 
and owr 



FlG. 41. Average annual precipitation for the United States.— By permis- 
sion, from Bernard, 19 in Hydrology, copyrighted 1942, McGraw-Hill Book 
Company. 

tected places, may remain throughout the growing season. The 
richest flora in best condition will usually be found at the margins 
of snow patches and in drainage lines below them. Ridges and 
raised ground are the first to be exposed at the beginning of the 
growing season, and there growth begins almost immediately. As 
the season progresses, more ground is exposed by melting snow, 
and plants there begin growth. Thus, at distances of a relatively 
few feet, may be found plants of the same species, that have 
flowered, fruited, and dried up, and, in the moist soil beside the 
snow, plants which have just begun their growth. 

The total annual precipitation of an area is only a rough indi- 
cation of moisture conditions for plant growth. A light rain of 
0.15 inches usually does not affect soil moisture, for most of it 
will be intercepted by vegetation and will evaporate quickly. 
That which reaches the soil will wet only the surface and like- 



CLIMATIC FACTORS : THE AIR 



95 



wise be lost to the air. Several inches of the total rainfall may, 
therefore, be of no significance whatever except to raise the hu- 
midity temporarily and reduce transpiration for a short time. If 
rain falls heavily for short periods, say two or three inches in the 
same number of hours, much of it will be lost by runoff, the 
amount varying with steepness of slope, nature of the soil, and 

Grassland Deciduous Forest Deciduous Forest Coniferous Forest 

Cheyenne. Wyo. Indianapolis. Ino. Richmond.Va Ottawa, Canada 



Jan. 




JULY 





FlG. 42. Annual precipitation patterns (based on averages) for several 
stations, which illustrate the relative amounts and distribution of precipitation 
throughout the year for areas supporting grassland, deciduous forest, and 
coniferous forest.— A dapted jrom Transeau. 2 



256 



amount and kind of cover. Again, the seasonal distribution of 
rainfall may be of much more importance than the total amount. 
If rainfall is uniformly distributed throughout the growing sea- 
son, moisture conditions may be far more favorable with twenty- 
five to thirty inches than they would be with forty to forty-five 
inches if the growing season is interrupted by one or more pro- 
tracted dry spells. If precipitation is regularly seasonal, the type 
of vegetation may be definitely limited. For instance, grasslands 
characterize those areas where rainfall is rather light and con- 
centrated in the spring and early summer. Winter rains with dry 
summers, characteristic of several coastal regions, support shrub- 
by vegetation. 

Measurement of Precipitation.— A standard rain gauge is a cyl- 
inder 8 inches in diameter and 20 inches high, which has a funnel 
built into the upper end that permits the water it catches to run 
into an inner cylinder with exactly one-tenth the cross-sectional 
area. The ratio of the outer to the inner cylinder being 10:1, the 
measurement of water collected in the tube must be divided bv 
ten if taken directly, or it can be measured with a standard 



96 



THE STUDY OF PLANT COMMUNITIES • Chapter V 



graduated rod. The 10:1 ratio makes accurate readings possible 
to 0.01 inch. Exceptionally heavy rains may overflow the tube, 
and the water in the large cylinder must then be poured over 
into the emptied tube for measurement. Two types of recording 
gauges are in use. 19 One registers increments of fall as a small 
bucket fills, tips, and records; the other, a weighing type, records 
accumulative precipitation as it falls. 



Chestnut-Chestnut i 
Longleof- 




r^V^ Tall Grass 
\/ / /\ Short Gross 



E353 Bunch Grass 



5223 Marsh Grass EZZg ChOPOTTOl 
WZZZZ Desert Savanna EZ22 Pacific 



Sagebrush 



Douglas Fir 



[Xm Creosote Bush ^^ °^ heT Western 
ESS3 Greasewood Forests 



K/H 



Forests 
Oak- Hickory 



^^ Oak- Pine 



FlG. 43. Isoclimatic lines of vapor pressure deficits and vegetation areas 
of the United States.— From Huffaker. vlt 



125 



For generalized field studies, the precipitation records from the 
nearest weather station may be quite satisfactory. However, there 
may be wide local variations, especially if the topography is 
irregular, and, in mountainous regions, only local measurements 
have real significance. In addition, under forest stands, the pre- 
cipitation reaching the soil will vary from stand to stand because 
of variation in interception. Thus, for intensive work, it is desir- 
able to maintain rain gauges at each site of study. Although stand- 
ard gauges are desirable, it is possible to obtain satisfactory 
records for comparison by using straight-walled jars or cans of 
equal diameter. 

Snowfall is measured at a point where the wind has not caused 
drifting or disturbance, and the equivalent value in rain is com- 
puted from samples of the snow. Depending upon the density of 



CLIMATIC FACTORS : THE AIR 



97 



the snow, the ratio may range from 5:1 to 50:1, but 10:1 is fairly 
average. Careful records of snowfall and water equivalents have 
not been generally kept until recently. In the western mountains, 
where melting snow may be the only source of water for distant 
low country, such records make possible forecasting of floods 
and, more particularly, the supply of water available for irriga- 
tion. 54 




FlG. 44. Precipitation-evaporation ratios for the United States calculated 
according to Transeau. 255 — By permission from Jenny, Factors of Soil Forma- 
tion, copyrighted 1941, McGraw-Hill Book Co. 

Atmospheric Moisture and Vegetation.— It should be clear that 
any single atmospheric factor is insufficient in itself to explain the 
distribution and survival of species or plant communities. Pre- 
cipitation records are only suggestive, for they must be inter- 
preted in terms of seasonal distribution, and they are not at all 
indicative of soil moisture conditions or of the evaporating power 
of the air to which a plant must be adjusted if it is to survive. 
The variation in the seasonal pattern of precipitation from place 
to place becomes particularly apparent when illustrated with 
twelve-point polygonal diagrams, 256 which make possible easy 
comparison of amount and time of rainfall by months. Evapora- 
tion alone is a poor criterion of ecological conditions since it does 
not take into account the amount of water supplied to the soil. 



98 



THE STUDY OF PLANT COMMUNITIES • Chapter V 




-30 



^WINTER 



-WINTER 



DECIDUOUS EOREST. 
BOREAL FOREST 



1 • 








o 


90- 






UJ 




*' % 




ir 




SUMMER N. 




=3 




""I 




I— 


80- 






cc 










~"\ / 




UJ 




\ ' 




Q_ 


70- 


\ / 




:> 




\ ,' 




UJ 




y 




1— 




A 




> 


60- 


1 1 i' \ 




_J 




\ ' \ 




3Z 




; I / \ 




I— 


50- 


• \ / \ 




o 




IV / H 




s 








40- 


» \ ' \ 




^ 




\ l y \ 




< 




i V' \ 




UJ 




v 1 ) 




s 


30- 


\ WINTER / 


SO DESERT SHRUB.... 
SAI-.FRRIISW 




20-f 

C 


1 2 


1 r — - — i 

3 4 5 6 



MEAN MONTHLY RAINFALLOnche; 



MEAN MONTHLY RAINFALL(inches) 



'. SUMMER 




WINTER 



SHORT GRASS. 
TALL GRASS- 



MEAN MONTHLY RAINFALL(inches) 



90- 



80- 



UJ 70- 
Q_ 



60 



50 



40- 



30- 



20 



SUMMER 




H, 



WINTER 



ALBUQUERQUE. N.M. 

EL PASO, TEXAS 



T 



2.5 



MEAN MONTHLY RAINFALL (Inches) 



Fig. 45. Three sets of composite climographs, which permit comparison 
of forest, desert, and grassland climates, as well as differences within these 
general types of vegetation. The fourth set, which shows the similarity of 
climates at stations in New Mexico and Texas, has been used as support for 
classifying El Paso grassland as short grass, of which Albuquerque is repre- 
sentative, and not desert grassland as some have done.— From Smith™ 

When .points of equal vapor pressure deficits are connected by 
lines on a vegetation map, 125 the zones come nearer to matching 
the distribution of vegetation tvpes on a regional basis than simi- 
lar ones based on evaporation. Seeking a single comprehensive 



CLIMATIC FACTORS : THE AIR 



99 



value that would include several factors operative in plant dis- 
tribution, Transeau 255 used the ratio of precipitation to evapo- 
ration (P/E) for plotting climatic zones. These zones match the 
limits of vegetation types remarkably well, but the method is 
limited by the availability of adequate and comparable evapora- 
tion data. 

A graphic method for distinguishing differences and similari- 
ties in atmospheric conditions is the climograph, in which mean 
temperature is plotted against mean relative humidity by months, 
and the points are connected to form highly distinctive twelve- 
pointed figures. Introduced by Ball 11 for indicating climate of 




FlG. 46. Cup anemometer, Weather Bureau type, for relatively permanent 
operation, and a Biram type anemometer, convenient for short-time measure- 
ments.— Courtesy Friez Instrument Division, Bendix Aviation Corporation. 



100 THE STUDY OF PLANT COMMUNITIES • Chapter V 

geographic areas, it has been variously used for comparing cli- 
mates in studies of the distribution, migration, and success of pop- 
ulations of man, birds, and insects. The system is subject to modi- 
fication and has been used also as a graph of temperature-precipi- 
tation (sometimes called a hythergraph). The latter method has 
been used 243 for characterizing climates of widely differing climax 
types in different parts of the world and for distinguishing grass- 
land climates in North America. 52 The method probably has not 
been given the use it deserves in plant studies. Because tempera- 
ture-relative humidity diagrams have been used with some suc- 
cess, it seems reasonable to suggest that similar graphs of tempera- 
ture-vapor pressure deficit might give even more distinctive pat- 
terns and might, therefore, be even more useful in detailed studies. 

WIND 

Air moves from a region of high pressure to one of low pressure, 
and the differences in pressure are largely the result of unequal 
heating of the atmosphere. The equatorial regions receive more 
heat than regions to the north or south; consequently, low pres- 
sures normally exist in the lower latitudes. The tendency, then, is 
for air to move from the poles toward the Equator, there to rise 
and return toward the poles. This pattern, although true in gen- 
eral, is modified by the deflecting action of the earth's rotation and 
by differences in temperature resulting from oceans and land 
masses. 

Continents in temperate zones tend to become very hot in sum- 
mer, and the resulting low pressures produce winds that blow 
inland. The cold of winter reverses the pressure, and winds tend 
to be outblowing. In mountainous areas or along sea coasts these 
seasonal trends may have daily variations again produced by tem- 
perature-pressure differences. Mountain valleys and slopes, which 
are often warmed rapidly during the day, produce valley breezes 
blowing upward. At night, the rapid cooling of bare high ridges 
results in a flow of cold air down the valleys. The contrast between 
day and night temperatures of land and water results in an off- 
shore breeze at night as the land cools rapidly and higher pressures 
result. During the day, the land again heats up rapidly above the 
temperature of the sea, and an inshore breeze develops that may 



CLIMATIC FACTORS : THE AIR 101 

be noticeable for several miles inland. This brief summary of fac- 
tors producing wind should serve to emphasize that air is almost 
constantly in motion and should suggest that, within limits, the 
general plan of motion is predictable for seasons and parts of the 
earth. 

Measurement of Wind.— Wind velocity is measured with some 
form of anemometer. The cup anemometer used by the United 
States Weather Bureau has three or four hemispherical or conical 
cups, each attached to horizontal arms that rotate on a vertical axis 
and thus drive a gear system, which turns indicator dials. These 
are readable in miles per unit of time, usually expressed as miles 
per hour. More elaborate instruments may be equipped with auto- 
matic recording devices. 

The cup anemometer is inconvenient to carry and operate in 
the field. In the Biram portable anemometer, a small fan drives the 
dial indicating air movement. The device is useful in small spaces 
and for short readings. Since it has no vane, it must be set to face 
the wind. 

Physiological-Anatomical Effects of Wind.— The movement of 
air being in general characteristic of all environments, plants are 
largely unaffected by it under average conditions. In certain situa- 
tions, however, wind may be an extremely important factor. Plants 
growing in habitats exposed to continuous winds of moderate 
velocity transpire more rapidly than unexposed individuals. If the 
prevailing winds are from one direction, the side of a plant toward 
the wind may be so desiccated that new growth is killed before it 
is well begun. Lateral buds taking over the growth may or may 
not survive, and a scrubby, matted growth develops on the wind- 
ward side. To leeward, the new shoots are protected by the rest of 
the plant, and growth goes on there, resulting, over a period of 
years, in asymmetric growth forms of amazing shape. Such one- 
sided growth is commonly found in exposed places at high alti- 
tudes in the mountains where otherwise upright plants may be 
prostrate and form mats fitting into hollows or behind protecting 
rocks. Not uncommonly a forest stand on the protected side of a 
ridge or in a ravine may appear as though every tree had had its 
tip sheared to an exact height limit. Again, this is due to the desic- 
cating effect of the prevailing wind. 



102 THE STUDY OF PLANT COMMUNITIES • Chapter V 

Asymmetric growth, matted vegetation, and sheared tops as 
seen along the coast are likewise produced by wind to some extent, 
but here an added factor plays a part. The wind picks up spray as 




FlG. 47. Prostrate and matted, wind-sheared trees {Firms albicaulis, Tsuga 
mertensiana) on a leeward slope near timber line, Mt. Hood, Ore. The 
twisted form is commonly termed Krummholz.—U . S. Forest Service. 

it comes in over the breaking waves. The spray may be carried 
several miles inland, especially in severe storms, but its major 
effects are most noticeable near the coast. The spray that strikes 
any obstacle is dropped there and, of course, the salt from the 
spray accumulates on that object. Few dune and coastal plants are 
completely tolerant to salt spray, but, fortunately, most strong 
winds are accompanied by rain, which minimizes the effects by 
dilution and washing. If a severe windstorm is not accompanied 
by or soon followed by rain, much vegetation will be injured or 
killed by salt spray even for some distance inland. 

Those plants growing near the beach are sprayed lightly almost 
daily and, as might be expected, show different degrees of toler- 
ance. This results in zonation of vegetation associated with expos- 
ure to the wind. 188 Undoubtedly salt spray is one of the strong 
factors in determining the make-up and distribution of all plant 
communities on coastal dunes. 270 

When trees grow on one side only, they may become so heavy 
as to uproot themselves, but usually the eccentric growth is slow 
enough to permit compensating anatomical changes, particularly 
in the trunk. Secondary growth may cease completely on the 



CLIMATIC FACTORS : THE AIR 103 

windward side of the trunk and increase proportionately on the 
leeward side, thus forming a brace under the added top. An ex- 
treme illustration is a section of trunk taken* from a Monterey 
cypress that grew on Cypress Point, just south of Carmel Bay, 
California. It is 74 inches in the diameter that grew parallel to the 
prevailing wind but is only 9 inches in the opposite diameter. Only 
50 growth rings were formed on the windward portion of the 
section, but the leeward portion (71 in.) has 304 rings. This section 
was taken 24 feet above the ground. 

Other physiological effects might be mentioned, but they are 
largely brought about within the plants themselves through adap- 
tations that serve to reduce the rate of transpiration through their 
effects on stomata. In the drier sections of the country, such as the 
plains and desert, the almost continuous dry winds increase tran- 
spiration rates materially and serve to accentuate the effects of low 
water supply. 

Physical Effects on Plants.— Most people have seen the effects of 
a strong wind (25-38 miles per hour) upon vegetation. It is not 
uncommon for dead branches to be torn from trees; an occasional 
tree, especially if overmature and diseased, may be blown down. 



(—■■■■'■ 




FlG. 48. Asymmetric growth of a live oak (Quercus virginiana) exposed 
to ocean wind and salt spray from the right. North Carolina coast.— U. S. 
Forest Service. 



*Collected by and in the possession of W S. Cooper, University of Minne- 
sota. 



104 THE STUDY OF PLANT COMMUNITIES • Chapter V 




FlG. 49. A white pine stand in New Hampshire after the storm of 1938. 
Such damage was prevalent over much of New England at the time- U. S. 
Forest Service. 



Closed forest stands usually suffer no major damage because the 
trees give support to each other. With greater velocities the wind 
becomes increasingly destructive. At gale velocities (39-54 m.p.h.) 
branches are broken, and a full gale uproots trees with ease. 

Many of the destructive storms along the Gulf coast approach 
hurricane speeds, and it is fortunate that they infrequently reach 
the mainland. The most destructive hurricane in recent years 
(1938) moved northward along the Atlantic coast and struck in- 
land at 70 miles per hour at Long Island and into west-central 
New England. The destruction in its path was extreme. Whole 
forests fell before the wind, the trees uprooted or broken off. An 
added factor in the destructiveness of this storm was the saturated 
soil, produced by a preceding period of heavy rain, which con- 



CLIMATIC FACTORS : THE AIR 



105 



tributed to the ease and amount of uprooting and wind throw. 
Storms of such force and destructiveness are rare in North Amer- 
ica, but lesser winds may cause considerable damage. When closed 
forest stands are thinned or selectively cut, the remaining trees are 
subject to wind throw for a number of years even though wind 
does not blow with great velocity. 

In addition to physiologically-produced flag forms of woody 
vegetation, there are those resulting from purely physical effects 
of wind. A study of asymmetric trees in the Columbia River 
Gorge 152 showed that, when branches are continually bent in one 
direction by prevailing winds, the branches become "wind trained" 
and hold their positions permanently. Some grew completely 
around the trunk from the windward to the leeward side. Still 
another cause of asymmetry was found here. Severe winter storms, 
coming largely from one direction, cause much breakage, espe- 
cially when accompanied by sleet, and almost complete pruning of 
branches on the windward side often results. 

Transportation by Wind.— We have already indicated how im- 
portant to precipitation are the vapor-laden winds moving inland 




FlG. 50. Wind throw often results when trees are uprooted, especially if 
on shallow or wet soil. Here is shown a giant Douglas fir in Washington 
whose torn-up root system had a spread of fifty feet— U. S. Forest Service. 



106 THE STUDY OF PLANT COMMUNITIES • Chapter V 















n 



FlG. 51. Flag-form trees in the Columbia River Gorge, Ore. (1-2) Storm- 
pruned Douglas fir, deformed by breakage and killing due to glaze storms 
and strong west winds of midwinter. (3-4) Wind-trained Douglas fir shaped 
by long-continued pressure of strong west winds of late spring and summer.— 
From Lawrence. 1 ' 32 



from large bodies of water and how transporting salt spray may 
be of local significance. Wind plays a more direct role in trans- 
porting pollen and in dissemination. 



CLIMATIC FACTORS : THE AIR 107 

Wind-borne Pollen.— Many pollen grains are light and small or, 
as in conifers, have bladder-like wings, which increase their buoy- 
ancy. As a result, they may be carried for great distances by the 
wind. The chances that an individual pollen grain will accomplish 
its function must be extremely small. This uncertainty is compen- 
sated for in quantity of pollen produced. Efdtman 97 gives the 
pollen production for several wind-pollinated European species 
from which the following are selected : Rumex cicetosct produces 
30,000 grains per stamen, Acer platanoides, 1,000; pollen output per 
staminate cone of gymnosperms may be judged by Pinus nigra, 
1,480,000; Ficea excelsa, 590,000; and Jwiiperns communis, 400,- 
000; production per flower of angiosperms ranges from Rumex 
acetosa, 180,000, through Tilia cordata, 43,500, to Acer platan- 
oides, 8,000. Such figures for single stamens and flowers serve to 
explain the continuous and enormous rain of conspicuous pollen 
that may fall in season, especially from conifers. Sidewalks, 
porches, floors, tables— everything in the vicinity of a coniferous 
forest— may be dusted with pollen. 

Not all noticeable pollen is locally produced, and a great deal of 
evidence has been accumulated to show irregular and normal dis- 
tributions. There is a story that, in the early days of the city of St, 
Louis, it was at one time continuously showered with a yellow 
dust, which gave residents some concern until botanists identified 
it as pollen of Pinus palustris transported from the coastal plain far 
to the south. Some quirk of pressure and wind was depositing the 
pollen upon St. Louis. 

There are numerous records of pollen being transported long 
distances. 97 Spruce, pine, and birch pollen was collected on light- 
ships in the Gulf of Bothnia thirty and fifty-five kilometers off the 
coast. Spruce pollen is carried from southern to northern Sweden. 
Peat samples taken in Greenland contained pollen of Picea mari- 
ana and Pinus banksiana, which must have originated on Labrador 
or southwestward. One of the most interesting studies of pollen 
transport was made by Erdtman as he crossed the Atlantic from 
Gothenburg to New York. Using a vacuum cleaner equipped with 
filters, he obtained a more or less continuous quantitative record 
of pollen in the air on the entire trip. Numbers of grains decreased 
with distance from land, but at no time did sampling fail to show 



108 THE STUDY OF PLANT COMMUNITIES • Chapter V 

some pollen. The evidence is to the effect that birch, pine, oak, 
willow, sedge, and grass pollen are carried in quantities for more 
than one thousand kilometers over the ocean. 

The amount of pollen in the air and the distance it is trans- 
ported is of significance to some plants but more so to many 
people who suffer from hay fever. Recently the kinds and num- 
bers of pollen grains in the air in many sections of the country 
are determined daily and made publicly available for the use of 
hay-fever sufferers. 

In general, wind-pollinated plants grow in the open or in ex- 
posed places. Even in a forest it is the trees of the upper strata that 
are characteristically wind pollinated; the flowers are small and 
inconspicuous, with simple or reduced structure. The corolla is 
often lacking, and there is an absence of bright colors, odor, and 
nectar. Stamens and pistils are commonly borne in different flow- 
ers, the stigmas are usually feathery, and the stamens are long and 
pendant. In spite of its apparent wastefulness, the system produces 
satisfactory results. 

Dissemination— Plants migrate from one point to another by 
means of spores, seeds, fruits, fragments of plants, or entire plants. 
The agent of transport may be water, animals, or wind, depending 
upon the various adaptations of the disseminules, which facilitate 
the movement. 

Dissemination by spores is characteristic of all plants except 
spermatophytes. Wind-disseminated spores, like pollen, are small 
and dry and may be transported great distances. Everywhere that 
pollen is carried, spores are found too. Their transportation over 
long distances can be of great ecological and economic impor- 
tance. A spore carried by a freak wind into distant territory may 
establish a species where it has never grown before, thus extending 
the range of the species and possibly necessitating adjustments 
within the community in which it develops. 275 The economic con- 
siderations are fairly obvious when it is remembered that fungi 
that produce diseases of both plants and animals are all propagated 
by spores. The fight against wheat rust is a case in point. When- 
ever a resistant strain of wheat is developed, it is immediately sub- 
ject to attack by mutating strains of the rust, whether these strains 
are of local origin or not. There is evidence that strains of rust 



CLIMATIC FACTORS : THE AIR 



109 



appearing in the Dakotas have come from wind-borne spores pro- 
duced as far away as Mexico. 246 

Seeds, fruits, and fragments of plants are effective -as dissem- 
inules in proportion to the devices that facilitate their transport. 
Wind dissemination is increased by the presence of winged struc- 
tures, bladder-like protrusions, or plumose extensions of the 




FlG. 52. Approaching dust storm near Springfield, Colo. (1937), which 
was typical of conditions in the "dust bowl" during the drought of the 1930's. 
— U. S. Forest Service. 

surface (see Fig. 93). Seeds, because of their small size, are apt to 
be carried farther than fruits, but for all, the kind of adaptation is 
an important factor in transport. The perfection of the parachute- 
like pappus is illustrated by the ubiquitous dandelion and related 
composites of field and roadside. Many winged fruits do not travel 
far because of their size, but often the wings (ash, elm, maple, 
basswood) are sufficient to assure transport beyond the shading 
and competitive effects of the parent tree. 

The transport of entire plants is well illustrated by the tumble- 
weeds (Salsola, Cycloloma). These have but a single main root, 
which, when broken at the ground surface, releases the spherical 
plant to roll before the wind until caught, perhaps in some fence 
corner. As it rolls, the seeds are gradually shed, sometimes miles 
from the place of growth. 

The pioneers in a new habitat usually have effective means of 
dissemination and an abundance of seed. The same is true of weeds 
of cultivated fields and waste ground. The more common and 



110 THE STUDY OF PLANT COMMUNITIES • Chapter V 




FlG. 53. Soil blowing out of a Kansas wheat field in the 1930's and piling 
up on highway, where fences and trees partially checked its movement.— 
U. S. Forest Service. 




FlG. 54. Road cut through a deep deposit of loess in iMissouri. The al- 
most vertical banks have stood for eighteen years without eroding.— U. S. 
Forest Service. 



CLIMATIC FACTORS : THE AIR 



111 




FlG. 55. Extensive active sand dunes on the coast of Oregon showing 
transverse ridges that have typical form with gradual slope to windward and 
an abrupt drop to leeward.— Photo by W. S. Cooper. 




FlG. 56. Blowout in Oregon coastal dune that was once completely stab- 
ilized by vegetation. This is a compound blowout as indicated by the partial- 
ly stabilized surface of an earlier blowout (lower right), which was again 
excavated to a lower level by later blowout.— Photo by W. S. Cooper. 



112 THE STUDY OF PLANT COMMUNITIES ■ Chapter V 



widespread a species is, the more efficient are the mechanisms that 
facilitate its dispersal, regardless of whether the agent be wind, 
animals, water, ice, or gravity. 

Wind and Soil— The, slightest air movement shifts dust particles 
from place to place, and increasing velocity results in the transport 






^^^^l^m^r^^^ 



(fa£j. 




FlG. 57. "Graveyard forest" near Florence, Oregon. Once a closed stand 
(probably mostly Finns contorta) growing on the soil layer which is broken 
through in the foreground, the forest was completely buried by the dune 
now appearing in the rear which subsequently moved on to uncover it again. 
The view is to leeward.— Photo by W. S. Cooper. 



of larger particles of soil in increasing amounts. Although fine 
materials are everywhere being shifted by wind, its greatest effects 
are noticeable in dry climates where there is a prevailing wind and 
a minimum of vegetation. During extended droughts, the culti- 
vated, semiarid regions of our Midwest and Southwest have at 
times become shifting seas of drifting soil, and the clouds of fine 
materials carried about in the air have given rise to the term, "dust 
bowl!' 

Over an extended period of time great quantities of materials 
may be transported and deposited by wind, as is demonstrated by 
the enormous deposits of loess in various parts of the world. This 



CLIMATIC FACTORS : THE AIR 



113 



fine-grained, fertile soil occurs in deposits from a few to fifty feet 
deep or more over thousands of square miles in the central Missis- 
sippi Valley region. Our richest farm lands in Iowa, Nebraska, and 
Kansas are on loess soils. The deposits occur along the Rhine in 
Europe and in the pampas of Argentina, and reach their greatest 
extent in Asia, particularly in north-central China. Loess probably 
originated during the glacial period as dust was swept up from 




Fig. 58. Coastal sand dune moving inland and encroaching on evergreen 
maritime forest near Kitty Hawk, N. C. Grasses in foreground have been 
planted.— Photo by C. E Korstian. 



the barren flood plains of glacial rivers and carried high into the 
air, from which it settled more or less uniformly over wide areas. 
Sand beaches and desert regions are commonly dry, free of 
vegetation, and swept by prevailing winds, which carry the soil 
along near the earth's surface. Any obstacle that checks the veloc- 
ity of the wind causes some of its load to be deposited and starts 
a mound or ridge called a dune. Some dunes grow, by the deposit 
of more sand, to a height of several hundred feet, but usually they 
are much smaller. Most of the sand is deposited near the crest or 
on the lee slope; this results in a characteristic gentle windward 
slope and a sharp drop on the lee slope, the steepness of which is 



114 THE STUDY OF PLANT COMMUNITIES * Chapter V 

determined by the angle of rest of the sand. Because wind fre- 
quently changes direction, dunes are rarely stable for long and 
present a constantly shifting pattern. Along sea coasts they tend 
to move inland as sand is carried from the windward side and 
dropped down the lee side. 




^aW***^''* 







&*■< 



FlG. 59. Planted grasses and brush fences set up on shifting sand as part 
of a dune-stabilization program developed by the Civilian Conservation 
Corps. The attempt was partially successful but was not followed up with 
later work, which would have added to its success.— Photo by C. R Korstian. 



A dune is never completely stable unless covered with a con- 
tinuous mat of vegetation. Should this mat be broken for any rea- 
son, a "blowout" results, which may enlarge and start again the 
shifting of the entire dune. Many cottage owners have learned this 
to their sorrow when— as has happened on Lake Michigan dunes— 
they have returned after a single year to find their summer homes 
almost completely buried under a shifting dune that had been stable 
for years. The encroachment of dunes on forest areas is not un- 



CLIMATIC FACTORS : THE AIR 1 1 5 



common. Whole forest stands may be buried and subsequently, 
with shifting winds, be uncovered again to expose "graveyard" 
forests of dead trunks and branches. 

The extensive dunes on the banks along the coast of the Caro- 
linas have in recent years become increasingly active because their 
cover was broken or reduced by overgrazing and other disturb- 
ances by man. Acres of maritime forest have been buried, build- 
ings have been destroyed, and channels in the waterways have 
been blocked. Here, as in the dust bowl, are problems that require 
drastic measures for solution, but such measures must take into 
consideration the ecological factors involved. Cover crops, strip 
cropping, mulching, and other modified methods of cultivation 
are now general practice in the dust bowl and promise to give 
some relief should an extended drought occur again. Long ago 
many European coastal dunes were planted with forests and ef- 
fectively stabilized. The Carolina dunes, though, occupy thousands 
of acres with almost bare sand on which forests cannot be planted 
until some stability is attained. Kill Devil Hill, the dune from 
which the Wright brothers made their historic first flight, was 
stabilized with grasses by Army engineers, after much effort and 
considerable cost, through use of sodding, seeding, and watering. 
Such methods are impractical on thousands of acres. The efforts 
of the Civilian Conservation Corps were at least partially success- 
ful. Taking only the native dune grasses, they transplanted them 
according to several spacing systems and with some regard to 
habitat variation over several hundreds of acres. Combined with 
plantings, brush fences were installed at regular intervals across 
the largest blowouts. A considerable part of their work has proven 
effective. 

GENERAL REFERENCES 

R. F. DAUBENMIRE. Plants and Environment. New York : John Wiley and 

Sons Co., 1948. 424 pp. 
W J. Humphreys. Fogs and Clouds. 
O. E. MEINZER. (ed.) Hydrology. New York : McGraw-Hill Book Co., 

1942. 712 pp. 

C. W THORNTHWAITE. Atmospheric Moisture in Relation to Ecological 

Problems. 
G. T Trewartha. An Introduction to Weather and Climate. 
H. B. WARD and W E. POWERS. Weather and Climate. 



1/ 



CHAPTER VI 



CLIMATIC FACTORS: RADIANT ENERGY 
TEMPERATURE AND LIGHT 

The sun is the source of the earth's radiant energy (insolation). 
This energy, radiating as waves, includes those wave lengths of 
the visible spectrum that we term "light" and those that lie just 
beyond the visible spectrum, called "heat" if slightly longer, or 
"ultraviolet light" if shorter. The amount of insolation reaching 
the earth is always reduced because of absorption by the atmos- 
phere (6-8 percent), and as much as 40 percent may be reflected 
by clouds. The remainder reaching soil or water on the earth may 
be further varied by such factors as distance from the sun at dif- 
ferent seasons, duration of radiation, and the angle of the rays with 
the earth's surface. The last determines the amount of air through 
which the rays pass, modifies the amount of reflection and absorp- 
tion, and likewise controls the amount of energy falling on a unit 
area simply by spreading or concentrating a given amount of 
energy over more or less space. With these things in mind, insola- 
tional variation with latitude and topography are more easily ex- 
plained. 

Insolation varies only slightly at the Equator because the angle 
of the sun's rays never exceeds 23 1 /2° from zenith, and the days 
are uniformly twelve hours long. Twice a year, on March 2 1 and 
September 21, called the equinoxes, the sun is at zenith at the 
Equator at noon and its circle of illumination exactly reaches the 
North and South poles simultaneously. After March 21, because 
of the tilt of the earth's axis, the North Pole comes progressively 
nearer to the sun until June 21, after which the shift is reversed 
to bring the pole back to the equinox position by September 21. 
The North Pole's movement away from the sun continues until 
December 21, after which it starts its shift back to the June posi- 
tion again. The shifting of the pole toward the sun causes the 
circle of illumination to extend far beyond the pole and results in 
continuous insolation at the pole during the June solstice, but, 

116 



CLIMATIC FACTORS : RADIANT ENERGY 



117 



since the December solstice results in diametrically opposite con- 
ditions, it represents a period without insolation. Conditions in the 
Southern Hemisphere are, of course, always exactly reversed. 
Thus, because of differences in insolation, we have seasons 





VERNAL EQUINOX 
MAR. 21 






SUN 



WINTER 

SOLSTICE 

DEC.2I 






AUTUMNAL EQUINOX 
SEPT 23 





Fig. 60. Diagrammatic representation of the changing position of the earth 
with respect to the sun and its relationship to insolation and change of sea- 
sons in the Northern Hemisphere.— Adapted from Trewartha. 



258 



marked by variation in length of day and temperature. Since the 
periodic differences in insolation become more marked with dis- 
tance from the Equator, the seasons likewise become more distinct 
with increasing latitude. The greatest total insolation, however, 
occurs at the Equator and decreases with distance from the Equa- 
tor in spite of the increasing length of day. Toward the poles, in- 
tensity of insolation is reduced because of the increasing angle of 
incidence. 



118 THE STUDY OF PLANT COMMUNITIES ' Chapter VI 

These introductory statements refer to insolation as a whole. We 
may now more conveniently consider separately the visible portion 
of the spectrum known as light, and those longer, invisible wave 
lengths known as heat, whose presence or absence are expressed 
as temperature. 




FlG. 61. Circle of illumination, areas of daylight and darkness, angles of 
sun's rays at different latitudes, and differences in areas affected and thickness 
of atmosphere penetrated at time of summer solstice.— Adapted from Ward 
and Powers. 265 

TEMPERATURE 

General Plant Relationships.— Each living thing is restricted to 
a definite temperature range, which may be quite dissimilar for 
different species and, depending largely upon the amount of water 
in the protoplasm, may vary for individuals of a species. The wide 
range of tolerance among species is illustrated, on the one hand, 
by subarctic conifer forests where -80° F. has been recorded and, 
on the other, by desert plants that withstand temperatures of 130- 
140° F. Dormant structures such as seeds and spores are practically 
without water and can, therefore, withstand the widest tempera- 
ture variations and extremes. 

Plant injuries from temperature changes are most often the 
result of freezing, which desiccates the tissues when the pure water 



CLIMATIC FACTORS : RADIANT ENERGY 119 

on the cell walls crystallizes in the intercellular spaces and con- 
tinues to crystalize as it is replaced from the vacuole and proto- 
plasm. Injurious chemical changes, such as the precipitation of 
proteins, may accompany the desiccation. Some species, however— 
especially subtropical ones— are often killed before temperatures 
fall as low as freezing. Temperature injuries cannot always be ex- 
plained in simple terms. 

It is obvious that there must be seasonal and other adjustments 
in some plants, which permit their survival as cold weather comes 
on. It is known, in this connection, that the concentration of the 
cell sap of most conifers increases in the fall. Gardeners make use 
of this characteristic, for young plants grown in greenhouses are 
"hardened" before they are set out and subjected to early spring 
temperature fluctuations. Such plants are most liable to injury 
when temperature changes are abrupt and extreme. On the other 
hand, many arctic and alpine species can grow, flower and fruit 
during a period when they are subjected almost daily to alternate 
freezing and thawing. 

Measurement of Temperature.— Accurate standardized glass 
thermometers are the most useful instruments for field studies. Air 
temperatures are usually taken in the shade with the thermometer 
exposed to the wind and away from the influence of one's body. 
Soil temperatures require a small well of some sort, or, when meas- 
urements are to be made periodically, a length of pipe may be 
permanently sunk to the desired depth. If the thermometer is 
suspended in the pipe by a string, it can be drawn up quickly and 
read before much change takes place. Soil temperatures at or very 
near the soil surface are difficult to obtain accurately with an ordi- 
nary thermometer because of the steep gradients from the surface 
downward, and upward into the air. The size of the thermometer 
bulb is sufficient to be affected by rather widely differing tempera- 
tures even when it is no thicker than 5 mm. Discrepancies have 
been observed as great as 1 1 ° C. between electrical (thermocouple) 
and ordinary thermometer readings at the surface. The errors are 
greatest in full sunlight and on dark soils. 86 It is under these con- 
ditions that the greatest care must be taken in placing the bulb. 

Continuous temperature records are obtainable with thermo- 
graphs. These usually consist of an expansion element attached by 



120 THE S TUDY OF PLANT COMMUNITIES * Chapter VI 

levers to a pen, which records on a graduated sheet revolving on a 
drum. Both air and soil thermographs are used and are also obtain- 
able in the same instrument, thus giving parallel records. If cost of 
these instruments prohibits their use where a comparative study 
of numerous stations is to be made, maximum and minimum ther- 
mometers are the simplest solution. Placed in pairs and read and 




FIG. 62. Soil-air thermograph, which records the temperatures of soil and 
air continuously on a revolving drum. The cable at right is about six feet 
long and terminates in a sensitive bulb (not shown), which can be placed at 
any level in the soil— Courtesy Friez Instrument Division, Bendix Aviation 
Corporation. 

reset at regular intervals, they give the useful values of maximum 
and minimum temperatures for the period of exposure. Above the 
reservoir in a maximum thermometer is a constriction through 
which, because of the force and volume of mercury involved, ex- 
pansion easily forces the liquid. Contraction, however, develops 
no pressure above the constriction, and the capillary column re- 
mains essentially at the level of its highest rise. As with a clinical 
thermometer, the column must be shaken or spun back down to 
the reservoir when a new reading is desired. The minimum ther- 
mometer has a small marker in its liquid, which, because of surface 
tension at the top of the column, is pulled down as the tempera- 
ture is lowered but is not raised with increasing temperature. Tilt- 
ing the thermometer will immediately bring the marker back to 
the top of the column in a new setting position. 



CLIMATIC FACTORS : RADIANT ENERGY 



121 



Temperature Records.— Because temperature is so extremely 
variable, isolated or even numerous single determinations may be 




Fig. 63. Maximum-minimum thermometers of a standard type for air tem- 
peratures. Installed in an instrument shelter, the holder permits whirling of 
the maximum thermometer for resetting.— Courtesy Friez Instrument Divi- 
sion, Bendix Aviation Corporation. 

completely useless. A continuous record is most desirable because 
it gives the duration of extremes and variations. Although extremes 
may be important in the reaction of a plant, their duration is apt 
to be what determines the plant's response. Therefore, a thermo- 
graph is desirable for thoroughly satisfactory work. The "mean 
temperature" as computed by the United States Weather Bureau 141 
is usually the average of the maximum and minimum for the day. 
This is not accurate or truly indicative of plant-temperature rela- 
tions because it ignores duration and is likely to run too high. The 



122 THE STUDY OF PLANT COMMUNITIES ' Chapter VI 

true mean is more nearly approached by averaging the hourly 
temperatures for twenty-four-hour periods. 

Annual mean temperatures are almost useless ecologically, for 
they do not indicate seasonal variation and duration. Temperate 
desert regions may have amazingly high annual mean temperatures 
and yet have winter frosts, which constitute an important limiting 
factor in the survival of certain species there. Subarctic areas may 
support forest vegetation because of the warm summers, yet mean 
temperatures may be so far below freezing that they suggest that 
little if any plant life would survive. It can be seen that mean 
monthly temperatures are desirable for evaluating ecological con- 
ditions, and this is equally true for monthly mean maximum and 
minimum values. Collectively, these indicate the extent of the 
growing season and the extremes to be expected during that time. 

Temperature Variations.— Since fluctuations of insolation result 
in fluctuations of temperature, seasonal and daily temperature 
changes, as with insolation, can be expected to follow a general 
pattern for any region. The pattern follows that of insolation but 
with temperature responses lagging behind changes in radiation. 
A daily maximum of atmospheric temperatures usually comes in 
midafternoon, and minimum temperatures occur just before sun- 
rise. Soil temperatures lag even more, for their maxima may not 
occur until 8:00-11:00 P.M. and minima may not be reached until 
8:00-10:00 A.M. This is, of course, due to the fact that soil is a 
poor conductor of heat. For the same reason, the soil surface, if 
unshaded, produces the highest temperatures for an area and like- 
wise has the widest range of temperatures. It is the subsoil tem- 
perature that follows the trend indicated above. With increasing 
depth, daily fluctuations are reduced until at two or three feet 
they are not apparent. Seasonal air temperatures also lag as is indi- 
cated by the usual hot days of July and the cold of January, both 
extremes coming after the June and December solstice. Soil tem- 
peratures follow seasonal atmospheric trends with a further lag. 

Since the total insolation decreases with distance from the Equa- 
tor, temperatures likewise decrease. Temperature zones, therefore, 
tend to run east and west, and the greater the latitude the lower 
the temperatures to be expected. 

There are, however, local and generalized exceptions. Large 



CLIMATIC FACTORS : RADIANT ENERGY 



123 




day night day night day night 

Time of Readings 



day 



NIGHT 



DAY 



FlG. 64. Maximum day and night soil temperatures taken on a sand dune 
at Beaufort, N. C. in August, 1947. Readings were made on successive days 
at 7:00 A.M. and 7:00 P.M. for night and day maxima, respectively. Tempera- 
tures were greatest at the soil surface and were successively less with increas- 
ing depth by day, but, at all depths at night, dropped as low, or lower than 
the maximum at eighteen inches. Minimum temperatures fluctuated within 
the range of 72-85° F. (difficult to show accurately on so small a scale). At 
eighteen inches the minimum was never more than, one degree below the 
maximum, but the difference between minimum and maximum increased up- 
ward to the surface where one minimum was as low as 72° F. 

bodies of water are slower to warm up and slower to cool than 
land because of the higher specific heat of water. In addition, they 
reflect much of the insolation, and what heat is absorbed is dis- 
tributed to much greater depths by water motion and convectional 
currents. As a result, temperature extremes are reduced around 
bodies of water as compared to those inland. The effect on plant 
distribution is particularly evident in the ranges of southeastern 
species, which often extend to the northern limits of the Atlantic 
coastal plain, where, undoubtedly, they are able to survive because 
of the maritime climate. The amelioration of temperatures is ap- 
parent about lakes as well as oceans, although to a lesser extent. 
The extremes of winter and summer temperature characteristic of 
the Dakotas are never experienced in lake-bounded Michigan, al- 
though latitudes are essentially the same. 



124 THE STUDY OF PLANT COMMUNITIES • Chapter VI 

The air near the earth's surface is warmed by absorption of 
insolation and reradiated heat from the earth. With increasing alti- 
tude the atmosphere becomes less dense and also contains less 
moisture and other heat-absorbing substances. Consequently, tem- 
peratures decline with altitude. Even the warm air rising from the 
earth is cooled by its expansion. Latitudinal temperature zones are, 
therefore, further disrupted by mountains where increasing alti- 
tude produces the same differences as increasing latitude. This is 
particularly noticeable on high mountains where, because of the 
combined effects of temperature and moisture, one may see zones 
of vegetation altitudinally arranged, which at lower altitudes are 
latitudinallv distributed over hundreds of miles. 

Just as latitudinal temperature zones are irregular, so are the alti- 
tudinal zones not perfect. Cold air drainage has been discussed, 
(p. 98.) It results in low night temperature in the valleys when 
tablelands and upper slopes are much warmer. 129 The areas may be 
distinctively marked by the vegetation they support. In moun- 
tainous country, orchards are frequently grown successfully at 
much higher altitudes on slopes than in valleys. 81 Slope and expo- 
sure disrupt mountain temperature zones even more. Since the 
maximum effectiveness of insolation comes only when it strikes 
a surface at right angles, the greater the variation from a ninety- 
degree angle, the less radiant energy will strike a unit area. In the 
Northern Hemisphere, therefore, a south-facing slope receives 
more insolation per unit area than a flat surface, and a north-facing 
slope receives less (see Fig. 67). Thus the same temperature con- 
ditions found on a tableland may occur at a higher altitude on a 
near-by south-facing slope and at a lower altitude on a north slope. 
The distribution of vegetation being correlated with temperature 
and the consequent moisture differences, a particular community 
will be found above its ordinary altitudinal range on south slopes 
and below it on north slopes, and the extent of this irregularity in 
zonation is affected both by the angle of the slope and its exposure. 
In Wyoming, Douglas fir from the montane zone may come down 
to 7,500 feet on north-facing slopes while mountain mahogany 
from the lower woodland zone may be found extending upward to 
better than 8,500 feet on south-facing slopes. In general, a vegeta- 
tion zone extends higher on the south side of a mountain than on 
the north side. 



CLIMATIC FACTORS : RADIANT ENERGY 



125 



I4O00 



12000 



10000 



u 
ui 

U. 



UJ 

O 



8000 



6000 



4000 



2000 




FlG. 65. A generalized profile of altitudinal zones of vegetation in the 
mountains of Utah, which illustrates the effects of northern and southern ex- 
posures.— Adapted jrom Woodbury . 



276 



Cover and Temperature.— Anything that absorbs or reflects in- 
solation before it reaches the earth will reduce both soil and atmos- 
pheric temperatures. Thus it is cooler in cloudy or foggy areas 
than in similar areas without clouds or fog, and any given area 
tends to be warmest on clear days. But, because heat radiated from 
the earth and clouds is held below a cloud blanket, the lowest tem- 
peratures also occur on clear days, and extremely low temperatures 
are not to be expected on cloudy days. Temperatures in and above 
bare soil, particularly dark soil, are higher than if that soil has some 
form of cover. Any type of vegetation must absorb some radiant 
energy and, consequently, reduce temperatures between itself and 
the soil, the reduction being proportionate to the closeness of the 
stand and how many strata compose it. Temperatures in forest 
stands in midsummer are usually ten degrees lower by day than in 



126 THE ST UDY OF PLANT COMMUNITIES * Chapter VI 

the open and ten degrees higher at night. Soil temperatures under 
forest are lower than in the open during the growing season and 
usually higher in winter. However, soil temperatures under de- 
ciduous forest are subject to considerable winter variation. 







FIG. 66. Effect of slope exposure is apparent in the desert, as elsewhere. 
Although species differences are not great, the south-facing slope at right sup- 
ports a much sparser, more widely spaced stand of sagebrush than the oppo- 
site slope. Washoe County, New. -Photo by W. D. Billings. 

Soil temperatures are further modified by dead or living cover 
on the surface. Any such cover reduces the range of extremes and 
the speed of variation. This amelioration of temperature may be 
important in the viability and germination of seeds and the sur- 
vival of seedlings. Particularly affected are the physical and physi- 
ological processes involving water, its movement and availability 
in the soil, and its absorption and transpiration by the plant. Also, 
when soil is frozen, the runoff from heavy rains is much increased. 
Studies in Arizona 123 showed daily minimum soil temperatures to 



CLIMATIC FACTORS : RADIANT ENERGY 



127 



be five degrees higher under forest litter in the fall of the year 
than in bare ground and the daily maximum to be seven degrees 
lower. The average diurnal range was eighteen degrees in bare soil 
and only six degrees under litter. In North Carolina, 165 litter re- 
duced the depth of frost penetration 40 percent, and, whereas the 



TABLE 7. The average day and night temperatures (°F) in three upland 
forest communities in central Iowa. Air temperatures in contiguous prairie 
are higher than those in shrub by about 10° (day) and 4° (night). From( 4 ). 



Community 


Time 


April 


May 


July 


August 


Shrub 


Day 


58.8 


65.1 


76.7 


73.9 




Night 


45.3 


52.8 


64.0 


61.1 


Oak-hickory 


Day 


57.5 


63.8 


80.1 


77.7 




Night 


42.2 


51.9 


70.4 


68.4 


Maple-basswood . . 


Day 


55.0 


60.9 


74.3 


70.0 




Night 


40.2 


51.8 


67.5 


61.0 



bare soil was frozen solidly, the soil under litter remained porous 
and loose, permitting deeper percolation during winter rains and 
thaws and causing very little runoff. The effects of snow as an 
insulator are much the same as are those of litter. 

Temperature and Physiological Processes.— There is probably 
for every species an average optimum temperature at which it 
grows most successfully, other factors being equal. Likewise there 
must be a maximum and a minimum temperature that it can with- 
stand. These limits may result from the temperature tolerances of 
the protoplasm peculiar to the species, but they may likewise 
result from responses of one or more physiological processes, 
which vary from species to species. 

The temperatures affecting germination might alone limit the 
range of a species. Among our cultivated crops, the minimum- 
maximum range of temperature for germination is 35°-82° F. for 
flax and 49°-115° F. for corn. The optimum for each, respectively, 
is 70° and 93°. That the center of production for flax is consid- 



128 THE STUDY OF PLANT COMMUNITIES • Chapter VI 

erably north of the center for corn is therefore not at all surpris- 
ing. 

Absorption of water is at a minimum when soil is frozen but 
increases, as do diffusion and capillary movement in the soil, with 
rising temperature. The optimum is surprisingly high as soil tem- 
peratures go, and the maximum approaches the boiling point in 
some instances. Absorption is reduced, more at low temperatures 
for plants that grow normally in warm soil than for plants that 
grow, at least part of the year, in cold soil. For example, cotton 
absorbs only 20 percent as much water at 50° as at 77° F. while 
collards absorb 75 percent as much at 50° as at 77° E 148 

Photosynthesis operates under a wide range of temperatures 
under natural conditions. Marine polar algae may live their entire 
lives at temperatures below 32° F. because the freezing point is 
depressed by the salts in the water. There is an often-quoted old 
report that spruce carries on photosynthesis at -22° E, but a re- 
cent study" using modern methods indicates that, although coni- 
fers do not lose their ability to carry on photosynthesis during 
midwinter, the species studied function only above 2 1 ° F. The 
process also goes on in desert plants at temperatures of 120° E or 
more. The effective temperature range, however, is usually be- 
tween 70° and 100° E With increase in temperature the rate in- 
creases steadily to the optimum and then drops abruptly to the 
maximum, which is not much in excess of the optimum. The rate 
of respiration also increases with temperature until at high tem- 
peratures the process becomes destructive of life. Vant Hoff's 
Law, which states that the speed of a chemical reaction doubles 
or more than doubles with each 18° F. rise in temperature, is ap- 
plicable within limits to reactions in organisms. In photosynthesis 
it holds reasonably well between about 41° F. and 77° F. Beyond 
these limits there is much variation. 

Growth, being a product of chemical and physiological proc- 
esses, follows the same pattern and is favored by relatively high 
temperatures. At temperatures near or above the maximum, the 
water balance is apt to be thrown off by excessive transpiration. 
Reproduction follows the same rule regarding temperature, but 
it is of interest that flowering and fruiting have higher optima 
than vegetative processes in the same plant. 



CLIMATIC FACTORS : RADIANT ENERGY 129 



LIGHT 

That portion of the sun's radiant energy which forms the vis- 
ible spectrum and which we commonly term "light" strikes the 
earth in quantities far in excess of the apparent needs of plants. 
Although green plants, with very few exceptions, are the only 
organisms that can directly convert this energy to their own use, 
they actually change to potential energy only about one percent 
of the light energy they receive. It has been estimated that, of the 
total solar energy falling upon a given field of corn during a grow- 
ing season, only 0.13 percent can be "stored" as potential energy. 
However, this also suggests that, to function normally, plants 
require much more light energy than they actually use. Not all 
wave lengths are equally usable. Green light is reflected or trans- 
mitted, while the longer wave lengths, in the red end of the spec- 
trum, are much more effective in photosynthesis than are the 
shorter lengths of yellow and blue. Not all species are equally 
efficient under equal illumination. Some require certain intensities 
and some need certain lengths of day or season to function nor- 
mally. To add to the difficulties of interpreting plant-light relation- 
ships, it is not always possible to distinguish between light effects 
and those of total insolation, which include heat and its influence 
on physiological processes. 

Light Measurements.— Ecological studies of light should not be 
casually undertaken in spite of the apparent simplicity of making 
measurements with modern instruments. As suggested above, plant 
responses and light values rarely bear a simple and direct relation- 
ship to each other. Whether or not these relationships can be in- 
terpreted may depend upon proper planning before making meas- 
urements. In addition, there are problems related to obtaining 
measurements for ecological purposes that must be considered. 

Chemical, illuminating, electrical, and heating effects of light 
are measurable, and for each a different type of instrument is 
used. 233 Field ecologists have largely abandoned the first two ap- 
proaches in favor of electrical measurements because of the recent 
perfection of compact, sturdy photoelectric apparatus with which 
accurate and rapid determinations can be made. These instruments 
are sensitive to approximately the same portion of the spectrum as 
is the human eye. Since they are selective instruments, there may 



130 THE STUDY OF PLANT COMMUNITIES ' Chapter VI 

TABLE 8. Light measurements, in foot candles, made with a Weston 
photometer in a mixed pine-hardwood stand between 12:00 and 1:00 P.M. 
when full sunlight was 9,500 foot candles. Readings taken along three lines, 
at three-foot intervals, at a height of three feet. After completion of a line, 
the measurements were repeated at the same points. Note the great variation 
in readings at the same points at different times (sun flecks) and that some 
points are apparently much less shaded than others. 



Line I 


Line 2 


Line j 


300 


500 


500 


400 


100 


100 


300 


500 


500 


300 


200 


400 


200 


300 


300 


400 


300 


100 


200 


300 


400 


200 


200 


600 


300 


300 


300 


200 


200 


200 


300 


200 


300 


100 


100 


200 


200 


200 


400 


3000 


200 


300 


100 


200 


3600 


2400 


200 


400 


2400 


200 


4400 


3600 


300 


300 


300 


200 


400 


200 


100 


200 


300 


200 


2000 


400 


200 


200 


200 


2400 


400 


400 


100 


200 


500 


200 


400 


300 


100 


200 


500 


200 


600 


400 


100 


100 


500 


200 * 


1000 


800 


100 


200 


300 


200 


200 


300 


200 


200 


300 


200 


200 


400 


1600 


200 


4200 


200 


300 


200 


100 


100 


200 


200 


1200 


5000 


100 


100 


200 


2100 


1600 


2400 


200 


100 


200 


600 


300 


4400 


100 


100 


300 


300 


300 


300 


100 


100 


400 


500 


100 


200 


300 


100 


200 


200 


300 


200 


200 


200 


400 


200 


800 


2000 


200 


100 


Aver. 532 


432 


832 


1140 


224 


200 



Average for the stand = 560 ft. candles. 5.9% of full sunlight. 

be some question of the advisability of generalizing as to plant 
responses in relation to the measurements they obtain. In most 
field studies this does not become a serious limitation because the 
usual objective is to compare relative intensities of light in two or 



CLIMATIC FACTORS : RADIANT ENERGY 131 

more situations or habitats. For this purpose, the photoelectric 
method is quite usable. 

The method has, however, other limitations, and its use requires 
certain precautions. Preferably two or more instruments should 
be available and the readings should be made simultaneously. Even 
so, readings should be made only on a clear day and, when pe- 
riodic observations are made, at the same time of day. Results 
should be expressed as percentages of full sunlight at the time 
when each observation is made. At sea level this would be approxi- 
mately ten thousand foot candles on a clear day at noon, but values 
as high as twelve thousand foot candles have been obtained in the 
clear air of high mountains. If for any reason the readings in the 
open are low on a given day, no further observations should be 
made. 

Because of its concave sensitive surface, the instrument can be 
operated in only one plane at a time. If readings are made simul- 
taneously at noon with the instrument in a horizontal plane, many 
complicating factors are automatically eliminated. The instru- 
ments are extremely sensitive to slight variations in light, and this 
necessitates numerous readings to arrive at average conditions. 
The slightest air movement shifts the position of leaves and per- 
mits bright sun flecks to come through a forest canopy. These 
flecks come and go, first at one point and then at another, and 
cannot be ignored in evaluating light in a stand. Their inclusion is 
best accomplished by making observations at a rather large num- 
ber of uniformly or randomly distributed predetermined points 
and averaging the results. In all instances, the instrument should 
be in the same position relative to the observer and the ground. 

A sensitive surface of spherical form is usually more desirable 
than a flat one. Where reflected light is appreciable, a sphere will 
record from all directions. If a continuous record is to be obtained, 
the sphere records accurately because one-half its surface always 
faces the sun regardless of its position. Several radiometers, which 
measure heat effects and are nonselective of wave lengths, are 
spherical in form and are advantageous in other respects. If a 
photoelectric cell is given more than a short exposure to strong 
light, the current it generates falls off because of solarization, but 
the radiometer can be exposed indefinitely without such effects. 



132 THE STUDY OF PLANT COMMUNITIES • Chapter VI 

It is, therefore, adaptable to continuous operation with a record- 
ing device. 

Such equipment is not always available to the field ecologist, 
but, even so, some form of measurement is far more dependable 
than an estimate. Good approximations of light intensity may be 
obtained with photographic light meters even though they are not 
calibrated in foot candles. Useful values are obtainable by expos- 
ing black and white bulb atmometers in pairs. When one pair is 
exposed in the open and differences from pairs in near-by habitats 
are expressed as percentages of the value in full sunlight, the re- 
sults may be quite as satisfactory as with more elaborate equip- 
ment. Since the atmometers would be operating continuously, 
they might even be more meaningful in terms of the vegetation. 

Light Variations.— The biologically important variations of light 
are those in intensity and quality. These occur periodically, re- 
curring seasonally and daily to a degree that is determined by 
latitude 140 as discussed under the general heading of insolation. 
Of course, altitude modifies the regional variations, and topog- 
raphy results in more localized variation through the effects of 
angle of slope and direction of exposure. Since the principles were 
previously discussed (p. 124), it should be sufficient here to pre- 
sent an illustration of how slope and exposure affect light in the 
southern Appalachian Mountains. 41 

Variation in quality of light is not so obvious as variation of 
intensity. Quality, however, is variable, largely because of the 
same factors that modify intensity, for the amount of absorption 
and diffusion by the atmosphere determines what wave lengths 
reach the earth. Clouds, fog, smoke, dust, or atmospheric moisture 
alone increase diffusion and absorption, and, as a consequence, dry 
regions receive more light than humid ones, and open country 
receives more light than smoky cities. The greater the diffusion, 
the higher the percentage of red light and the lower the percent- 
age of blue reaching the earth. 

A local variation of far greater general ecological importance is 
that produced by vegetation of one stratum upon that of a lesser 
stratum beneath it. Because plants growing in the shade of others 
receive only the light that is not absorbed or reflected, they must 
be adapted to functioning with reduced light intensity (often re- 



CLIMATIC FACTORS : RADIANT ENERGY 



133 




6AM 8AM 10AM NOON 2 PM 4 PM 6 PM 

TIME OF DAY 



FlG. 67. Intensity of radiation received at different times of day on (A) 
south, (B) north, and (C) east slopes in the southern Appalachians, on June 
21 and on December 21. For S. exposure, in summer, the 20 percent slope 
receives greatest radiation because it forms an angle of almost 90° with the 
sun's rays at noon. In winter, when the sun is low, the 100 percent slope re- 
ceives more radiation than the 20 or the 40 percent slope. For N. exposures, 
in summer, 20 percent slopes receive almost as much radiation as 20 percent 
south slopes. In December, 100 percent N. slopes are in complete topographic 
shade but 100 percent S. slopes receive 48 percent of maximum radiation at 
noon. Curves for west slopes would be mirror images of those for east slopes. 
—From Byram and Jemison.** 

duced to 15 percent or less) of somewhat different quality (re- 
duced red and blue light) than those in full sunlight receive. Con- 
sequently, there are species representing a wide range of tolerance 
to shade, for no forest is so dense that nothing can grow beneath 
it, even when there is a reduction to 1 percent or less of full sun- 
light, as under some tropical forests. The reduction of light in- 



134 THE STUDY OF PLANT COMMUNITIES * Chapter VI 

tensity under a forest .canopy is probably of more ecological im- 
portance than the change in quality. 

Shade Tolerance.— The ability or inability of certain plants to 
grow normally when shaded, as on the forest floor, has several 
practical considerations. When a forest stand is thinned or clear- 
cut, the new stand that appears will, in general, be determined by 
the kinds of seedlings and saplings already present at the time of 
cutting. These species may or may not be desirable, and the ques- 
tion of how to encourage or inhibit them, depending upon circum- 
stances, has led to much study and theorizing on the causes of 
shade tolerance. 

Since light is obviously reduced under a forest stand, it was 
once assumed rather generally that light is the controlling factor. 
Studies of "trenched plots" under forest stands gave results inter- 
preted by many workers as indicating a greater significance for 
water since, within these plots, shade-intolerant species for a time 
grew well when root competition for water and nutrients was 
eliminated by cutting off the roots of the dominant trees. 146 ' 253 
Extensive investigations of conifer reproduction in the Lake States 
indicate that, for each light intensity, growth could be increased 
by reducing root competition and that at each level of root com- 
petition growth could be increased by increasing light. 234 Obser- 
vations of the reproduction of certain southern pines 190 indicate 
that these shade-intolerant species may successfully meet extreme 
root competition if light is sufficient. It would seem that the suc- 
cessful growth of a seedling under a forest canopy may depend 
upon its ability to manufacture enough food with the light avail- 
able to grow enough roots to meet the competition of the trees 
established there. Undoubtedly, shade tolerance cannot be ex- 
plained on the basis of a single factor. 

Physiological Responses.— When the supply of food in an or- 
ganism falls and remains below what is required for respiration 
and assimilation, the organism cannot continue to function nor- 
mally and must eventuallv die. Since a green plant produces its 
carbohydrates through photosynthesis, the process must proceed 
at a rate sufficient at least to satisfy the immediate needs of the 
plant if growth is to be normal. Light, which provides the energy 
for photosynthesis, is sufficient during the growing season to sup- 



CLIMATIC FACTORS : RADIANT ENERGY 



135 



ply plant needs anywhere on the earth. In fact, light intensities 
may be too high for some plants to grow in full sunlight, their 
seedlings being especially subject to injury. Such plants might well 
be restricted to habitats with partial shade; if their photosynthetic 




FlG. 68. Trenched plot in a loblolly pine stand (40 T 50 yr.) four years after 
initiation (see Fig. 12). Contrast vegetation on trenched plot with floor of 
surrounding forest and control plot in foreground.— Photo by C. F. Korstian 



146 



efficiency is insufficient to maintain them in forest shade, they 
might thrive in regions where light intensity is reduced by cloudi- 
ness or fog. Probably the range of a species is rarely determined 
by light intensity alone, however, for it must be remembered that 
light effects are apparent in several processes and activities, which 
can rarely be considered independently. The production of chlor- 
ophyll, the opening and closing of stomata, and the formation of 
auxins are examples of light-conditioned phenomena with widely 
differing effects, but these activities must be considered in rela- 
tion to each other when interpreting plant responses. 

The production of chlorophyll, although (with a very few ex- 
ceptions) accomplished only in the presence of light, is perhaps 
more apt to become limiting or significant in high than in low 
light intensities. Available evidence indicates a greater production 



136 THE STUDY OF PLANT COMMUNITIES * Chapter VI 

of chlorophyll with decreasing light intensity and an ability of 
most plants to produce chlorophyll at light intensities considerably 
below those necessary for effective photosynthesis. 

The opening and closing of stomata can usually be correlated 
with light, but there are enough exceptions to give warning against 
generalizations or interpretations based on the principle of alter- 
nate opening and closing with light and darkness. In some plants, 
stomata may open at night; in others, light seems not to be a con- 
trolling factor. Where stomatal movement seems directly respon- 
sive to light, other factors may at any time become more impor- 
tant and modify or counteract the effects of light, as when stomata 
close during the day if the water supply is insufficient. However, 
stomatal movement is usually correlated with light changes and, 
when other conditions are favorable, is apparently caused by tur- 
gidity changes in the guard cells resulting from metabolic activity, 
which varies with light. The opening and closing in turn may 
modify effects of light by varying gas exchange related to photo- 
synthesis and rate of loss of water by transpiration. 

The production of certain auxins or growth-controlling sub- 
stances in plants is inhibited by light. As a result, through them, 
size, shape, movements, and orientation of parts may be influenced 
by light. A plant grown in complete darkness, since it produces a 
maximum of auxins, elongates excessively, with poorly differenti- 
ated tissues throughout and with almost no supporting structure. 
These characteristics in an intermediate condition are often rec- 
ognizable in plants grown in heavy shade, as under a forest canopy 
or in close stands where plants shade each other. Such plants tend 
to be tall and spindly with widely spaced nodes and relatively few 
leaves. The better the light, the stouter and more compact the in- 
dividual will be. 

Should illumination be one-sided, the increased production of 
auxins on the shaded side usually stimulates sufficient extra elonga- 
tion on that side to turn the growing portion of the stem toward 
the light. Some species— sunflower, for instance— are so sensitive 
to such differences of light that the floral portions shift from east 
to west with the sun daily as differential elongation in the stem 
progresses from one shaded side to the other. 

The orientation of vegetative parts is such that every leaf re- 



CLIMATIC FACTORS : RADIANT ENERGY 137 

ceives a portion of the light available. Genetic differences deter- 
mine whether the leaves are exposed in the form of a rosette or 
in a mosaic pattern, or whether they are supported by a spirelike 
central axis or several spreading branches, each of about equal 
size. The variations within such a general plan probably result 
from effects of auxins on growth of petioles and secondary 
branches. 

Leaves normally become arranged with their broadest surface 
exposed outward and upward on the side of the plant where they 
grow. This results in a maximum exposure to the available light 
at that point. However, plants growing under conditions of ex- 
cessive light, especially where there is reflection from light-colored 
soil, not uncommonly have their leaves in a profile position, which, 
of course, reduces the light to which they are exposed. Turkey 
oak (Quercus catesbaei), which grows on sand dunes in the south- 
eastern United States, regularly develops a twist in the petiole 
that turns every blade vertically. The leaves of wild lettuce (Lac- 
tuca scariola) are vertical when grown in full sunlight but do not 
change from a horizontal position in the shade. Several so-called 
compass plants have leaves that are not only vertical but that also 
face east and west, exposing only their edges to the sun's rays at 
midday. 

Plants growing in close stands characteristically lose leaves and 
usually branches from below when the light penetrates insuffi- 
ciently to maintain necessary photosynthesis. Most monocots with 
grasslike leaves and underground stems are unaffected because 
their upright linear leaves permit light to penetrate to their bases. 
In forest stands, this self-pruning may be economically impor- 
tant. Conifers that self-prune grow tall and straight with few 
knots and smooth grain. In contrast, those with dead branches 
down to their bases are difficult to handle and produce much less 
valuable wood when finally cut. 

Leaves grown in full sunlight tend to be smaller, thicker, and 
tougher than leaves grown in the shade. This is particularly no- 
ticeable in plants of the same species and may also be observed on 
the same plant. A forest-grown tree may have sun leaves at the top 
and shade leaves near the base, or in the interior of its crown. 

Certain structural differences are associated with the two types 



138 THE STUDY OF PLANT COMMUNITIES ' Chapter VI 



of leaves. Intense light results in elongated palisade cells and often 
the production of two or more layers of them. Conversely, weak 
illumination favors the production of sponge cells. A leaf that, 
with average illumination, has a single layer of palisade and several 






FlG. 69. Seedling of turkey oak (Quercus catesbaei), a sandhill species, 
whose leaves have already assumed the vertical position they maintain 
throughout life. 

layers of sponge cells might have had, in intense light, two or three 
layers of palisade and a proportionate reduction in sponge tissue. 
In reduced light the sponge tissue is increased at the expense of 
the palisade. In extreme cases there may be no palisade or no 
sponge tissue. The thickness of cutin and the amount of support- 
ing tissue in the veins are likewise greater or less depending upon 
light intensity. These characters affect the relative toughness of 
the leaf. 

What forces cause a developing cell to elongate at right angles 
to the leaf surface to form palisade or parallel to the surface to 
form sponge tissues, cannot be stated with any certainty. The 
causes may not be entirely controlled by light, for unfavorable 
moisture conditions favor palisade production as does poor aera- 
tion. Sucker sprouts from stumps often produce leaves of the shade 



CLIMATIC FACTORS : RADIANT ENERGY 



139 



type in full sunlight, probably because of the favorable water bal- 
ance maintained by the extensive root system of the tree. Certain 
advantages of shade leaf development are more obvious than the 
causes. 

In strong light, cells elongate parallel to the light source. The 




FlG. 70. The anatomical characteristics associated with so-called sun and 
shade leaves of two chaparral species. (A) Arctostaphylos tomentosahom nor- 
mal xeric habitat, (B) from mesic habitat. (C) Adenostoma jasciculatum from 
normal xeric habitat, (D) from stump sprout. Note differences in thickness of 
leaf and cuticle, and proportion of palisade to sponge tissue.— From Cooper™ 

more intense the light, the deeper its penetration into the leaf and 
the more layers of palisade there will be. Desert and alpine plants 
may have the mesophyll entirely made up of palisade cells. Leaves 
subject to reflected light from below commonly have palisade on 
the lower surface as well as the upper, and leaves growing ver- 
tically regularly have palisade on both sides. 

When illumination is intense, chloroplasts arrange themselves 
along the side walls, and thus in palisade cells they receive a mini- 
mum of direct insolation. On the other hand, with weak light the 
chloroplasts tend to appear along the walls at right angles to the 
light source, and the form of sponge cells permits exposure of 
more chloroplasts to the greatest effectiveness of available light. 



140 THE STUDY OF PLANT COMMUNITIES ' Chapter VI 

There are added advantages ( in the thinness and greater area of the 
shade leaf since both maximum exposure under conditions of re- 
duced light and penetration of light to a high proportion of in- 
ternal cells are thus assured. 

Since reduced light favors elongation, vegetative growth, and 
delicacy of structure, it can readily be understood why several 






A b c 

Fig. 71. Structure of leaves of broad sclerophyll forest trees (A) Castan- 
opsis chrysophylla, (B) Quercus agrifolia, (C) Quercus durata. Note com- 
pact structure, multiple layers of palisade, and tendency for all mesophyll to 
be palisade-like. Note also struts of mechanical tissue from epidermis to epi- 
dermis.— From Cooper™ 

garden crops used either for leaves or roots are best grown in 
spring and fall or in regions with many cloudy days. A number of 
leaf crops are grown under artificial shade. The point is well illus- 
trated by the production under artificial shading of the large thin 
leaves of tobacco needed for cigar wrappers. 

Since intense light inhibits vegetative growth and favors, or is 
actually necessary for, flowering and fruiting, it is not surprising 
that centers of grain and fruit production characteristically have 
much clear, cloudless weather during the growing season. Here, 
too, is a partial explanation of the reduced size of alpine and arctic 
plants, which produce large and numerous flowers. Likewise it 
helps explain why trees in the open often fruit more prolifically 
than those in a closed stand, where overtopped individuals rarely 
produce a seed crop. 

Photoperiod.— A number of seasonal biological phenomena long 
have been accepted as such, without much concern as to causes. 
Violets, miliums, bellworts and many other wildflowers blossom 



CLIMATIC FACTORS : RADIANT ENERGY 141 



in the spring, but asters, goldenrods, and chrysanthemums are ex- 
pected to flower in late summer or fall. When a fruit tree occa- 
sionally blossoms in the fall, the occurrence is considered unusual. 
The controlling factor in such periodic phenomena was not recog- 
nized until Garner and Allard 104 published results of their studies 
of photoperiodism, or responses of organisms to the relative length 
of day and night. Their investigations developed from difficulties 



FlG. 72. The effect of long day (15 hours), left, and short day (9 hours), 
right, on flowering of henbane {Hyoscyamus niger), a long-day plant. All 
plants received 9 hours of natural radiation. The supplemental light of the 
15-hour lot was obtained from 100-watt incandescent lamps, which gave an 
intensity of only about 30 foot candles.— Photo by courtesy of H. A. Borth- 
ivick, Bureau of Plant Industry, U. S. Dept. Agr. 

experienced in growing new varieties of tobacco and soy beans in 
the vicinity of Washington, D. C. The tobacco grew vigorously 
and did not flower under field conditions, but in the greenhouse, 
during the winter months, it flowered and fruited abundantly. 
The soy beans flowered and set fruit at about the same date in late 
summer regardless of how long they had been in the vegetative 
condition, as determined by plantings spaced at wide intervals 
during the spring and early summer. When the length of daylight 
period was shortened for these plants by enclosing them in a dark 
chamber for a few hours each day, the tobacco flowered very 
soon and the formation of seeds in the soy beans was hastened 
materially. 

Some Applications '.—It can readily be seen why garden plants 
grown for vegetative parts, if they are long-day species, develop 
best in spring and late fall and, if grown in summer, bolt to form 



142 THE STUDY OF PLANT COMMUNITIES • Chapter VI 




Fig. 73. An Abelia hedge in late fall that (left) ceased growth and hard- 
ened normally everywhere except section under boulevard light. Here, be- 
cause of the extended photoperiod, the plants continued to grow and put out 
new shoots, which were killed by the first heavy frost (right).— From 
Kramer. 1 * 1 

flowering structures. The differences in photoperiodic response 
between varieties may be the sole reason for success or failure of 
a crop at a particular latitude and is an excellent reason for know- 
ing one's seed stock and its potentialities. Flowering shrubs and 
herbs, too, if grown beyond their normal latitudinal range, may be 
pampered and kept alive but often fail to flower because the length 
of day is unsuitable, or may invariably flower too earlv in the 
spring or too late in the fall. 

The cessation of growth and subsequent "hardening" of ever- 
green woody plants are initiated in response to length of day. If 
plants are put out within range of street lamps, some winter-killing 
mav be anticipated. Street trees of several species retain their leaves 
on the side illuminated by street lamps long after dormancy and 
complete leaf fall on the opposite side, which does not have sup- 
plemental light. 171 On the Duke University Campus, lamp posts 
are regularly spaced in a long Abelia hedge, and every winter frost 
injury results within a certain distance of each lamp because the 
plants here do not go into dormancy. 147 



CLIMATIC FACTORS : RADIANT ENERGY 143 



Commercial greenhouses are making use of supplementary light- 
ing and controlled period of illumination to bring crops into 
flower on special days or to produce maximum vegetative growth. 
Growing a crop for its vegetative parts in one latitude for which 
seeds must be produced in another latitude is now common prac- 
tice. 

Ecological Significance?— It is thus apparent why many plants 
in the tropics, where the light period is almost constantly twelve 
hours, flower throughout the year and, likewise, why so few 
plants in the United States, even in the South, have this character- 
istic. It is apparent, too, that arctic species must be long-day plants 
and why they rarely flower when brought farther south. Also, 
short-day species could not survive in the tropics since they would 
not reproduce. Species requiring high temperatures and long days 
to mature are definitely limited in their northern range. The for- 
mation of abscission layers in leaves of trees and their decline in 
physiological activity are initiated in response to shortening days, 
not to reduced temperature. Therefore, at or beyond the northern 
limits of their range, trees may be killed by frost because they are 
not yet sufficiently dormant to withstand low temperatures when 
they occur. 

It should not be assumed that plant distribution is primarily de- 
termined by length of day. Many species are little affected by it. 
Also temperature fluctuations have been shown to modify photo- 
periodic requirements and responses in several species. Photo- 
period is just another factor, which may operate with temperature, 
moisture, and light to determine the range and distribution of a 
species. 

GENERAL REFERENCES 

H. A. ALLARD. Length of Day in Relation to the Natural and Artificial 

Distribution of Plants. 
R BURKHOLDER. The Role of Light in the Life of Plants. 
R. F. DAUBENMIRE. Plants and Environment. New York : John Wiley and 

Sons Co., 1948. 424 pp. 
W J. Humphreys. Ways of the Weather. 

H. L. SHIRLEY. Light as an Ecological Factor and Its Measurement. 
U. S. DEPT. Agr. Climate and Man. 
H. B. Ward and W E. Powers. Weather and Climate. 



CHAPTER VII 



PHYSIOGRAPHIC FACTORS 

SOIL 

Land masses of the earth are covered by an unconsolidated sur- 
face mantle of mineral particles derived from parent rock by proc- 
esses collectively called weathering. The depth of the mantle is 
variable depending upon disturbances and time, while its physical 
and chemical properties depend upon the nature of the parent 
rock and the weathering agencies that may have affected it. This 
inorganic material may be termed soil but is usually not so con- 
sidered until organic materials have accumulated from organisms 
that have lived in or upon it. 

Soil Formation.— Weathering may result in purely physical 
change, as when rock masses are broken into smaller and smaller 
sizes, or may be of a chemical nature, producing changes in com- 
position of the material. The two processes function together nor- 
mally. Disintegration is largely accomplished by physical agents, 
such as water, wind, ice, and gravity, and by expansion and con- 
traction resulting from temperature changes. The first four agents 
are functional through the erosive action of the load of cutting 
material they transport and are, therefore, effective in proportion 
to speed of movement or to force and pressure. The effects of 
temperature are the most widespread although not always con- 
spicuous. Differential expansion and contraction of rock materials 
result in cracking, which is especially marked when temperature 
changes are abrupt. The widest temperature fluctuations occur in 
arid regions and at high altitudes where their effectiveness is indi- 
cated by consistently coarse and angular soil particles. To a lesser 
extent the process goes on everywhere. Prying action of plant 
roots and excavating or burrowing by animals may contribute to 
disintegration, but these activities are certainly of greater impor- 
tance in their facilitating of chemical processes. Openings in the 
soil increase aeration and the percolation of water. Shifting the 

144 



PHYSIOGRAPHIC FACTORS 



145 




Fig. 74. Wind-swept alpine habitat in Utah with typical coarse, angular 
soil particles and little organic material. Krummholz at left is of Picea en- 
gelmanni and Pinus flexilis (see also Fig. 47).— U. S. Forest Service. 

soil about exposes new particles to chemical action and likewise 
helps to incorporate organic matter. 

The chemical or decomposing processes all tend to result in in- 
creased solubility of soil materials, which, in solution, may then be 
available for the use of plants but are also subject to leaching, or 
washing out, of the surface layers by rain water. Both oxidation 
and hydration, the addition of oxygen or water to a compound, 
are common and result in softening of rock. Carbonation, or the 
taking up of carbon dioxide, produces carbonic acid merely by 
union with water, and the acid is an effective solvent of many 
rocks. Water itself is a weak solvent, and, with the addition of 
carbonic acid, which is always present, its action is much increased. 
Decaying vegetation, when present, also contributes acids that 
facilitate solution. In solution, salts ionize and the relative effective 
concentrations of the basic and the acid radicals thus formed de- 



146 THE STUDY OF PLANT COMMUNITIES ' Chapter VII 

termine whether the soil solution will be alkaline or acid in reac- 
tion. 

These and other chemical processes operating more or less con- 
tinuously, together with physical processes, constitute weathering, 
which produces soil material that retains few characteristics of 
parent rock. However, soil is not a product of these processes 
alone, for biological activity also contributes to its formation. Or- 
ganic material is an essential part of soil, and its decomposition and 
incorporation are accomplished largely by microorganisms, whose 




Fig. 75. A soil well that illustrates a soil profile (White Store sandy loam) 
in which the A horizon is very thin, the sandy gray-white A horizon is 
sharply distinguished from the plastic red clay of the B horizon, and the 
rocky C horizon shows in the bottom— Photo by C. F. Korstian. 



PHYSIOGRAPHIC FACTORS 147 



numbers and activities increase as more complex organisms, par- 
ticularly higher plants, gradually occupy the surface. 

Soil Profile.— Processes resulting in the formation of soil mate- 
rial also contribute to soil development. As weathering proceeds, 
fine materials in suspension and solution are carried downward by 
percolating water to a lower level, where they gradually accumu- 
late. As a soil develops, therefore, a rough stratification becomes 
apparent in which the horizons characteristically have different 
physical and chemical properties. These horizons, collectively 
called the soil profile, are designated and recognized as follows: 

A Horizon. The upper layer of soil material from which 
substances have been removed by percolating water. 

B Horizon. The layer below the A Horizon in which these 
materials have been deposited. Layer of accumulation. 

C Horizon. The underlying parent material, relatively un- 
weathered and not affected as above. 

Litter accumulated on the surface of the mineral soil may be 
termed the Ao Horizon. It is often convenient to subdivide the 



ROOT DISTRIBUTION ON OUTSIDE WALL OF SOUTH TRENCH 
TRENCHEO PLOT NO 2 JUNE 10,1932. 



A, 



A > • : — 1 5 s" 

9 5' 



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B. • •.••. - 215* 



360* 
LEGEND 
• = root.0'-or ..root 0.2*- 0.3" ■ SCALE , 
.»roo10.f-0.2' •=root0.3'-0.4* "cot 



FlG. 76. An illustration of root distribution in soil horizons and of a 
method for mapping roots in the wall of a soil well.— From Korsticm and 
Coiled 

major horizons as Ai and A2, Bi and B2, etc. Ai is a particularly 
useful subdivision, for it is applied to the portion of the A horizon, 
distinguishable by its darker color, in which organic material has 
become incorporated. 

Soil profiles may be observed in any fresh road cut. When 
studied in connection with vegetation, a rectangular pit is usually 



148 THE STU DY OF PLANT COMMUNITIES • Chapter VII 

dug some four to six feet long, and wide enough to stand in com- 
fortably. One face is kept vertical and cut cleanly to observe the 
horizons— and possibly the root distribution. Depth of the pit is, 
of course, determined by local conditions and position of the par- 
ent material. 

Soil-Plant Relationships.— Soil must provide plants with an- 
chorage, a supply of water, mineral nutrients, and aeration of their 
roots. Not all plants require these essentials to the same degree, but 
unless all are present to some extent the average plant cannot be- 
come established. On this basis, soil has four major components : 
(1) mineral material derived from parent rock, (2) organic sub- 
stances added by plants and animals, (3) water, and (4) soil air. 
These components vary in amount and proportion from place to 
place, and the variation may be a significant factor in determining 
the distribution of species and vegetation types. 

Local Soil Variations.— Size of soil particles (soil texture) and 
shape of particles, which determines how they fit together (soil 
structure), may vary markedly within short distances. Texture 
and structure primarily affect the plant through their influences 
on air and water in the soil. Organic materials, in addition to 
modifying soil structure, are the source of plant nutrients that 
may be quite unavailable from mineral sources. 

These variables are a product of the manner in which the soil 
originated and the time involved in its development. Great areas 
of the earth are covered with soils that overlie the parent rock 
from which they were formed. These are sedentary soils, whose 
materials are termed residual, if of mineral origin, or cumulose, 
when deposited as organic matter. If soil material has been brought 
to its present location by some agency such as wind, water, grav- 
ity, or ice, it is said to be transported and will accordingly have 
distinguishing characteristics. 

Soils Formed in Place — Residual materials are most weathered 
at the surface and become progressively more like the parent rock 
with increasing depth. Where parent rocks differ in hardness or 
solubility, the resulting soils will differ. Fine-textured clayey soils 
may represent the leached residue of easily soluble rock, such as 
limestone, or may be the individual particles that made up a fine- 
grained hard rock. When the parent rock contains a high propor- 



PHYSIOGRAPHIC FACTORS 



149 



tion of hard, insoluble material like quartz, its soils will be sandy 
or even coarser. 

Cumulose materials may be mixed with mineral soils in any 
proportion or may have accumulated as almost pure organic 
masses. The latter are illustrated by peat bogs, which are common- 




FlG. 77. A wide flood plain in an old river valley whose alluvial soils con- 
stitute the best farming land in the region. Hiawassee River, Tenn— U. S. 
Forest Service. 

ly made up of plant remains that only partially decayed and were 
added to year after year until the lake or pond in which they grew 
was completely filled. Found most abundantly in lakes produced 
by glacial topography, the peat accumulations are likewise great- 
est where temperatures are low enough to limit the activities of 
organisms that produce decay. 

Transported Soils— On the great part of the earth's surface 
covered with residual soil, the effects of transporting agents are 
commonly noticeable only locally. But, to the ecologist, these lo- 
calities are of interest because the soil conditions are usually differ- 
ent enough to cause vegetational differences too. 

Except for loess, discussed elsewhere (p. 112), soils of aeolian 
origin are usually sandy deposits, which wind picked up from 
wide exposed beaches of lakes or oceans. Normally occurring as 
dunes, they usually form unfavorable habitats because of the low 
water-holding ability of sand, its relative sterility, and because the 



150 THE STUDY OF PLANT COMMUNITIES * Chapter VII 

dunes are subject to blowouts should the surface cover of vegeta- 
tion be incomplete (see Figs. 55, 56). In contrast, stabilized dunes 
of arid or semiarid regions form relatively favorable habitats be- 
cause almost all the water that falls upon them is available for 
plant use. 

Alluvial soils have been deposited by streams, which, as trans- 
porting agents, are effective in proportion to their velocity and 
the size of particles involved. Since currents are rarely constant, 
the size of transported particles varies, and deposits are always 
noticeably stratified. Alluvial soils are characteristic of lowlands 
that formed as deltas in or at the mouths of streams or as flood 
plains along streams that periodically overflow their banks. The 
greater the distance from the main channel of the stream, the finer 
the texture of the soil materials deposited. Alluvial deposits usually 
make desirable agricultural land if properly drained, and, because 
of favorable moisture conditions, they usually support the richest 
natural flora of a region. 

Colluvial materials are transported by gravity. Except in regions 
of rugged topography or in mountainous areas, they are rarely ex- 




FlG. 78. Colluvial cones, still in formation in Colorado. Only in such 
rugged mountain topography is gravity of direct significance in soil trans- 
port.— U. S. Forest Service. 



PHYSIOGRAPHIC FACTORS 



151 



tensive. Generally, they occur as talus slopes at the bases of cliffs 
from which the material has fallen. They are usually potentially 
good soils because they are mixtures of coarse and fine materials, 
often originating from several kinds of rocks, and organic matter 
is likewise mixed with the mineral components. The favorableness 
of the habitat is primarily determined by the moisture supply, 
which is strongly variable, depending upon exposure. 

Glacial ice plucks and gouges quantities of soil material from 
whatever surface it traverses. Carried in the ice, these materials 
are ground, pulverized, and mixed until they are deposited as 
moraines at the limit of advance or dropped as the ice recedes. 




FlG. 79. Shrinkage upon drying as illustrated by some Piedmont soils. 
Samples obtained in place (see Fig. 83), then initially saturated with water 
and oven-dried. B horizon clays— (1) Orange, (2) White Store, (3) Tirzah; 
A horizon sandy loam— (4) White Store. Such shrinking and swelling in the 
B horizon affects soil aeration and water movement.— From Coile. 04 



152 THE STUDY OF PLANT COMMUNITIES * Chapter VII 

The glacial debris is heterogeneous in composition and texture, 
and the depth of its deposit is highly variable. Drainage is imper- 
fect, but melt water from the receding ice is plentiful. Its early, 
rapid, and haphazard flow results in the transporting and assorting 
of a large amount of soil material, which, as drainage lines become 
established, is deposited to form topographic and soil features as- 
sociated with glacio-fluvial activity. The water-assorted soils de- 
posited in the valleys of glacial streams or carried from terminal 
moraines to form outwash plains are characteristic. 

Although glacial deposits may include weathered rock and some 
organic material, these are usually not abundant in the beginning. 
Weathering and the establishment of vegetation at first proceed 
slowly on glacial soil, but as they progress, a generally good, pro- 
ductive soil is formed. The soils of the northeastern United States 
and most of Canada are almost entirely of glacial origin. 

Soil Texture.— One of the most useful bases for classifying soils 
is that of size of particles. The local variations discussed above are 
all reflected in soil texture, which in turn has much to do with soil 
moisture, aeration, and productivity. 

The standard classification in the United States is that of the 
United States Department of Agriculture, which recognizes the 
following sizes of soil particles by name: 

Name Diameter, mm. 

Fine gravel 2.00 -1.00 

Coarse sand 1.00 —0.50 

Medium sand 0.50 -0.25 

Fine sand 0.25 -0.10 

Very fine sand 0.10 -0.05 

Silt 0.05 -0.002 

Clay < 0.002 

The percentage weight of these size classes in a soil sample is 
determined by mechanical analysis. The larger classes may be 
separated satisfactorily by means of sieves, but the fractions of 
small size are determined by the pipette method 182 or, better still, 
the use of a hydrometer. 26, 27 > 28 Both methods are based upon the 
differential rate of settling of particles in water. 

After mechanical analysis, accurate textural description is pos- 



PHYSIOGRAPHIC FACTORS 153 

sible by using the names for the fractions singly or in combina- 
tion. The soil classes are named primarily for the predominating 
size fraction, 87 but when many sizes are present, the term, loam, 
is introduced. Thus a soil may be termed gravel or clay if either 
of these sizes is present almost exclusively, but if gravel or clay 
merely predominates and is mixed with several other size classes, 
the soil is called gravelly loam or clay loam. 

A knowledge of the textural grade of a soil suggests numerous 
other characteristics of that soil. With experience, even a rough 
estimate determined by "feel" is useful, for texture indicates other 
physical properties, particularly those affecting moisture, aera- 
tion, and workability. 

Soil Structure— The arrangement of soil particles becomes es- 
pecially important when small size classes are involved. Sands have 
single-grain structure, but silts, and more particularly clays, tend 
to have particles aggregated in clumps. Aggregation is largely 
caused by the colloidal portion, less than 0.001 mm., of the clay. 
Just as clay soils with their tremendous internal surface swell when 
wet, they also contract as they dry. The minute particles are 
drawn together by cohesive forces in large or small aggregates 
whose size and shape affect drainage, percolation, erosion, and 
aeration (Fig. 79). 

If the granular structure is lacking or destroyed by mismanage- 
ment, as when trampled by livestock or worked too wet, the soil 
puddles or bakes into hard solid masses, and shrinkage results in 
the formation of deep cracks. In a loam soil or one with a high 
organic content, these undesirable features are reduced while the 
desirable characteristics produced by colloids are retained. 

Organic Content— The amount of organic material in soil mav 
greatly modify its physical characteristics as determined by the 
mineral components. In addition, organic material is the major 
source of certain plant nutrients, especially nitrogen, so that fer- 
tility and productiveness are usually correlated with it. 

Under natural conditions, organic matter in soil is derived from 
remains of plants and animals. Mostly these remains accumulate 
on the surface of the mineral soil to form a layer of litter, which, 
if sufficiently thick, may reduce the effects of insolation, check 
erosion, and prevent compacting resulting from precipitation. 



154 THE STUDY OF PLANT COMMUNITIES * Chapter VII 

When decomposition of litter does not exceed accumulation, the 
Ao horizon has a surface stratum of undecomposed twigs and 
leaves, which is termed the L layer. Beneath this is a stratum of 
decomposing but still identifiable plant remains, which is marked 
by fungal hyphae in abundance and is called the F or fermenta- 
tion layer. In contact with the mineral soil there may be an H or 
humus layer if the climate is sufficiently cool and moist. The term, 
humus, is applied to material decomposed beyond obvious recog- 
nition. Soil animals and percolating water carry the humus into the 
soil where, through further decomposition, its chemical constitu- 
ents are slowly released for use by succeeding generations of or- 
ganisms. 

When a distinct layer of humus (H layer) is present with a 
rather abrupt transition to mineral soil, the humus type may be 
designated as mor. If there is no distinct layer of humus but rather 
it is mixed with the surface mineral soil, the humus type is mull. 120 

Local variations in amount, nature, and rate of decomposition 
of humus are to be expected. Evergreen leaves do not decompose 
as readily as deciduous ones, nor do they have the same chemical 
composition. 264 Even the leaves of deciduous species do not all 
yield the same decomposition products. Organisms causing decom- 
position may be active and abundant in one habitat but quite 
incapable of living in another because of such factors as tempera- 
ture, moisture, and aeration. Consequently, humus may be un- 
equally effective in different habitats, and soils of similar origin 
may have quite different productive qualities. 

Regional Soil Variations.— Climate, which varies with latitude 
and longitude, includes the important factors in soil formation, 
especially temperature and rainfall. Within a climatic area, differ- 
ences in parent material and topographic position often are re- 
flected in soil variations, which may be chemical or physical. Such 
variations are most pronounced where parent rock is newly ex- 
posed or where soil materials have weathered but slightly, as below 
a receding glacier. After longer exposure the developing soils be- 
come much more alike, and the longer the time involved, the less 
noticeable will be differences related to local conditions. Evidence 
is sufficient to indicate that, within a climatic area, soil develop- 
ment progresses toward a particular kind of soil and profile regard- 



PHYSIOGRAPHIC FACTORS 155 

less of the origin or nature of the materials; likewise, that the 
ultimate soil group for similar climatic regions will be the same. 

Since climatic conditions determine the activities and kinds of 
organisms of a region and these organisms in turn contribute to 
soil development, it is not surprising that vegetation types and soil 
types are closely related. The development of a soil is paralleled 
by vegetational changes, the vegetation contributing to soil ma- 
turation and the soil controlling the rate of progressive succession 
of plant communities, until a mature soil for a given climate sup- 
ports a climax community of organisms. Mapping soils on the basis 
of mature profile and mapping vegetation on the basis of climax 
vegetation should produce closely similar results. 

The recognition of climatic soil types originated in Russia. The 
approach is well illustrated by Glinka's (1927) grouping of the 
great soil groups of the world primarily on a climatic basis. Ac- 
ceptance of the idea has become rather general although sometimes 
in modified form. The use of specific climatic factors, such as the 
relationship between precipitation and evaporation, for delimiting 
effective climate produces regions that correspond closely to the 
major soil groups. 131 In the United States, 169, 17 ° soils are most 
often grouped on the basis of mature profiles. Since only the ma- 
ture profile is considered, it is a recognition of the same basic ap- 
proach used by those determining regional limits through climate, 
although it requires that the profile must exist in reality, not as a 
potentiality. 

Profile Development —Three major processes of soil develop- 
ment are concerned in the production of the profiles characteristic 
of different climatic conditions. 

Podsolization occurs typically in humid, cold temperate regions 
where rainfall exceeds evaporation and where vegetation produces 
acid humus. The acid decomposition products from the litter in- 
crease the solvent power of the plentiful percolating water so that 
soluble materials and colloids are almost completely removed from 
the surface soil, which is, therefore, of single grain structure at 
maturity. Although podsolization occurs under hardwood and 
pine forests, its strongest development takes place where spruce, 
fir, or hemlock are dominant. The process is partially a product 
of the vegetation, for the content of bases in the needles of these 



156 THE STUDY OF PLANT COMMUNITIES ' Chapter VII 



trees is notably low, and decomposition products of the litter they 
produce always give an acid reaction. 

Laterization is characteristic of tropical conditions with high 
temperatures and abundant rainfall. It is essentially the leaching 
of silica from the surface soil. The low acidity produced by de- 



.. .. 





FlG. 80. The layer of calcium accumulation in a pedocal soil under sage- 
brush desert as shown in a road cut in Nevada— Photo by W. D. Billings. 

composition of tropical litter promotes the solution of silica as 
well as alkaline materials. After laterization, the surface soil is high 
in iron and aluminum, which are not removed by the process. 

Calcification may occur anywhere but is most important in 
regions with low rainfall unevenly distributed throughout the 
year and with temperatures producing a relatively high rate of 
evaporation. Under these conditions, a permanently dry stratum 
may develop in the profile below the depth to which rainwater 
penetrates. Carbonates produced by carbonation in the surface 
layers, as well as those that may be present in the original soil ma- 
terial, are carried downward in solution toward this dry layer. 



PHYSIOGRAPHIC FACTORS 



157 



When the water is removed by plants or evaporation, the carbon- 
ates are left behind, at or above the dry layer, depending upon the 
depth of penetration of the moisture at the time. 

Climatic Soil Types.— On the basis of absence or presence of a 
lime carbonate layer formed by calcification, the mature profiles 
of all soils of North America fall into two groups : pedalfers, with- 
out the layer; pedocals, with the carbonate layer. The two condi- 
tions occur regardless of the nature of parent material or its geo- 
logical origin, and their distribution is obviously controlled by 
climate. Soils of eastern North America are all pedalfers, for the 
unfavorable balance between rainfall and evaporation necessary to 
carbonate deposition does not occur here. West of about the 99th 
meridian (a line through the center of the Dakotas to the pan- 




FlG. 81. General distribution of the important zonal soil groups of the 
United States. After Kellogg, 136 from Klages, Ecological Crop Geography, by 
permission of The Macmillan Company, publishers. 

handle of Texas), where annual precipitation is normally less than 
twenty inches a year, mature profiles almost invariably show 
pedocal characteristics except where climatic conditions are vari- 
able, notably in the mountains and in parts of California. Climate, 
vegetation, and soil have corresponding distributions. The pedal- 



158 THE STUDY OF PLANT COMMUNITIES * Chapter VII 

fers occur principally in association with forest regions, while the 
pedocals do not support forests but are typically covered with 
grassland or desert. 

Pedalfers.— Although mature soils lying east of the line marking 
the western boundary of the prairie are usually of this type, they 
vary considerably. The range of temperatures within the area is 
so great that podsolization is characteristic in the north and lat- 
erization in the south with intermediate conditions represented 
between. The following zonal climatic soil groups, therefore, 
occur in eastern North America. 

Tundra Soils : Far northern soils with shallow profiles and 
high proportions of undecomposed organic materials. 

Podsol Soils : Northeastern United States and extending 
north and northwestward into Canada. Distinct horizons with 
a thick Ao, white or gray leached A over a brown B horizon 
with its accumulation of aluminum and iron. 

Gray -Brown Podsolic Soils : A wide band across east-cen- 
tral United States. Like podsol but with thinner Ao horizon 
and less leaching of the A, which is grav-brown over a brown 
B horizon. 

Red and Yellow Soils : Southeastern United States where 
humid, warm climate produces both podsolization and later- 
ization. Colors bright, low in organic matter, high in clay, 
strongly leached. Yellow soils in the sandy, poorly drained 
coastal plain; red soils in the well-drained Piedmont. 

Prairie Soils .'Western margin of the pedalfers. Intermediate 
between forest and grassland soils. Black or dark brown with 
brown subsoils that differ little in texture from the surface. 

Lateritic and Laterite Soils : Subtropical and tropical. Rep- 
resent extreme in mineral weathering. Leached of silica. 

Pedocals.— Zonation of these soils from north to south has not 
been recognized as for pedalfers. Moisture being more effective 
than temperature in producing variation in pedocals, the con- 
spicuous zones lie in a north-south position. Their location and 
brief characterization follow : 

Chernozem Soils : A broad band extending from Canada 
into Mexico just west of the Prairie Soils. Rich in organic 



PHYSIOGRAPHIC FACTORS 159 

matter. Black soils with brown or reddish calcareous subsoils. 
Strong carbonate horizon but normal horizons indistinct. 

Br oil'?! Soils (also known as Chestnut Soils) : Bordering 
Chernozems to the west and developed under successively 
drier conditions, they contain successively less organic matter 
westward and southward and become lighter in color, as in- 
dicated by their division into Dark Brown and Brown Soils. 
Occupy mainly the area usually called the Great Plains. 

Gray Soils : Desert and semidesert soils largely in the Great 
Basin and southward. Gray with yellowish to reddish cal- 
careous subsoils. Negligible organic content. Weathering 
largely physical. 

"Within these climatically determined soil regions, are local varia- 
tions that, because of time and topography, bear no resemblance 
to the mature soil type. Swamps and bogs, islands and flood plains, 
salt and alkali flats, or merely immature soils on steep slopes— all 
are illustrations of local conditions that must be disregarded in 
considering the broad aspects of climatic control of soil develop- 
ment. 

The climatic classification of soils is useful because it makes 
possible broad considerations of regional problems. It is logical 
because it bases the major categories upon mature conditions, 
which remain stable with the climate, and makes possible the ex- 
planation of local variations, which represent merely stages of 
profile development. Best of all, it has world-wide application. 
Enough investigations have now been made to show that the same 
general soil types are repeated in those parts of the world where 
climatic conditions are duplicated. Thus, it has been feasible to 
devise several schematic representations of the relationship of tem- 
perature and moisture to soil formation that are reasonably ap- 
plicable anywhere. A relatively simple climatic system 251 is shown 
below, in which temperature-evaporation and precipitation-evap- 
oration relationships are used as criteria of climatic control. It 
serves to emphasize the importance of moisture in pedocal devel- 
opment and grassland areas but shows that temperature is more 
effective where pedalfers develop with the forests they support. 

Vegetation and Soil Development.— The close similarity be- 
tween the distribution of major vegetation types and climatic soil 



160 THE STUDY OF PLANT COMMUNITIES ■ Chapter VII 



DRY COLD 



THE DISTRIBUTION OF; 



CLIMATIC 

TYPES 



DRY HOT 



2.VEGETATIONAL 
FORMATIONS 



3. MAJOR ZONAL 
SOIL GROUPS 



WET COLD 



PERPETUAL SNOW AND ICE 


TUNDRA 




TAIGA 






C SUBARCTIC ) 






Q 


Q 






O 


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3 


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5 CLIMATE 

a. 

5 



PRECIPITATION 



WET HOT 





PERPETUAL SNOW AND ICE 




TUNDRA 


TAIGA 






to 


IO 








UJ 


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10 


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VEGETATION 



PERPETUAL SNOW AND ICE 


TUNDRA SOILS 


PODSOLS 


•0 

_i 

O 

to 

1- 

K 
UI 

(0 
UI 

a 


CO 

_i 
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<0 

z 

$ 

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GREY-BROWN 
PODSOLIC SOILS 


LATERITIC RED 
& YELLOW SOILS 


LATERITES 
RED SOILS 



SOILS 



FlG. 82. Schematic representation to show the interrelated distribution of 
climatic types, vegetational formations, and major zonal soil groups— After 
Blumenstock and Thornthwaite. 2 * 



types has been mentioned. It has also been suggested that the char- 
acteristics of a mature profile are partially produced by the vege- 
tation or that they are possible only because of the kind of vege- 



PHYSIOGRAPHIC FACTORS 161 

tation supported by that soil in the given climate. This point 
should be further emphasized. Newly formed soil material has no 
profile and bears no resemblance to the mature soil of the region. 
It cannot support the vegetation that grows on a mature soil, but 
the plants that can grow upon it contribute to its development, 
probably most effectively through their decomposition products, 
and so, in time, the resulting soil changes permit other plants to 
grow. There results, sometimes over a long period, a succession 
of edaphically controlled vegetation types leading ultimately to a 
climatically controlled community. Paralleling the plant succes- 
sion are changes in the soil— called soil development— -which are 
primarily possible because of the plants and which lead to the 
mature profile, also controlled by climate. Soil development and 
vegetational development are intimately related and together are 
controlled by climate. 

SOIL WATER* 

Soil water probably affects plant growth much more commonly 
than any other soil factor. It follows, therefore, that a basic under- 
standing of what causes differences in amounts and availability of 
soil moisture and what such differences may mean to a plant is 
ecologically necessary. 

Classification of Soil Water.— A simple, arbitrary system of 
classification that divides soil water into four general categories is 
sufficient for most ecological purposes. 

1. Gravitational water occupies the larger pores of the soil 
and drains away under the influence of gravity. For a 
short time after a heavy rain or irrigation, the soil may be 
completely saturated with water, the air in it having been 
displaced from the noncapillary pore spaces between the 
particles. Under the influence of gravity, the free water 
soon percolates downward through the soil toward the 
water table unless prevented by some barrier, such as a 
hardpan or other impermeable layer. Within two or three 
days after a rain, all the gravitational water usually drains 
out of at least the upper horizons of the soil, and the pore 



*Much of this section is adapted from a review by Kramer, 149 which in- 
cludes an extensive bibliography. 



162 THE STUDY OF PLANT COMMUNITIES • Chapter VII 

spaces become refilled with air. If the soil remains sat- 
urated with gravitational water for several days, serious 
injury to root systems may result from lack of oxygen 
and accumulation of excess carbon dioxide. Hence gravi- 
tational water is of little direct value to most plants and 
even may be detrimental. 

2. Capillary water is held by surface forces as films around 
the particles, in angles between them and in capillary 
pores. Immediately after gravitational water has drained 
away the capillary water is at its peak, and a soil is then 
said to be at its field capacity. Much of this film water is 
held rather loosely and is readily available to plants, but 
some of it, which is held by colloidal material and which 
is in the smallest pores, is relatively unavailable. It is in this 
connection that the size of particles becomes important. A 
cubical sand grain one millimeter on the edge has a surface 
of only 6 square millimeters, but if it were divided into 
cubes of colloidal size, 0.1 micron on the edge, the total 
surface resulting would be 60,000 square millimeters. The 
increase in surface and angles between particles would 
thus increase tremendously the total capacity for holding 
capillary water. However, the water available to plants 
does not increase proportionally, for the greater curva- 
ture of the films and the sharper angles sufficiently increase 
the force with which water is held to materially increase 
unavailable water. 

3. Hygroscopic water is held in a very thin film on the sur- 
face of particles by surface forces and moves only in the 
form of vapor. The moisture remaining in air-dry soil is 
usually considered as hygroscopic and is, in general, un- 
available to plants. Distinction between this and capillary 
moisture is difficult, for exposure of soil to increasingly 
moist atmospheres may increase the water content even 
to saturation. 

4. Water vapor occurs in the soil atmosphere and moves 
along vapor pressure gradients. It is probably not used 
directly by plants. 

Origin of Soil Water.— Precipitation in the form of rain, hail, or 



PHYSIOGRAPHIC FACTORS 163 

snow is the ultimate source of water found in the soil, but not all 
precipitation becomes soil water. The steeper a slope, the more 
water will run off from its surface before it can enter the soil. 
Excessive precipitation in a short period of time results in greater 
runoff than that following a gentle rain, since infiltration cannot 
keep pace with the rate of fall. If soil becomes saturated and pre- 
cipitation continues, little, if any, will enter the soil. A larger pro- 
portion of water from slowly melting snow is apt to enter the soil 
than from an equal amount of rain. Infiltration into a fine-textured, 
clayey soil is slower than into a coarse-textured, sandy soil, and a 
compact mineral soil absorbs water more slowly than a loose soil 
or one with a high organic content or heavy litter. The particles 
of a bare mineral soil tend to pack at the surface when rained upon 
for only a few minutes and thus reduce the rate of infiltration (see 
Fig. 39). Variation of local conditions may, therefore, modify the 
effectiveness of a given amount of precipitation. 

Movement of Soil Water.— Water moves downward in quantity 
during and immediately after rain or irrigation. Later it may move 
upward or laterally to some extent when evaporation and use by 
plants reduces the amount near the surface. Its principal movement 
occurs as a liquid in capillary films or through noncapillary pores, 
but some movement also occurs in the form of vapor. Gravity, 
hydrostatic pressure, and capillary action are the forces involved, 
and movement may be the result of interaction of all three. 

The rate at which infiltration takes place is at first determined 
by surface conditions. When they are favorable, practically all of 
a light rain is absorbed. Within a half hour or less, however, ab- 
sorption declines and is then controlled by conditions in the lower 
horizons, where percolation may be very slow. Movement of 
gravitational water through the soif is controlled by the number, 
size, and continuity of the noncapillary pores through which it 
percolates. Drainage is rapid in coarse-textured soils, but in clays 
movement is slow since the pores are small and may be blocked by 
the swelling of colloidal gels or by trapped air. Channels left by 
earthworms or other animals and those left by dead roots greatly 
facilitate downward movement. If there is no impermeable hard- 
pan layer and if the water table has not been raised too near the 
surface, all gravitational water drains from surface strata within 



164 THE STUDY OF PLANT COMMUNITIES • Chapter VII 

two or three days after a rain leaving the soil water content at 
field capacity. 

A simple explanation of the movement of capillary water may 
be entirely adequate for most ecological purposes. Since capillary 
water forms a continuous, thin film around soil particles and in 
the small spaces and angles between them, it is obvious that sur- 
face tension of the water creates inward pressure in the film and 
that water, therefore, tends to move from regions with thicker 
films to regions with thinner films. An explanation with broader 
applications considers- the difference in attraction for water be- 
tween two portions of soil having different moisture contents and 
expresses this attraction or force as capillary potential— -that is, the 
force required to move a unit mass of water from a unit mass 
of soil. Various methods of measuring this force indicate that the 
potential is directly related to the water content and that there is 
no change in the state of water as moisture content is reduced 
from field capacity to an oven-dry condition, but merely an in- 
crease in energy required to move it. On this basis, the boundaries 
between gravitational, capillary, and hygroscopic water are too 
indistinct to be recognized. That these boundaries are indistinct is, 
in fact, true regardless of the point of view. Such relatively simple 
considerations seem entirely satisfactory for an adequate under- 
standing of plant-water relationships, although recent studies of 
soil moisture by soil physicists have become increasingly technical. 

Movement of capillary water is closely related to soil texture. In 
wet soils, it is rapid in sand and slow in clay, but the rate is re- 
versed as soils dry out. Capillary rise, or the distance that capillary 
force will move water, is much greater in clay than in sand al- 
though the rate of movement is less in clay. The rate is surpris- 
ingly slow at all times and probably is quite insufficient to main- 
tain an adequate film on the soil particles from which a root is 
removing water. The water coming to a root by capillary action 
does not at all equal the amount made available in new films that 
the root contacts because of its elongation and production of new 
root hairs. When soil water is below field capacity, capillary move- 
ment is probably insufficient to replace the film on particles from 
which roots of an actively transpiring plant are removing water. 
The continuous elongation of these roots with the production of 



PHYSIOGRAPHIC FACTORS 165 

new root hairs brings them in contact with new films and helps 
to keep up the supply of necessary available water. 

Movement of water vapor is along vapor pressure gradients, 
which are affected by temperatures and vapor pressures of the air 
and the different soil horizons. There must, therefore, be some 
movement in all soils, but its effects are most noticeable in semi- 
arid regions where there is no connection between the water table 
and capillary water near the surface. In winter or in any cool pe- 
riod, water vapor moves upward from the warmer subsoil and 
cools and condenses in the surface layers. When temperatures rise 
at the surface, evaporation takes place into the air, and the total 
ground water is reduced. Usually the surface soil is warmest in 
summer and results in downward movement of vapor with con- 
densation at lower levels. If the surface soil is cooler than the air 
above it, water vapor may move into the soil and condense there 
in quantities sufficient to be of significance under semiarid condi- 
tions. 

Water Lost to the Atmosphere.— The loss of water from soil to 
the air by evaporation varies with the factors affecting the steep- 
ness of the vapor pressure gradient. Temperature, humidity, and 
movement of the air, as well as temperature and moisture content 
of the soil, are factors, which in turn are modified by exposure, 
cover, and color of the soil. Probably the loss of water by evapora- 
tion is much less than is commonly supposed, for numerous studies 
indicate that there is little capillary rise to replace water lost by 
evaporation unless the water table is within a few feet of the sur- 
face. In those areas where water lost by evaporation might be 
critical, the water table lies so deep that precipitation rarely wets 
the soil down to it and, consequently, the upward rise is of no 
consequence. In general, the loss of water by evaporation seems 
mostly to be from the top foot of soil. Under natural conditions, 
this probably affects few species and is rarely of significance. 

In agriculture, water lost by evaporation has been the subject of 
much argument, particularly with regard to the effects of cultiva- 
tion. Evaporation from a dry soil surface is much less than from a 
moist one because diffusion through soil is very slow. Since a dry 
soil surface can be moistened only by an upward capillary move- 
ment of water if no rain falls, it has been maintained that cultiva- 



166 THE STUDY OF PLANT COMMUNITIES ' Chapter VII 

tion of the surface must reduce loss by evaporation since it pre- 
vents capillary movement. It is now known that, unless the water 
table is very near the surface, capillary rise is negligible under any 
circumstances. This being true, the dust mulch, or cultivated sur- 
face, has little to support it. In fact, if the surface capillary water 
is not connected with the water table, as is frequently true under 
irrigation, cultivation for a mulch probably increases the loss of 
water. Organic mulches seem to be more effective in reducing 
water loss, probably because they shade the soil and reduce its 
temperatures, increase the distance of diffusion from soil to air, 
and protect the soil from the drying effects of wind. 

Water lost to the atmosphere through transpiration far exceeds 
that lost by evaporation. Whereas evaporation seems to be effective 
only in the surface soil, plants remove water from considerable 
depths. Studies of orchard soils in different parts of the country 
indicate that all readily available water may be removed to a depth 
of three to six feet in three to six weeks, depending upon atmos- 
pheric conditions and the kind of soil. Sandy soils, of course, are 
exhausted more quickly than clayey soils. The relative losses by 
evaporation and transpiration are illustrated by experiments, 262 in 
which water was lost from a bare soil surface in a tank at the rate 
of 4.7 pounds per square foot during one growing season, while a 
four-year-old prune tree removed water from a similar tank at the 
rate of 416 pounds per square foot of soil surface. An acre of de- 
ciduous fruit near Davis, California, used eight acre-inches of 
water in six weeks in midsummer. Corn grown in Kansas requires 
some fifty-four gallons of water per plant to mature. If this were 
applied at one time, as by irrigation, it would cover a cornfield to 
a depth of about twelve to fifteen inches. Plants growing natural- 
ly have similar requirements. The knowledge that transpiration is 
the chief means of reducing capillary water in the soil has led to a 
consideration 142 of what kinds of plants on watersheds will least 
reduce the supply of water by transpiration and still prevent ero- 
sion. 

Soil Moisture Constants.— To compare the moisture character- 
istics of soils or to discuss them with respect to plants, quantita- 
tive expressions of hydro-physical properties are a necessity. These 
properties, determined under fixed conditions, are called constants. 



PHYSIOGRAPHIC FACTORS 



167 



The hygroscopic coefficient is the moisture content, expressed 
as a percentage of the dry weight, of a soil in equilibrium with an 
atmosphere of known relative humidity. The value is difficult to 
obtain with accuracy and is of little use to plant scientists. 




\\^' —Inner Cylinder 

I 



— Cutting Cylinder 



062 




FIG. 83. Sampler for obtaining undisturbed soil for determining volume- 
weight, air space, and water holding capacity. A counter-sunk steel plate or 
a block of wood placed on the cylinder prevents it from being battered 
when driven into the soil with a sledge hammer. The inner cylinder (see Fig. 
79) is removed with the sample (600 cc.) and is covered with tightly fitted 
lids for transportation.— After Coiled 



168 THE STUDY OF PLANT COMMUNITIES • Chapter VII 

Maximum "water holding capacity is the water held by a sat- 
urated soil. It may be determined by weighing a unit volume of 
soil before and after it has been immersed in water for twenty-four 
to forty-eight hours. 

Field capacity is the amount of water a soil retains after all 
gravitational water is drained away. Soils in the field attain this 
condition within one to five days after a rain except when the 
water table is near the surface or saturation extends to a depth of 
many feet. After prolonged rain, soil may be assumed to be at field 
capacity if samples taken at eight- to twelve-hour intervals have 
essentially the same moisture content. 

It is now common practice to express most soil moisture values 
on a volume basis. In addition, it is desirable that most of these 
values should apply to the soil as it lies in the field. It is, therefore, 
advisable to obtain undisturbed samples of a certain volume and to 
make all determinations without modifying the structure of the 
samples. Such samples may be obtained with metal cylinders, 63 
which, when forced into the soil, cut a sample of exact volume, 
which is then enclosed with airtight lids. Rocky soils may make it 
impossible to obtain undisturbed samples. It then becomes neces- 
sary to use special techniques, which, although they give much 
the same results, require more time and pains than are ordinarily 
necessary. 163 Some investigators obtain all their samples only when 
the soil is at field capacity. This system has several advantages, 
such as eliminating the problems related to swelling on wetting, 
simplifying sampling, and giving a value for field capacity that is 
strictly determined by field conditions. When soils are dry, it is 
often possible to soak them, in place, and permit them to come to 
field capacity before sampling. 

Capillary capacity (water holding capacity) is the water re- 
tained against the pull of gravity. Although this appears to be 
essentially what is meant by field capacity, it is a value determined 
under laboratory conditions and may run slightly higher than field 
capacity. The saturated samples of undisturbed soil used for de- 
termining maximum water holding capacity are permitted to drain 
over sand for a fixed time, usually two hours, and the weight of 
water retained, expressed as a percentage of the volume of the 
sample, is termed the capillary capacity. 



PHYSIOGRAPHIC FACTORS 169 



When the maximum water holding capacity, the field capacity, 
or capillary capacity, and the dry weight of an undisturbed sam- 
ple are known, it is relatively simple to calculate pore volume, air 
capacity, volume weight, and specific gravity of soil material. 162 

The moisture equivalent denotes the water content of soil that 
has been subjected, usually for thirty minutes, to a centrifugal 
force of one thousand times gravity in a soil centrifuge. Its deter- 
mination is simple if equipment is available. Within limits, it bears 
a constant relationship to certain other soil moisture values or, at 
least, suggests what these values should be. Its ratio to field ca- 
pacity is near unity, but the relationship is least constant with 
coarse-textured soils. In many soils the moisture equivalent is 1.84 
times as great as the water left in those soils when plants wilt. Un- 
available water can, therefore, be approximated from the moisture 
equivalent. The ratio of moisture content to moisture equivalent 
(relative wetness) can be used to make comparisons between soils 
or soil strata of different textures where moisture content alone 
would mean little in terms of plants because of variation in avail- 
ability. 

The permanent wilting percentage should be considered as the 
moisture content of the soil at the time when the leaves of plants 
growing in that soil first become permanently wilted. Because it 
has not always been so considered, there have been various other 
terms (wilting point, wilting coefficient, wilting percentage) ap- 
plied to the concept, and not all investigations have produced the 
same results. Briggs and Shantz 35 first emphasized the importance 
of this soil moisture condition to plant growth and called it the 
"wilting coefficient!' Their procedure was to grow seedlings in 
glass tumblers of soil sealed with a mixture of paraffin and vase- 
line. When the leaves wilted and did not recover overnight in a 
moist chamber, the moisture content of the soil was determined 
by oven drying at 105° C. and calculated as a percentage of the 
dry weight. It is generally agreed that permanent wilting marks 
the soil water content at which absorption becomes too slow to 
replace water lost by transpiration. 

Briggs and Shantz came to the conclusion that soil texture alone 
determines moisture content at which plants wilt permanently, 
regardless of the species, their condition, or the environmental 



170 THE STUDY OF PLANT COMMUNITIES * Chapter VII 

conditions. This conclusion was not immediately acceptable to 
everyone, and numerous studies were made to check its validity 
with different kinds of plants of different ages under a variety of 
conditions. It is now generally agreed that permanent wilting of 
any species occurs at the same water content of a soil of a certain 
texture regardless of the age of the plant or environmental condi- 
tions under which it grew. Uniformity of results is assured if non- 
cutinized herbaceous plants are used and if permanent wilting of 
the lowest pair of leaves is used as an end point. This eliminates the 
problem of recognizing the onset of permanent wilting and varia- 
tions related to the ability of some plants to live much longer than 
others after the onset of wilting. 

Briggs and Shantz also concluded that it was possible to calcu- 
late the wilting point from the moisture equivalent because the 
following relationship held in their soils : 

... ~ . moisture equivalent 

wilting coefficient == 3 

5 1.84 ±0.013 

Although this often holds true, it does not apply to all soils. Studies 
in different parts of the country indicate that the ratio ranges at 
least from 1.4 to 5.65. Attempts to relate the moisture content at 
the time of wilting to other variables have been equally unsatis- 
factory, and it, therefore, appears that its determination is most 
reliable when observed directly. Because the expression, "wilting 
coefficient" has been so often associated with calculated values, it 
is logical to restrict it to that usage and to apply the term, "per- 
manent wilting percentage'/ to determinations made by direct ob- 
servation. 

Readily available water is that which can be used by plants for 
growth and is, therefore, the moisture above the permanent wilt- 
ing percentage. This includes gravitational water, but its rapid 
drainage makes it of little consequence. The remaining usable 
water is in the range from field capacity or moisture equivalent 
down to the permanent wilting percentage. This range is narrow 
in sand and wide in clay. Obviously, the wider the range, the 
longer plants can resist drought and, in cultivation, the less fre- 
quently irrigation is necessary. The rate at which water moves 
from soil to an absorbing surface is strongly indicative of plant- 
soil moisture relationships at the time. An indication of the avail- 



PHYSIOGRAPHIC FACTORS 171 

ability of soil water to the plant may be obtained with porous soil 
point cones, 158 whose rate of absorption is taken as the basis for 
evaluating water supplying povcer of the soil. 

Availability of Soil Moisture to Plants.— Gravitational water is 
readily available to plants only when present in a saturated soil, a 
condition that rarely continues long enough to be of importance. 
Normally, then, readily available water is that capillary water in 
the range between field capacity and the permanent wilting per- 
centage. It is usually lowest in sand, and highest in clay. The fol- 
lowing values for readily available water are found in some North 
Carolina soils : 149 sand, 2 percent, sandy loam, 14 percent, clay, 19 
percent. However, this generalization does not always hold, for 
some clays may have high field capacities but also have high wilt- 
ing percentages. A California clay with a moisture equivalent of 
31 percent was found to have a wilting percentage of 25 percent, 
and, therefore, it could contain only 6 percent of available water. 
Such a soil would store less water for plant use than many sandy 
soils, and plants growing in it would suffer from drought much 
sooner than its soil texture would indicate. This also explains why, 
in contrast to the usual situation, sand dunes in deserts have more 
favorable moisture conditions than the surrounding clay soils. 
When both are at or near the wilting percentage, as they fre- 
quently are, a typically light rain provides little or no available 
water in the clay but does provide some in sand, in addition to 
penetrating more deeply, because of the lower wilting percentage 
of sand. 

Whether or not all available water is equally available to plants 
is not entirely agreed upon. The evidence from a variety of 
sources seems to favor a decreasing availability as the supply is 
reduced toward the permanent wilting percentage and particu- 
larly in the lower half of the range of available water. Another 
factor affecting the availability of soil water is the concentration 
of the soil solution, which, if high, may have a toxic effect on 
plants and also modify their osmotic activity. Soil temperature, 
too, may be effective. Water supplying power may be reduced by 
half when soil temperature is lowered from 77° E to 32° F. Prob- 
ably the increase in viscosity of water at low temperatures reduces 
the rate of movement from soil to absorbing surface. 



172 THE STUDY OF PLANT COMMUNITIES • Chapter VII 

Measurement of Soil Moisture.— For ecological purposes, it is 
of prime importance to know how much soil water is available for 
plant use and often to be able to follow its variations from day to 
day throughout a growing season. Because of soil variation, it is 
usually desirable to have determinations from numerous places in 
a stand and usually from more than one stratum in the soil. It is 
undesirable to use sampling methods that disturb any considerable 
amount of the soil or injure roots in the experimental area, and, 
again, any expression of soil moisture should preferably refer to a 
unit volume of sample obtained in an undisturbed condition. The 
last qualification is advisable because interest is in the volume of 
water available to roots occupying a given volume of soil, rather 
than weight of water in a given weight of soil. 

It should be clear from our previous discussion that, to inter- 
pret soil moisture conditions, several soil moisture constants are 
necessary and that some physical analyses of the soil may be de- 
sirable. A single collection of samples from each local area of study 
may suffice for these purposes. Thereafter, some method must be 
fixed upon, which, within the time available to the worker, will 
give as adequate a notion as possible of the variations in soil mois- 
ture content of the experimental areas. Finally, it must be possible 
to express the soil moisture data in terms of what is available to 
plants. 

Methods currently in general use are of two types : ( 1 ) deter- 
mining the actual content of water, (2) measuring the forces with 
which water is held or the rate at which it is supplied to an ab- 
sorbing surface. 

The actual content of water is determined by weighing samples 
before and after drying to constant weight in an oven at 105° C. 
The loss in weight, representing the water content, is expressed as 
a percentage of the dry weight or, if the samples are undisturbed, 
on a volume basis. The disadvantages of the method are numerous. 
Sampling takes time and disturbs the soil, the samples must be 
transported, weighing and drying are time-consuming, and a con- 
tinuous record is impossible. However, the method has its uses, 
and, where only a few determinations are wanted, it is undoubt- 
edly the procedure to use. Note, too, that it requires no equip- 
ment that is not ordinarily available. 



PHYSIOGRAPHIC FACTORS 



173 



Several electrometric methods have been adapted to the meas- 
urement of soil moisture. All require calibration in terms of the 
wilting percentage of the soil involved but thereafter permit rapid 
determinations at short or long intervals and direct translation of 
measurements into available water. The method that seems to be 
most in favor at present is the measurement of resistance between 
two electrodes imbedded in gypsum blocks and buried in the 
soil. 29 The resistance varies inversely with amount of soil water 
and also with soil temperature. Other methods measure dielectric 
constant, or electrical or thermal capacity of the soil, values that 
vary with changes in soil moisture. 

Two physical measurements, making use of (1) tensiometers 207 
and (2) soil point cones, 158 have been used successfully. 

A tensiometer measures the tension existing between the soil 
and the soil water. It consists of a porous cup set in the soil and 
connected to a manometer by a tube of small diameter. Water in 
the instrument makes connection through the porous cup with the 
soil water, from which equilibrium tension is transmitted to the 
mercury of the manometer. Since the height of the mercury col- 
umn indicates the tension in the soil, the manometer can be cali- 
brated for a wide range of soil moisture values, and readings can 
be taken at any time and translated directly into values for avail- 



TABLE 9— Percentage composition of oxygen and carbon dioxide in soil 
air extracted at different depths in a silty loam soiK 30 ). Note that the percent- 
age of 2 decreases and of CO2 increases with depth both winter and sum- 
mer but that subsoil aeration is far better when the soil is dry in summer than 
when it is wet in winter. 



Depth 


Oxygen 


Carbon dioxide 










(feet) 


Winter 


Summer 


Winter 


Summer 


1 


19.4 


19.0 


1.2 


2.4 


2 


11.6 


17.4 


2.4 


3.7 


3 


3.5 


16.7 


6.6 


5.0 


4 


0.7 


15.25 


9.6 


8.55 


5 


2.4 


12.95 


10.4 


11.85 


6 


0.2 


11.85 


15.5 


11.9 



174 THE STUDY OF PLANT COMMUNITIES * Chapter VII 

able water. The instrument is accurate for values ranging from 
zero tension to approximately 0.85 atmosphere of tension, or from 
saturated soil to a reduction of 80 or 90 percent of available 
water. 217 Approaching the wilting percentage, its values cannot be 
wholly trusted. 

Soil point cones are small, hollow cones of porous porcelain, 
which can be inserted into the soil with a minimum of disturbance 
so that each has an equal area of surface in contact with the soil. 
The amount of water absorbed by the cone is determined by 
weighing and is taken as a measure of the water supplying power 
of the soil. In some types of studies, this value alone is sufficient to 
make comparisons between soils without any further analyses be- 
ing necessary. It is also indicative of moisture conditions, for, at 
the wilting percentage, it approximates 0.085 g. in two hours. 

SOIL ATMOSPHERE 

Organisms and Soil Atmosphere.— It was pointed out earlier 
that air is a component of soil (p. 148). Both the amount and com- 
position of this air are of importance to plants. Most plants re- 
quire a well-aerated soil for growth and even for survival. Many 
seeds will not germinate unless well aerated even though tempera- 
ture and moisture are favorable. Healthy roots must carry on res- 
piration continuously, which means that oxygen must be present 
in the soil. At the same time, their activity produces carbon dioxide 
and carbonic acid, which tend to accumulate. To some plants, 
the increase in the proportion of carbon dioxide is more injurious 
than the decrease of oxygen. Since all microorganisms present are 
likewise using oxygen and releasing carbon dioxide, the balance of 
the two cannot be maintained unless there is a free exchange of 
gases with the air above the soil. If aeration is good, this may be 
accomplished by diffusion from the air. However, in any soil the 
proportion of oxygen decreases and that of carbon dioxide in- 
creases with depth, and the proportion of oxygen is not as great 
in soil as in air even when conditions are most favorable. 

Relation to Growth and Distribution of Roots.— Since aeration 
becomes poorer and oxygen decreases with depth of soil, these 
conditions may limit the depths to which roots can grow. The 
deepest root penetration is in well-aerated soils. Species growing 



PHYSIOGRAPHIC FACTORS 



175 



in wet lowlands are invariably shallow-rooted, for here aeration is 
poorest because the soil is periodically or continuously saturated 
and the only available oxygen may then be in solution. These 
shallow-rooted species will usually grow well in uplands, but, if 
the naturally deep-rooted species are moved to lowlands, they do 
not do well or may actually die. Thus aeration may determine the 
rate of growth, an element of importance in forest stands, and may 
be the factor controlling the type of vegetation. 

Soil Aeration and Plant Adaptations.-Well-aerated soils may 
have an air capacity of 60 to 70 percent by volume, a condition 
determined primarily by structure and scarcely affected by tex- 
ture. The amount of air varies, of course, with the water content 
of the soil, for air is forced from the spaces in the soil that become 
occupied by water. 

Thus continuously saturated soil is poorly aerated, and the mud 
under a pond probably has the poorest aeration of any plant habi- 




FlG. 84. Some types of lacunar tissue found in stems of emergent and 
other aquatic vascular plants. (A) Cortex of water milfoil (Myriophyllum). 
(B) Ground parenchyma throughout stem of a rush (J uncus). (C) Same for 
a sedge (Cyperus). 



176 THE STUDY OF PLANT COMMUNITIES * Chapter VII 

tat. Most species growing in such habitats have adaptations that 
serve to counteract poor aeration. Many have large, continuous 
spaces— lacunar tissue— in their stems and roots permitting storage 
and free movement of gases within the plant. In emergent and 
floating-leaved species, these spaces are connected directly with 
the atmosphere through the stomata. Submerged leaves of aquatics 
are invariably finely dissected or extremely delicate, conditions 




Fig. 85. Cypress swamp (Taxodium distichum) in the coastal plain of 
South Carolina. Note buttressed, somewhat planked bases of trees and an 
abundance of cypress knees, whose uniform height marks average high-water 
level.— U. S. Forest Service. 



that bring a majority of the cells in contact with the water, from 
which they must obtain oxygen in dissolved form. A few sub- 
merged species produce pneumatophores, or special branches that 
extend above water and give direct connection with the air through 
lacunar tissues. In addition to shallow root systems, a number of 
swamp trees have other characteristics in common. Enlarged or 
buttressed bases and plank roots are frequent, especially in south- 
ern swamps, and the "knees" of cypress are in the same category. 
That these structures facilitate aeration has not been conclusively 
demonstrated, but their formation seems to be in response to alter- 
nate inundation and exposure to air. 150 



PHYSIOGRAPHIC FACTORS 



177 



Determination of Volume and Composition.— Total pore space 
or pore volume (in c.c.) is equivalent to the weight of water (ing.) 
in the soil at saturation, for water then is assumed to occupy all 
the space in the soil. Actually not all air can be replaced by water, 
and the small amount of air remaining at saturation represents 
what is available to roots regardless of circumstances. Air capacity 
is the amount of air in soil that has been drained of all gravitational 
water. It is, therefore, equal to the difference between pore vol- 
ume and the weight of water at field capacity. Since total water 
holding capacity and field capacity are constants, it follows that 
pore volume and air capacity are soil-air constants. The actual air 
content is not at all constant, for it varies inversely with the water 
content. Soils with a high air capacity are in general well aerated, 
but, after prolonged rain or flooding, they may for a time be 
poorly aerated because water fills so much of their internal space 
that actual air content is low. 



TABLE 10— Porosity, field capacity, and air capacity of some soils with 
different textures. After 14 from Kopecky. 



Character of soil 



Compact heavy clay . . . . 

Clay loam 

Compact loam 

Very fine sand 

Friable loam 

Friable fine sandy loam 



Particles 






smaller 


Total 




than 


pore 


Field 


o.oi mm. 


volume 


capacity 


86.7% 


47.6% 


48.0% 


67.2 


41.1 


46,1 


46.5 


34.9 


41.1 


48.4 


39.3 


49.3 


42.6 


37.1 


49.3 


39.6^ 


34.6 


49.5 



Air 
capacity 

0.4% 

5.0 

6.2 
10.0 
12.2 
14.9 



The composition of soil air may be determined with a portable 
gas-analysis apparatus. 113 The sample of air can be pumped from 
the soil through a sampling tube 211 or some similar device, 30 or it 
can be withdrawn from a unit volume of undisturbed soil. The 
total percentage of oxygen and carbon dioxide in the soil is usu- 
ally very nearly that found in air, and, in general, an increase of 
one results in a proportional decrease of the other. 



178 THE STUDY OF PLANT COMMUNITIES * Chapter VII 

SOME CHEMICAL FACTORS 

Soil Acidity.— Regardless of the nature of their parent material, 
soils tend to become acid in reaction if precipitation is sufficient 
to cause downward percolation of water during much of the year. 
This is largely the result of leaching of soluble basic salts. To illus- 
trate, calcium carbonate is relatively insoluble in water but reacts 
with carbonic acid, ever-present in soil water, to form readily 
soluble calcium bicarbonate. This, of course, is leached from the 
surface soil by percolating water. Although the leaching bicar- 
bonates may be re-precipitated at any time the soil dries out, they, 
nevertheless, tend always to move downward. Thus the surface 
horizons tend to be low in basic materials and may have a highly 
acid reaction because of the acids produced by chemical and 
biological activity in progress there. The surface strata have the 
largest accumulation of organic matter, which yields acid prod- 
ucts upon decomposition, the greatest numbers of soil organisms 
whose activities may produce acids, and the most active chemical 
changes in the mineral components, also contributing to acidity. 
Consequently, acidity is normally greatest at the surface and de- 
creases in the lower horizons of the soil. 

A solution is acid when the concentration of hydrogen ions 
(H + ) exceeds that of hydroxyl ions (OH), and it is alkaline if 
there are more OH~ ions than H + ions. If the two concentrations 
are equal, as in pure distilled water, the reaction is neutral. Since 
the concentration at neutrality is known, an expression of the H + 
ion concentration in a solution indicates its degree of either acidity 
or alkalinity. 

Because H + ion concentration involves numbers too cumber- 
some for ordinary use, negative logarithms of the numbers are 
substituted and preceded by the expression pH. Neutrality is ex- 
pressed as pH 7.0, indicating a solution that is 0.0000001 (or 10~ 7 ) 
normal in H ions. A pH value below 7 indicates a greater concen- 
tration of H ions, or acidity, and a value larger than 7 indicates 
alkalinity. Since the pH values are logarithmic, the relationships 
between them are geometric and acidities at pH 5.0, 4.0 and 3.0 
are respectively 10, 100, and 1000 times as great as at pH 6.0. The 
pH of most soils will normally fall between 3.0 and 9.0, and, in hu- 
mid regions, the range to be expected is considerably less, perhaps 
no greater than pH 4.0 to 7.5. 



PHYSIOGRAPHIC FACTORS 179 

Under ordinary conditions, the hydrogen ions themselves prob- 
ably have little direct effect upon plants, but degree of acidity of 
the soil may have a regulatory effect upon chemical processes that 
do influence growth. Increased acidity may reduce availability of 
nutrients, as when phosphorous combines with aluminum and iron 
to form insoluble phosphates. High acidity may, apparently, pro- 
duce toxic effects, but these are not caused by H ions. It is more 
likely that they result from soluble compounds of aluminum and 
iron, which form in increasing amounts as the H ion concentration 
rises. Since lime is a required nutrient, its characteristically low 
content in acid soils may be of more importance than the degree 
of acidity. Numerous soil organisms are sensitive to changes of 
acidity, and, if their activities are inhibited, decomposition of or- 
ganic matter may be retarded, nutrients may not be released, and 
nitrification and nitrogen-fixation may be checked. 

With such a variety of things that may be affected by soil 
acidity, it should be suspected that a simple relationship between 
pH and plant responses does not exist. Studies of soil pH and plant 
distribution bear this out, for, in general, if the environment is 
favorable and necessary nutrients are available, most species can 
tolerate a rather wide range of pH. At the same time, many of 
these species reach their best development or are most abundant 
within a restricted portion of that range of pH. It should be clear 
that, even under such conditions, pH alone cannot be the limiting 
factor. 

Determinations of pH may be made colorimetrically by the use 
of indicator solutions or electrometrically with a potentiometer 
and a glass electrode. 206 A very useful approximation may be made 
with a universal indicator, which, when placed in the soil solution, 
takes on a color corresponding to a particular pH value. This is 
handy in the field since it requires no more than a small bottle of 
indicator and a pocket-size porcelain plate on which permanent 
color standards are painted. More accurate colorimetric deter- 
minations require a series of indicators whose colors correspond 
to overlapping pH ranges. When electrometric equipment is avail- 
able, it is preferable because of its accuracy. 

Exchangeable Bases.— Ecologists have given relatively little at- 
tention to the ways in which the mineral nutrients of the soil 



180 THE STUDY OF PLANT COMMUNITIES * Chapter VII 



affect plant distribution and growth of wild species. An important 
part of the mineral nutrition of native and cultivated vegetation is 
derived from the exchangeable bases or cations adsorbed on the 
surfaces of the soil colloids. When these vary considerably in 
amount or kind, there may be marked differences in the type of 
vegetation or at least in rate of growth. For example, it has been 
shown that, in soils derived from hydrothermally altered rocks in 
the Great Basin, sagebrush and its associated species fail to grow 
because of the very low percentage of exchangeable bases as com- 
pared with the normal brown soils of the sagebrush zone. 22 

The colloidal portion of the soil is composed primarily of alum- 
ino-silicates. These colloidal particles are almost always negatively 
charged, and upon their surfaces are adsorbed great numbers of 
cations. These cations are principally H + , Ca ++ , Mg ++ , K + , and Na + , 
named in the decreasing order of tenacity with which the cations 
are held. The hydrogen ion is held more tightly than calcium and 
replaces calcium more readily than calcium will replace hydrogen. 
This same relationship holds between calcium and magnesium, and 
so on down the series. The displaced cation usually enters the soil 
solution. This phenomenon, in which one cation may replace an- 
other on the colloidal particle, is called base exchange. 

Plants are almost entirely dependent on this process of base ex- 
change for their supply of calcium, magnesium, and potassium. Of 
the anions, only POi is held to any extent by colloidal adsorp- 
tion, the other anions, such as NO.r, being readily soluble in the 
soil solution and therefore, readily leached. One source of the H 
ions that can displace the bases and make them available is the car- 
bonic acid formed when carbon dioxide from root respiration is 
released into the soil solution. This was shown experimentally for 
the calcium ion. 132 Another common source is the organic acids 
derived from humus. 

Soils differ widely in their ability to supply cations because of 
the effects of climate, parent material, and vegetation. The maxi- 
mum amount of exchangeable cations a soil can hold is called 
the base exchange capacity of the soil. Obviously, a soil high in 
colloids will have a high capacity as compared with one low in 
colloids, as, for example, a sand. Even the kind of clay may make 
a great difference in the base exchange capacity of a soil. For ex- 



PHYSIOGRAPHIC FACTORS 



181 



ample, kaolinite has a very low capacity compared to clays of the 
montmorillonite group, which have relatively high capacities. 

Since soils are constantly losing some of their adsorbed bases 
due to replacement by H ions, the soil is rarely, if ever, saturated 
with bases to its capacity. The degree of saturation at any given 
time is known as the percentage of base saturation of the soil. The 
base exchange capacity of a soil minus the percentage of base sat- 
uration is theoretically equivalent to the percentage hydrogen 
saturation of the soil, since hydrogen is the replacing ion. 

Both climate and vegetation have great effects upon the amounts 
of exchangeable bases present in soils. On soils derived from the 
same parent material, sugar maple-beech-yellow birch forest 
maintains a soil at a higher percentage of base saturation than that 
under a red spruce forest. 53 This seems to be due largely to the 
ability of the hardwoods to absorb calcium from the subsoil and to 
add it to the surface soil by leaf fall. 



Ca Ca K Ca K 



Ca Mg Ca K 






Ca H Ca Mg 


H H 






Colloid 


H Mg 





H 
H 
H 



K Mg Ca Na Ca 



Ca Na 



Ca H Ca 



Arid region 

Desert soils 

Arid brown soils 

Chestnut soils 



Transition zone 
Chernozems 



Humid region 

Gray-brown- 

podsolic soils 

Podsols 



Many investigators have shown the relation between precipita- 
tion, percentage base saturation, and pH. In brief, it may be stated 
that, in regions of high precipitation, the bases are readily replaced 
by hydrogen ions and then leached from the soil. The excess of 
hydrogen ions results in lowering the pH and creating an acid soil. 
Such conditions prevail in the cool, moist, coniferous forests of 
the north. Just the opposite conditions prevail in the soils of arid 
regions where low precipitation and scanty vegetation combine to 
allow the bases to remain on the colloids, thus maintaining a hio-h 



182 THE STUDY OF PLANT COMMUNITIES * Chapter VII 

percentage saturation and pH. These relationships are represented 
schematically on page 181. 131 

Inhibition of Growth by Plant Products.— That certain plants 
produce soil conditions inhibiting the growth of other plants is 
probably true. 273 Over a hundred years ago it was argued that crop 
rotation was necessary for this reason and that fallowing of land 
favored the next crop because it permitted the leaching of harmful 
excretions or by-products of decomposition resulting from the 
previous crops. Today we cannot entirely ignore this line of rea- 
soning, for explanations of the benefits of rotation and fallowing 
based upon nutrient deficiencies are not always adequate. Like- 
wise, there is some evidence that toxic substances are released in 
the soil as excretions, 215 or when external root cells are sloughed 
and decompose, 209 or when the plants disintegrate after death. 

A number of grasses inhibit growth of other plants. In lawns, 
certain strains of bluegrass almost completely check the growth of 
white clover. 1 Walnut inhibits the growth of a number of herbs. 
Fairy rings of both fungi and higher plants may be the result of 
toxic products produced by the plants, for other explanations do 
not always suffice. If water, supplied in excess to flats of experi- 
mental plants, is permitted to percolate through the soil and is then 
used as the water supply for other plants, the latter are frequently 
inhibited in growth even under the most favorable conditions. 17 
Extracts from decomposing plant remains have produced similar 
results. Apparently toxic or growth-inhibiting substances are pro- 
duced by a number of plants, which may affect germination of 
seeds and growth of seedlings, or even of mature plants of the 
same or other species. Some species are affected, others are not. 
Whether higher plants are affected directly is not always clear. 
Perhaps effects upon soil organisms and their activity in turn af- 
fect the higher plants. 

The subject is controversial, and some evidence is conflicting. 
The limited information that is available is often derived from 
observation of agricultural soils and cultivated plants. Cultivation, 
probably because of better aeration, reduces the effectiveness of 
inhibiting substances, and the problem is practically eliminated by 
crop rotation and the compensating effect of fertilizer. It is, there- 
fore, not surprising that investigators have turned to other things. 



PHYSIOGRAPHIC FACTORS 183 



In natural soil, however, these artificial modifications are absent, 
and, consequently, in view of the possible implications in inter- 
preting associations of species or the causes of succession, it is sur- 
prising that the subject has not been given more attention. 

Alkalinity.— Soils with an alkaline reaction have usually orig- 
inated from limestone, dolomite, or marble in which calcium car- 
bonate is the basic mineral. The CaCOa tends to neutralize acids 
that appear in the soil, and the degree of alkalinity is proportional 
to the solubility of the limestone. Dolomite contains more MgCOs 
than CaC03, and gypsum is largely CaSCh, but the soils they form 
contain CaC03, and their floras are essentially similar to that of 
limestone. In our arid West, soils are often alkaline in reaction be- 
cause of the sodium ions, which accumulate as sodium hydroxide 
(NaOH). 

Neutral or alkaline soils favor the activities of most soil organ- 
isms and the availability of nutrients for higher plants. At the same 
time, the tendency of soil colloids to aggregate and produce crumb 
structure in the presence of lime results in soil structure with 
water, air, and temperature conditions favorable to plant growth. 
Thus most cultivated crops do best on soils with a pH ranging 
close to neutrality. Native plants, in general, respond similarly, but 
there are exceptions, which require, on the one hand, high concen- 
trations of CaC03 or, on the other, extremely acid conditions re- 
gardless of other factors. 

Not all species found growing in calcareous soils are calciphiles. 
The distribution and occurrence of many show no correlation 
with alkalinity of the soil. A considerable number of these widely 
distributed species may, however, grow more luxuriantly when on 
calcareous soil. Some, although not restricted to the habitat, will 
be found there characteristically. These are true calciphiles. There 
are, in addition, obligate calciphiles, which grow only in cal- 
careous habitats. 

The exceptional vigor on calcareous soils of otherwise wide- 
spread species may result simply from the improved aeration, 
moisture, or nutrient conditions produced by lime. Calciphiles 
may grow on other than alkaline soils if competition from non- 
calciphiles is not too great. The less favorable are the general con- 
ditions for growth, the more the calciphiles are restricted to their 



184 THE STUDY OF PLANT COMMUNITIES * Chapter VII 

alkaline habitat, and, as a result, at or near the limits of their ranges 
they often appear as obligate calciphiles. 

Salinity.— Under conditions of poor drainage and high tempera- 
ture, much of the water deposited in low places evaporates and 
leaves behind the salts it has carried from the soil of surrounding 
slopes. If precipitation is seasonal and alternates with extreme 
drought, there is insufficient leaching to prevent accumulation of 
these soluble salts, which then form alkali soils, so called regard- 
less of the salt involved. Alkali soils of various kinds occur in all 
parts of the world and are common in the arid portions of western 
North America. Lowlands bordering the oceans are subject to 
periodic inundation with sea water and, consequently, contain 
relatively high concentrations of salts. 

Plants that can tolerate the concentrations of salts found in 
saline soils are termed halophytes. How they survive where ordi- 
nary plants have little chance has been the subject of much debate. 
If not actually dry, these saline habitats may be termed "physi- 
ologically dry" because of the high concentrations of salts, which 
would limit osmotic activity and, consequently, absorption of 
water by the ordinary plant. The morphological and anatomical 
characteristics usually appearing in plants of arid regions are com- 
mon in plants of saline habitats. Succulence is particularly general. 
Yet these xeromorphic characters have been shown to be relatively 
ineffectual in maintaining low transpiration rates in halophytes. 
They must then be able to absorb water in spite of the high salt 
concentrations, and this is possible because of their own high salt 
contents. 

Not all species are equally tolerant, and, therefore, they will 
often be found in zones adjusted to the concentrations of salts in 
the soil and the plant. Flat areas with uniform salt concentration 
may support a constant group of species over their entire extent. 
The number of species tolerant to salinity is not great and many 
of the same genera are found in all parts of the world where similar 
conditions occur (e.g., several Chenopodiaceae). Because certain 
species in alkali areas are tolerant to definite ranges of salt concen- 
tration and, in addition, to particular salts, they may be rather 
positive indicators of soil conditions. There are other species that 
are not so limited. In some, the concentration of the cell sap ad- 



PHYSIOGRAPHIC FACTORS 



185 



justs itself to changes in the soil and permits growth under a 
variety of conditions. Some can tolerate only small amounts of salt 
and do better in its absence, while a few others absolutely require 




Fig. 86. Margin of a saline flat in the Smoke Creek Desert, Nev. The 
shrub at the margin is the relatively salt-tolerant greasewood (Sarcobatus 
vermiculatus). Extending farther into the playa is salt grass (Distichlis 
stricta), which is more tolerant but soon also fades out until nothing grows 
over most of the area— Photo by W. D. Billings. 

salt to survive, some even requiring a fairly high concentration. 
The extreme in salinity is illustrated by portions of the Great Salt 
Lake area in Utah where salt concentrations are so great that no 
vascular plants can grow. 

TOPOGRAPHY 
Although topography affects vegetation indirectly by modify- 
ing other factors of the environment, it has nevertheless a signifi- 
cant influence upon all plant communities. If an area is so level that 
topographic variations are practically nonexistent, then, other fac- 
tors being equal, uniform vegetation may be anticipated through- 



186 THE STUDY OF PLANT COMMUNITIES • Chapter VII 

out. Normally, however, such areas of any extent are rare, and 
slopes, bluffs, and ridges with different exposures, lowlands, drain- 
age lines, and depressions are present. 

Such irregularities in topography produce light, temperature, 
and moisture conditions that differ greatly between north and 
south slopes or ridges and depressions. The effect of exposure on 
these individual factors having been previously discussed (p. 124, 
132), it is necessary here only to emphasize that vegetation on 
slopes is the resultant of interaction of light, temperature, and 
moisture differences. South-facing slopes receive more light, have 
higher temperatures, and are drier than the average site in the 
area, while north-facing slopes receive less light, are cooler and 
moister than the average. Of course, these differences vary with 
degree and extent of slope, but, in general, the environment of 
north and south slopes differs sufficiently to maintain distinctive 
vegetative types. 

Apart from the interaction of the factors mentioned above, 
slopes affect runoff and the amount of soil water and, likewise, the 
possibility of erosion. 

Since water always moves toward depressions, they are invari- 
ably moister than uplands and usually support distinctive vegeta- 
tion. If topography is immature, as in the northeastern United 
States, drainage is relatively poor and depressions contain ponds 
or lakes supporting aquatic vegetation. Some lakes fill with sedi- 
ment, marl, and organic materials to form bogs, which likewise 
have their characteristic species. With more mature topography, 
depressions are connected by streams, which make drainage far 
more effective. Even so, the streams are usually bordered by flood 
plains supporting vegetation requiring more favorable moisture 
conditions than obtain upon the uplands. 

The greatest differences in vegetation associated with local 
variations in topography can usually be correlated with moisture, 
either in respect to an excess or to a deficiency. If the latter, 
adaptations that facilitate absorption or restrict transpiration are 
likely to characterize the plants. In a region where moisture is 
rarely a critical factor, slope and exposure produce scarcely no- 
ticeable differences in vegetation. This occurs only under condi- 
tions where a combination of fog, clouds, or rain maintains a hu- 



PHYSIOGRAPHIC FACTORS 187 



mid atmosphere, low transpiration rates, and a plentiful supply of 
water. 

In addition to local topographic effects are those of a regional 
nature associated with mountains. The increase in precipitation 
and decrease in temperature with increasing altitude result in 
vegetational zonation. Within these zones, the local effects of to- 
pography again become apparent so that zones lie at higher alti- 
tudes on a south than on a north slope and the species of a particu- 
lar zone will be found extending downward in ravines and upward 
on ridges. 

A mountain may affect conditions for growth at some distance 
from itself. Some mountains are centers over which rain clouds 
form and from which they often move to provide moisture for 
surrounding lowlands. At the same time, streams starting in moun- 
tains and fed by precipitation there, flow down to valleys below. 
Other mountains act as barriers when they lie at right angles to the 
prevailing winds, for all the moisture may fall upon the mountain 
and none be left for the area beyond. This explains the lack of 
moisture in the Great Basin. The prevailing winds coming from the 
Pacific lose their moisture over the Coast Ranges and the Sierra 
Nevada. 

Finally, it is probable that mountains act as barriers to the nat- 
ural migration of some species that are unable to compete with the 
flora upon the mountain or to withstand the successive changes of 
environment associated with increasing altitude. 

GENERAL REFERENCES 

L. D. Baver. Soil Physics. 

K.D. GLINKA. The Great Soil Groups of the World and Their Development. 

H. JENNY. Factors of Soil Formation. 

C. E. KELLOGG. Development and Significance of the Great Soil Groups of 

the United States. 
R J. KRAMER. Soil Moisture in Relation to Plant Growth. 
C. F. MARBUT. Soils of the United States, in Atlas of American Agriculture. 
U. S. Dept. Agr. Soils and Men. 



CHAPTER VIII 



BIOLOGICAL FACTORS 

Associated organisms having mutual relationships to each other 
and to their environment are recognized as a community. Many, 
if not all, of the organisms in a community are thus not only a part 
of the community but also a part of the environment of every 
other organism there. The dominants obviously compete with 
each other and with subordinate individuals. At the same time, 
they provide conditions that permit the survival of lesser organ- 
isms, which, though quite inconspicuous, may yet markedly affect 
the permanence of the community as a whole. Both plants and 
animals are factors of the environment of any community, and 
man is not the least of these factors. 

PLANTS AS FACTORS 

Competition.— It has been shown that, within a community, 
competition occurs between individuals of the same species, or 
between different species, whenever some requirement of the 
organisms is available in amounts insufficient to supply all demands 
adequately. Each organism involved in competition is a factor in 
the environment of all other organisms so involved. The effects of 
competing organisms upon each other are more apt to result from 
their influence upon physical or physiological conditions of the 
environment (such as available water or nutrients, light, tempera- 
ture, humidity, and air movement) than they are from direct ac- 
tion. An extreme example of direct competition as a factor is that 
of the strangling fig, a liana of tropical forests, which climbs to the 
tops of the dominant trees that support it. Eventually the tree is 
killed as the pressure of the vine about its trunk increases. When 
the tree falls, the vine may pull down numerous other trees over 
whose tops it has sprawled. The community, however, is only 
locally disturbed and soon readjusts itself, for the forest is climax 
and these giant lianas are a part of it. 

The introduction of new species into a community, by man or 

188 



BIOLOGICAL FACTORS 



189 



other agents, usually results in failure because the plant cannot 
meet the competition of the normal species, which are adapted to 
each other and their environment. However, an occasional species 
reverses the rule, establishes itself as a part of the community, and 
often produces community changes. Japanese honeysuckle was 
introduced in the southeastern states manv years ago and has 




FlG. 87. Japanese honeysuckle in bottomland hardwood forest. When the 
vine is as dense as this, few tree seedlings come up through it. If they do, 
they are soon pulled over and the honeysuckle forms mounds upon them, as 
at the left.— Photo by L. E. Anderson. 

spread widely. In lowland woods particularly, it sprawls over all 
the low vegetation and climbs well up into the trees. Under favor- 
able conditions, it almost excludes low herbs and shrubs. When a 
tree seedling grows through it, the vine climbs upon it and bends 
down the slender stem, which, under the mass of honeysuckle, 
soon dies. Such lowland stands frequently have practically no tree 
reproduction beneath them. It is a matter of ecological interest as 
to how the natural development of these stands will progress. An 
economic aspect must be considered by the forester who is inter- 
ested in regeneration of trees or planting these areas after cutting, 



190 THE STUDY OF PLANT COMMUNITIES ■ Chapter VIII 




FlG. 88. Dead chestnut, killed by blight, in a forest stand of which they 
once were important members. Cherokee National Forest, Tenn.— U. S. For- 
est Service. 



for, unless the land is cultivated, the honeysuckle cannot be elim- 
inated without considerable trouble. 

Parasites.— A parasite is completely dependent upon its host for 
its existence and thereby becomes a factor in the environment of a 
community. When conditions are favorable for the host, a certain 
amount of parasitism can be tolerated with little apparent effect. 
Parisitic fungi and bacteria are almost constantly present but cause 
no serious disturbance of a community unless conditions become 
unusually favorable for their increase. Then they may cause death 
of enough hosts to produce a change in dominance or to destroy 
the community. Such occurrences are usually local and may be 
followed by gradual recovery of the original community. How- 
ever, when a parasite is introduced from afar, it may be so effec- 
tive in its new environment that disaster results. 166 Chestnut blight 
has practically eliminated chestnut in the eastern United States, 
and oak is now dominant where oak-chestnut occurred before. 
Dutch elm disease 62 is gradually spreading from New England, 
where it first appeared, although its spread has been somewhat re- 
tarded by the drastic procedures used to check it. 

Parasitic seed plants are not usually of much ecological signifi- 



BIOLOGICAL FACTORS 



191 



cance, but they are always of interest because of their peculiarities 
and relatively local distribution. A considerable range of degree of 
parasitism is possible. 78 The common dodder (Cuscnta) is repre- 
sentative of those parasites (holopar ashes) completely dependent 
upon their hosts, but the mistletoes and others are termed partial 
parasites because they are green and can manufacture food. Some 
species are attached to their hosts at a single point of contact, often 
by roots. A number of Scrophulariaceae are of this type. Others 
twine or sprawl over the host plant and are connected to it at in- 
tervals by absorbing structures called haustoria, whose conducting 
systems may be in intimate contact with xylem and phloem of the 
host. Still others may be contained within the host and show only 
their reproductive structures externally. Effects upon the host are 
obviously physiological, and reduction of growth and vitality are 
usually apparent. Abnormal growth is also common in the pres- 
ence of a parasite. It is often manifested as bushy masses, called 
"witches brooms" or is occasionally found in twisted, flattened, 
or distorted branches. Parasitic seed plants have little effect upon 




FIG. 89. A stand of scrubby oak infested with mistletoe (?horadendron 
ftavescens).—U S. Forest Service. 



192 THE STUDY OF PLANT COMMUNITIES * Chapter VIII 




FIG. 90. A striking witches'-broom on a young red pine in Michigan. 
U. S. Forest Service. 



BIOLOGICAL FACTORS 193 

community structure in comparison with the drastic changes that 
may result from infestation with pathogenic fungi or bacteria. 

Epiphytes.— These include a wide variety of plants, all of which 
depend upon larger plants for physical support only. Algae, fungi, 
mosses, liverworts and lichens may be found growing on bark or, 
in some instances, even on leaves. Often their occurrence seems 
correlated only with the general humidity of the atmosphere in 
particular habitats, but they are frequently associated with certain 
communities and not with others, and, within a community, they 
may be distributed systematically. Some may grow only on the 
bark of certain trees and, even more specifically, only in patterns 
related to drainage of water down that bark. 200 Others may be 
found only at the base, middle, or top of a tree trunk, and this may 
be correlated with moisture content of the bark. 23 The occurrence 
of the moss Tortula pagorimi 8 illustrates how specific a habitat 
may be required by some epiphytes. This moss has been found 
only in close proximity to man's habitations and then almost exclu- 
sively on the trunks of elm trees. The epiphytic lichens associated 
with evergreen forests of boreal and alpine regions are distinctive 
and characteristic. 

In and near the tropics, higher and less variable humidity per- 
mits a greater variety of epiphytes to survive, and vascular species 
increase. In temperate regions, drought-resistant species, such as 
polypody ferns, are found occasionally, but farther south, first on 
swamp trees only and then almost anywhere, epiphytic vascular 
plants become the rule. Orchids, bromeliads, and ferns are espe- 
cially abundant. Structures that catch or conserve water are char- 
acteristic of many of these species. Stratification at different levels 
in the forest, as controlled by light, air movement, and water sup- 
ply, is common, and succession of epiphytic communities may be 
observed as organic "soil" is accumulated. 181 Occasionally their 
weight may increase sufficiently to break down the branches sup- 
porting them. Such massive growths as are produced by the well- 
known Spanish "moss" (Tillandsia) of the southeastern United 
States must reduce the normal foliage and its functioning (see Fig. 
8). In general, however, the epiphytes and their "hosts" seem sur- 
prisingly well adapted to their relationship. 

Symbiosis.— The most generally accepted concept of symbiosis 



194 THE STUDY OF PLANT COMMUNITIES ■ Chapter VIII 



includes only the relationship of intimately associated, dissimilar 
organisms that live together to their mutual advantage. By append- 
ing descriptive adjectives, the concept has been expanded by some 
to include almost any relationship between organisms whether ac- 
tually in contact or merely in competition with each other (e.g., 
cattle grazing in a meadow would illustrate antagonistic nutritive 
disjunctive symbioses 167 ). But the conservative interpretation rec- 
ognizes only a few plant symbionts as significant in community 
life. The intimate association of unicellular blue-green algae with 
a fungus mycelium, termed a lichen, is an example of plant sym- 
bionts that is familiar to all who have any botanical interest. 
Lichens, however, can hardly be considered of general importance 
in community relationships. Although they often play a part in 
the establishment of communities on bare rock, they probably in- 
fluence mature, stable communities very little. Fungi and bacteria 
living symbiotically on plant roots are less noticeable but of far 
more importance. 

Mycorhiza — When a root and the mycelium of a fungus grow 
together, the fungus may form a feltlike layer around the root 




FlG. 91. Transverse sections of mvcorhizal roots of forest trees: (1) en- 
dotrophic, (3) ecto-endotrophic, others all ectotrophic. (1 and 4) Psendo- 
tsuga imicronata, (2 and 3) Pinus vmrrayana, (5) Popuhis tremuloides, (6) 
Picea rubens.— After McDougall and Jacobs. 



1G8 



BIOLOGICAL FACTORS 195 

and penetrate the spaces between cells (ectotrophic mycorhiza), 
or the fungus may occur within the cortical cells of the root only 
(endotrophic mycorhiza). Such root-fungus relationships are far 
more common than was once supposed. It is known that they 
occur on most forest trees and shrubs and that many herbaceous 
plants may have them. They form during periods favorable to 
root growth and are practically restricted to the young roots in 
the surface strata of the soil. 

Whether mycorhizas represent a mutualistic relationship or 
merely parasitism on the part of the fungus has been strongly 
argued by numerous investigators. The conflicting evidence makes 
interesting, if somewhat confusing, reading. However, the evi- 
dence that mycorhiza must be present for the successful growth 
of many species, particularly forest species, is sufficient to suggest 
that the mycorhizal condition is desirable under most situations 
even though the reasons are not too obvious. 

Pot cultures of certain tree seedlings in poor soil have been un- 
satisfactory until inoculated with mycorhizal fungi. On a larger 
scale, unsuccessful forest nurseries on prairie soil or long deforested 
agricultural soil have been saved by bringing in small amounts of 
forest soil, which started the formation of mycorhiza. Tree seed- 
lings transplanted without mycorhiza to treeless areas have been 
saved from gradual death by the application of small amounts of 
soil containing mycorhizal fungi. 

Several members of the heath family (azalea, rhododendron, 
blueberry) are dependent upon the presence of mycorhiza that 
cannot tolerate alkaline conditions. Disappearance of mycorhiza 
leads to death of the plants, and consequently, the soil must be 
acid for successful propagation of these species. 

Many orchid seeds germinate normally only in the presence of 
mycorhizal fungi and were difficult to propagate until it was 
found that proper nutrient media could compensate for the ab- 
sence of the fungus. Such evidence indicates that, regardless of 
what the fungus may take from the root, the vascular plant is 
benefited by the presence of the mycorhiza or may actually be 
dependent upon it. Probably the benefit is derived through some 
nutritional improvement provided by activities of the fungus. 

Nodules— Certain saprophytic bacteria, living free in many soils, 



196 THE STUDY OF PLANT COMMUNITIES ■ Chapter VIII 





FlG. 92. Two seedlings of Psychotria punctata, about three and one-half 
months old. The plant on the right is normal both as to growth and the pres- 
ence of bacterial nodules dotting every leaf. The one on the left, grown bac- 
teria-free, has reached its maximum development.— From Hwmn. 



127 



enter the root hairs of most legumes when available and produce 
a proliferation of cortical cells sufficient to appear as a small 
nodule on the root. Although the plant provides food for the bac- 
teria and produces the nodule in which the bacteria multiply, the 
relationship is truly symbiotic. These nitrogen-fixing bacteria are 
able to take free nitrogen from the air, unavailable to most 
plants, and to combine it with other elements to form compounds 
that can be used by the plant during its lifetime. After death of the 
plant, the accumulated nitrogenous compounds are released in the 
soil and are used by other plants growing there. Legumes and 
nitrogen-fixing bacteria are, therefore, important factors in main- 
taining soil fertility in natural or cultivated soils. Plant commu- 
nities becoming established on poor sites, such as eroded slopes, 
invariably include a number of legumes, which are, of course, 
particularly adapted to colonizing sterile or nitrate-depleted soils 
and contributing to their improvement. Agricultural practice in- 
cludes legumes in most crop rotations, and worn-out lands are 
rebuilt by cropping with legumes of some sort. 

Nodules produced by bacteria are found on the roots of a few 
plants in families other than Leguminosae, but they are not of the 
same type. Nodules containing bacteria are also formed on leaves 
of a number of tropical plants, mostly in the family Rubiaceae. 



BIOLOGICAL FACTORS 197 



These bacteria are associated with the plant tissues in all stages of 
development from seed to maturity, but nodules form only on 
leaves. Although these bacteria have been credited with nitrogen- 
fixing ability, it is certain that the plants are not dependent upon 
them for their nitrates. Certain products of their presence are 
necessary, however, for without the bacteria, seedlings do not ma- 
ture. 127 The relationship is, therefore, truly symbiotic (Fig. 92). 

Other Soil Flora.— In addition to the symbiotic fungi and bac- 
teria, great numbers of bacteria, fungi, and algae occur free in the 
soil. Their importance to natural plant communities cannot be 
evaluated accurately, but their significance is indicated by their 
general functions of making nitrogen available by fixing it, or 
releasing it with other nutrients through their activities in decom- 
posing organic matter. 

The fixation of nitrogen as nitrates by free soil organisms is 
known to be accomplished by a number of bacteria under both 
aerobic and anaerobic conditions and even in practically sterile 
soils. Some are inhibited by acidity or chemical constituents of the 
soil, and temperature ranges may affect their activity, but, in gen- 
eral, some are present almost everywhere. Certain algae are also 
thought to be capable of nitrogen fixation. 

All nitrates appearing in the soil from sources other than fixa- 
tion are the products of organic decomposition, particularly of 
proteins. The breakdown involves a series of chemical changes 
accomplished by a succession of bacteria and fungi. The first of 
these causes the proteins to break down into the less complex pro- 
teoses, peptones, and amino acids. This digestive process allows 
the bacteria and fungi to use a part of the nitrogen for themselves, 
and, in so doing, they release ammonia as a waste. Few plants can 
use ammonia directly, and many are injured by its accumulation 
in the soil. Ammonification is followed by nitrification, in which a 
group of nitrite bacteria convert the ammonia to nitrites by partial 
oxidation. Subsequently, the activities of nitrate bacteria cause 
further oxidation and the formation of nitrates. Now, finally, the 
nitrogen is usable by higher plants. Digestion of proteins, am- 
monification, and nitrification must all take place before organic 
nitrogen can be used by plants, and the succession of bacteria must 
be present if the processes are to occur. 



198 THE STUDY OF PLANT COMMUNITIES ■ Chapter VIII 

The activities resulting in available nitrates produce partial 
breakdown of organic materials, which are further decomposed 
by other bacteria and fungi acting upon the remaining nonprotein 
plant materials. The partially decomposed plant remains, or hu- 
mus, may be broken down completely in a single season if rela- 
tively high temperatures and sufficient moisture occur most of the 
year and permit more or less continuous functioning of the organ- 
isms. If the organisms can operate for only a few summer months, 
the deposition of litter usually exceeds the rate of decomposition, 
and humus tends to accumulate. 

ANIMALS AS FACTORS 

Pollination.— Insects are by far the most important animals in- 
volved in pollination, and bees, wasps, moths, and butterflies are 
particularly concerned. A few birds, especially hummingbirds, 
contribute to pollen transport, and even some small crawling ani- 
mals may be effective at times. Most animal-pollinated flowers 
have certain characteristics in common, such as conspicuousness 
in size and color and the production of an odor as well as nectar. 
It has been shown that all of these characters serve more or less to 
attract insects. In general, the flowers are more elaborate than 
those of wind-pollinated plants, and they have characters usually 
interpreted as of more modern origin. 

Devices that insure insect pollination are common and often of 
intricate design. Adaptations may occur in both insect and flower 
limiting pollination of a particular species to a single type of insect. 
Some adaptations are so extreme as to produce complete depen- 
dence of plant and insect upon each other. 

Dissemination.— Plant parts, called disseminules, give rise to new 
individuals in new places. Their food content is an attraction to 
various animals, which, consequently, often act as agents of dis- 
semination. Many seeds that are eaten are indigestible and retain 
their viabilitv after they are dropped at considerable distances 
from their sources. Others, not immediately eaten, are carried by 
birds, rodents and even ants to places of storage or concealment, 
where they may germinate. Of course, great numbers of seeds are 
eaten or destroyed by animals, but dissemination from seed sources 
is a partially compensating factor. 



BIOLOGICAL FACTORS 



199 



Vegetative structures may be effective in the same way. Aqua- 
tic animals, such as muskrat, tear up rhizomes and bulbs, some of 
which float free and establish new communities elsewhere. In this 
connection, it is worth mentioning the importance of water as an 
agent of dissemination, especially of floating propagules, even 
though they do not retain their viability for long when saturated. 

Finally, the hooks, spines, and other devices characteristic of 
many seeds and fruits insure their attachment to almost any ani- 
mal contacting them and thus make possible their transport for 




FlG. 93. Structural modifications of seeds and fruits that facilitate dissem- 
ination by wind or animals. (1) The parachute fruit of common dandelion 
(Taraxacum) ; (2) winged fruit of dock (Rumex pulcher); (3) the silky- 
haired seed of milkweed (Asclepias mexicana); spiny, hooked, and awned 
fruits of (4) sandbur (Cenchrus paucifloriis) , (5) cocklebur (Xanthium 
canadense), (6) red-stem filaree (Erodium cicutarium), (7) beggar's-tick 
(Widens frondosa).— By permission, from Weed Control by Robbins, Crafts, 
and Raynor, copyrighted 1942, McGraiv-Hill Book Company. 



200 THE STUDY OF PLANT COMMUNITIES ' Chapter VIII 

some distance. Animals with long, soft hair are the most effective 
agents. The clothing of man is likewise well adapted to such 
transport, as anyone knows who spends time in the field during 
late summer and fall. Some of these devices are simple hooks, ef- 
fective because of sharpness or strength; others are elaborate 
structures with several features insuring their transport. The fruits 
of awn and needle grasses are illustrative, since they have sharp- 
pointed, retrorsely-barbed fruits, which easily penetrate cloth, 
fur, or wool, and an awn which twists with changes of moisture 
and thus pushes the fruit forward to a secure anchorage. These 
may cause severe damage to grazing animals by penetrating skin, 
lips, or even internal organs. 

Soil Animals.— The microfauna of the soil, concentrated in the 
upper strata, consists of great numbers of protozoa, nematodes, 
and rotifers. In addition, there are various macroscopic worms and 
insects. 263 In general, the numbers of animals vary in response to 
the same factors affecting the microflora, and the greatest numbers 
are always found in soil with high organic content. All contribute 
to organic decomposition and use a part of the products for food. 
Several protozoa probably consume bacteria, and some nematodes 
are parasitic on the roots of plants, causing much trouble in culti- 
vated soils where they are present. 

Of the macroscopic fauna, earthworms are most active. Their 
constant burrowing facilitates aeration and drainage and their use 
of fresh or partially decomposed organic matter as food contrib- 
utes to decomposition. Since mineral matter is also ingested in 
feeding, the earthworm moves quantities of soil about, and this 
tends to mix mineral and organic materials. In cultivated soils this 
has no great significance, but for natural soils the advantages are 
obvious. Earthworms are found in the best soils and best sites but 
rarely in poor soils. It would appear, then, that they serve to make 
good soils better but that poor soils derive little from them. 

A very high proportion of all insects spend part of their lives in 
the soil. Their larvae tunnel through the soil and, thereby, con- 
tribute to organic decomposition and distribution. 

Larger Animals.— The principal effect of larger animals upon 
plants results from grazing or other feeding habits. Carnivorous 
animals affect communities onlv indirectly by keeping down the 



BIOLOGICAL FACTORS 



201 



population of herbivores and thus maintaining a balance in food 
relationships. In spite of this, the feeding by herbivores may some- 
times be excessive enough to cause serious disturbance or even 
destruction of community structure. 

Under natural conditions, grazing was undoubtedly greatest 
when buffalo ranged throughout our grasslands. Locally, as around 
water holes, their feeding and trampling sometimes destroyed 




FlG. 94. Distinct browse line on stand of ironwood resulting from deer 
feeding on low branches. Note the uninterrupted view under stand, and ab- 
sence of shrubs and tree seedlings. Such damage commonly results when deer 
population is high, and especially when winter supply of food is inadequate. 
— U. S. Forest Service. 

most of the vegetation but otherwise probably did little damage 
since they were constantly on the move and distributed themselves 
where grazing was best. Moderate grazing by cattle does not 
change the essential nature of a grassland community. A succession 
of dry years in the time of the buffalo could have resulted in local 
conditions similar to those in overgrazed pasture areas today. 

Deer and moose similarly have little effect on grassland or for- 
est, where they browse, unless there is an overpopulation. Then, 
especially as a result of winter browsing, the complete destruction 
of tree reproduction might be possible. 

Prairie dogs may consume all the forage for some distance about 
their villages. The total consumption of food bv such relatively 
small animals is sufficient to reduce considerably the value of a 
range for larger herbivores. The same may be said for jack rabbits, 
but their feeding is less localized. 



202 THE STUDY OF PLANT COMMUNITIES • Chapter VIII 

The feeding of cottontail rabbits ordinarily affects natural vege- 
tation but little. However, if a peak in their fluctuating population 
comes at the time of a bad winter with much snow, they can do 
serious damage to seedlings and even to larger trees from which 
they eat the bark. Because of selective feeding, snowshoe rabbits 
may change the course of forest succession. 68 




FlG. 95. Injuries to seedlings and saplings resulting from feeding by ro- 
dents and larger animals may strongly influence the development of stands 
and the nature of future vegetation. (1) Young ponderosa pine girdled by 
porcupine. (2) Scotch pine browsed by deer the year after planting. All 
needles and buds eaten. (3) A pine seedling eaten back by rabbits in three 
successive winters. Such seedlings can never make normal trees.— U. S. Forest 
Service. 



Rodents that eat bark by preference may cause considerable 
damage, especially if their feeding is selective as to species. Porcu- 
pines are in this category, and beavers are even more destructive 
because their activities are concentrated around their dams. Here 
they cut down and strip the bark from the trees they most prefer 
nearest their ponds and then gradually extend their operations to 
surrounding slopes. Their dams, too, affect conditions locally, for 
they maintain ponds that sometimes flood large areas, modify 
drainage, and even affect the water table. This may sometimes be 
desirable, sometimes not. 

Man.— The effects of man upon vegetation are fundamentally 
similar to those of lower animals. The greater the concentration of 



BIOLOGICAL FACTORS 



203 



population, the greater the modification of natural communities 
by use and destruction. Whereas man was once essentially a de- 
pendent in community structure, he is now more and more be- 
coming the dominant organism everywhere. By cultivation, he has 
eliminated natural vegetation from vast areas. Logging, even with- 




FlG. 96. Center of a burned swamp in iMaryland that once supported ma- 
ture cypress-gum forest. Intense fire destroyed the forest and burned deep 
into the peaty soil, which had accumulated through the years. Rebuilding 
soil in the depressions, now filled with water, will require many years and 
numerous generations of plants.— Photo by G. E Beaven. 

out subsequent cultivation, has changed the forests, and stands 
equaling the original virgin forests will probably never occupy 
most logged areas again. Cities, highways, airfields, and similar 
products of man's living mean serious disturbance of natural vege- 
tation. Drainage and irrigation projects, canals, road fills, and dams 
result in soil moisture changes that promote the development of 
quite different communities. Many similar disturbances can be 
noted as a result of animal activities but always on a more local- 
ized scale and consequently with less permanent effects. 

Fire is not peculiar to man's activities and, undoubtedly, oc- 
curred here and there in North America before the white man 
came. However, the conditions provided by lumbering operations, 
and the constant use of fire, often with too little concern for its 



204 THE STUDY OF PLANT COMMUNITIES • Chapter VIII 

effects, have made it an important factor associated with man's 
presence. Local small fires occur almost everywhere occasionally, 
and the destruction of vegetation followed by gradual replace- 
ment is characteristic. Under the right conditions, fire may be so 
common as to become a major factor controlling the vegetation of 
a region. This is true of much of the coastal plain of the south- 




FlG. 97. What fire can do to a mountain forest. Such fires are usually fol- 
lowed by erosion, and it requires years for the re-establishment of forest 
vegetation. Coconino National Forest. Ariz.— U. S. Forest Service. 

eastern United States. 105 Prolonged dry periods and little attempt 
to control fire in these flatlands result in most areas burning almost 
every year. Only fire-resistant species predominate and only a 
limited degree of vegetational development is possible before fire 
occurs again and sets back that development. As a result, grassy 
savannahs with longleaf pine are characteristic instead of the po- 
tentially possible hardwood forests. In parts of California, fires 
have resulted in an increase of the fire-resistant chaparral and a 
proportionate decrease of forest. Similar illustrations may be 
found in many parts of the world. 
The immediate economic loss from an intense forest fire is 



BIOLOGICAL FACTORS 



205 



paralleled by other less obvious losses. Such fires in the temperate 
zones may destroy practically all the humus accumulated through 
the years and necessitate the slow rebuilding of the soil before 
forest can occupy the area again. Leaching and erosion, which 
follow such fires, may delay revegetation for years. Thus the pro- 
ductivity of the soil may be indefinitely impaired. 




FlG. 98. A subalpine flat denuded by intense fire that killed all trees and 
burned off organic material down to mineral soil. The fire occurred many 
years before picture was taken and it is obvious that it will be many more 
years before the soil is sufficiently rebuilt to support forest.— U. S. Forest 
Service. 



It is of interest that light, controlled burning has been found 
beneficial for certain purposes. On some grazing land, certain 
undesirable species may be kept down or eliminated to the ad- 
vantage of more palatable plants. More vigorous growth of certain 
forage types is sometimes obtained after light burning in the 
proper season, probably because of the nutrients released and made 
available. It would appear that under some circumstances fire 
could be used as a beneficial tool. 117 

Man, like lower animals, transports seeds and fruits, but to far 
greater distances and with resulting changes in vegetation of a 



206 THE STUDY OF PLANT COMMUNITIES • Chapter VIII 





: 






t 



^ 5 * 



« 



FlG. 99. An introduced weed, tumble mustard (Sisymbrium altissmtum), 
dominant over the entire extent of a sagebrush burn, one year after the fire. 
Washoe County, Nev.— Photo by W. D. Billings. 



more drastic nature. It is hard to believe that 60 percent or more 
of our weeds are not native but introduced species that have come 
from all parts of the world. 176 Some were brought in as orna- 
mentals and almost immediately escaped and spread from gardens. 
Others came in accidentally with seeds of desirable plants. Many 
introductions have been useful and extremely valuable. Most of 
our cultivated plants have been improved by crossing with strains 
of foreign varieties at some time, or they were themselves original- 
ly introduced. In recent years, such introductions are not made 
haphazardly. 

Unfortunate experiences with unconsidered or accidental intro- 
ductions can be listed for all parts of the world. The water hy- 
acinth, introduced from South America, has spread throughout 
the lowland waterways of our southern states where it chokes 
canals, impedes drainage and navigation, and destroys wildlife. A 
similar problem has resulted with the introduction of Elodea in 
the low countries of Europe. Animals may cause similar difficul- 
ties, as the spread of the introduced English sparrow and the star- 
ling in the United States. The muskrat has become a pest in central 



BIOLOGICAL FACTORS 



207 



Europe, and rabbits, introduced into Australia, increased to 
enormous numbers in only a few years. 

Natural communities are made up of groups of species adapted 
to living together. The numbers and sizes of individuals are de- 
termined by the entire complex of environmental factors. If a 
species is eliminated, others of the community may increase and 
take its place, or there may then be opportunity for an incidental 
species to become a part of the community. Usually, if a species is 
introduced, it does not reproduce and gradually dies out. Occa- 
sionally, an introduced species has the necessary characteristics to 
compete successfully and to reproduce regularly. Then adjust- 
ments must be made within the community and a new balance 
among its members must be established. Such a species might even 
become a dominant, and then the adjustments would result in a 
new community. The prickly pear (Opuntia inermis), introduced 
in Australia, became a dominant and made useless more than thirty 
million acres in Queensland alone. 




FIG. 100. Massed water hyacinth covering the water in Louisiana swamp- 
land. The dusting by airplane is part of an experimental eradication program. 
-Courtesy of Department of Wildlife and Fisheries, Louisiana. 



208 THE STUDY OF PLANT COMMUNITIES ' Chapter VIII 

When man has tampered with the balance among the species of 
a community by eliminations or introductions, he has not always 
considered the possible effects upon the community as a whole. If 
large carnivores are destroyed, herbivores increase, and, if their 
reproductive capacity is great, they may soon become so abundant 
that their grazing destroys the community or changes it radically. 
If a predator is introduced whose prey is some native species that 
is a pest, the predators may eliminate the pest and then become 
pests themselves. 10 

Only a few examples are necessary to illustrate these points. The 
Indian mongoose was introduced into Haiti, Jamaica, and other 
West Indian islands to rid them of rats and snakes. This the mon- 
goose did most effectively, but its numbers increased, and, with its 
natural prey disappearing, it turned to robbing birds' nests of eggs 
and young. Now it is practically impossible to raise poultry there. 
The gypsy moth was accidentally introduced into Massachusetts 
when it escaped from cultures being reared to test its silk-produc- 
ing ability. It is now a serious pest of fruit and shade trees in most 
of the eastern United States although much money and effort have 
been expended to control it. On the other hand, introductions of 
about sixty foreign predators or parasites of the gypsy moth have 
resulted in the establishment of a dozen or more that are aiding in 
its partial control. The destruction of coyotes in some western 
states has resulted in such marked increase of rabbits that their 
winter feeding on tree seedlings modifies vegetational develop- 
ment (see Fig. 95). 

On game reserves where predators have been eliminated and no 
hunting is permitted, the population of herbivores, such as deer, 
usually increases rapidly. When the number of deer exceeds the 
natural carrying capacity of the region, a shortage of food results 
during unfavorable seasons. Then, especially in winter, many ani- 
mals die unless they are fed bv man. As a result of supplementary 
feeding, the population is still larger the next season, and the prob- 
lem is not solved. Controlled hunting is now permitted on several 
such reserves where the population capacity has been determined. 
The effects on the vegetation of such overcrowding are very con- 
spicuous. All young woody plants protruding above snow are 
eaten off, and the lower limbs of young trees, even conifers, are 



BIOLOGICAL FACTORS 



209 



"pruned" to the height the animals can reach, standing on their 
hind legs. Obviously, community structure and development in 
such areas is completely out of balance. 

Disturbance of natural communities should not be undertaken 
without a reasonable appreciation of the end results. Management 
or manipulation of the balance among species of a community may 





FlG. 101. Drained swampland in the Everglades of Florida. Many acres of 
these muck soils are producing winter truck crops in quantities, now that 
problems of drainage, tillage, and fertilizing have been worked out.— U. S. 
Soil Conservation Service. 



frequently be possible but should offer the best prospects of suc- 
cess when the ecology of the individuals and the community is 
well understood. 

Man's unconcern for natural resources built up through the 
years has led to economic losses and a reduction of those resources, 
which only time can replace. Soil erosion, quite unnecessary if 
cropping is properly handled, had reached a shameful point before 
we began to do anything about it. Only recently have we at- 
tempted to correct overworking of poor soils, mismanagement of 



210 THE STUDY OF PLANT COMMUNITIES * Chapter VIII 

others, overgrazing, and other destructive practices. Contour 
plowing, strip cropping, terracing, and similar procedures check 
runoff, hold water, and permit the rebuilding of rundown soils. 
On wild lands and some submarginal cultivated lands, the re-estab- 
lishment of natural vegetation is being encouraged where it should 
never have been removed. Application of ecological principles in 
such reclamation has generally paid good dividends. 

Not only has man disturbed or destroyed natural vegetation, 
but he has also modified the environment, sometimes to his ad- 
vantage. By irrigation or drainage, the soil moisture has been so 
modified that great acreages have been brought under his control. 
Enormous dams hold water in artificial lakes. When this water is 
properly supplied to the surrounding soils, it transforms worthless 
desert to highly productive agricultural land. Elsewhere drainage 
systems put into lowlands have changed swampy, untillable soil to 
some of the best truck and farming acreages. Not all drainage 
projects have been profitable, however, especially those of muck 
lands. Not all are equally productive, and cost of maintaining 
drainage of some mucks is out of proportion to the crop yields. 
Many such projects have been abandoned— to the joy of sports- 
men and conservationists, who objected to the extensive destruc- 
tion of homes and feeding grounds of all kinds of wildlife associ- 
ated with these swamps. 

GENERAL. REFERENCES 

R. M. ANDERSON. Effect of the Introduction of Exotic Animal Forms. 

J. M. Coulter, C. R. Barnes and H. C. Cowles. A Textbook of Botany. 

(Vol. II : Ecology, pp.485-964.) 
H. C. HANSON. Fire in Land Use and Management. 
XV. A. McCUBBIN. Preventing Plant Disease Introduction. 
S. A. WAKSMAN. Principles of Soil Microbiology. 



Part 4 - Community Dynamics 



CHAPTER IX 



PLANT SUCCESSION 

j 

HISTORICAL BACKGROUND 

When a cultivated field is permitted to lie fallow, it produces 
a crop of annual weeds the first year, numerous perennials the sec- 
ond year, and a community of perennials thereafter. In forest 
areas, the perennial herbs are soon superseded by woody plants, 
which become dominant. After any disturbance of natural vege- 
tation—such as cultivation, lumbering, or fire— a similar sequence 
of communities occurs with several changes in the dominant vege- 
tation through the years. 

Such relatively rapid vegetational changes are familiar to most 
people today and must have been observed hundreds of years ago. 
It was not until the seventeenth century, however, that any syste- 
matic study of such changes was made, and those studies dealt 
primarily with the development of peat bogs. Bog studies were 
continued in the eighteenth century, and, in addition, some at- 
tempt was made to apply the principles to burned and disturbed 
upland areas. It was then that the term, succession, was first ap- 
plied to the vegetational changes involved. During the nineteenth 
century, succession was considered rather frequently but invari- 
ably as incidental to other problems. Several writers hinted at the 
importance of succession in all habitats, but it was not until 1885 
that a regional study of vegetation in Finland was made in which 
succession was recognized as fundamental to all community de- 
velopment. 

Between 1890 and 1905, the modern concepts of succession 
were clarified through the efforts of several writers. Two, whose 
influence has been as great as any, were Americans. In the first 
comprehensive application of successional principles in the United 

211 



212 THE STUDY OF PLANT COMMUNITIES * Chapter IX 

States, Dr. Henry C. Cowles (1899) described the development of 
vegetation on the sand dunes of Lake Michigan. Later (1901) he 
described the vegetation of Chicago and vicinity, as it is related to 
physiography, in so logical a fashion that a pattern for studies of 
community dynamics was established. His papers also served to 
stimulate similar investigations by others. Beginning at about the 
same time, the publications of Dr. F. E. Clements, then working in 
Nebraska, included much that served to shape our present con- 
cepts of succession. The culmination of his ideas appeared in his 
exhaustive treatment of the entire subject of plant succession, 56 
which remains a basic source of reference today. 

THE CONCEPT 

Plant communities are never completely stable. They are char- 
acterized by constant change, 73 sometimes radical and abrupt, 
sometimes so slow as to be scarcely discernible over a period of 
years. These changes are not haphazard, for within a climatic 
area, they are predictable for a given community in a particular 
habitat. This means, of course, that similar habitats within a cli- 
matic area support a sequence of dominants that tend to succeed 
each other in the same order. Contrasting habitats do not support 
the same sequence of communities. As a result, any region with 
several types of habitats will have an equal number of possible suc- 
cessional trends. 

CAUSES 

A detailed consideration of the relationships of organisms to 
their environment should make it clear that major changes in the 
composition of a community can only follow changes in the en- 
vironment. The specific, immediate cause of a particular change 
of species may not always be obvious because of the interrelation- 
ship of controlling factors. Two general types of habitat change 
may cause differences in the community. Development of the 
community causes parallel developmental changes of the environ- 
ment, and physiographic changes can likewise modify the envi- 
ronment materially. 

Developmental changes of the environment result from reac- 
tions upon the habitat by the organisms living there. To illustrate : 
Accumulation of litter affects runoff, soil temperature, and the 



PLANT SUCCESSION 



213 



formation of humus; this, in turn, contributes to soil development, 
modifies water relations, available nutrients, pH, and aeration, 
and affects soil organisms. Thus every organism in a community 
may have some reaction upon the habitat. By these reactions, the 
habitat becomes changed and consequently is less favorable to the 
organisms responsible for the changes, while, at the same time, it 



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ROCK SHORE SUCCESSION 



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OELTA SWAMP SUCCESSION 



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XERARCH SUCCESSIONS 



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PRIMARY SUCCESSIONS 



FlG. 102. A diagram of the trends of succession for the principal habitats 
on Isle Royale, Lake Superior. This is one of the early complete condensa- 
tions of a successional story for an entire region. On this pattern, similar dia- 
grams have been worked out for many sections of the country. Note that the 
system shows at a glance the kinds of habitats in which succession originates, 
the interrelationship of trends, and the major dominants in each of the stages 
of succession. Study of the diagram should help to clarify concepts of suc- 
cession and climax. It must be remembered that not all trends progress with 
equal speed.— After Cooper. 10 



has become more favorable for species that could exist there 
previously only with difficulty. Under the changed conditions, 
new species are able to compete successfully with the established 
species and often even to replace them. 

The habitat may also be modified by forces quite apart from 
the effects of organisms. A flood plain or swamp may become bet- 
ter drained as a stream cuts more deeply into its channel. Silting in 
of a lake or pond raises the level of mineral soil. Chemical changes 
in the soil may result from leaching or accumulation of salts. Such 
modifications of the habitat also produce vegetational changes. 



214 THE STUDY OF PLANT COMMUNITIES ■ Chapter IX 

These two types of causes of succession are commonly in opera- 
tion at the same time, and their effects cannot always be readily 
separated. Since they both result in vegetational change, it seems 
unnecessary to distinguish between their effects in a general con- 
sideration of plant succession. 

KINDS OF SUCCESSION 

Primary succession is initiated on a bare area where no vegeta- 
tion has grown before. It may be observed on glacial moraine ex- 
posed by recession of the ice, a new island, an area of extreme 
erosion, newly deposited volcanic ash or rock, or any similar habi- 
tat newly exposed to colonization. Such habitats are apt to be 
unsuitable to the growth of most plants, and, consequently, the 
pioneers that do establish themselves must have adaptations per- 
mitting survival under extreme conditions. Moisture relationships 
usually control their ability to invade the new area. If the habitat 
is extremely dry, it is described as xeric; if wet, hydric; and if 
intermediate, mesic. The successional trends are similarly referred 
to as being xerarch, hydrarch or mesarch succession. 

Whatever the condition of the initial habitat, reaction of vege- 
tation tends to make it more favorable to plants and always results 
in improved moisture conditions. Thus xeric habitats become 
moister and hydric ones become drier as succession progresses. 
Because of the diversity of habitats upon which succession may 
begin, there are an almost equal number of possible pioneer com- 
munities. Within a climatic area, however, the variety of commu- 
nities decreases as succession progresses because the trend is to- 
ward mesophytism from both hydric and xeric habitats. Thus 
unrelated habitats may eventually support similar vegetation and 
may even undergo identical late stages of succession. 

Secondary succession results when a normal succession is dis- 
rupted by fire, cultivation, lumbering, wind throw, or any similar 
disturbance that destroys the principal species of an established 
community. To what extent the development of vegetation on the 
secondary area resembles primary succession is determined by the 
degree of disturbance. Although the first communities that de- 
velop may not be typical of primary succession, the later stages 
again are similar. When disturbance is extreme, as after severe fire, 



PLANT SUCCESSION 



215 





FlG. 103. An illustration of relatively rapid secondary succession. The 
fire that destroyed this Oregon forest (above) did not appreciably affect the 
soil organic matter and was not followed by erosion. As a result, Douglas fir 
soon became established and, when fourteen years old, formed a closed stand 
10 to 15 feet tall (below).— U. S. Forest Service. 



many of the effects of previous vegetation upon the habitat are 
eliminated, resulting in a slow vegetational development. After 
wind throw or lumbering, many of the products of community 
reaction remain and succession is rapid. If seedlings and young 
trees are not destroyed, progress of succession tends to exceed that 
of the original trend. 



216 THE STUDY OF PLANT COMMUNITIES ■ Chapter IX 

Most of the settled parts of North America have little evidence 
of primary succession today, and even unsettled areas have largely 
been disturbed by grazing or lumbering. Thus primary succession 
must often be interpreted in terms of small and often poor exam- 




FlG. 104. Hydrarch succession as illustrated by girdles of vegetation 
around a shallow lake in northern Minnesota. In what remains of open water 
are submerged and floating-leaved aquatics, the pioneer angiosperms. A mar- 
ginal, floating sedge mat is gradually filling the lake with peat and advancing 
over the water. On the mat are a few bog shrubs, behind which is a girdle of 
tamarack forming a closed stand. The oldest part of the bog is marked by 
the spires of black spruce, which succeed the tamarack. On the upland, be- 
hind the spruce, is a mixed white pine-hardwood forest. Eventually, the en- 
tire depression will be a peat-filled bog supporting a forest of black spruce. 

pies of what once occurred. Studies of secondary succession may, 
however, have the greatest practical value because we are in- 
volved with secondary successions in any problem of applied 
ecology; yet their interpretation may be partially dependent upon 
an understanding of primary successions. 

Representative Successions.— Because water and bare rock rep- 
resent the extremes in types of habitats upon which succession is 
initiated, the growth form of early stages of each is remarkably 
similar everywhere and even genera and some species are often 



PLANT SUCCESSION 



217 



duplicated regardless of the region. It is, therefore, possible to pre- 
sent a general description of such successions, which can be ap- 
plied almost anywhere and which will illustrate what we have just 
discussed. 




FIG. 105. Hydrarch succession illustrated by swamp vegetation. The zone 
of cattails occupies the partially flooded, muddy margins. When soil builds 
up or drainage improves, bog shrubs (buttonbush, alder, willow) appear as 
in the middle background. On wet, but drained soil a swamp forest of mixed 
hardwoods develops as in background.-Ptoo by H. L. Blomquist. 

Hydrarch succession progresses in response to better moisture 
conditions in combination with improved aeration. Initiated in a 
lake, pond, or stream margin where water movement is not too 
great, the pioneer vascular plants are submerged aquatics with 
thin, dissected, or linear leaves. Their depth of growth is limited, 



218 THE STUDY OF PLANT COMMUNITIES • Chapter IX 




FlG. 106 (A). Xerarch succession as illustrated by vegetational develop- 
ment on granitic rock in the Piedmont of the southeastern states. Early stage 
(upper) of mat formation initiated by the pioneer moss (Grimmia laevigata) 
upon which a lichen (Cladonia leporina) is well established. As mat thickens 
(lower), herbs come in, with eventual Andropogon spp. dominance. 1 



186 



on the one hand, by light penetration of the water and, on the 
other, by a zone of floating-leaved species. These latter (water 
lilies, etc.) exclude submerged species by shading but cannot move 
into the zone of submerged forms until the bottom is built up or 
the water level falls. In still shallower water, emergent species pre- 
dominate. These have their roots and rhizomes in the mud and 
extend upward into the air (rushes, reeds, cattails, sedges). The 



PLANT SUCCESSION 



219 



close growth in this zone serves to hold sediment, and the bulk 
results in substantial accumulation of partially decomposed or- 
ganic matter. When filling is sufficient, shrubs can survive on the 
built-up soil. Finally, the soil will be firm enough and sufficiently 
raised above the water table to support lowland trees, which may 
eventually give way to a community similar to that of uplands. 
This entire sequence can sometimes be seen as a more or less 



wm. 





FlG. 106 (B). Shrub stage of rock succession, mostly Rhus copallina here. 
Note fringe of Andropogon, smaller herbs, and finally mosses at periphery 
(upper). Tree stage (lower) on an old mat, forming an island on bare rock. 
Oak-hickory forest in background is growing on shallow soil overlying 
rocks. 186 



220 THE STUDY OF PLANT COMMUNITIES ■ Chapter IX 

continuous series of zones girdling a lake that is gradually filling 
in. Borings of the soil under any zone will show the partially de- 
composed remains, in vertical sequence, of each of the previous 
stages of succession that contributed to the development of that 
zone. 

Xerarch succession on rock follows a definite pattern, whose 
progress is controlled by the rate at which soil forms and accumu- 
lates. Pioneers on rock surfaces are either lichens or mosses ca- 
pable of growing during the brief periods when water is available 
to them and lying more or less dormant through the usually longer 
periods of drought. The pioneer lichens are crustose and foliose 
types, which usually contribute little to succession since they are 
not mat-forming. 186 However, they do probably cause corrosion 
of the rock surface and thus provide some anchorage for other 
species. Pioneer mosses, on the other hand, are in tufts or clumps, 
which catch dust and mineral matter from wind and water. This 
material, combined with the remains of mosses, forms a gradually 
thickening mat with a periphery of young plants that spreads over 
bare rock (and the pioneer lichens) and with a central area that 
may become thick enough to support foliose lichens (Cladonia 
especially), larger mosses such as Polytrichum, or often species 
of Selaginella. Such bushy plants catch and hold still more mineral 
material, and their death adds much organic soil to the mat. 

When soil has built up sufficiently to provide the necessary an- 
chorage and water-retaining ability, seed plants appear on the 
mats. A number of hardy, annual herbs, often weeds of field and 
garden, appear first and are followed by biennials and perennials, 
of which grasses are most abundant. Later a shrub stage becomes 
dominant, which usually includes some species of sumac (Rhus) 
and several ericaceous shrubs. By this time, the mats may be sev- 
eral inches or a foot thick and then trees make their appearance. 

Just as a series of girdles of vegetation usually surrounds a lake 
and indicates the sequence of succession from open water to solid 
ground, so the progress of succession on rock may be seen as a 
series of girdles of vegetation from the periphery to the center of 
an old mat. Pioneers are at the outer margin of the mat, and each 
successive stage of dominance is nearer the center where, on the 
thickest soil, trees may be present. 



PLANT SUCCESSION 



221 




FlG. 107. Herb stages in secondary succession on abandoned upland fields 
in the Piedmont of the southeast. (1) Horseweed dominance on a field 
abandoned one year. (2) Aster dominance indicating two years of abandon- 
ment. (3) Broom sedge (Andropogon) dominance in a field abandoned five 
years, and young pine well established. 

The early stages of these two successional trends are apt to be 
extremely slow, but later stages speed up considerably as reaction 



222 THE STUDY OF PLANT COMMUNITIES • Chapter IX 




FlG. 108. Forest stages of old-field succession (continuing Fig. 107). (1) 
Fully stocked 15-year loblolly pine, which has eliminated all old-field herbs 
and under which hardwood seedlings may be found, (2) 26-year pine, under 
which saplings of gum, red maple, and dogwood are noticeable, (3) 50-year 
pine stand in which hardwoods, including oak and hickory, have formed an 
understory, (4) oak-hickory climax forest, of the type that could develop on 
an old field after 200 years or more. 183 — Photo (1) by C. E Korstian. 



PLANT SUCCESSION 223 



of the vegetation becomes more effective. The final changes, after 
tree dominance, are again very slow. Changes of currents or drain- 
age in the lake and wind throw or fire on the rock may disrupt 
either of the trends and result in secondary succession. The result 
of succession in both habitats is, however, a gradual change in the 
direction of habitat conditions that are relatively mesic for the 
climate of the region and a community adapted to such conditions. 

RATE OF SUCCESSION 

If succession is to be recognized as universal and occurring in all 
habitats, it becomes necessary to ignore time to some extent. A 
mesic habitat in a given climate will obviously produce a forest 
much more quickly than a xeric one, especially if the initial habi- 
tat is bare rock. Yet the potential ultimate communities of the two 
sites are the same, for all successions in a climatic area progress 
toward communities of mesophytes. Two habitats of apparently 
similar characteristics might support the same successional se- 
quence, but progress of the successions might be at different rates 
because of the type of soil and the difference in its response to 
reaction. Or, if seed sources were not equally available to both 
sites, one might develop more rapidly than another. This could 
result from an oversupply of seed, producing overstocking of cer- 
tain species and consequent delay in development of the next stage 
because of competition; on the other hand, poor seed sources or a 
series of poor seed years might materially delay the initiation of a 
community that otherwise could have started. This should make it 
clear that the rate of succession is extremely variable. Pioneer 
stages of primary succession are commonly very slow because 
they can progress only with soil development. An extreme exam- 
ple is probably that of succession on bare rock, which must wait 
not only upon soil development but also upon the disintegration 
of the rock for soil formation. In contrast, the pioneer stages of sec- 
ondary succession, especially on abandoned fields, are remarkably 
rapid, for often the dominants change every year for several years. 

STABILIZATION AND CLIMAX 

All successional trends lead toward relative mesophytism within 
a climatic area. This explains why related successions parallel each 



224 THE STUDY OF PLANT COMMUNITIES • Chapter IX 

other in their mature or late stages. Eventually, all successsional 
trends lead to a single community, which is composed of the most 
mesophytic vegetation that the climate can support and whose mois- 
ture relations are average, or intermediate, for the region as a 
whole. This community, determined by the climate, terminates 
succession and is called the climax community or climax for that 
climatic area. It is capable of reproducing itself, and, since it rep- 
resents the last stage of succession, it cannot be replaced by other 
communities so long as the climate remains the same. It is, there- 
fore, a stable community in which the individuals that become 
overmature and die are replaced by their own progeny, leaving 
the character of the community unchanged. 

Uniformity and Variation of Climax.— Since climax is determined 
by climate, the distribution and range of a particular climax should 
be an indication of a region in which effective climatic factors are 
equivalent. Climax is a product of all the interacting factors of cli- 
mate and is, therefore, a better expression of the biological effec- 
tiveness of climate than man can obtain by physical measurements, 
which he must interpret. This is well illustrated by the similarity 
of prairie vegetation over an area with an extremely wide range 
of several factors, particularly of temperature from north to south. 

On this basis, it might be assumed that a climax would be uni- 
form throughout its extent. This is true only in part. Certain 
variations are to be expected, which are related to the great extent 
of climax regions and the history of different parts of these re- 
gions. The extent of deciduous forest climax results in transitions 
to both coniferous forest and grassland. These transitions are not 
abrupt, and the composition of the climax community is affected 
for some distance. The deciduous forest likewise illustrates how 
the time element may be involved in variation. Most of its north- 
ern extent lies on glacial soils and topography and has occupied the 
area only in relatively recent times. Unglaciated areas to the south 
supported deciduous forest throughout the period of glaciation 
and still do today. Thus there are differences in age of vegetation, 
topography, and soils, all of which contribute to variation in the 
deciduous climax. 47 

The obvious uniformity of vegetation in a climax region is in 
the life form of the dominants, which is definitely a product of 



PLANT SUCCESSION 225 



climate. Thus the major climax regions are easily recognized : 
grassland, desert, and semidesert with shrubs predominating; and 
forest climaxes that are boreal, deciduous if temperate, or broad- 
leaved evergreen if tropical. In addition to life form there is uni- 
formity of genera among the dominants of a climax. Variations of 
the dominant species, as well as dependent ones, are a product of 
the environmental variations discussed above. 

The major climaxes are distinguishable on the basis of physi- 
ognomy or life form of the dominants alone. Such climaxes are 
termed formations.™ Floristic variation within a formation is usu- 
ally sufficient to produce two or more recognizably distinct cli- 
max communities, which, following Clements, would be called 
associations. Although distinct, the associations of a formation are 
at the same time bound together by one or more species present in 
all associations and by the constant presence of some dominant 
genera throughout. Thus the associations of a formation are quite 
obviously similar and related. 

Just as associations are recognizable subdivisions of formations, 
there are distinguishable variations within associations. These geo- 
graphical variants that make up the association are called facia- 
tions. 60 They are recognizable by differences in the abundance or 
relationships of the dominants. Faciations may be further subdi- 
vided into local variations, called lociations. Further subdivision is, 
of course, possible and often desirable. The various systems of 
classification and the terminologies that have been used make for 
more detail and controversy than can be presented here. 

Because, unfortunately, the term, association, is constantly used 
in more than one sense, it deserves further mention. The systems 
of classifying communities, as supported by the various schools 
of thought, almost invariably include the term. Although not 
always in agreement among themselves, European ecologists con- 
sistently consider associations as basic units of classification that 
can be grouped into categories of successively higher rank. Thus 
lociations, as mentioned above, might be given associational rank 
in such a system. The use of the term here is in an absolutely 
contrasting sense in that it makes it a community of the highest 
rank, inclusive of, and divisible into, numerous lesser categories. 
It has been suggested that, to avoid conflict, the use of the term 



226 THE STUDY OF PLANT COMMUNITIES * Chapter IX 

in this sense be indicated by referring to climax associations or 
major associations, but this has not been generally accepted as yet. 
An attempt was made to standardize the use of the term at a recent 
International Botanical Congress, but, even so, the rulings have not 
been completely accepted. For a summary of some of the diverse 
points of view and some applications of the term, reference should 
be made to Conard's 67 discussion of plant associations and its ap- 
pended bibliography. 

Types of Climax.— In a climatic area, all succession is in the di- 
rection of a community that can maintain itself permanently, and 
there is only one such community for the region as a whole. How- 
ever, succession is often halted temporarily in almost any stage of 
its progress, and sometimes is halted almost permanently in late 
stages. Diseases, fire, insects, or man may produce conditions that 
prevent completion of succession and hold it indefinitely at some 
stage preceding the climax. Edaphic or physiographic conditions 
may be such that succession cannot proceed to completion. Al- 
though such communities may appear to be as stable and perma- 
nent as climax, they cannot be considered as such because they are 
not controlled by climate. 

This is the monoclimax hypothesis. In contrast is the polyclimax 
view, which recognizes edaphic, physiographic, and pyric cli- 
maxes within a climatic area. The conflict between the two views 
lies in the interpretation of the concept of climax. Actually, the 
same communities are recognized by both but under different 
terminology. Since the basic concept of climax implies one ultimate 
community controlled by climate, the monoclimax view is con- 
sistent with the meaning of the term. When used in conjunction 
with a few precise terms, 60 which are discussed below, it is ade- 
quate for explaining all climax variations. 

Subclimax.— When, in any succession, a stage immediately pre- 
ceding the climax is long-persisting, for any reason, it can be called 
subclimax. It may be the result simply of extremely slow devel- 
opment to climax, or of any disturbance, such as fire, that holds 
succession almost indefinitely in its subfinal stage. In the eastern 
United State, most pine forests are subclimax to hardwood climax 
because of the relatively slow elimination of pine in the progression 
toward hardwood dominance. In the coastal plain, subclimax pine 



PLANT SUCCESSION 227 



forests are maintained indefinitely by the constantly recurring 
fires to which the pines are resistant and which keep down hard- 
woods. 

Disclimax.— When disturbance is such that true climax becomes 
modified or largely replaced by new species, the result is an ap- 
parent climax, called disclimax. The disturbance is usually pro- 
duced by man or his animals and the introduction of species that, 
under the existing conditions, become the dominants over wide 
areas. The prickly pear cactus thus has formed a disclimax over 
wide areas in Australia. A grass, Bromus teetotum, forms a discli- 
max in much of the Great Basin where, because it burns readily, it 
facilitates fires, which reduce dominance of desert shrubs and in- 
crease the area of grass. The short grasses of the Great Plains were 
long considered as climax but now are generally considered as 
disclimax resulting from grazing and drought, which have prac- 
tically eleminated the midgrass climax. The ravages of chestnut 
blight illustrate how disclimax may result from disease. Oak-chest- 
nut climax is today an oak disclimax. 

Postclimax and Vr e climax. —Ps. climatic area is normally bor- 
dered, on the one hand, by one that is drier and warmer and, on the 
other, by one that is moister and cooler. The contiguous climates 
are, therefore, either less favorable or more favorable to plant 
growth. As a result, each has its own climax, distinct in species and, 
often, in growth form. On a large scale, this is apparent in latitudinal 
zonation from the tropics to the arctic. Often it is noticeable in the 
climaxes along a line from oceanic or maritime climate to the in- 
terior of a continent. It is most conspicuous on mountains where 
altitude produces a zonation of climates and climaxes. Each of the 
climatic areas in such a sequence has a bordering climate with a 
more favorable water balance, usually on the north, toward the 
coast, or at higher altitudes; while the climate to the south, toward 
the interior, or at lower altitudes, usually is less favorable. 

For any particular climax the contiguous climax produced by a 
more favorable climate, usually cooler and moister, is termed post- 
climax, and the one produced by less favorable conditions, usually 
drier and hotter, is termed preclimax. To illustrate on a broad basis, 
deciduous forest climax has grassland as preclimax and northern 
conifer forest as postclimax. At the same time, deciduous forest 



228 THE STUDY OF PLANT COMMUNITIES • Chapter IX 

holds a postclimax relationship to grassland that has desert as pre- 
climax. The use of the concept is not restricted to formations as 
illustrated above since it is just as applicable to associations, even 
within the same formation. For example, within the deciduous 
forest formation oak-hickory is preclimax and hemlock-hardwood 
is postclimax to the beech-maple association. Likewise, oak-hick- 
ory is preclimax and beech-maple (or hemlock-hardwood) is post- 
climax to the oak-chestnut association. 

Should the present phase of relatively stable climates be inter- 
rupted, the climate of any given area would undoubtedly tend to 
become more like that of one of its contiguous areas and a migra- 
tion or shift of climax would result. Such a shift occurred during 
the glacial period when the northern coniferous forest moved 
southward, and the northern extent of the deciduous forest was 
proportionately constricted. When the climate ameliorated, the 
ice receded, and again, the ranges of the climaxes were readjusted. 
When such shifts occur, remnants of the previous dominants are 
left behind in locally favorable habitats where they may maintain 
themselves indefinitely as relicts of a previous climax. These relicts 
are either preclimax or postclimax depending upon their relation- 
ship to contiguous climaxes and the direction of the climatic shift. 
The habitats in which they survive must have edaphic or physio- 
graphic characteristics that differ so markedly from the average 
for the region that conditions for growth are similar to those of a 
contiguous climatic area. Deep valleys or canyons with steep bluffs 
and contrasting exposures, poorly drained flood plains, bogs, ridges 
of rock or gravel, areas of deep sand or other peculiar soil condi- 
tions are specific examples. 

Where there have been shifts of climax, it is apparent that pre- 
climax and postclimax communities should occupy such habitats. 
Not all preclimax and postclimax communities, however, need be 
relicts. Within the general range of a climax, there are bound to be 
local habitats such as those mentioned above that will continue 
indefinitely to be somewhat more favorable or less favorable, wet- 
ter or drier, than the conditions controlled by climate in the region 
as a whole. As a result, when vegetational development proceeds 
to a condition of stability on such a site, it will have characteristics 
of the contiguous more or less favorable climate. Such localized 
stable communities are likewise postclimax or preclimax for the 



PLANT SUCCESSION 229 



region. In or approaching transition zones, such areas are partic- 
ularly noticeable, and here, especially, application of the concept 
greatly simplifies interpretation of climax. 

Since communities such as these exist to some extent in every 
climatic area, they must be recognized. As mentioned earlier, not 
all ecologists agree as to their interpretation. Some, with the poly- 
climax view, describe them variously as edaphic or physiographic 
climaxes. This is open to the general criticism that, by definition, 
there can be but one climax for a climatic region. Use of preclimax 
and postclimax is a necessary part of the monoclimax view but is 
consistent with the meaning of climax. At the same time, it shows 
relationshiDS with contiguous and past climaxes. 

METHODS OF STUDYING COMMUNITY DYNAMICS 

Determination of Climax Formations.— The major climax re- 
gions (formations) are fairly obvious, and their number and ap- 
promixate limits have been accepted for some time. Each has its 
distinctive physiognomy or life form that makes for clear demar- 
cation. An additional number of criteria corroborating the appar- 
ent unity based upon physiognomy have been applied. 

Tests of climax that have been used in fixing formations 00 are 
briefly summarized below. Both static and developmental criteria 
must be met. 

Static Criteria 

1. Life form must be uniform throughout. 

2. All associations must include one or more of the same 
or closely related species as dominants or subdominants. 

Developmental Criteria 

3. Late stages of succession must be essentially identical 
for a climax; and distinct from those of another climax. 

4. Postclimax should show relationships to contiguous cli- 
max or subclimax. 

5. Historical records as to composition and structure must 
conform to the modern picture. 

a. Recent historical— old records and land surveys. 

b. Historical development reconstructed from pollen 
statistics. 

c. Geological record, physical history, and fossils. 



230 THE STUDY OF PLANT COMMUNITIES • Chapter IX 

Recognition of Local Climax.— The variations of a formation 
(associations) are not always immediately obvious, particularly in 
areas of transition from one association to another. Because of dis- 
turbance by man, the climax vegetation once present in virgin 
stands over wide areas has practically disappeared. We now, there- 
fore, must rely upon small samples of climax vegetation, often 
disturbed; or, when even these are lacking, we must determine the 
climax on the basis of studies of succession. There may, therefore, 
be different interpretations, and errors are possible. To illustrate : 
It was generally believed for years that short grasses constituted 
the climax of the plains. Added evidence and reinterpretation in- 
dicated to many ecologists that mid-grasses are climax and short 
grasses are disclimax maintained by modern grazing under the 
conditions of periodic drought. 

A climax association must, of course, conform to the criteria 
that delimit the formation of which it is a member. To check these 
criteria, it becomes necessary to know the successional trends of 
the vicinity in detail, to know the composition and structure of 
the postulated climax and subfinal stages of succession, and to dis- 
tinguish preclimax and postclimax communities and habitats. Thus 
it becomes necessary to know something of related associations as 
well as the one involved. Finally, the history of the region, both 
recent and geological, is desirable for proper interpretation of ob- 
servations. 

The climax must be a community capable of maintaining itself 
indefinitely under existing climatic conditions. It must be the final 
community in all successional trends in the region except those 
isolated instances of edaphic or physiographic variation producing 
preclimax or postclimax by compensating for climate. It must 
recur throughout the area under average conditions, or the evi- 
dence from succession must indicate its potential presence. 

General Procedure in Local Study.— The desirability of fa- 
miliarity with the area as a whole has been emphasized. Observa- 
tion and note-taking should proceed at the same time that literature 
is searched to learn the historical aspects of the area and the rela- 
tionships of its flora to that of surrounding climaxes. With con- 
tinued observation, certain ideas will develop as to probable and 
possible successional relationships and the relative position of dif- 



PLANT SUCCESSION 



231 



Quercus alba 




8 1 








\ ■ 




a t~ 


\ » 


if I 






9 / ] 
t / J 
$ / / 



Quercus stellata 
D 





/ 


V \ 




/ 


\ \ 


B | 


/ 


\ \ 
\ 1 


A 1 


\ 1 

\ 1 

\ 1 

\ I 

\ \ 


/' I 

// J 
It 1 



Carya spp 
D 



B L -L 




A 

\ I 1 


' / 1 
/ / / 


\ V 





sc 



sc 



Quercus coccinea 
D 




Quercus borealis 
var maxima 




Quercus rubra 
D 







1 V 




\ 11 

\ M 


I / 

I I J 




u J 



Quercus marilandica 
D 



i ! 

• i 
i I 

li 

I 





SC 

FlG. 109. A phytographic comparison of the overstory species found in 
the two oak-hickory climax variants of the North Carolina Piedmont. 183 
Values for the white oak type are indicated by solid lines, for the post oak 
type by broken lines. D— percent of total tree density, F— frequency percent, 
SC— percent of four size classes (overstory, understory, transgressives, seed- 
lings) in which the species was found. Zero is the center, 100 percent the 
periphery of the circle. Only quantitative data can give information such as 
illustrated by these phytographs. 

ferent habitats. Such methods alone have produced some excellent 
interpretations of vegetational dynamics. General conclusions may 
be as good as any obtained otherwise. However, there are reasons 
why supporting data are most desirable. 



232 THE STUDY OF PLANT COMMUNITIES • Chapter IX 



Frequency in Percent 

llyrs 22yrs 3 I y r s 3 4 yrj 42yrs 75yr$ 110 yrj Oak - 

90 i i Hickory 




B Oak and Hickory Trees 
\/////A Oak and Hickory Reproduction 
[XX^ Pme Trees 

Pme Reproduction 




25- 

20- 

* 15- 

I 10- 

5- 
0. 




35 

95 

100 

290 
295 

300 



10 



Frequency in Percent 

Fig. 110. Relationships of trees and reproduction of pine and oak-hickory 
in old-field succession in North Carolina as shown by their density and fre- 
quency in successive ages of pine dominance leading to oak-hickory climax. 
Frequency is indicated by width of columns, density by height. Such phyto- 
sociological representations clarify relationships that might otherwise go un- 
recognized. 

It is often possible for honest observation to be wrong, and only 
quantitative and qualitative data will demonstrate the discrepancies. 
Again, such data may bring to light pertinent information that 
could not be realized by observation alone. When questions of 
"why" "when" or "how" come up, they can be most satisfactorily 
answered with absolute data. 

These things were soon realized by some early students of suc- 
cession, and quadrat methods were introduced as a part of their 
procedure. Early methods of sampling, however, were rarely ade- 



PLANT SUCCESSION 233 



quate. Unfortunately, sampling methods in successional studies 
were not improved as rapidly as they should have been. Perhaps 
students of community dynamics were too much concerned with 
an overall picture rather than detail. As a result, much desirable 
information was not obtained and now may not be available be- 
cause vegetation has been destroyed. 

Phytosociological Methods in Studies of Succession.— The 
static point of view long held by many Europeans led naturally to 
an interest in the detail of community composition and structure. 
Sampling methods were an essential part of their work, and, as a 
result, these methods were studied and revised for efficiency and 
effectiveness. Their objectives and uses were outlined in our dis- 
cussion of analysis and description of plant communities. It was for 
this purpose that they were developed, but they need not by any 
means be restricted to static studies. How successfully they can 
be applied to special successional situations is well illustrated by 
Billings' 20 study of secondary succession and soil changes on aban- 
doned fields. It is likewise possible to adapt phytosociological 
analytical methods to a comprehensive vegetational study involv- 
ing all the major successional trends of a region. 183 Herein lies an 
application for phytosociological methods that has so far been 
given too little attention. In addition to putting on record the 
sociological characteristics of the various communities involved, 
the same data can be used for clues to solution of stubborn dy- 
namic problems, to substantiate observations, and as proof of con- 
clusions. 

GENERAL REFERENCES 

S. A. CAIN. The Climax and Its Complexities. 

F. E. Clements. Plant Succession : An Analysis of the Development of 
Vegetation. 

F. E. CLEMENTS. Nature and Structure of the Climax. 

W S. COOPER. The Fundamentals of Vegetational Change. 

J. PHILLIPS. Succession, Development, the Climax, and the Complex Organ- 
ism : An Analysis of Concepts. 



CHAPTER X 



THE DISTRIBUTION OF CLIMAX COMMUNITIES 
PRESENT DISTRIBUTION OF CLIMAXES 

In the early nineteenth century, Humboldt drew attention to 
the importance of climate in determining the distribution and 
range of species, and Grisebach showed the possibilities of using 
communities, instead of species, as units of study. These were the 
beginnings of modern descriptive plant geography, which deals 
with the extent and distribution of vegetation types, particularly 
climaxes, and the reasons they occur where they do. The complex 
nature of climate necessitated from the first separate consideration 
of its components, and this led to oversimplified explanations of 
plant distribution based upon single factors. Even Warming, 266 to 
whom we are indebted for shaping the foundations of much of 
our modern ecological philosophy, was confident that communi- 
ties and their responses are primarily controlled by water. Among 
the early geographers, Schimper 213 deserves special mention be- 
cause he emphasized what is now generally recognized, namely, 
that a complex of interacting factors determines vegetation. There 
is still no simple means of expressing the effectiveness of the com- 
plex. 

Merriam's 173 attempt to correlate all vegetational distribution 
with temperature is illustrative of the search for a single factor 
whose quantitative value would express climatic conditions. He 
showed that zones with similar summer temperature character- 
istics frequently have similar vegetation, but, unfortunately, he 
assumed that because there was a correlation there must also be a 
cause and effect relationship. His generalizations are, therefore, not 
acceptable, and too many exceptions remain unexplained. 

A persistent search was made by Livingston and his associates 159 
for a single quantitative value of physiological significance, which, 
when plotted to indicate isoclimatic lines, would closely match the 
distributions of major vegetation types. Summer evaporation rates, 
temperature coefficients, and temperature indices based upon 

234 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 



235 



physiological responses all were tried. The most successfully ap- 
plicable value he found 156 was one that combined a physiological 
temperature index, precipitation, and evaporation. Actually this 
was a refinement of the precipitation : evaporation ratio proposed 
earlier, 255 but it is scarcely more useful. These and other studies 
serve to emphasize the complexity of plant-environmental rela- 




1 



m TUNDRA 

ED BOREAL FOREST 

E%1 HEMLOCK-HARDWOOD F. 

ES3 DECIDUOUS FOREST 

E-7H3 S.E.EVERGREEN FOREST 

Fg~\ GRASSLANDS 

r?rl DESERT CRASS AND SCRUB 

r^il DESERT 

V77ZK COASTAL FOREST 

rrrrm rocky mt. forest 

E^ WET AND DRY TROPICAL F. 



FlG. 111. General ranges of the principal vegetation types of North Amer- 
ica.— By permission jrom Transeau, et al (1940)~ 5: Harper and Brothers, pub- 
lishers. 



236 THE STUDY OF PLANT COMMUNITIES * Chapter X 

tionships and the impracticality of expressing them as a function 
of a single variable. This becomes even more obvious when influ- 
ences such as length of day, winter temperatures, and the season 
of precipitation are considered. 

CLIMAX REGIONS OF NORTH AMERICA 

The vegetation maps available for North America 230 ' 236 ' 268 serve 
to emphasize by their similarities that the major vegetation types 
are fairly obvious, but their differences in detail indicate disagree- 




FlG. 112. Alpine tundra in the Colorado Rockies— U. S. Forest Service. 

ment on the interpretations of climax relationships, especially 
within formations. An understanding of the bases for different 
interpretations can best be obtained by study of the many papers 
dealing with local investigations of vegetation. There are, how- 
ever, several of a more comprehensive nature, 230 ' 231 > 118 which give 
more detail than can be presented here. 

The concept of climax formations and associations was discussed 
earlier (Chap. 9). Classification of North American vegetation on 
this basis is altogether logical, particularly if the point of view is a 
dynamic one. The system shows modern successional and climatic 
relationships but is based as well upon past history of the climaxes. 
Although growth form is the apparent major basis of classification, 
dynamic factors are given equal consideration. 

Below are listed the major climax formations of North America. 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 237 



These, together with their associations, are discussed in the section 
that follows. The formations restricted to the mountains of the 
west occur in altitudinal zones whose relationships should be clear- 
ly understood. Consequently, discussion of these zonal formations 
is centered about each of the principal mountain ranges rather 
than considering each zone separately, throughout its extent. 

Climax Formations of North America 
Tundra 

Tundra Formation 

Forest 

Coniferous 

Boreal Forest Formation 

Subalpine Forest Formation 

Montane Forest Formation 

Pacific Coastal Forest Formation 
Deciduous 

Deciduous Forest Formation 
Woodland 

Woodland Formation 
Scrub 

Broad-Sclerophyll Formation 

Sagebrush Formation 

Desert Scrub Formation 
. Grassland 

Grassland Formation 
Tropical Formations 

Tundra Formation.-Tundra lies between the northern limit of 
trees and the area of perpetual ice and snow in the far north, or 
above timber line in high mountains. In North America, it forms 
a broad band completely across the continent, and it also occupies 
the narrow low coastal area around most of the periphery of 
Greenland. It occurs on mountains as far south as Mexico if their 
altitude is sufficient to produce a timber line. Thus it is limited in 
its northern or upward extent by ice and bounded on the southern 
or lower margin by boreal or subalpine conifer forest. 

Vegetation is low, dwarfed, and often matlike, and includes a 



238 THE STUDY OF PLANT COMMUNITIES • Chapter X 



high proportion of grasses and sedges. Even the woody plants, in- 
cluding willows and birches, are prostrate. The herbs are mostly 
perennial and of a rosette type, producing relatively large flowers, 
often with conspicuous colors. A4osses and lichens may grow any- 
where and in favorable habitats form a thick carpet with the low 
herbs. The number of species is small compared with floras of 
temperate climates, and, even within the tundra, the number de- 
creases northward. Most of the genera and numerous species are 
to be found throughout the Northern Hemisphere wherever 
tundra occurs. 

The uniformity of the flora is undoubtedly related to the pe- 
culiarities of environment. The growing season is short and its 
temperatures are relatively low. The depth to which soil thaws 
in summer is of great importance. Light is continuous throughout 
the growing season in the arctic, and is intense and high in ultra- 
violet rays in alpine habitats. Precipitation is largely in the form 
of snow and varies greatly. Drying summer winds, which are 
characteristic, produce high rates of evaporation and transpira- 
tion. As a result, water is often a critical factor, especially inland 
away from moist coasts. Local marked differences in vegetation 
are commonly related to minor variations in topography and the 
differences they produce in drainage and retention of snow. The 
poor, haphazard drainage associated with new topography is ap- 
parent everywhere. 

Arctic Tundra— Although the flora of the tundra is fairly well 
known, its communities and their successional relationships have 
not been sufficiently studied. 20 * In contrast with temperate vege- 
tation, many species may occur in any type of habitat, and several 
that appear to be climax may also be pioneers in the newest of 
habitats. Even climax is not agreed upon, possibly because observa- 
tions have been made at widely separated points. Interpreted in 
terms of Greenland vegetation, Cassiope heath appears to be cli- 
max, and a Sedge-Dryas dominated community, of equal extent 
but on drier sites, is preclimax. 185 Two subclimaxes are frequent. 
Any habitat with sufficient moisture, whether it be pond margin, 
seepage area, or boggy ground, eventually is covered with a thick 
moss mat supporting several herbs of which cotton grass (Eri- 
ophorum spp.) is most conspicuous. Xerarch succession on rock 



CLIMAX COMMUNITIES: PRESENT DISTRIBUTION 239 



exposures eventually results in a lichen-moss mat, which may con- 
tinue almost indefinitely. 

Important climax dominants are Cassiope tetragona, one or more 
species of Vaccinium, Arctostaphylos alpina, Empetrum nigrum, 
Andromeda polifolia, Ledum palustre, Rhododendron lapponicum, 
and species of Betula and Salix. These and other species occur in 
varying combinations and degrees of importance. 

Practically all habitats support some of the many species of 
Carex, of which the commonest include Carex capillaris, C. nar- 
dina, and C. rupestris. The preclimax sedge community invariably 
includes Elyna bellardii in abundance. Some grow in mats, some 
are in clumps, but all are dwarfed. The same can be said for the 
grasses, which, although relatively abundant and widespread, are 
restricted to a few genera, of which Festuca and Poa are espe- 
cially well represented. Many of the conspicuous herbs previously 
mentioned are included in the numerous species of one of the 
following genera : Saxifraga, Potentilla, Ranunculus, Draba, Cer- 
astium, Silene, Lychnis, Stellaria, Castilleja, and Pedicularis. Con- 
spicuous and widespread species typical of tundra are Oxyria 
digyna, Papaver spp., Dry as octopetala, and Epilobium latifolium. 

Alpine Tundra— Mountains high enough to have a timber line 
support tundra, whose upward extent is limited by the snow line. 
In the east, as a consequence, tundra is found only on a few high 
peaks in New England. Farther south, the Appalachians are not 
of sufficient height to support tundra. That on Mt. Washington 
is representative of the type and is essentially similar to the not far 
distant arctic vegetation. 

Alpine tundra in the western mountains mostly lies far to the 
south of the arctic and is consequently found at high altitudes 
only. In the Canadian mountains, it is found as low as 6,000 feet, 
but southward its altitudes grow progressively higher. In the 
Rocky Mountains of Colorado, it is well developed between 
11,000 and 14,000 feet. In the Sierra Nevada, where many peaks 
are higher, the snow line is lower, and thus, tundra lies mostly 
between 10,500 and 13,000 feet. 

When climate changed and terminated the glacial period, vege- 
tation similar to modern tundra must have followed the ice as it 
receded northward. This left only these high peaks and ridges 



240 THE STUDY OF PLANT COMMUNITIES • Chapter X 

where tundra could survive as relicts. The relict vegetation obvi- 
ously belongs to the Tundra Formation because of the growth 
form and the duplication of characteristic genera as well as many 
species. The greater importance of grasses and the presence of 
numerous endemics in the western mountains suggest that both 
the Sierran and Petran tundras might be classed as associations of 
the Tundra Formation. 

Boreal Forest Formation.— This great forest, often called "taiga" 
in its northern extent, spans the continent in a broad band to the 
south of the tundra. Along the Atlantic coast it extends from 
Newfoundland on the north to the New England states on the 
south. Westward, the southern boundary touches the Great Lakes 
region, trends northwestward across Saskatchewan and along the 
Rocky Mountains, and then to the Pacific coast in Alaska. The 
band is, therefore, narrowed abruptly in the far west although it 
extends much farther to the north there than it does over much of 
the continent. 

Climate is scarcely less severe than that of the tundra. The short 
growing season from June through August is cool, and winters 
are very cold. Precipitation is moderate, averaging perhaps twenty 
inches, except on the east coast where it may be forty inches. The 
precipitation : evaporation ratio is, however, favorable because of 
the low temperatures. The topography is almost entirely that pro- 
duced by glaciation. Lakes are scattered everywhere, and many of 
them have filled to form extensive bogs or muskegs. The mineral 
soils are either thin and residual, overlying the rock masses ex- 
posed by glaciation or, along the southern boundary, deep moraine 
and outwash. All are immature and often poorly drained. Sub- 
soils, in the bogs especially, may not be frost-free even in mid- 
summer. 

Climax— The climax forest of white spruce and balsam fir is 
best developed in and about the St. Lawrence river valley where 
the trees reach maximum size and grow in close stands under a 
variety of conditions. Here, and over much of the range, Picea 
glanca and Abies balsam e a form dense stands under whose canopy 
there are relatively few dependent or secondary species. Paper 
birch (Be tula papyrifera) is a constant associate although it is 
successional after fire or disturbance and often occurs as subclimax 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 241 

in pure stands. Characteristic tall shrubs are Viburnum alnifolium 
and V. cassinoides. Typical lesser plants on the shady forest floor 
are Aster acuminatum, Dryopteris dilatata, Oxalis montana, Clin- 
tonia borealis, Cornus canadensis, Maianthemum canadense, Aralia 
nudicaidis, Coptis trifolia, and Chiogenes hispidula. 

With increasing distance from the St. Lawrence center, both 
westward and northward, the number of species declines. Balsam 




FlG. 113. Interior of boreal white spruce-balsam fir forest as it appears in 
northern Michigan.— U. S. Forest Service. 

fir is completely absent along the northern boundary and in most 
of the western range of the type. Beyond the range of fir, the 
subclimax species, otherwise found in bogs or on burned areas, 
often appear with white spruce as climax. Along the northern 
transition tamarack (Larix laricina) may take an essentially climax 
position as does the black spruce (Pice a mariana), especially on 
high rocky ground. Both are bog species farther south. To the 
west, paper birch and jack pine (Finns banksiana) have climax 
characteristics although both are definitely subclimax nearer the 
center. 

Successions.— Primary succession occurs mainly on bare rock 
or ir lakes. 70 The former is initiated by xerophytic mosses and 



242 TH E STUDY OF PLANT COMMUNITIES * Chapter X 

lichens, which, after mat formation, lead to a heath mat stage. In 
the western part of the range, this is followed by the xerophytic 
jack pine, or black spruce to form a subclimax, but eastward white 
spruce-balsam fir may come in directly. Jack pine also occupies 
extensive areas of sand plains and gravelly soils. 




Fig. 114. Typical stand of jack pine (Pinus banksiana) on sand or gravel 
soils in northern Michigan.— U. S. Forest Service. 

Bog succession is everywhere apparent in the many lakes that 
are filling up. The usual submerged and floating-leaved aquatics 
are commonly followed by sedges and grasses, which may form 
a floating mat upon which a bog-shrub stage develops. This may 
include Chamaedaphne calycidata, Andromeda polifolia, Almis in- 
cana, Ledum groenlandicmn, and Vaccinium spp. Larch is the 
commonest tree to come in after shrubs, followed by black spruce 
or, in less acid bogs, sometimes Thuja occidentalis. Any of these 
species may maintain their dominance for long periods, but they 
can be superseded by climax. 

Secondary succession is usually caused by fire. If the burn is so 
severe that all humus is consumed, leaving bare rock, primary suc- 
cession may be repeated. If a dry peat bog burns, it usually fills 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 



243 



with water again, and succession is reinstated at the aquatic stage. 
More often a burn results in pure stands of paper birch, which 
eventually give way to climax. Wind throw and lumbering of 
climax stands may also result in birch or aspen dominance but 
sometimes are followed directlv by climax species. 




FlG. 115. Aspen stand (Populus tremuloides) at forty-five years of age in 
northern Alinnesota. Its successional nature is clearly shown by the well- 
developed understory of spruce and fir.— U. S. Forest Service. 

Transitions— The, northern border is abrupt, but the line is ir- 
regular depending upon topography. Forest extends far into the 
tundra in sheltered valleys, and tundra appears on the high ridges 
well within the forest area. Timber line seems to be advancing in 
Alaska, retreating in eastern Canada, and remaining more or less 
stable in the interior. The southern transition is to deciduous for- 
est in the east and to grassland in the west. From New England to 
Minnesota, the transition is marked by pure stands of white pine 
(Finns strobns), a subclimax of long duration. In the lake states 
red pine (P. resinosa) and jack pine may also occupy similar po- 
sitions on less favorable sites. Scattered individuals of white pine 
especially tend to persist well into the climax. Through much of 
the eastern transition, spruce, fir, and hardwoods may grow in 



2 44 THE STUDY OF PLANT COMMUNITIES * Chapter X 

mixture or in alternating stands. The transition to grassland in the 
Middle West is marked by aspen (Populus tremuloides) llb in a 
band some fifty miles wide. In spite of fluctuations produced by 
fire, grazing, and drought, the trees persist and, in some instances, 
seem to have advanced into the grassland. In the west, along the 
Rockies, the subalpine Abies lasiocarpa is associated with Pice a 




FlG. 116. Interior of red spruce-Fraser fir forest in the southern Appala- 
chians. Compare with Fig. 113.— U. S. Forest Service. 

glauca, and northward in Alaska there is a merging with the 
northwestern coastal forest. 

Appalachian Extension — -On the higher mountains of the Ap- 
palachian system, the northern conifer forest extends as far south 
as the Great Smoky Aiountains of North Carolina. The growth 
form and associated species are in every way similar to the main 
body of the formation, but, from New Brunswick southward into 
New England, red spruce (Picea rubens) tends increasingly to re- 
place white spruce. Still farther south, Fraser fir (Abies fraseri) 
takes the place of balsam fir so that the dominants in the southern 
Appalachians are ecologically equivalent to those elsewhere in the 
formation but are taxonomically distinct. It seems reasonable to 
consider the Appalachian extension as a distinct association whose 
limits are marked by Picea rubens. A northern and southern facia- 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 245 



tion are suggested by the presence of Abies balsamea and Be tula 
papyrifera in the north but the substitution for them in the south 
of Abies fraseri and Betula lutea. 

The compensating effect of latitude is apparent in the altitudinal 
limits of the association, which increase southward. In the north- 
ern range of red spruce, it may be found anywhere, as is true of 




7. Mixed hardwood forest in Indiana. Large trees are white oaks.— 



U. S. Forest Service. 



fir. Southward, the approximate lower limit of spruce-fir forest on 
Alt. Katahdin is 500 feet; in the White Mountains, about 2,500 
feet; in the Adirondack Mountains, 3,000 feet; in the Catskills, 
3,500 feet; and in the Great Smoky Mountains, almost 5,000 feet. 
Deciduous Forest Formation.-This formation occupies all of 
the eastern United States except southern Florida. Its northern 
transition to conifer forest extends into Canada along a line from 
northern Minnesota to Maine. On the west, forest gives way to 



246 THE STUDY OF PLANT COMMUNITIES • Chapter X 



Grassland as precipitation : evaporation ratios become less favor- 
able. The irregular line of transition runs northward from eastern 
Texas with thirty-five inches of precipitation, to central Minne- 
sota where precipitation falls to twenty-five inches. 

The great extent of the deciduous forest includes soils and to- 
pography of diverse nature and origin. The northern portion was 




FIG. 118. Sugar maples (160-200 years old) in beech-maple forest associa- 
tion, Pennsylvania— U. S. Forest Service. 

glaciated. There are mountains in the east. The great valleys of 
the Mississippi and Ohio Rivers are included as are the Piedmont 
Plateau and coastal plain of the Atlantic and Gulf coasts. Any and 
all kinds of topography as well as soil types are, therefore, repre- 
sented. 

Climate is temperate with distinct summer and winter, and all 
parts are subject to frost, one of the few environmental factors 
that applies throughout. Precipitation varies from sixty inches in 
the southern mountains to less than thirty inches northwestward, 
but it is everywhere fairly well distributed throughout the year. 
The ratio to evaporation is most favorable in the north, the east, 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 247 



and in the mountains and becomes decreasingly favorable ap- 
proaching the transition to prairie. 

The southern Appalachians represent the oldest exposed land 
surface in the region. Here the deciduous forest is more complex 
than in any other part. Practically all of the species found else- 
where in the deciduous forest are represented, as well as several 




FlG. 119. Sugar maple-basswood forest, illustrating the climax for much 
of southern Wisconsin and Minnesota— U. S. Forest Service. 

others. Numerous endemics occur as associates. Most of the trees 
also attain their greatest size here. Away from the mountains, the 
number of species declines, and habitat requirements become of in- 
creasing importance. It is believed that a forest similar to the pres- 
ent one has existed here since Tertiary time. Such evidence is 
taken to mean that the southern Appalachians are a center of origin 
for much of the widespread deciduous forest. The distribution and 
nature of the several associations of the formation give additional 
supporting evidence. In general, with increasing distance from 
the center, the associations are made up of fewer species and yet 
all are bound together or interrelated by several species that range 
throughout. 

Mixed Mesophytic Forest Association.-Thtonghoxit the Ap- 



248 THE STUDY OF PLANT COMMUNITIES * Chapter X 

palachian and Cumberland plateaus, the numerous species of this 
climax grow in varying combination. Fagus grandifolia, Aes cuius 
octandra, Magnolia acuminata, Tilia spp., Liriodendron tidipifera, 
Acer saccharum, Quercus alba, and Tsuga canadensis are the most 
abundant trees, but there are twenty or twenty-five other species, 
any of which may have climax status. The differing sensitivity of 




FlG. 120. Seventy-year-old jack pine with a strong understory of balsam, 
indicating the trend that succession may take in the Lake States region.— 
U. S. Forest Service. 

the species to minor variations in environment result in their oc- 
currence in all kinds of combinations, which may be referred to 
as association-segregates. 32 The best indicators of the association 
are large trees of basswood (Tilia heterophylla) or buckeye (Aes- 
culus octandra). 

The association prevails in the Cumberland and southern Al- 
legheny mountains and in the adjacent Cumberland and Allegheny 
plateaus. 33 Away from this center, there is a progressively increas- 
ing tendency toward restriction to the most favorable habitats. To 
the south, the association is seldom found except in the moist coves 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 



249 



of the high Appalachians. To the west, southwest, and east it is 
found only in ravines and deep valleys. To the northwest, it is 
represented in southern Ohio by a mixed hardwood forest of far 
fewer species. 

Beech-Maple Association— The northward extension of the 
mixed mesophytic forest shows an increasing importance of beech 




FlG. 121. Virgin white pine (Pinus strobus) forest in Connecticut, of the 
type that once occurred over wide areas in the northeast.— U. S. Forest Serv- 
ice. 



(Fagns grandifolia) and sugar maple (Acer saccharum). North of 
the boundary of Wisconsin glaciation, they are the climax species 
over an area west of the Alleghenies from New York to Ohio and 
up into Wisconsin. 31 Virgin forest in Michigan showed beech pre- 
dominating over maple, and associates included red maple (A. 
rubrum), elm (Ulmns america?ia), red oak (Quercus borealis var. 
maxima) and black cherry (Primus serotina).^ The original for- 
ests of southwest Michigan, as reconstructed from land survey 
records, were beech-maple on good sites and oak-hickory on 
coarse soils with poor moisture conditions. 139 This conforms with 
present conditions and can be interpreted as climax and preclimax. 
Maple-Basszvood Association- -The natural range of beech does 



250 THE STUDY OF PLANT COMMUNITIES ' Chapter X 

not extend to the northwest limits of the deciduous formation. 
Beech is replaced in the climax by basswood (Tilia americana), 
beginning in Wisconsin and continuing into Minnesota. 95 Other- 
wise the community is changed very little. 

Hemlock-Hardwoods Association — between the northern con- 
iferous forest and the deciduous forest lies a transitional association 




&.L2 



FlG. 122. Virgin hemlock (Tsuga canadensis) as it once occurred in the 
hemlock-hardwoods association of the northeast and in mountain coves 
southward.— U. S. Forest Service. 



of which hemlock (Tsuga canadensis) is an important and con- 
stant member, together with beech and sugar maple, and, in lesser 
numbers, yellow birch (Betida Intea), white pine, basswood, elm, 
white ash (Fraxinus americana), red oak, and other species. The 
association, which extends from northwestern Minnesota through 
the Lake States to Nova Scotia, has been given various names by 
authorities with different points of view. It is the area throughout 
which occurred the magnificent pine forests of the recent past— 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 



251 



now mostly decimated by fire and lumbering. Where pine was 
dominant, Finns strobus tended to occur on sites with more favor- 
able moisture conditions than the sand plains and ridges occupied 
by P. resinosa. By some 268 these pure stands of pine are considered 
to be climax, but many more ecologists agree that the pines are 
successional species occupying inferior sites for long periods as 




FlG. 123. The oak-chestnut forest that once occupied the lower slopes of 
much of the Appalachian system.— U. S. Forest Service. 

subclimax. That white pine especially carries over into the hard- 
wood climax 180 is undoubtedly true. Its long life and relatively low 
numbers suggest that these trees in the climax should be regarded 
as relicts even though they can maintain their numbers by repro- 
duction under openings appearing in the hardwood canopy. 161 

Postclimax forests of the northern conifers— tamarack, black 
spruce, white cedar (Thuja occidentalis)— occupy the many bogs 
throughout the area. The extensive areas denuded by lumbering 
and fire are today largely occupied by second-growth forests of 
aspen or pine. 

Oak-Chestnut Association.— As the mixed mesophytic forest 
becomes restricted to special habitats to the east and southeast of 
its center, the slopes and uplands are occupied by what was, until 



252 T HE STUDY OF PLANT COMMUNITIES • Chapter X 

recently, oak-chestnut forest. The almost complete elimination 
of chestnut (Castanea dentata) by blight has left practically none 
of the original forest that extended along the mountains from 
southern New England to Georgia. Chestnut oak (Quercus mon- 
tana) and scarlet oak (Q. coccinea) are important species today. 
Tulip poplar, red and white oaks, and some hickory are common 




FlG. 124. Savannah-like transition from deciduous forest to grassland. Bur 
oak predominates in these scrubby clumps of trees on the Anoka sand plain 
northwest of Minneapolis. Note blowout in sand dune in process of restabili- 
zation by Hudsonia— Photo by W. S. Cooper. 

associates. None of this association remains in its original state 
today, for the remnants untouched by extensive lumbering opera- 
tions have been modified by the ravages of chestnut blight. 

Pitch pine (Finns rigida) is the important successional species 
throughout the range, but shortleaf and Virginia pine (P. echinata, 
P. virginiana) are increasingly noteworthy southward. 

In its southern extent, the association is restricted to the moun- 
tains, occupying most of the favorable slopes. Northward it is 
found on progressively lower sites, occurring as far east as Long 
Island. 66 Through the foothills of the mountains, it grades into the 
oak-hickory climax of the bordering Piedmont Plateau. 

Oak-Hickory Association— In all directions from the deciduous 
forest center, except northward along the mountains, precipitation 
decreases and becomes less effective. This results in dominance by 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 



253 



the drought-resistant oak-hickory association, which consequently 
occurs as a fringe around all the margin of the formation except 
toward the north. Oak-hickory climax ranges through much of 
the Piedmont Plateau and the Atlantic and Gulf states coastal plain 
in an arc that widens westward to eastern Texas. North from east- 
ern Oklahoma it may become savannah-like where it grades into 
prairie, but it is more or less continuous to western Minnesota. 




Fig. 125. Typical longleaf pine savannah (Georgia) as maintained by al- 
most annual burning. Note that the only apparent ground cover is wire grass 
(Aristida), which is an important factor in facilitating fire— U. S. Forest 
Service. 

Northwest of the Appalachian center, in unglaciated parts of 
Ohio and Indiana, oak and hickory occur in combination with 
numerous other species, forming .a mixed mesophytic forest cli- 
max, which suggests, by its similarity, that the mixed mesophytic 
association may still be expanding its range. Throughout the asso- 
ciation, various combinations of oak-hickory may occur as pre- 
climax. Postclimax communities of mixed forest may be found 
within the oak-hickory area on sites, such as old flood plains, where 
moisture may be exceptionally favorable. 183 Beech, sugar maple, 
willow oak (Quercus phellos), overcup oak (Q. lyrata), swamp 
chestnut oak (Q. prinus), and shagbark hickories are indicator 
species. 



254 THE STUDY OF PLANT COMMUNITIES * Chapter X 

The dominants of oak-hickory forest are not the same through- 
out its extensive range, but several species occur consistently. 
Quercus alba, Q. bore alls maxima, Q. velutina, Q. stellata, Q. 
marilandica, Carya cordijormis, C. ovata, C. alba, and C. laciniosa 
are species that may be found in the climax anywhere. Other oaks 
and hickories with more restricted ranges may be in association 




FlG. 126. Slash pine savannah after protection from fire for only a few 
years. With continued protection, the pine will soon form a closed stand 
with shrubs and hardwoods forming an understory— U. S. Forest Service. 

and produce local variations. Shingle oak (Q. imbricaria), not so 
important in the east, should be added for the western forest from 
Arkansas and eastern Oklahoma 37 northward. 4 Bur oak (Q. ma- 
crocarpa) is the characteristic tree of the sometimes extensive sa- 
vannah-like transition from forest to grassland, as well as along 
the rivers in the prairie, from Texas to Minnesota. Constant sub- 
ordinate species are sourwood (Oxydendrum arbor eum), dog- 
wood (Cornus florida), black gum (Nyssa sylvatica), and sweet 
gum (Liquidambar styraciflua). 

Because of the amount of abandoned land throughout the east- 
ern and southern range of the association, old field succession is 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 255 

particularly noticeable, and subclimax pine stands are conspicuous 
(see Figs. 108 and 110). Virginia pine (Finns virginiana) predom- 
inates in the northern Piedmont, but southward and westward 
shortleaf (P. echinata) and loblolly pine (P. taeda), usually in pure 
stands, precede the climax in secondary succession on uplands. 
Successional trees in lowlands are sweet gum, tulip poplar (Lirio- 




FlG. 127. Scrubby, open oak forest (mostly 0- catesbaei and Q. cinerea) 
of the southeastern sandhills areas. The open stand and expanses of bare 
white sand are typical.— Photo by H. L. Blomquist. 

dendron tulipifera), sycamore (Platanus occidentalis), river birch 
(Betida nigra), red maple, elms (Ulmus spp.), ash (Fraxinus spp.) 
and hackberry (Celtis spp.). 

Fire and Swamp Subclimaxes of the Coastal Plain— The coastal 
plain, once covered by the sea, extends from New Jersey down 
into Florida and along the Gulf to Texas as a low-lying, relatively 
level area, mostly overlayed with sandy soil. Drainage is poor, re- 
sulting in much swampy ground, but any raised area between 
streams is apt to be very dry for a part of each year. The height 
of the water table during the wet seasons and the amount of fire in 
dry seasons are fundamental factors in determining the nature of 
the vegetation. 

From the pitch pine barrens of New Jersey through loblolly 
pine and longleaf and slash pine in the more southern states, fire 



256 THE STUDY OF PLANT COMMUNITIES ■ Chapter X 

maintains pine dominance, usually in open stands, called savannahs, 
with the highly combustible wire grass (Aristida stricta), for 
ground cover. These stands owe their origin and maintenance to 
their resistance to fire. 53a If protected from fire, they would un- 
questionably be replaced by oak-hickory dominated forest. 269 No 
extensive areas exist where fire has been excluded for more than a 




FlG. 128. Interior of a Florida hammock. 

relatively few years. The successional evidence is clear enough, 
however, and pine in the coastal plain must therefore be classed as 
a fire-maintained subclimax within the oak-hickory association. 

A possible preclimax is the scrub oak-hickory forest found on 
sand dunes near the coast and inland. Turkey oak (Quercus cates- 
baei), margarete oak (Q. margaretta), blue jack (Q. cinerea) and 
black jack oak (Q. marilandica) are dominants. Wire grass may 
be present, but often the sand is bare, glaring white in the sun, 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 257 

except for a few characteristic herbs. These include Euphorbia 
ipecacuanhae, Jatropha stimulosa, Stipulicida setacea, Polygonella 
polygama and Selaginella acanthonota. 272 

Undrained, shallow depressions in savannahs form upland bogs 
or pocosins, sometimes acres in extent, in which evergreen shrubs 
predominate. Ilex glabra, Myrica cerifera, Cyrilla racemiflora, 




Fig. 129. Southern white cedar bog (Chamaecy parts thyoides) in New- 
Jersey. Note well-drained site.— U. S. Forest Service 

Persea borbonia, P. pubescens, Magnolia virginiana, and Gordonia 
lasianthus are representative of the numerous tall shrubs or small 
trees. With them are usually a large number of ericaceous shrubs 
of smaller size. All are commonly overgrown with lianas, of which 
Smilax laurifolia is most abundant. The presence of Pinus rigida 
serotina in the bogs explains its name of pocosin pine. Sphagnum 
is the usual ground cover. 

It is at the margins of pocosins and in wet savannahs in North 
Carolina that the venus fly trap (Dionaea muscipirta) is found, 
sometimes in great abundance but never continuously over an ex- 
tensive area. With it several other insectivorous plants may occur. 
Species of Sarracenia, Drosera, and Pinguicula are common. 

The hammocks of Florida, in contrast with pocosins, are mesic 



258 THE STUDY OF PLANT COMMUNITIES ■ Chapter X 

habitats raised somewhat above surrounding wetter areas. Over 
much of Florida their dominants suggest postclimax to oak-hick- 
ory, but toward the southern tip of the state, the species are more 
and more subtropical. 

Any shallow depression in the flatland of the lower coastal plain 
fills with water. Permanent standing water results in open 




FlG. 130. A4aritime live oak forest (Quercus virginiana) on Smith's Island, 
N. C. Once characteristic of the banks and islands of the south Atlantic and 
Gulf Coast, much of it has been destroyed because of neglect. Note the 
dunes at right, which were once forested.— Photo by C. F. Korstian. 

marshes, 198 sometimes miles in extent, dominated by rushes and 
grasses. If flooding is not continuous, subclimax swamp forests de- 
velop. Bald cypress (Taxodium distichum), which dominates 
where water normally stands most of the year, occupies stream 
and lake margins or entire lakes to the exclusion of other trees. 
Gum swamps are usually flooded only seasonally. Nyssa bi flora 
and Nyssa aquatica are the important species, 114 with ash (Fraxinus 
profunda, F. caroliniana), bald cypress, and red maple as associates. 
The less the flooding, the greater the number of pocosin species 
that may be present. 16 

Still another forest of undrained areas is formed by Chamaecy- 
paris thyoides, which occurs on peat bogs where it apparently be- 
comes established only after fire occurs when the water table is 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 259 



high. Although the stands have subclimax characteristics, there is 
evidence that they may be succeeded by species characteristic of 
pocosins. 38 These valuable trees have been cut so systematically 
that they remain only as small sample stands or in relatively inac- 
cessible places. 145 

Perhaps the most extensive bog and swamp forests still remain- 
ing in virgin condition are to be found in parts of the Dismal 
Swamp in Virginia and in the Okefenokee Swamp in Georgia. 

The plant communities of the banks 155 and islands 197 along the 
coast, as well as a narrow fringe of the coast itself, are distinctive 
enough to merit more discussion than can be given them here. The 
effects of salt spray on vegetation were considered earlier, (p. 102.) 
Live oak (Quercus virginiana) is the most important tree of the 
forested areas, and the associated shrubs include Myrica cerifera, 
Ilex vomitoria, Batodendron arboreum, and several others, mostly 
evergreens. 199 ' 271 Thus, this maritime climax forest is an evergreen 
variant of the oak-hickory association. 

Rocky Mountain Forest Complex.— Changes of environmental 
factors with altitude and the resulting zonation of vegetation on 
mountains have been discussed earlier (see Fig. 66 and related 
text). The great height of the Rocky Mountains provides condi- 
tions for a discontinuous alpine zone on the peaks, a subalpine 
zone, a montane zone, and a zone of woodland forest, which grades 
into the surrounding desert or grassland. These zones are recog- 
nizable by their distinctive climax vegetation over an area extend- 
ing latitudinally from northern Alberta to the southern end of the 
Sierra Madre of northern Mexico and from the Black Hills of 
South Dakota on the east to the eastern foothills of the Sierra 
Nevada and the eastern slopes of the Cascades on the west. 

Climaxes with so great an areal extent would be expected to 
vary somewhat in different parts of their ranges, especially as to 
associated species. The zones are not always continuous, nor are 
they always all present. Near the northern limits of the area, the 
lower zones run out and the upper zones are found at relatively 
low altitudes. Southward all zones are, of course, found at succes- 
sively higher altitudes. Because the prevailing winds are from the 
west and carry with them oceanic climatic influences, the entire 
eastern slope of the Rocky Mountain system has different growing 

1 J J o t> 



260 THE STUDY OF PLANT COMMUNITIES ■ Chapter X 

conditions from those of the west slope and, accordingly, dif- 
ferences in vegetation. Within the system, the individual ranges 
likewise have similar east-west slope differences. North and south 
exposures produce marked irregularities in zonation. Narrow val- 
leys permit the dominants of one zone to extend downward into a 
lower zone, and high dry ridges allow upward, fingerlike projec- 
tions of dominants into continuous higher zones. Cold air drainage 
locally causes marked disruption of the zonal pattern. 

The factors operative in producing and controlling vegetation 
and its zonation in the Rockies have been studied in a number of 
localities just as there have been many local studies of the vegeta- 
tion. An unusually complete review and synthesis of all these in- 
vestigations is available 85 with an extensive bibliography. What 
follows is largely an adaptation from this report. 

Vegetation Zo7ies.—The zonal climaxes may be grouped as fol- 
lows : 

Alpine Zone 

Tundra Climax (discussed earlier— p. 239-240) 
Subalpine Zone 

Engelmann spruce— Subalpine fir climax 
Montane Zone 

Douglas fir climax 

Ponderosa pine climax 
Foothills (Woodland) Zone 

Pinon-Juniper climax 

Oak-Mountain mahogany climax 

Each of these types of vegetation extends over an altitudinai 
range of about two thousand feet, where fully developed, and is a 
true climax. The foothill zones narrow down and disappear en- 
tirely in the north where the upper zones are found at progres- 
sively lower altitudes. 

Near the upper and lower limits of a zone, the characteristic 
species are more and more restricted to special habitats. Upward, 
the climax species do best on south-facing slopes, which are warm- 
er and drier than the general climate. Thus, in its upper transition 
area, each association shows its preclimax relationship to the climax 
of the next higher zone. At its lower limits, the association tends 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 261 



to be restricted to moist and cool sites and extends into the next 
lower zone only in such habitats. It, therefore, holds a postclimax 
relationship to the climax below. Subalpine and alpine zones tend 
to be drier and colder than the zones below, and, consequently, 
preclimax and postclimax relationships may be reversed above the 
montane zone. 




FlG. 131. Virgin Engelmann spruce (Picea engelmamii), with some alpine 
fir (Abies lasiocarpa) of the subalpine zone in Colorado.— U. S. Forest Service. 

Subalpine Spruce-Fir Climax— From timber line downward for 
about two thousand feet, the climax forest is made up largely of 
Engelmann spruce (Picea engelwmnnii) and subalpine fir (Abies 
lasiocarpa), which grow in dense stands. The spruce is the larger 
and more abundant tree. In Arizona, New Mexico, and southward, 
Abies lasiocarpa var. arizonica is as important as A. lasiocarpa. In 
Montana and northern Idaho, mountain hemlock (Tsuga merten- 
siana) is often found in the zone, and still farther north, approach- 
ing the merging with northern conifer forest, Picea glauca and 
A. lasiocarpa may grow in association. 

Subordinate species vary far more than do the dominants. On 
the relatively dry eastern slope of the central Rockies, ground 



262 THE STUDY OF PLANT COMMUNITIES • Chapter X 

cover is sparse and made up largely of dwarf Vacciniums, while 
the moister west slope has an abundance of bryophytes and herbs. 
Northward, the bryophytes increase until they practically cover 
the ground, and the vascular plants, both herbs and shrubs, also 

increase. 

The most conspicuous succession in the subalpine zone follows 
fire and may result in subclimax stands of lodgepole pine (Finns 




FIG. 132. Dense aspen stand (Populus tremidoides) that came in after fire 
in the subalpine zone in New Mexico. Spruce reproduction underneath.— 
U. S. Forest Service. 

contorta var. murrayana), aspen (Populus tremidoides), or Doug- 
las fir (Pseudotsuga taxifolia). Progression to climax is extremely 
slow. Lodgepole pine is absent in the southern Rockies, but else- 
where aspen is favored over the pine on moist sites, and after light 
fires it has an advantage, probably because of its ability to regen- 
erate from sprouts. Near timber line, burned areas are revegetated 
directly by climax. 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 



263 



The transition from subalpine forest to alpine tundra is usually 
gradual with a thinning out of trees, which here commonly have 
the dwarfed and distorted form known as Krummholz. Character- 
istic of timber line are several trees that cannot survive in the 
tundra above and cannot compete with climax species below, 
where they are only found on dry and windswept ridges. Foxtail 




FlG. 133. Foxtail pine (Pinas aristata) Krummholz at timber line of the 
subalpine zone in Colorado— U. S. Forest Service. 

pine (Pinus aristata) occupies this position in the southern Rockies, 
limber pine (P. flexilis) in the central Rockies, whitebark pine (P. 
albicaulis), and alpine larch (Larix lyallii) in the northern Rockies, 
except in the far north where lodgepole pine occurs at timber line. 
Douglas Fir Climax.— Below the subalpine zone, Douglas fir 
(Pseudotsuga taxifolia) is the climax dominant, growing in such 
dense stands that subordinate species are negligible. As in the sub- 
alpine zone, climax associates differ in the north and south. In the 
central and southern Rockies, white fir (Abies concolor) and blue 
spruce (Picea pungens) are found in relatively small numbers and 
mostly on moist sites. In the north, grand fir (Abies grandis) is an 
associate west of the continental divide and principally on west 
slopes. East of the divide, Picea glauca of the northern conifer 



264 THE STUDY OF PLANT COMMUNITIES ■ Chapter X 




FlG. 134. Montane zone climax forest of Douglas fir (Pseudotsuga taxi- 
folia) and white fir (Abies concolor) in Colorado.— U. S. Forest Service. 



forest shares dominance with Douglas fir and extends southward 
through the montane zone as far as the Black Hills. 

Dry, exposed ridges in both the montane and subalpine zones 
support open stands of pine, including several species characteris- 
tic of timber line. P'mus strobiformis is important in the south. P. 
aristata occurs in northern Arizona and southern Utah and Colo- 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 265 

rado, while P flexilis is more common northward to where P. albi- 
caidis takes over in the northern Rockies. 

Fire in the Douglas fir climax results in the establishment of 
lodgepole pine or aspen stands, which bear the same relationships 
here as in the subalpine zone. 

Ponderosa Pine Climax.— Below the Douglas fir, is a belt in 
which Pinus ponderosa or a close relative forms a relatively open 
climax forest that becomes savannah-like with decreasing altitude. 
The widely spaced trees form little shade so that the ground cover 
is made up of grasses, among which numerous species of Festuca. 
Agropyron, Poa, and Muhlenbergia are important. Between the 
zone of Douglas fir and the drier, lower altitudes with pure stands 
of ponderosa pine is a fairly broad transition where the two trees 
may share dominance. 

Although the climax is termed ponderosa pine, the species is 
dominant only in the northern Rockies to the west of the con- 
tinental divide. Elsewhere it is replaced by or in association with 
closely related varieties whose ecological characteristics are sim- 
ilar. Pinus ponderosa var. scopidorum is the important tree on the 
east slope in the north and throughout the zone southward. In the 




Fig. 135. Subclimax stand of lodgepole pine in Montana.— U. S. Forest 
Service. 



266 THE S TUDY OF PLANT COMMUNITIES • Chapter X 

southern Rockies, the substitutes are P ponder osa var. arizonica, 
P leiophylla, and P. latifolia. 

The only exceptions to ponderosa pine dominance are found 
along streams and drainage lines where narrow-leaved cottonwood 




FIG 136 Climax forest of ponderosa pine (Pinus ponderosa) in typical 
open stand. Montane zone, Arizona.-L7. S. Forest Service. 

(Populus angustifolia), the commonest tree, forms postclimax 
stands with P. acuminata and P sargentii in association. Aspen 
(P tremuloides), in glades, and box elder may also occur frequent- 
ly on these moist sites. Although fires are common, in dry summers, 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 267 



favored by the grasses of the forest floor, they are rarely severe 
enough to kill the fire-resistant older trees. That pine seedlings are 
destroyed is indicated by the even-aged groups of saplings, each 
of which can be related to a series of summers that were free of 
fire. Severe fires in the upper part of the ponderosa pine zone may 
be followed by stands of lodgepole pine. Lumbering and over- 
grazing often result in the development of a dense scrub made up 
of species from the oak-mountain mahogany zone. 




FlG. 137. Characteristic open stand of piiion- juniper, and the transition 
from sagebrush desert —U. S. Forest Service. 

Vinon-Juniper Climax- This open forest of widely spaced, small 
trees (ten to thirty feet) forms the lowest coniferous zone in the 
Rockies and, on many of the low ranges of the Great Basin, repre- 
sents the only zone present. It is, therefore, typical of the inter- 
mountain region as well as forming a distinct zone in the southern 
Rockies. Although it is fairly constant in appearance and charac- 
teristics over its wide range and extensive acreage, several species 
with restricted ranges are involved. The junipers include Juniperus 
scopulorwn, J. monospermy J. utahensis, J. occidentalism J. pachy- 
phloea, J. mexicana, and others, and the pinons, or nut pines, are 
varieties of Finns cembroides (edulis, monophylla, parry ana). 

The type extends from northern Mexico along the west slope 
of the Rockies to the Snake River in Idaho, beyond which it con- 
tinues into southern Alberta with piiion replaced by limber pine. 
Along the east slope, its northern limit is in Colorado although it 
is represented northward through Wyoming by Juniperus scop- 



268 THE STUDY OF PLANT COMMUNITIES • Chapter X 

ulorum, often with sagebrush in association. Pinon-juniper is com- 
pletely lacking in Sierran zonation, which goes directly from 
Artemisia and Furshia to Pinus ponder os a. However, almost with- 
out exception, it occurs on every westernmost range and mountain 
of the Great Basin, often lying just across a valley from the base 
of the Sierra. 



■ ■-■•,:.•.-.:.-.•.-. ;v(j(.mv. : j*;K 



mmis&m 




FlG. 138. An example of the scrub oak-mountain mahogany zone in the 
foothills near Colorado Springs, Colo. Quercus gcmibellii predominates here 
with Cercocarpus parviflorus and Rosa arkansas as associates. Although the 
scrub is sometimes taller, its open, irregular distribution is typical.— P/joto by 
R. B. Livingston. 

The openings between trees support a grass cover (Bouteloua, 
Stipa, Agropyron, Poa) and numerous other herbs, together with 
a few shrubs (Ceanothzis, Cercocarpus, Purshia, Coivania, Ar- 
temisia, Opuntia) characteristic of the next lower zone. Over- 
grazing or fire may result in the temporary dominance of these 
shrubs. 

Oak-Mountain Mahogany Climax .—The transition from the 
conifer forest of the lower slopes to the treeless plains and pla- 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 269 

teaus may be marked by a zone of broad-leaved scrub. The zone is 
widest and best developed in the southern Rockies, narrows and 
becomes discontinuous in the central Rockies, and fades out en- 
tirely farther north. The components of the community vary, but 
oaks (Quercus gambellii, Q. gimnisoni, Q. undulata, Q. fendleri, 
Q. emoryi, and others) are the largest (up to thirty-five feet) and 
most conspicuous dominants in the south. North of the latitude 
of Denver, Colorado, the oaks are spottily represented, and moun- 
tain mahogany (Cercocarpus parviflorus, C. ledif otitis, etc.) is 
dominant. Other important associates include Rhus triloba, Pur- 
shia tridentata, Fallugia paradoxa, Amelanchier spp., and Sym- 
phoricarpos spp., any of which may assume local dominance. The 
vegetation does not form a continuous cover but occurs in dense 
clumps, or even as individual plants, separated by areas of grass- 
land or desert vegetation. 

The Black Hills — Although they are now isolated, the Black 
Hills are geologically and ecologically related to the Rockies. 
They deserve especial mention because of their mixture of eastern, 
western, and northern species. Because the altitude is only a little 
over seven thousand feet, the montane zone is chiefly represented. 
There is no Douglas fir present. Instead, Picea glauca, which ex- 
tends southward from Canada along the east slope of the Rockies 
as an associate of Douglas fir, here is the only dominant on the high 
slopes at the southern limit of its range. Paper birch from the 
northern conifer forest is also present. Ponderosa pine dominates 
most of the lower slopes, which include most of the area, and 
lodgepole and limber pine in small numbers are additional repre- 
sentatives from the Rockies. Species from the eastern deciduous 
forest are ash, hackberry, elm, birch, and bur oak, of which only 
the last attains any size. The scrubby appearance of the commu- 
nity, as well as its distribution along the lower margin of the coni- 
fer forest, suggests the oak-mahogany zone of the Rockies. 85 

Sierra Nevada Forest Complex.— The area here considered in- 
cludes the southern portion of the Cascade Mountains and the 
Sierra Nevada, which together extend from Oregon southward 
along the eastern boundary of California as the innermost ranges 
of the coastal mountain system. The long west slope of the Sierra 
rises gradually to altitudes of 14,000 feet and more, but the east 



270 THE STUDY OF PLANT COMMUNITIES ■ Chapter X 

slope drops abruptly to the floor of the Great Basin, which lies at 
about 4,000 feet. At the base of the west slope, there are only ten 
to fifteen inches of rainfall and a long, unbroken, dry summer sea- 
son. Upward precipitation increases, temperatures decrease, the 
dry summer season shortens, and a larger proportion of precipita- 
tion falls as snow. 




FlG. 139. Interior of the red fir (Abies magnified) forest that occupies 
most of the subalpine zone of the Sierra Nevada. 



Because the general north-south axis of the range lies across the 
path of the prevailing westerly winds, climatic conditions for the 
region as a whole are influenced by them and east slopes are much 
drier than west slopes. Winter precipitation makes up 80-85 per- 
cent of the total, and at high elevations, most of the moisture falls 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 271 



as snow (thirty-five to seventy feet in the subalpine zone). The 
greatest total precipitation occurs in the middle slopes, between 
5,000 and 7,000 feet, which support the luxuriant mixed coniferous 
forest of the montane zone. The subalpine zone coincides with the 
altitudes of greatest snowfall, where precipitation equals about 
forty to fifty inches a year. 




Fig. 140. Lodgepole pine at 8,800 feet in the subalpine zone, Carson Range 
of the Sierra Nevada.— Photo by courtesy of the Agricultural Extension Serv- 
ice, Univ. of Nevada. 



Subalpine Zo?ie.— This zone extends through an altitudinal range 
of little more than 1,000 feet, its limits, varying with latitude, be- 
ing between 6,500 and 9,500 feet. The climate may be described 
as cool, winter-wet, summer-dry, with a short growing season. 

Red fir (Abies magnified) is the important climax species, grow- 
ing in dense stands and making up 80-90 percent of the forest. 189 
Of the associated species, none is an important component of the 
climax. Although western white pine (Pima monticola) is con- 



272 THE STUDY OF PLANT COMMUNITIES * Chapter X 

stantly present in small numbers, it is only a minor constituent. 
Lodgepole pine (Finns contorta) is often present, especially in wet 
meadows, but its role is primarily successional. Mountain hemlock 
(Tsnga mertensiana) and white fir (Abies concolor) occur in an 
extremely irregular fashion. Of the shrubs, which are few, Kibes 
viscosissimum and Symphoricarpos rotimdifolins are the most 




FlG. 141. Virgin forest in the Sierran montane zone of California, in this 
instance made up of sugar pine (Pinus lambertiana), ponderosa pine (P. pon- 
der -osa), and white fir (Abies concolor).— U. S. Forest Service. 



abundant and most constantly represented. The herb flora is also 
sparse. Constant species are Chrysopsis breweri, Fedicnlaris semi- 
bar bata, Gayophytnm ramosissimum, Firola picta and Monardella 
odoratissima. The yellow-green lichen (Evernia vidpina) is con- 
spicuously present on the trees throughout the zone. 

Although the altitudes in the Sierra are often greater than those 
of the Rockies, conditions are severe and timber line is lower, 
varying from about 7,000 feet in the north to some 10,000 feet in 
southern California. The characteristic trees are Finns albicaidis, 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 



273 



P. flexilis, and P. balfoiiriaiia. 2 * 1 On exposed, bare, rocky slopes, 
Juniperus occidentalis is common at timber line and, especially on 
the west slope, at much lower altitudes. 

The upper margin of the red fir forest does not commonly ex- 
tend to timber line but, instead, grades into a relatively narrow 




Fig. 142. Giant redwoods (Sequoia gigantea) of the Calaveras grove, Se- 
quoia, Calif— U. S. Forest Service. 



274 TH E STUDY OF PLANT COMMUNITIES * Chapter X 

zone of Finns contorta-Tsuga mertensiana dominance. Although 
P. contort a is successional to Abies magnified at lower altitudes, it, 
with hemlock, has climax characteristics in this zone. This would 
suggest three f aciations for the subalpine zone, namely, white bark 
pine or timber line f aciation, lodgepole pine-hemlock through the 
upper part of the zone, and red fir, which, from the lower margin 
upward, occupies the major part of the zone. 

Montane Zone —The altitudinal range of this zone lies between 
about 2,000-6,000 feet in the Cascades, 4,000-7,000 feet in the cen- 
tral Sierra, and 5,000-8,000 feet or more in the south. Five or six 
principal species have climax characteristics and may appear in 
any combination at any altitude. However, the upper and lower 
parts of the zone tend to have consistent vegetational differ- 
ences. 69 ' 144 White fir (Abies concolor) is usually the important 
dominant in the upper part of the zone, sometimes in pure stands, 
and decreases markedly at lower elevations. Lower down, incense 
cedar (Libocedrus decurrens), predominating on the most favor- 
able sites, sugar pine (P. lambertiana) , Jeffrey pine (P. jeffreyi), 
ponderosa pine, and Douglas fir are the species of importance. 
Douglas fir is more abundant in the north than in the south. 151 
Sugar pine and Jeffrey pine are more conspicuous than ponderosa 
pine at the upper altitudes, a logical arrangement since the latter 
is the most drought-resistant of the major species. 

Fire subclimaxes are formed by Pinns attenuata, P. muricata, and 
P. radiata in different parts of the range 268 although preceded by 
dense chaparral communities of species of Arctostaphylos, Ceano- 
thus, Rhamnus, etc., which may last for years. 

Included in the montane zone, on the western slope, are the for- 
ests of giant redwood (Sequoia gigantea), at altitudes of 4,500- 
6,000 feet. Once widespread, they now occur only southward 
from the latitude of San Francisco in a disrupted zone. Their pres- 
ent best development is in the central Sierra where they reproduce 
but do not spread. Sugar pine, ponderosa pine, and incense cedar 
are common associates. 

Foothills (Woodland) Zone —As in the Rockies, the vegetation 
of the lower slopes and foothills is made up of coniferous and 
scrub associations, but they are not as sharply separated here. The 
zone ranges between about 1,500 and 4,000 feet. In the upper part, 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 



275 



digger pine (P. sabiniana) and blue oak (Q. donglasii) are the dom- 
inants, forming typical open, or woodland, stands. The lower alti- 
tudes are characteristically covered with close-growing, ever- 
green scrub, or chaparral, in which Ceanothns spp. and Arctosta- 
phylos spp. predominate. Common associates are several scrub 
oaks (Q. r wislize7?i, Q. chrysolepis, Q. dumosa) y Aesculns calif or- 




FlG. 143. Characteristic open oak woodland of the Sierran foothills. Se- 
quoia National Forest, Calif.— U. S. Forest Service. 

nica, Rhamnus calif ornica, and numerous other species are repre- 
sented. 

East Slope.— Although the same zones are present on both the 
west and east slopes, many of the generalizations made above must 
be qualified for the east slope because of its less favorable condi- 
tions. The red fir forest occurs only in restricted areas on the east 
slope, such as in the Carson Range east of Lake Tahoe and locally 
in the northern Sierra. The subalpine zone is represented, there- 



276 THE STUDY OF PLANT COMMUNITIES • Chapter X 

fore, largely by the timber-line pines and patches of lodgepole 
pine. The montane and foothill zones extend to high altitudes, and 
the vegetation is poorly developed. Finns jeffreyi is the important 
species of the montane zone in which the open forest has little 
resemblance to that of the west slope. The woodland forest is 
practically absent. Although pifion- juniper occurs as a major zone 
on the next ranges across the valley, it is not found on the east 
slope of the Sierra except where an occasional high spur extends 
eastward. The scrub zone is sometimes made up of oak and moun- 
tain mahogany as in the Rockies, but is more often represented by 
species from the desert below (Artemisia, Purshia, Chrysotham- 
nus, etc.), which, especially on areas of disturbance or fire, may be 
found high up in the montane zone as well as on the lower slopes. 

Pacific Conifer Forest.— This area parallels the coast from north- 
ern tree limits in Alaska southward to central California. Coastal 
mountain ranges with varying altitudes are included throughout 
its length. The climate, tempered by the Pacific Ocean, is mild 
and without extremes. Although Alaskan winters are cold, subzero 
temperatures are uncommon along the coast. Southward, tempera- 
tures are progressively less severe until, in Oregon and California, 
frosts are rare. Precipitation is adequate to heavy (30 to 150 
inches), and the humidity is always high, producing an extremely 
favorable P/E ratio. The southern part is winter-wet with no 
snow; here fog compensates for the summer drought. Northward, 
the summer dry season shortens until, in Alaska, there is none. 
Northward, too, there is an increase in the proportion of precipi- 
tation falling as snow. In the higher mountains, it may be entirely 
snow with falls as great as sixty to sixty-five feet a year. 

The coastal forest is primarily montane in character, although 
ranging from sea level to altitudes of 5,000 feet. Only in the United 
States, as in the Cascades, and for a short distance into British 
Columbia does it include a subalpine forest. Here it is well de- 
veloped, but the dominants are derived from the Rockies (Abies 
lasiocarpa), the Sierra (Tsnga mertensiana) , as well as the coastal 
forest (Abies amabilis, A. nobilis). Northward, the zone becomes 
fragmentary or disappears entirely. 

Species of the coastal forest are most fully represented in the 
general vicinity of Puget Sound, and the best development of the 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 277 



forest is indicated by the luxuriant vegetation of the Olympic 
Peninsula. Here the ranges of all the major species overlap and 
most of the trees attain their maximum size. The climax dominants 




FlG. 144. Douglas fir (Pseudotsnga) and western arborvitae (Thuja pli- 
cata) in the coastal montane forest. Snoqualmie National Forest, Wash.— 
U. S. Forest Service. 



are western hemlock (Tsuga heterophylla), arborvitae (Thuja pli- 
cata), and grand fir (Abies grandis). Subordinate broad-leaved spe- 
cies and many herbaceous species are associated in abundance. 133 
Douglas fir, which reaches its greatest size here, is the most abun- 
dant and widespread species, but it occupies drier sites, is relatively 



278 THE STUDY OF PLANT COMMUNITIES • Chapter X 




FlG. 145. Pacific coastal forest in California showing redwood (Sequoia 
sempervirens) predominating and Douglas fir in association. Conspicuous 
subordinate species are Lithocarpus densiflora, Rhododendron californicimi, 
Gaidtheria shallon, Vaccinium spp., Polystichum sp.— U. S. Forest Service. 



intolerant of shade, and is the major dominant after fire. It is, 
therefore, subclimax in nature. 124 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 



279 



To the north of the Puget Sound region, Sitka spruce (Ficea 
sitchensis) becomes increasingly important as the forest becomes 
more closely associated with coastal conditions. Although it has 
subclimax characteristics near its southern limits, Sitka spruce be- 
comes, with Tsuga heterophylla and T. mertensiana, an important 
climax dominant in the northern extension of the forest. 75 At its 
extreme limit in Alaska, the coastal and boreal forests merge and 
both P. sitchensis and P. glanca are found at timber line advancing 
into the tundra. 110 

Southward, the important species of the Puget Sound center 




&«s? 



FIG. 146. Successional community of western white pine (Pinus monti- 
cola) and western larch (Larix occidentalis) in Idaho. Understory of Thuja 
plicata and Tsuga heterophylla —U . S. Forest Service. 

extend down the low coastal mountains into Oregon with Port 
Orford cedar (Chamaecy parts laivsoniana) as an added climax 
species and Douglas fir of relatively greater importance. 195 Along 
the coast, however, Sitka spruce is replaced by redwood (Sequoia 



280 THE STUDY OF PLANT COMMUNITIES ' Chapter X 



sempervirens), which, in pure stands, closely follows the limits of 
the fog belt 71 to below San Francisco and fades out southward. 

If the ranges of the principal species of the Puget Sound area 
are mapped, they appear in the form of a peninsula extending east- 
ward across northern Washington and southern British Columbia 
and expanding north and south on the west slope of the Rockies. 85 
The coastal dominants extending into this area are Tsuga hetero- 
phylla, Thuja plicata, and Pseudotsuga, which occupy a zonal posi- 
tion between the normal Douglas fir and spruce-fir zones of the 
Rockies. Although the importance of hemlock and arborvitae de- 
creases eastward and Douglas fir increases, the zone remains dis- 
tinctive largely because of the species peculiar to the forests de- 
veloping after fire. The two principal successional trees are western 
larch (Larix occidejitalis), which is endemic to the peninsula area, 
and western white pine (Finns monticola) , which grows more 
abundantly here than anywhere else. The presence of Abies 
grandis in association with these species indicates the coastal af- 
finities. 

Daubenmire 85 points out that this eastward overflow of coastal 
species marks an area in which steady winds blow inland from the 
coast, following a well-developed storm track, and thereby extend 
the coastal climate far inland. This theory is supported by the 
superior development of the coastal species in the peninsula on 
westward slopes at intermediate altitudes and their occurrence in 
the Rockies only in the storm path and west of the continental 
divide. 

Broad-Sclerophyll Formation.— As the name indicates, major 
species in both associations of this formation have thick, hard, 
evergreen leaves. One climax is dominated by trees and termed 
broad-sclerophyll forest. The other is a shrub climax called chap- 
arral. Both reach their best development on the coastal ranges of 
southern California, but their ranges extend from southern Oregon 
southward through the coast mountains, as well as through the 
Sierra Nevada foothills, into Lower California. Several of the 
species are found on the east slopes of the Sierra, and some appear 
in the desert woodland zone on the lower slopes of the Rocky 
Mountains. 

The climate of the sclerophyll region is mild-temperate to sub- 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 



281 



tropical with long, dry summers and heavy winter rainfall. Total 
precipitation is not less than ten or more than thirty inches, and, 
of this amount, no more than 20 percent falls in summer. In this 
area, desert vegetation appears where precipitation is less than ten 
inches, and, if it is over thirty inches, conifer forest is dominant. 72 
The two climaxes may be found in alternating patches in almost 
any part of their more or less coinciding ranges. However, chap- 
arral occupies the greatest area and is climax in the south where it 




Fig. 147. Broad sclerophyll forest (Quercus agrifolia, Arubutus, etc.) on 
north-facing slope (foreground and right). Chaparral on south-facing slope 
(left). Santa Lucia Mountains, Calif.— Photo by W. S. Cooper. 

grades into desert, and sclerophyll forest is climax in the north and 
at the margin of montane conifer forest where its variations may 
be a part of the woodland zone. Where found together, the two 
communities bear no successional relationship to each other. The 
forest consistently appears on north slopes and the better sites, 
chaparral on south slopes and drier sites. The forest is postclimax 
in the south, and chaparral is preclimax in the north. 

Sclerophyll Forest— The important evergreen forest trees are 
Quercus agrifolia, Q. chrysolepis, Q. ivislizeni, Lithocarpus densi- 
flora, Umbellularia californica, Arbutus menziesii, Castanopsis 
chrysophylla, and Myrica californica. Several deciduous trees are 
almost as characteristic, as are a number of shrub and herb associ- 
ates. The dominants may occur in various combinations related to 
altitude and exposure. 



282 THE S TUDY OF PLANT COMMUNITIES * Chapter X 

Chaparral- -This community extends its dominance over a wide 
area and a diversity of habitats, and its composition is proportion- 
ately diverse. It includes at least forty species of evergreen shrubs 
with varying degrees of dominance and importance, which may 
occur in many combinations but which invariably form low, dense 





FIG. 148. Chaparral in the Santa Lucia Mountains, Calif. Smooth cover at 
top, mostly Adenostoma. Light-colored shrubs in shallow ravine at left, Arc- 
tostaphylos glauca. Grades into broad sclerophyll forest in deep ravine at 
right.— Photo by W. S. Cooper. 

thickets. The most important and constant species is chamiso 
(Adenostoma jasciciilatnm). The numerous species of manzanita 
(Arctostaphylos) are scarcely less characteristic, and of these A. 
tomentosa is the widest ranging. Others with high constancy are 
Heteromeles arbutifolia, Ceanothus cimeatus (9 other spp.), Quer- 
cus dumosa, and Cercocarpus betulaejormis. 

Fires.— The long, dry summers and the nature of sclerophyllous 
vegetation make frequent fires the rule. A study in the Santa 
Monica mountains showed that chaparral stems were mostly about 
twenty-five years of age, and a stand without fire for fifty years 
was considered old. An ordinary fire causes chaparral to sprout 
profusely, and then, come back to normal within ten years. 12 Fire 
usually favors the extension of chaparral at the expense of sclero- 
phyll forest. Too frequent fires, however, may cause the death of 
chaparral and its replacement by grassland. Undoubtedly, the orig- 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 283 

inal extent of sclerophyll dominance has been much reduced by 
fire, since, once they are destroyed, the return of the sclerophyll 
species is long delayed. 

Desert Formations.— The major area of the North American 
desert extends from southeastern Oregon and southern Idaho 
southward through the Great Basin, including most of Nevada 
and Utah except high elevations, continues southward into south- 
ern California and western Arizona, down most of the peninsula 
of Lower California and, on the mainland, through Sonora as far 
south as the Yaqui River. The highlands of eastern Arizona and 
western New Mexico interrupt the continuity of desert, but from 
south-central New Mexico, there is almost continuous desert 
through eastern Chihuahua and most of Coahuila in Mexico. 238 

In spite of the great extent of this area, there are certain environ- 
mental features characteristic throughout. Precipitation is low and 
erratic; temperatures of air and soil are extremely high by day and 
drop abruptly at night; atmospheric humidity is usually low, and 
bright sunny days are the rule. These factors serve to explain why 
predominating plants are those that can survive desiccation with- 
out injury or that store water in their succulent tissues. This is not 
to imply that desert vegetation is uniformly similar throughout. 
Climatic differences, associated with latitude and altitude, are ac- 
companied by differences in species and life forms. Locally, the 
physical differences in topography, exposure, and soils produce 
distinct vegetational variations just as in moister climates. Finally, 
there are numerous undrained depressions into which the water of 
winter rains flows and, upon evaporation, deposits the silts and 
clays it has transported as well as salts of various kinds. The re- 
sulting mud flats (playas) in themselves constitute a special habitat 
with associated species, but the nature and concentration of salts 
in the soil is even more effective in controlling the communities 
there. 

Four desert areas are distinguishable on the basis of regional en- 
vironments and, likewise, by the nature and importance of the 
major dominants : 238 namely, the Great Basin, Mojave, Sonoran, 
and Chihuahua deserts. In each of these areas there are communi- 
ties that occur with minor variations wherever conditions are not 
extreme. These may be recognized as climax. Other communities, 



284 THE STUDY OF PLANT COMMUNITIES * Chapter X 

which seem equally permanent, are found only in special habitats. 
Succession, as ordinarily conceived, is almost nonexistent since re- 
action of the vegetation is negligible. Unless there is marked dis- 
turbance, most communities remain indefinitely unchanged and 
dominant in their special habitats. It seems best, therefore, to pre- 
sent the characteristics distinguishing the four deserts and to indi- 
cate the dominant vegetation in different habitats with less than 
the usual emphasis upon climax or its relationships. What follows 




,-&:■ .■,■*. . '■ . » ■■■■■>■• -■■■■ ■■'■'■'' v- :V * %■ 



FIG. 149. Sagebrush desert (Arte?nisia tridentata) northwest of Reno, 
Nev.— Photo by W.D. Billings. 



is adapted almost entirely from Shreve's 238 excellent summary of 
desert vegetation except for the distinction made here between 
Sagebrush and Desert Scrub Formations. 

Sagebrush Formation- Great Basin Desert— There is physio- 
graphic, climatologic, and vegetational unity throughout all the 
Great Basin area north of southern Nevada and southern Utah. 
The wide valley floors, lying at about 4,000 feet, are interrupted 
by numerous ridges, often rising to more than 8,000 feet, and de- 
pressions of the playa type. The meager rainfall, four to eight 
inches, is heaviest in spring but may come in any season. Tempera- 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 285 



tures are not as high as farther south, and frosts are common. The 
combination of lower temperatures, lower evaporation rates, and 
better distribution of rainfall explains the use of the term, "semi- 
desert;' for the area. Likewise distinctive is the growth form of the 
dominants, made up largely of shrubby chenopods and composites, 
which further supports the desirability of its recognition as a for- 
mation distinct from the scrub of the southern desert. 




-"I 




'V-S4^^^^*^-.-^- : i^ / >^&.^Zte./.l»f*.A*:~^><-x~*-S,J* f ~M.^*«r*X 



Fig. 150. Typical dry desert expanse with shadscale (Atriplex conferti- 
folia) dominance. Mineral County, Nev. Characteristic gravelly desert pave- 
ment shows here .— Photo by W. D. Billings. 

The two major communities are simple, with few dominants in 
each, and often extend uninterrupted for miles. The sagebrush as- 
sociation is dominated by Artemisia tridentata (common sage- 
brush), which is climax in the northern portion of the Great Basin 
or at relatively high altitudes. The shadscale association, with shad- 
scale (Atriplex confertifolia) and bud sage ( Artemisia spines c ens) 
as its important species, ranges through the south and at low alti- 
tudes. In its northern and eastern distribution, shadscale is found 
on heavy lowland soils containing some alkali, but, to the south, it 
is climax on gray desert soils with a shallow carbonate layer and 
regardless of salts. Sagebrush tends to occur on brown soil, either 



286 THE STUDY OF PLANT COMMUNITIES * Chapter X 

sandy or clayey, with the carbonate layer at a deeper level and 
with a minimum concentration of salts. 21 

The controlling effect of salts on community structure has been 
amply demonstrated for different parts of the area. 21 ' 98 > 134 Zonal 
patterns around playa lakes are the same everywhere (see Fig. 86). 
Where flooding is periodic and salt content excessive, vegetation 



~~i 




FlG. 151. Creosote bush (Larrea divaricata) with Franseria dumosa in as- 
sociation as is typical of much of the Mojave Desert. Numerous desert an- 
nuals can be seen.— Photo by W. D. Billings. 

is absent or dominated by glasswort (Salic ornia spp.) or iodine 
bush (Allenrolfea occidentalis). With somewhat less salt, shadscale 
and greasewood (Sarcobatus vermiculatus) or red sage (Kochia 
vestita) are dominant. Away from the playas on soils with a mini- 
mum of salts, sagebrush may be the major species. 

Many other species occur, of course. They are mostly semi- 
shrubs with the same growth form. There are numerous species of 
Atriplex and Artemisia. Chrysothamnus puberulus, Grayia spin- 
osa, Coleogyne ramosissima, Eurotia lanata, Purshia tridentata, and 
others are variously associated with the major species or sometimes 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 287 



assume dominance under local special conditions. Several species 
of Ephedra are characteristic. 

Desert Scrub Formation.- Mojave Desert— This, the smallest of 
the desert units, lies almost entirely in California below the south- 
ern end of the Sierra Nevada. Physiographic conditions are similar 



r 






FlG. 152. Joshua tree (Yucca brevifolia), characteristic of the northern 
Sonoran desert, particularly in the transition from creosote bush dominance 
to shadscale of the sagebrush formation.— Courtesy Univ. of Nevada Agri- 
cultural Extension Service. 

to the Great Basin but elevations are generally lower (1,000-4,000 
feet). The irregular precipitation of less than five inches is dis- 
tributed over fall, winter, and spring. Summers are very hot and 
dry. The area includes Death Valley, with a minimum elevation of 
480 feet below sea level. Its infrequent maximum rainfall is two 



288 THE STUDY OF PLANT COMMUNITIES * Chapter X 

inches, and official records show at least one period when tempera- 
tures held above 100° E for 538 days. 192 

Conditions are not too different from those of the Great Basin 
although somewhat more extreme. This is borne out by the vege- 
tation, which includes many of the same species, their distribution 
controlled here, too, by soil texture and salt concentration. Certain 
character species do stand out, however, and this justifies the vege- 
tational distinction from the Great Basin. At the upper elevations 
(3,000-4,000 feet) and in the transition from sagebrush, with maxi- 
mum precipitation (near five inches), Joshua tree (Yucca brevi- 
folia) is conspicuous. With decreasing altitude and precipitation, 
creosote bush (Larrea divaricata), with burro weed (Franseria 
dumosa) in association, becomes the major dominant. This com- 
munity occupies 70 percent of the total area of the iMojave Desert. 

Sonoran Desert.— The lowlands around the Gulf of California 
in Mexico and Lower California, which lie chiefly below 2,000 
feet, constitute the Sonoran Desert. Much of the area is made up 
of dunes and sand plains. Precipitation is extremely uncertain, not 
exceeding two to four inches in the vicinity of the Gulf, although 
increasing some with altitude. Its effectiveness is counteracted by 
the extremely high temperatures. 237 




Fig. 153. Sahuaro (Carnegiea gigantea), the giant of the columnar cacti 
that characterize the uplands of the Sonoran Desert.— U. S. Forest Service. 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 289 



The low plains are dominated by Larrea-Franseria, with various 
associates, as in the Mojave Desert. Because drainage here is not 
internal, margins of streambeds support a distinctive mixed com- 




FlG. 154. Mesquite (Prosopis chilensis), a common ground-water indica- 
tor in the desert scrub formation —Courtesy Univ. of Nevada Agricultural 
Extension Service. 



munity including species of Prosopis, Cercidium, Olneya, Dalea, 
etc. In the higher elevations of Arizona and northern Sonora 
(1,000-2,000 feet), there is a great mixture of species and life 
forms. Although numerous species characteristic of the other des- 
erts are present, Cercidium microphyllum is a dominant with num- 
erous arborescent and columnar cacti, including Carnegiea giga?i- 
tea, Lemaireocereus schottii, and many species of Opuntia. The 
variable topography of the peninsula of Lower California supports 
an equally variable flora including many species. Near the coast, 
there are more leaf succulents than in any of the other desert 
areas. 

Chihuahua Desert— Extending from southern New Mexico 
southeastward to western Texas and down into Mexico, much of 
this area is interrupted by high mountains and lies between 4,000 



290 TH E STUDY OF PLANT COMMUNITIES • Chapter X 

and 6,000 feet. Precipitation varies with altitude from three to 
twelve inches and falls largely in summer. Temperatures are some- 
what lower than in the Sonoran Desert, and frosts are not un- 
common. 

Under these conditions, the communities are not as complex as 
those of the Sonoran Desert or as simple as those of the Great 
Basin, but there is much regional variation. Shrubs and semishrubs 
predominate with a great variety of inconspicuous stem succulents 
in association. Ocotillo (Fonquieria splendens), which is found 
throughout the area, creosote bush (Larrea tridentata), and mes- 
quite (Prosopis juliflora) are the only three species common in 
the Sonoran Desert that are also important and widespread here. 
A number of species are conspicuous because of size or unusual 
form. Yucca, Nolina, and Dasylirion are large semisucculents. 
Agave and Hechtia are particularly abundant leaf succulents. 
Leafless, green-stemmed trees, columnar cacti, and Dasylirion 
longissimwn with its six-foot, linear leaves, are examples of locally 
important species of striking appearance. 

Grassland Formation.— Grasses are climax dominants over all 
the vast area extending from southern Saskatchewan and Alberta 
to eastern Texas, and from Indiana and the western margin of 
deciduous forest westward to the woodland zone of the Rockies. 
Separated from this major area are the Palouse region of Washing- 
ton and the grasslands of the great valley of California. The for- 
mation has the greatest extent of any in North America and con- 
sequently grows under a great diversity of conditions. This is 
possible because of the growth form of the species, their long pe- 
riod of dormancy, and the fact that their moisture requirements 
are critical only in spring and early summer. 

The eastern transition to forest is marked by an annual precipi- 
tation of thirty to forty inches from Texas to Indiana and twenty 
or twenty-five inches farther north. A high proportion of this 
precipitation falls as spring rain, but westward, as the total de- 
creases to about ten inches near the Rockies, the proportion fall- 
ing in spring and summer also decreases. Temperatures are equally 
variable. In the north, the growing season is cool and short, and 
subzero temperatures occur for long periods in winter. In the 
southern part of the range, frosts may be almost unknown, and 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 291 



extremely high summer temperatures are characteristic. Through- 
out the formation, late summer dry spells with high temperatures 
and drying winds are the rule, but, if there is sufficient moisture 
for the grasses during the spring growing period and summer ma- 
turation, such extremes affect them but little because of their long 
period of dormancy. The hot season with limited precipitation is 
probably of great importance in maintaining grassland climax 
against the advance of forest. 

The increasingly severe moisture conditions from east to west 
are accompanied by changes in the dominant species whose com- 
binations are distinguishable as associations of the formation. Three 
major regions are recognizable either by climate or vegetation, or 
both. Their limits, climate, and vegetation have been summarized 
and the important regional and local studies of grassland have been 
listed in a concise presentation by the late Dr. J. R. Carpenter. 52 
This condensation of grassland information could well be used as 
the starting point for any consideration of the nature and distribu- 
tion of grassland. The great number of classifications attempted 
for grassland communities and the disagreements as to major dom- 
inants and most important species implied by the terminology sug- 
gest the complexity of the formation. Probably, too, there is a 
suggestion of much more variation regionally than might at first 
be supposed. Of necessity, we are restricted here to a simple pres- 
entation. On this basis, the discussion will deal with only three 
major associations, which may be termed Tall Grass Prairie, Mixed 
Prairie, and Short Grass Plains. Some authorities recognize as many 
as seven associations, 57 and, even then, most of these can be di- 
vided into several faciations. Furthermore, a detailed discussion 
must recognize within each faciation the usually distinct upland, 
slope, and lowland variations. 

Tall Grass Prairie— Sometimes called "true prairie" this associa- 
tion borders the deciduous forest, receives the most rainfall, has 
the greatest north-south diversity and the greatest number of 
major dominants of the association. Bunch grasses are the con- 
spicuous species, for many of them grow in excess of six feet tall, 
but sod-forming species are also dominants. Because of the gen- 
erally favorable climatic and soil conditions, most of the area is 
cultivated and little of the original vegetation remains today. 



292 THE STUDY OF PLANT COMMUNITIES • Chapter X 




FlG. 155. Tall grass, or true prairie, community in which Andropogon 
scoparius, Bouteloua gracilis, and Sporobolus heterolepis are the most impor- 
tant species.— Photo by R. B. Livingston. 

The long list of major dominants includes tall grasses, such as 
Stipa spar tea, Andropogo?i furcatus, and Sorghastrum nutans; 
medium grasses, such as Andropogon scoparius and Bouteloua 
curtipendula; and the short grasses, Bouteloua gracilis and B. hir- 
suta. The association of dominants with topography should be in- 
dicated at some point even though it is impossible to recognize it 
throughout our discussion. The following groupings are not un- 
common for Tall Grass Prairie. 



UPLANDS 

Agropyron repens 
Bouteloua gracilis 
B. curtipendula 
Andropogoji scoparius 
Poa pratensis 
Sorghastrum nutans 



SLOPES 



LOWLANDS 



Poa pratensis 
Sorghastrum nutans 
Koeleria cristata 
Andropogon furcatus 
Stipa spartea 



Poa pratensis 
Sorghastrum nutans 

Andropogon furcatus 

Agrostis alba 
Spartina pectinata 
Panicum virgatum 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 293 



This distribution is not the same everywhere. In the north, 
Koeleria and Stipa appear in the uplands, and Poa and Sorghastrum 
do not appear at all. In the central region, Panicimi virgatum, Birt- 
bilis dactyloides, and Boutelona hirsnta are added to the uplands 
and Sporobolus heterolepis and S. cryptandrus to slopes. The 




Fig. 156. Mixed grass prairie in which Bouteloua gracilis, Stipa co?nata, 
and Calamovilja longifolia are the principal species. Colorado.— Photo by R. 
B. Livingston. 



southern faciation, sometimes regarded as a separate association, is 
even more distinct, especially because of added species in the up- 
lands, such as Stipa leucotricha, Andropogon saccharoides, A. 
tener, and A. ternarius. 

There has been much discussion and study of the eastern mar- 
gin of Tall Grass Prairie, its extension as a "peninsula" into Illinois 
and Indiana, and the isolated areas farther east, particularly in 



294 THE STUDY OF PLANT COMMUNITIES * Chapter X 



Ohio. Because here it includes somewhat different combinations 
of species, some of which are dominant, the community is re- 
garded by some as a separate association. The predominating tall 
grasses, as well as other basic similarities, make it reasonable to 




FIG. 157. Mixed grass community in Arizona in which grama grasses pre- 
dominate.— U. S. Forest Service. 



others to consider the prairie peninsula as a f aciation of Tali Grass 
Prairie to which it bears a postclimax relationship. The soils with- 
in the peninsula are prairie soils although the climate is now that 
of forest climax. The community may, therefore, be regarded as 
preclimax to the forest, maintained by edaphic conditions. 

Mixed Grass Prairie.— Although the mixed grasses occupy an 
area between that of the tall grasses and short grasses and the dom- 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 295 

inants are derived from both these communities, it is generally 
agreed that there is sufficient unity and distinctness to justify as- 
sociational rank. Important dominants throughout the area are 
Bouteloua gracilis, B. hirsuta, Andropogon scoparius, and, except 
in the north, Bulbilis dacty hides. In the north, Koeleria cristata, 
Stipa spartea, and 5. comata are added dominants, which suggest 
the recognition of a northern faciation. Other important species 
included among the dominants are Andropogon furcatus, Sporo- 
bolus cryptandrus, and several species of Stipa. 







FlG. 158. Short grass plains pastured to sheep in Wyoming.— Photo by W. 
D. Billings. 

The western limit of the association may be taken as the line 
where tall grasses disappear and beyond which only short grasses 
are dominant. Since the tall grasses require available soil moisture 
to a depth of twenty-four or more inches during their active 
growing season, the limit of mixed grass prairie is a line beyond 
which precipitation is insufficient to provide moisture to this 
depth. The eastern limit is not as sharply defined but is also de- 
termined by soil moisture, since mixed prairie is marked by prairie 
grasses in bunch-grass habit sharing dominance with permanently 
established short grasses. 3 Thus the area forms a strip from Sas- 
katchewan through the central Dakotas, Nebraska, Kansas, and 



296 THE STUDY OF PLANT COMMUNITIES * Chapter X 

western Oklahoma into Texas. The sand hills of Nebraska are an 
exception, for here soil conditions are such that postclimax tall 
grasses predominate. During protracted dry periods, the short 
grasses increase at the expense of the moisture-requiring tall 
grasses. 267 Thus the boundaries of the association are not particu- 
larly static and are represented by a wide transition zone. 




FlG. 159. Short grass range in Colorado under average grazing and con- 
sequently in good condition.— U. S. Forest Service. 

Short Grass Plains— Westward from the Mixed Grass Prairie to 
the woodland zone of the Rockies, the xeric short grasses are dom- 
inant. On the basis of exclosure studies and other observations, the 
climax nature of short grasses has been questioned, and the com- 
munity has been described as disclimax resulting from overgrazing 
an area that would otherwise support mixed prairie. 268 This inter- 
pretation is gradually gaining favor. Regardless of terminology, 
the short grasses are, at present, dominant over the entire area. 

The most important species are Bouteloua gracilis and Balbilis 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 297 

dactyloides, except north of the Dakotas, where the latter is ab- 
sent. Several faciations are recognizable that result from combina- 
tions of the major dominants with Stipa comata, or Agropyron 
smithii, or Aristida longiseta. 

The desert plains area extending from western Texas across 
northern Mexico and southern New Mexico and Arizona supports 
short grasses, which, although including different species, are re- 
lated to short grass dominance. Several species of Bonteloiia and 
Aristida predominate. Overgrazing has greatly increased the num- 
bers of desert shrubs here, and these include Larrea, Gpuntia, 
Flourensia, and several others of which widely spaced individuals 
occur everywhere. 

Other Grassland Climax— There is evidence that the great valley 
of California was once dominated by grasses, which, because of 
fire and grazing, have been eliminated except for relict areas. The 
latter suggest that the dominants were bunch grasses, which pro- 
duced grassland similar in appearance to mixed prairie. Through- 
out most of the area it appears that Stipa pulchra was the principal 
species, except near the coast. Today introduced annual grasses 
occupy most of the remaining grassland areas, especially species 
of Avena, Bromus, Festuca, and Hordeum. 

The rolling hills of the Palouse region, as well as most of eastern 
Washington and Oregon and eastward into Idaho, supported 
prairie grasses before being cultivated for wheat production. Al- 
though numerous species characteristic of other grassland areas are 
present here, the major dominants are distinctive, including Agro- 
pyron spicatum, Festuca idahoensis, and Elyimis condensatus. Pos- 
sibly much of the sagebrush dominance in this region is only the 
result of grazing, and certainly the dominance of the annual, 
Br omits tectorum, results from fire and grazing as it does south- 
ward in the Great Basin. 

The Palouse and California grasslands, in contrast with the major 
areas, are products of winter, rather than spring and summer, 
precipitation. 

Aspect Dominance.— Probably no other formation has such 
marked variations in appearance through the growing season. 
Since not all the grasses mature at once, there are times when 
simple observation might lead to incorrect conclusions as to their 



298 THE STUDY OF PLANT COMMUNITIES • Chapter X 

relative importance. Associated species other than grasses, often 
called forbs, may be seasonally so conspicuous as to obscure the 
grasses and, temporarily at least, to give the appearance of dom- 
inance. 

Tropical Formations.— The truly tropical vegetation of North 
America, which occurs only in southern Mexico and Central 
America, probably includes as great a diversity of communities as 
is usually found in temperate climates. The major controlling fac- 
tor in this diversity is moisture, as affected by topography, expos- 
ure, and seasonal distribution. Although numerous local studies of 
the vegetation of American tropics have been made, it is only re- 
cently that a comprehensive classification of the plant communities 
has been attempted in the light of modern concepts. 15 

A misconceived but popular idea of tropical vegetation is un- 
doubtedly one which can best be placed in the category of rain- 
forest-in-its- jungle-form. But such tangled masses of vegetation 
are found only on areas of disturbance and "True rain forest al- 
ways gives the impression of the vault of cathedral aisles!' 15 It is 
made up of many species of tall broad-leaved, evergreen trees in 
several strata with the tallest sometimes rising ninety feet to the 
lowest branch. Undergrowth is sparse, lianas are few, and epiphy- 
tes are not abundant near the ground. Apparently, after disturb- 
ance of any kind, such forests are replaced by a tangled jungle of 
growth that is almost impenetrable. The rain forest is not wide- 
spread because conditions for its development are by no means 
everywhere available. It occurs where temperatures are fairly con- 
stantly high, precipitation is plentiful (over two hundred inches in 
some areas), and on good sites with proper drainage but with a 
continuous supply of available water. 

It should be re-emphasized that not all tropical vegetation is 
rain forest, and to this should be added that not all broad-leaved 
evergreen forest is rain forest. The presence in the tropics of 
mountains of sufficient height to have permanent snow on their 
peaks insures altitudinal zonation similar to that of temperate re- 
gions. These mountains may interrupt moisture-bearing winds and 
so maintain desert conditions. Seasonal deciduous forests, pine for- 
ests, and even tundra are to be found on their slopes. The major 
variations in American tropical vegetation have been grouped into 



CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 299 

six formations, each of which may be divided into from two to 
nine associations. 15 

1. Rain Forest (Optimum formation) 

2. Seasonal Formations 

3 . Dry Evergreen Formation 

4. Montane Formation 

5. Swamp Formation 

6. Marsh or Seasonal Swamp Formation 

The subtropical climate of the southern tip of Florida and the 
Gulf coast down into Mexico permits the growth of numerous 
species with tropical characteristics and affinities. The palms, the 
many broad-leaved evergreens, the mangroves, the many epiphytes 
and lianas, and the sometimes jungle-like masses of vegetation are 
all suggestive of tropical conditions. 



GENERAL REFERENCES 

E. LUCY Braun. The Undifferentiated Deciduous Forest Climax and the 

Association Segregate. 
J. R. Carpenter. The Grassland Biome. 
E E. CLEMENTS. Plant Indicators : The Relation of Plant Communities to ■ 

Processes and Practice. 
R. E DAUBENMIRE. Vegetational Zonation in the Rocky Mountains. 
J. W HARSHBERGER. Phy to geographic Survey of North A?nerica. 
B. E. Livingston and E Shreve. The Distribution of Vegetation in the 

United States, As Related to Climatic Conditions. 
H. L. SHANTZ and R. ZON. The Physical Basis of Agriculture : Natural 

Vegetation, in Atlas of American Agriculture. 
V E. SHELFORD (ed.). Naturalises Guide to the Americas. 
E SHREVE. A Map of the Vegetation of the United States. 
J. E. Weaver and F. E. Clements. Plant Ecology. 



CHAPTER XI 



THE DISTRIBUTION OF CLIMAX COMMUNITIES 
SHIFTS OF CLIMAXES WITH TIME 

The present distribution and limits of climax communities are 
not necessarily static, nor have they been in the past. Looked at in 
terms of geological time, changes of climate must be recognized 
that were so extreme that vegetation must likewise have changed 
radically. Within relatively recent geological time, glaciation of 
northern North America obviously must have produced such 
changes in climate that disruption of then existing lines of vegeta- 
tional distribution were inevitable. Advance of the ice southward 
resulted in constriction of vegetational zones and retreat of species 
and growth forms as the climate changed. With the recession of 
the ice, there was again a northward advance of species and a re- 
adjustment of plant communities as the glaciated area was reoc- 
cupied by vegetation. Probably there were several minor advances 
and retreats of vegetation correlated with the shifting ice fronts 
and the similarly varying climate. 

Within historical time, there have been major shifts of climate 
producing conditions that may have had serious effects on vege- 
tation. There is evidence that early Norsemen who colonized 
Greenland were able to carry on a primitive sort of agriculture on 

lands alone the southern coast. Between the twelfth and the four- 
th 

teenth centuries the climate there deteriorated rapidly so that 
summers became shorter and colder, the soil remained frozen, and 
the colonists disappeared. Today, as for some time past, the reced- 
ing glaciers in Greenland indicate an increasingly favorable cli- 
mate. Receding glaciers in Alaska have been similarly interpreted. 76 
In recent years, conifer forest has been advancing into the tundra 
in Alaska. 110 Periodically, prairie vegetation is invaded for some 
distance by forest, and although drought often eliminates such ad- 
vances, they may be permanent or, at least, appear so. 

That climates have changed over long periods of time cannot 
be questioned, and that slow change continues today in certain 

300 



CLIMAX COMMUNITIES : SHIFTS WITH TIME 301 

areas is undoubtedly true. With climatic change, vegetational 
change is to be expected. Some modern changes are easily recog- 
nized, as indicated above. In highly populated areas, the changes 
may be much less obvious because natural vegetation has been 
disturbed by man. 

PALEO-ECOLOGY 

This phase of ecology deals with the history of vegetation, es- 
pecially the reconstruction of past climaxes and climates, their 
rise, decline, and migration over long periods of geological time. 
Its basic source materials are derived from paleontology and geol- 
ogy and must be interpreted in terms of what is known of the 
ecology of modern organisms. 

Tracing changes in modern climax vegetation is a complex proc- 
ess involving the use of every kind of evidence available. E B. 
Sears' 218 reconstruction of the natural vegetation of Ohio and its 
prehistoric development 221 illustrates how historical records and 
pollen statistics may contribute evidence. A. M. Raup's 203 study of 
New England climate and vegetation utilizes still other sources of 
evidence. Archaeology, zoology, botany, and geology all were 
drawn upon in a variety of ways before he concluded that New 
England had had a warmer climate within recent years— probably 
no more than a thousand years— and that the trend has since been 
to the cooler and moister, with parallel vegetational changes. 

Knowing that climates have changed, one may be equally cer- 
tain that vegetation has varied accordingly. Major alterations in 
vegetation may likewise be assumed to indicate modification of 
climate. In some instances, however, such shifts have been inter- 
preted as purely successional in nature, a point not to be ignored 
since succession has gone on in the past as it does today. Change 
within historical time, if still in progress, may be observed, or may 
become apparent from detailed quantitative and qualitative studies 
of transition areas. 39 A less reliable source of information is the 
historical literature not always dependable, unfortunately, because 
of the limited knowledge of the early writers. It is, nevertheless, 
a source from which much of value can be learned, 36 ' 203 particu- 
larly when the information is drawn from several sources and is 
correlated with other kinds of evidence. 

The difficulties of reconstructing the vegetational picture dur- 



302 THE STUDY OF PLANT COMMUNITIES ' Chapter XI 

ing early historical time are as nothing compared with those in- 
volved in determining prehistoric climaxes. 50 Fossils, variously pre- 
served, are the chief source of our knowledge of ancient floras, 
many of which have disappeared completely. Considering that 
different species and even parts of the same plant are unequally 
preserved, it is surprising that we know as much of these old 




FlG. 160. Interglacial forest relicts on beach below high tide, Glacier Bay, 
Alaska. These hemlock stumps, probably several thousand years old, repre- 
sent forest that lived before the last major advance of ice, which buried them 
under glacial debris (above beach). Tide action has again exposed the stumps. 
—Photo by W. S. Cooper. 1 



75 



floras as we do. Certainly we know that there have been extreme 
climatic changes on various parts of the earth and that with them 
have come modifications in vegetation, which sometimes elimin- 
ated entire floras. 

More recent vegetational history has been given greater atten- 
tion because of its direct relationships to our modern flora and, 
possibly, because it offers greater probability of solution. Post- 
glacial climate and vegetation have been studied more intensively, 
therefore, than those of preglacial time. Plant remains, buried and 
preserved between layers of glacial drift, have yielded much in- 
formation on the amount of time involved, the climate, and the 



CLIMAX COMMUNITIES : SHIFTS WITH TIME 



303 



vegetation. These deposits, often preserved in a natural state as 
wood, leaves, fruits, or seeds, have been uncovered by erosion, 
excavation, and even in driving wells at considerable depth. Such 
findings have been fortuitous largely, since the deposits do not 
occur generally and, when stumbled upon, must be brought to the 
attention of those interested if they are to be of any scientific 




FlG. 161. Well-preserved Pleistocene plant remains found in silt or peat 
layers buried under 10 to 12 feet of undisturbed moraine in Minneapolis, 
Minn. (1) Collier gon giganteum, (2) Neocalliergon integrijolium, (3) Picea 
sp., wood structure almost perfect, (4) Picea sp., wood structure distorted by 
pressure, (5) cone of Picea glanca, (6) cone of Picea mariana, (7) cone of 
Larix laricina—From Cooper and Foot. 11 

value. As a result, the information they have yielded is fragmentary 
and discontinuous both in time and space. 

A more promising approach to the problem began with the 
study of the nature and composition of the strata of plant remains 
and other sediments that have accumulated in lakes and ponds as 
peat or related material. 82 These strata may give almost continuous 
records back to glacial time, and, since deposits are distributed 
over wide areas, their study makes possible the correlation of find- 



304 THE STUDY OF PLANT COMMUNITIES * Chapter XI 

ings, particularly regarding climate, in one place with those in an- 
other. Obviously such studies can not be entirely satisfactory since 
they indicate vegetation only within the bogs themselves, or at 
their immediate margins, and bog vegetation is not of the climax 
type. 

POLLEN ANALYSIS 

When, in 1916, von Post presented the results of his studies ot 
pollen preserved in Swedish peat deposits, an entirely new ap- 
proach to the reconstruction of prehistoric vegetation was begun. 97 




EBERBACH , 

" MM— ■ 




FlG. 162. A type of sampler frequently used for pollen studies of peat and 
marl deposits. It consists of a jacketed plunger that completely closes the 
sharpened end of the jacket. After it is pushed down to sampling depth, 
using the four-foot extension rods, it is drawn upward a few inches. This 
partly withdraws and locks the plunger in the upper part of the jacket. 
Then, when forced downward, the jacket cuts a ten-inch sample core- 
Courtesy of Eberbach and Sons Company. 



Wind-borne pollen is deposited everywhere and much of that 
which falls in a lake is preserved in its sediments because of the 
low rate of oxidation. Since the pollen of most dominant trees is 
wind-borne, the pollen deposited at any one time should include 
that of the important tree species in the general vicinity and the 
numbers of grains of a species should be indicative of the relative 
importance of that species in the surrounding forest at the time. 
Because pollen grains of a species are constant in size and form, 
genera, and sometimes species, can be identified positively. Conse- 



CLIMAX COMMUNITIES : SHIFTS WITH TIME 



305 



Z 



LJ <J 



7 1 



8- 



9- 



I 
.1 



r 
i- 

a 

LJ 

CI , 



12 



13- 



?! O" 

uj o 

a *> < w i- i 

~i — 



D 
O 

a 

LJ 

3 
O 



D r 

u - 

CD a 



3 i 

< 2 



< 

LJ 

2 »■ 



< - 

■ 

>o 

a. * 

< . 

U i O u 



GO 
< 



< i 



CO 



50 



l l l i i i — rr 

SO 



r- i i i i — i i i i — i i i i — i i i i — rrr 



C3 



C2 



B 



A2 



A I 



50 



FlG. 163. An example of a common form of pollen diagram, which also 
illustrates what we know of the vegetational history of the northeastern 
United States although derived from one place (Upper Linsley Pond, North 
Branford, Conn.) Zone A indicates a spruce-fir forest; the high values for 
pine are attributed to over-representation resulting from its light weight and 
its abundant production. In the northeast, a secondary maximum for spruce 
(A-2) is not uncommon in this period and is thought to represent a local 
readvance of retreating ice. Zone B, a warm dry period, shows a pine maxi- 
mum and the beginning of warmth-loving, deciduous trees. Then followed 
deciduous dominance over a long period, in which hemlock-oak were first 
important (C-l), then oak-hickory (C-2), and, with cooler moister condi- 
tions (C-3), an increase of chestnut, followed by a reappearance of spruce in 
some localities.— Fr om Deevey* 9 

quently, if samples of lake deposits are taken from the bottom up- 
ward to the present surface of the sediment, the pollen content of 
successive strata should indicate the nature of the forest, as to 
genera and their relative abundance, throughout the period of ac- 
cumulation. 

Sediments on lake bottoms as well as peat deposits have been 



306 THE STUDY OF PLANT COMMUNITIES " Chapter XI 

studied. Samples must be taken with care to prevent contamina- 
tion, and several types of augers have been devised for the pur- 
pose. With these, cores can be cut that, placed end to end, form a 
continuous column of material for the entire depth of the deposit. 
Borings are made in summer under most conditions, but, since it 
is desirable to have them from the deepest part of the depression, 
it is often advantageous to make them in winter from the frozen 
surface. 

Identification and counting of the pollen grains must be done 
under a microscope. This necessitates treatment of the samples 
with one of the several methods recommended 48 to eliminate for- 
eign material and to concentrate the grains. Identification is facili- 
tated by reference to illustrations 274 and by comparison with a 
series of grains taken from modern plants. What constitutes an 




NORTHERN MtNNESOTA 
AND WISCONSIN 





NORTH-CENTRAL 
STATES 



NORTHEASTERN 
OCEANIC 



FlG. 164. Schematic pollen profiles that show the general picture of what 
is known of vegetational history for the eastern United States. F— fir, G— 
grassland complex, H— hardwoods except oak, O— oak, P— pine, S— spruce. 
Depth shown vertically, percentages horizontally. Although there are differ- 
ences relatable to continental and maritime climates, there is regional similar- 
ity in the indications of a middle warm, drier period, and the suggestion of 
subsequent cooler, moister conditions leading into the present, as well as the 
shift toward early proportions of species in the upper portions of the dia- 
grams. Succession may be a factor in these latter shifts.— After Sears. 2 



219 



CLIMAX COMMUNITIES : SHIFTS WITH TIME 



307 



adequate sample in the count of grains is not agreed upon by all 
investigators but fewer than 150-250 grains are rarely counted. 

When the proportions for genera are known for each stratum, 
they are represented in a standardized form, known as a pollen 
diagram, in which pollen spectra— the relative importance of each 
genus in a stratum— are plotted on horizontal lines, one spectrum 
above another to show the progressive changes for genera, which, 
are shown on vertical lines. A pollen diagram is no more than a 
means of visualizing the pollen spectrum of a section— a vertical 
series of samples from the bottom to the surface of a deposit. 
Changes in the spectra from the bottom upward are, of course, to 
be correlated with time. 

The shortcomings and pitfalls of pollen analysis as a method of 
determining past vegetation and climate are appreciated by all 
who have used it. 48 ' 97 There are sources of error in methods, in 
records, which may be incomplete, and in identifications which 
may not always be correct, and interpretations may be based upon 
inadequate data. Because of its simpler flora and greater amount 
of study, the pollen spectrum for Europe is better established and 



Age in 
years 

2O0O 



400O 



6000 



6000 



10,000 



12000 



14.000 



16.000 



16.000 



Period itr 
Cooler 

Moijler 



Per/od n'i 
Maximum 

Warmth 
and 

Dryness 




Period li 

Increasing 

Warmth 

and 
Dryness 

Period / 
Cool-Moisf 



Pmus 
monhcola 




Pjeudolsupa 
la*, i folia 



Tsuga 
hererophy/la 



% eo 



to 



60 



20 



20 



-hO 



oO 



eo 



4-0 



FlG. 165. A composite of ten pollen profiles from the Puget Sound region, 
which is indicative of postglacial climate and vegetation in the northwest 
although not typical of all areas as to species. The volcanic ash level, present 
in all northwestern profiles, is considered to be of common age. Such com- 
posite profiles, because they eliminate the sharp fluctuations from level to 
level found in individual profiles, give a better picture of the trend of post- 
glacial vegetation.— From Hansen. 



115 



308 THE STUDY OF PLANT COMMUNITIES * Chapter XI 

accepted than in North America. Most of our studies have been 
made in areas where bogs are common and within a reasonable 
range of accessibility of an individual or his students : the north- 
west, the north-central states, and the northeast. There is still 
much to be done within the glacial area to complete the picture. 

It is somewhat surprising that investigators are in as close agree- 
ment as they are. Most generally accepted is a postglacial climatic 
series beginning with increasing warmth, followed by a period of 
maximum warmth and drought, followed by a period of decreas- 
ing warmth, the present. 224 This is applicable to both Europe and 
America. Some students would subdivide these three major pe- 
riods, claiming that greater refinement is possible. Others contend 
that their data contain no evidence of a warm dry period in North 
America. 

More studies are certainly necessary in North America before 
agreement can be reached as to all phases of the basic normal pollen 
spectrum and its meaning in terms of climate. Several scattered 
studies have been made of deposits beyond the limits of glaciation, 
and these offer real possibilities. Likewise, there must be more ef- 
forts to correlate all sources of contributing evidence, 89 ' 203 a truly 
paleo-ecological approach : floristic, vegetational, zoological, geo- 
logical, archeological, as well as evidence from pollen analysis. 

DENDROCHRONOLOGY 

Another bioclimatic approach to past history was originated by 
an astronomer. Dr. A. E. Douglass, when he began studies of an- 
nual growth rings of trees in an attempt to correlate their differ- 
ences with climatic variations, presumably related to solar activity. 
Cross-dating, or matching the growth patterns year by year, for 
modern trees in Arizona was first accomplished in 1904, but its 
significance was not fully appreciated until several years later. 91 
Then a chronology was established from modern times back to 
A.D. 1400 by matching ring records of modern trees to the ex- 
terior ring records of earlier trees and so on with trees that grew 
still earlier. When these records were matched with rings in beams 
taken from ancient pueblos, the records became complete back 
to A.D. 1299, then to A.D. 700 and, more recently, successively to 
A.D. 643, A.D. 500, and finally to A.D. 11. Recent finds suggest 



CLIMAX COMMUNITIES : SHIFTS WITH TIME 



309 



that the chronology will be carried even further back. 119 Some of 
the record was completed and some of the cross-matching was 
made possible by fragments of wood from ancient pueblos and 
some even with charcoal, which was better preserved than wood. 
It should be noted that an even longer chronology has been worked 
out for the giant redwoods, which is complete for 3,000 years. 

When the pueblo dendrochronology was completed, it was a 
major contribution to archaeology since some thirty prehistoric 
ruins were immediately given absolute dates, and later hundreds 
more were dated. This usefulness of the method was immediately 
apparent to archaeologists, who accepted it and adapted it to their 
purposes. At the same time, their findings in archaeology have 
contributed to the establishment of dendrochronology as a means 
of studying past climate. 

Recent ring studies in moist cool regions indicate that no better 
climate than the arid Southwest could have been selected for the 
initial investigations. In extremely dry regions, growth and size of 
rings are closely related to annual precipitation and the correlation 
is not complicated by light or temperature effects. It is now known 
that in the north, or at high altitudes, tree growth is most respon- 
sive to temperature and that in temperate regions with adequate 
rainfall, both temperature and moisture factors are reflected in 
the rings. 90 

This does not mean that tree-ring studies are successful only in 
arid regions but rather that their interpretation may be more dif- 



TO PREHISTORIC 
TIMES 



THIS YEAR'S 




TREE CUT 
THIS YEAR 



SECTIONS FROM HISTORIC BEAMS 



FlG. 166. A diagram illustrating how the bridge method is used to extend 
knowledge of dated rings, an important part of the building of complete and 
continuous chronologies. The usual desirable overlap is fifty years.— After 
Glock. 107 



310 THE STUDY OF PLANT COMMUNITIES * Chapter XI 

ficult elsewhere. That cross-dating and correlation with climatic 
variation is possible in moist-temperate climates was demonstrated 
by Douglass' studies in Europe and several parts of the United 
States. In the Mississippi drainage area, the deviation from the 
normal annual precipitation has been shown to affect ring growth 
more than total precipitation, but the relationship is modified by 
temperature and wind as they influence evaporation. 119 Ring 
growth in New England has been shown, in a chronology from 
hemlock, to have close correlation with climate as indicated by 
exceptional and poor crop years. 164 

Since tree-ring analysis was originally begun with the hope that 
it would show solar-terrestrial relationships, it was natural that, 
with the establishment of long, dated chronologies, the data should 
be studied for cyclic characteristics. Permanent periods, or those 
of fixed length, showed no correlation; therefore, this idea was dis- 
carded for one of cycle complexes in which any obvious or sig- 
nificant recurrence of variation in data was considered to be cyclic. 
On this basis, definite relationships were demonstrable between 
sunspot activity in the past and terrestrial climate as recorded in 
certain long-time chronologies of tree rings. An eleven- (ten to 
twelve) year cycle is especially pronounced throughout the old 
records and continues to be borne out, in a general way, for mod- 
ern conditions. During periods of sunspot maximum, drought is 
characteristic, and sunspot minimum is associated with excessive 
precipitation. Application of the method to climatic prediction 
may be possible as more long-time meteorological records are an- 
alyzed and as more tree records of great length are worked out to 
show the nature of prehistoric climates on many parts of the earth. 

THE RELICT METHOD 

As for several other phases of dynamic ecology, we are indebted 
to Dr. E E. Clements for recognizing the potentialities of the relict 
method, for demonstrating its usefulness, and for a clear and com- 
plete exposition of the entire subject. The brief discussion that 
follows can hardly avoid being a condensation of his ideas. 58 

"In the ecological sense, a relict is a community or fragment of 
one that has survived some important change, often to become in 
appearance an integral part of the existing vegetation!' The con- 



CLIMAX COMMUNITIES : SHIFTS WITH TIME 



311 



cept may be applied to individuals or a species, but is more often 
used for communities. It may be used to describe delayed or lag- 
ging stages of succession, but it has far greater usefulness in con- 
nection with climax vegetation. 

The usefulness of relicts lies in their indicator value of past con- 
ditions of habitat and vegetation as well as of the causes underly- 
ing changes that have occurred elsewhere in the area. A relict 



! 




FlG. 167. Relict (postclimax) black spruce forest in a Minnesota bog.— 
U. S. Forest Service. 



community having remained relatively unchanged because of pe- 
culiar local conditions is an actual sample of, or shows strong 
similarities to, previous vegetation. At the same time, the peculiari- 
ties of the relict habitat are indicative of environmental conditions 
previously characteristic of the area as a whole and may, therefore, 
be suggestive of why vegetation changed generally there. 

Relict communities occur where local edaphic, topographic, or 
biotic factors differ sufficiently to compensate for the effects of 
environmental conditions obtaining generally. Thus altitude, ex- 
posure, or soil may provide locally unusual moisture conditions. 
Ridges, streams, and lakes may constitute barriers to fire. Peculi- 
arities of drainage may result in swamps, bogs, and low flood 



312 THE STUDY OF PLANT COMMUNITIES ■ Chapter XI 

plains. Any such condition may be effective in maintaining relict 
communities, which, in terms of climate, could not be anticipated. 
They are the relicts indicative of shifts of climax and climate over 
long periods. 




FlG. 168. Effect of grazing on mixed prairie in central Colorado. Short 
grass, to left of fence, is typical over much of the region but, where cattle 
are excluded, to right, mixed prairie develops.— Photo by R. B. Livingston. 



A quite different kind of relict is one that is maintained by man, 
purposely or otherwise, after he has destroyed or modified the 
picture of climax generally. Overgrazing, cultivation, and lumber- 
ing have destroyed or modified climax over extensive areas to such 
a degree that its recognition and interpretation, even though its 
destruction was within historic time, are dependent upon rem- 



CLIMAX COMMUNITIES : SHIFTS WITH TIME 



313 



nants of the former vegetation. Such relicts may be found in 
fence rows, along railroad right-of-ways, in old cemeteries, and 
in any areas long undisturbed and may yield much information 
about the past. The deliberately protected areas, such as game and 
wildlife preserves, natural areas, Indian reservations, and national 
parks, offer still more possibilities because of their extent, fre- 
quently included virgin areas, and relative permanence. 




FlG. 169. Postclimax community of ponderosa pine occurring as an iso- 
lated island in sagebrush desert wherever the special local soil conditions exist 
in Nevada. Often disjunct from nearest ponderosa pine forest of the Sierra 
by fifty miles or more .— Photo by W. D. Billings. 22 



Relicts related to climatic change are most abundant in the 
transitions from one climax to another but may likewise be found 
well within the general range of a climax, provided the local con- 
ditions are present that maintain the necessary compensating fac- 
tors. Usually the local conditions are a result of topography, 
which, through its effects on precipitation, drainage, tempera- 
ture, and air movements, permits the relict to survive. The result- 
ing relict communities are the postclimaxes and preclimaxes pre- 
viously discussed in detail. With an understanding of the concept 
of postclimax and preclimax, their presence greatly simplifies the 
interpretation of shifts of climate and climax in the past. The pres- 
ent condition of the relict, if free from disturbance, may furnish 



314 THE STUDY OF PLANT COMMUNITIES • Chapter XI 



strong evidence of the present degree of climatic stability. Judg- 
ment of such evidence must, of necessity, be tempered by what is 
known of climatic cycles. In parts of the West, precipitation may 
be several times as great during a period of sunspot minimum as 
that during sunspot maximum. The condition of vegetation, par- 
ticularly in relict communities, must be interpreted accordingly. 

GENERAL REFERENCES 

S. A. CAIN. Pollen Analysis as a Paleo-Ecological Research Method. 
S. A. CAIN. Foundations of Plant Geography. 

F. E. CLEMENTS. The Relict Method in Dynamic Ecology. 

A. P DACKNOWSKI. Peat Deposits and Their Evidence of Climatic Change. 

G. ERDTMAN. An Introduction to Pollen Analysis. 

W S. Glock. Principles and Methods of Tree-Ring Analysis. 
R B. SEARS. Climatic Interpretation of Postglacial Pollen Deposits in North 
America. 



Part 5 • Practical Consider ati 



CHAPTER XII 




APPLIED ECOLOGY 

Man is rapidly becoming the earth's dominant organism. To an 
increasing extent, natural communities survive because he tolerates 
them, are modified to suit his purposes or fancy, or are destroyed, 
sometimes through his carelessness, but usually so that land may 
be used for agriculture, industry, or other activities. His domi- 
nance is of a different order from that characteristic of com- 
munities in nature, for, with his knowledge and technology, his 
activities are often so extreme and so rapid that their effects are 
like those of a series of catastrophic natural events. Thus he may 
not only destroy or modify natural communities, but he may also 
frequently modify the environment to a great extent. Suggestive 
of a different form of environmental modification are the recent 
experiments with rain-making by the use of dry ice. All this is 
necessary from our modern point of view and will continue, per- 
haps at an accelerated rate, as populations increase and the earth 
is more completely occupied and used. 

Natural communities and their environments, particularly the 
soil, are natural resources. When they are destroyed or modified, 
they may reappear only after a Jong period of time or, with ex- 
treme disturbance, this may even be impossible. It becomes in- 
creasingly apparent that future generations may require these 
natural resources and likewise that man has been most wasteful of 
them, especially in modern times. A problem today, which will 
become greater in the future, is that of how to use such natural 

D 

resources to the fullest extent without jeopardizing their con- 
tinued availability for future needs. The problem is fundamentally 
ecological. Its solution depends upon the comprehension and ap- 
plication of ecological principles. 

315 



316 THE ST UDY OF PLANT COMMUNITIES • Chapter XII 

Since man is becoming the dominant organism and also is 
gifted with thought processes, his dominance should be such that 
he turns natural laws to his advantage or, at least, does not permit 
them to work against him. It is in this connection that applied 
ecology becomes useful. The characteristics and distribution of 
natural communities, the nature of the environment, and the inter- 
relationships of organisms and environment are subject to natural 
laws, which the ecologist seeks to recognize and verify. The more 
completely the pattern of these interrelated processes is under- 
stood, the greater the probability that man will remain a per- 
manent dominant, assuming that he restricts his activities to the 
limits of these laws. Only if biological laws are recognized in full 
can we hope to rebuild the natural resources we have destroyed, 
or even maintain those still available to us. 

If we knew the ecology of all natural vegetation and that of all 
crop plants, strong recommendations for land use could be made 
in terms of its greatest contribution to society. Not only could 
agricultural, forestry, and grazing lands be positively recognized, 
but the details of management for maximum continuous production 
could be recommended with certainty. Quite obviously, ecologi- 
cal knowledge has not accumulated to this extent. The ecology of 
natural vegetation is still inadequately known, and the ecology of 
cultivated plants has not been sufficiently studied. If the ecologist 
is to contribute successfully to the direction of man's activities as 
a dominant, there is still much that must be learned. On the other 
hand, even though knowledge is incomplete, ecology has much 
to contribute that has not been fully utilized in applied fields. 
What is known should be applied when man destroys or modifies 
natural communities. Much progress has been made in the use of 
ecological principles in several fields, but their potential applica- 
tion is still great. 

FORESTRY 

The early history of lumbering in North America indicates, on 
the part of lumbermen, a complete disregard for forests as a 
natural resource and little concern for the future. Foresters have 
long been conscious of this improdigal attitude although until re- 
cently they were usually unable to change the lumberman's 
methods or point of view. Through the years, forestry has be- 



APPLIED ECOLOGY 317 



come a respected profession as the necessity for scientific manage- 
ment has become apparent. An important part of a forester's train- 
ing is forest ecology, or silvics, in which he learns the scientific 
background upon which silvicultural practices are based. 

A generally accepted definition of silviculture states that it is 
that branch of forestry dealing with the establishment, develop- 
ment, care, and reproduction of stands of timber. 254 More often 
than not, the silviculturist aims to control the establishment and 
development of forests so that they will be made up predomi- 
nately of economically desirable species or so that merchantable 
timber will be produced in a minimum of time. Or, he may be 
interested in results not directly related to the production of 
lumber. Cultural operations may point to erosion control, water- 
shed protection, dune stabilization, game encouragement, or rec- 
reational purposes, 244 or, if in the West, to a better balance be- 
tween timber production, watershed control, and use of the forest 
for range purposes. If his methods are scientific, they will be 
based upon reasons derived from silvics. Consequently, the more 
completely forest ecology is understood, the more successful 
should be its application in silviculture. 

Since the practice of silviculture almost invariably involves at- 
tempts to control forest communities and their development, a 
knowledge of successional trends and the climax of the region is 
all important. Knowing the principles of succession, it should be 
obvious that the simplest form of management would be one that 
least modifies the natural development of vegetation. To main- 
tain a successional community indefinitely requires considerable 
effort, if it can be done at all, but the nearer the desired forest type 
is to the climax, the easier it should be to maintain it. These may 
seem to be obvious generalizations, but they have not been, and 
are not, fully appreciated or applied. 

In the past, artificial forest types have been attempted under a 
great variety of conditions. Species have been planted outside the 
limits of their natural ranges, even including several introduced 
from other continents. Often such trees are grown in pure stands 
or, if not, then in combination with native species to make quite 
unnatural communities. Even more common have been the at- 
tempts to grow species on sites to which they are not naturally 



318 THE STUDY OF PLANT COMMUNITIES * Chapter XII 

adapted. The situation in New England is illustrative. Here the 
original forest has long been gone, and reforestation and silvi- 
cultural programs have been in progress for some time. Introduc- 
tions include Scotch pine, European larch, and Norway spruce 
from Europe, and white spruce from the northern conifer forest. 
Red pine and white pine have been grown at the fringe of their 
range in pure stands on rich, heavy soils instead of the sandy soils 
on which they naturally occur. 

The production of artificial forest types in New England can, 
as a whole, be described as unsuccessful. S. H. Spurr and C. A. 
Cline, 245 pleading for the application of ecological principles, say 
that older trees are often of poor form, and growth is likely to 
decline sharply in later years. Very few artificial stands have been 
profitably brought to maturity. Furthermore, these types are 
especially susceptible to damage— from insects and other animals, 
from disease, and from the elements. Norway spruce is severely 
attacked by the white pine weevil; exotic larch plantations may 
be severely damaged by the porcupine and squirrel; red pine, 
south of its natural range, is particularly susceptible to Tympanis 
canker and to attacks by the European pine-shoot moth; crooked- 
ness of Scotch pine has been attributed to frost damage; weevils 
do more damage to white pine on heavy than on light soils. These 
authors admit that eventually, if sufficient knowledge is acquired, 
artificial types may be grown successfully. For the present, they 
cannot be recommended for New England because of previous 
lack of success, the risk involved, and the high cost of production. 
Probably similar generalizations can be made for most of the 
forest regions of North America but with less evidence because 
there has not been as much experimenting elsewhere. Although 
forest species have been successfully introduced into new areas, 
as, for example, the eucalyptus into California, the results in New 
England are suggestive that such experimenting might be of 
dubious value and certainly would not yield the necessary in- 
formation except at great cost over a long period of years. 

If only natural forest communities are to be the objective, there 
are two general types to be considered. Silviculture is usually 
given consideration only after the old forests have been destroyed 
and, not uncommonly, after much of the land has been used for 
agriculture and subsequently abandoned. Under these conditions, 



APPLIED ECOLOGY 



319 




FlG. 170. Eight-year-old plantations of pine on the same soil type (Duke 
Forest, Piedmont of N. C.) to compare growth of northern species, (1) red 
pine, and (2) white pine, with that of native loblolly pine (3). The pictures 
speak for themselves.— Photos by W. R. Boggess. 



320 THE STUDY OF PLANT COMMUNITIES * Chapter XII 

the abandoned land supports various early stages of secondary 
succession, and cutover land is in late successional or subclimax 
forest. The problem then becomes one of cultural practices de- 
signed (1) to maintain the temporary forests of successional na- 
ture or (2) to permit stands to develop to climax or near-climax 
conditions. 

The relatively short-lived successional communities often in- 
clude as dominants the most valuable trees (e.g., pine where hard- 
woods are climax) and, because of their rapid growth, the most 
desirable commercial species growing in the region. But, because 
of their successional position, when these species are removed, 
they are replaced by other species, representing later stages of 
succession, whose seedlings were there and released by the cut- 
ting. The problem of maintaining dominance of such temporary 
species has been given much study, but it is by no means solved. 
Without expensive cultural operations, usually including planting 
and periodic weeding, these temporary types cannot be main- 
tained indefinitely. Even though the productiveness of a desired 
species in a stand may be extended by various types of cutting and 
treatment, its replacement is inevitable. Almost invariably, the 
succeeding stand tends to be nearer the climax than its predecessor 
and will include a higher proportion of economically less desir- 
able species. Where successional species are fire resistant, there is 
the possibility of using controlled burning to hold back succession 
and maintain dominance of the temporary type. Under these 
conditions a temporary type could be cut selectively and provide 
a continuous yield. The merits of the method have been argued 
and are being tested for the longleaf pine forests of the coastal 
plain of the southeast. 

The alternative would be to allow all forest land to develop to- 
ward the climax or at least to near-climax conditions. Once estab- 
lished, such forest would require a minimum of silvicultural 
attention. Continuous production would be assured, and with 
judicious selection of species for cutting, undoubtedly the pro- 
portions of desirable and undesirable species could be controlled. 
Additionally, permitting natural development of stands should 
result in a distribution of species in the habitats to which they are 
best adapted. Different conditions of soil, exposure, and moisture 



APPLIED ECOLOGY 321 



would support stands of different composition, but presumably 
these species would be making their best growth although a mini- 
mum of management would be involved. This is not to imply that 
silviculture is unnecessary. For example, artificial planting is fre- 
quently economically justifiable since it assures uniform stocking 
and even-aged stands and may speed stand development by several 
years. If there are few seed sources of desirable species, succession 
may be so long delayed, by shrub stages perhaps, that planting be- 
comes a necessity. 

Silviculture is usually desirable and sometimes a necessity, but 
it should be emphasized that its practices, to be most effective, 
should be governed by ecological knowledge. The less cultural 
practices tend to modify the natural trends of succession, and the 
more nearly the desired forest is to the climax of the region, the 
less the effort and expense there will be in developing and main- 
taining it. Here is an economic reason for learning the nature of 
virgin forest wherever it still remains and for determining all that 
is possible of its variations with habitat. Similarly, successional 
trends must be known in detail for every major soil type and situa- 
tion if cultural practices are to be adjusted accordingly. Secondary 
successions are of major importance these days, and they can be 
worked out for any region. Climax forest in virgin condition is 
rapidly disappearing and usually only remnants remain for study. 
Their characteristics should be recorded at every opportunity. 
When possible, representative portions of these virgin forests 
should be saved intact for future study. 

RANGE MANAGEMENT^ 

The objective of range management is to produce the highest 
possible forage yield while the condition of the range is maintained 
or actually improved. To this end, the methods of ecology have 
been used to such an extent that range management is largely ap- 
plied ecology, and just as silvics is the basis of silviculture, so is 
range ecology the basis of range management. 

Range ecology has, on the one Hand, concerned itself with the 
purely ecological concepts of regional climaxes with grazing value 
and the patterns of succession for each. On the other hand, there 
has been the practical consideration of the quality and type of 



322 THE STUDY OF PLANT COMMUNITIES * Chapter XII 





•^ajfi&ijfc^^^^^* 




^** >* «** 



&~ 



I 



FIG. 171. Illustrations of blue grama-oak savannah range that tell their 
own story of good and poor range management. (Above) "One of the finest 
demonstrations of range and livestock management in the southwest!' (Be- 
low) Range depleted by overuse and poor management. Note differences in 
condition of cattle, amount of forage, ground cover, and erosion.— U. S. 
Forest Service. 



APPLIED ECOLOGY 



323 



forage provided by each of the communities and of how they may 
be controlled or modified to advantage. Only suggestions of the 
nature of the research on these problems can be given here, but 
they should indicate the degree to which ecology is contributing 
to the solution of range problems. 




FlG. 172. To permit grazing to continue until range is entirely depleted 
and gullying has reached such extremes is obvious mismanagement, but it 
happens all too frequently. Note absence of gullies under protection of oak 
tree.— U. S. Forest Service. 



The seasonal variations of major species have been studied in 
terms of grazing value. Competitive relationships of grasses and 
forbs (associated herbs) have been investigated as well as their 
relative palatability. The effects of grazing on community struc- 
ture have been given much attention, particularly with regard to 
criteria for the recognition of excessive use and the time and con- 
ditions necessary for recovery to normal. As a result, the carrying 
capacity of many forage types is well known, even for different 
seasons of the year. With regard to range condition and carrying 
capacity, the effects of rodents have been studied as well as the 
effects of predators upon the rodents. Effects of drought have 
been given much attention as well as the rates with which ranges 
recover from drought, and, in this connection, the water require- 
ments of important individual species have been determined. In 



324 THE STUDY OF PLANT COMMUNITIES • Chapter XII 

the consideration of water, the effects of different types of cover 
on runoff, flooding, erosion, and water supplies have been studied 
in detail. In attempts to rebuild depleted and eroded ranges, there 
have been studies of artificial seeding and planting to speed recov- 
ery. As in forestry, numerous foreign species have been tested with 
some successes (e.g., crested wheat grass) in an attempt to improve 
conditions. 




'9n 



*^f 



.JKf v^^ 




FlG. 173. Two years before, this Idaho range supported only Wyethia and 
sage. Seeding with timothy, smooth brome, and clover, and protection for 
one year produced this abundance of forage at the end of the first grazing 
season.— U. S. Forest Service. 



Because grazing is a part of every question in range ecology, the 
exclosure method is an important technique in range research. 
Exclosures are especially useful for testing experimental condi- 
tions, but they, or equivalent isolated areas, are likewise necessary 
for determining:- the nature of climax and related successional 
communities. In experimental studies, exclosures, in combination 
with grazed areas around them, are one of the better means of de- 
termining the effects of conditions in progress on that range. If 
causes are to be investigated, they are tested separately, each with 
its controlled treatment, on individual plots within an exclosure. 
Such treatments may include clipping (for grazing), burning, 
trampling, seeding, etc. As indicated earlier, the installation of 



APPLIED ECOLOGY 325 



exclosures of sufficient size, which will keep out rodents and yet 
will not alter microclimate, presents numerous difficulties. Conse- 
quently comparative studies on ranges supporting different, but 
known, animal units are coming into use whenever possible be- 
cause they do not require exclosures. 

When the results of such studies are evaluated and expressed in 
general terms, it becomes apparent that several principles have 
been established that appear to be universally applicable. 59 From 
an ecological point of view, these principles, determined by ex- 
periment, would seem to be self-evident since they conform to 
ecological theory. It must be remembered, however, that these 
things were originally theory and now can be stated as principles 
supported by experimental evidence. The testing was necessary to 
establish them as tried bases for range management. In grasslands, 
no less than elsewhere, succession is operational, and all trends 
constantly proceed toward the climax unless they are modified by 
disturbance or are held in check by an unfavorable swing of cli- 
mate, as during a series of dry years. Grassland is a climatic life 
form, which maintains itself in the absence of disturbance and 
which, if destroyed, reappears when the disturbance is removed. 
All evidence indicates that perennial grasses become dominant and 
eliminate annual grasses, forbs, and shrubs in the absence of graz- 
ing, fire, or similar destructive agencies. The grasses of a particular 
climax are adapted to its climate and usually have an advantage in 
terms of competition over introduced ones. 

From the above, it becomes apparent that, as in forestry, prac- 
tices of management which least disturb the natural balance of 
grassland and its environment are most desirable. Those that take 
into consideration the trends of succession and local climax are 
likely to be most successful at the same time that they require a 
minimum of expended effort. Although a few exotic species have 
proved to be easier to propagate than native ones, the introduction 
of foreign species for range improvement or erosion control is 
likely to be unsatisfactory unless those species are to be given 
extra care or special cultural conditions. In fact, there is evidence 
that seeding of native species should be done only with locally 
produced seeds since the species may consist of geographic physi- 
ological races. 



326 THE STUDY OF PLANT COMMUNITIES • Chapter XII 

The establishment of general principles is being followed by 
more and more intensive studies of local variations in communities 
and environments. The productivity of most range lands has been 
reduced by man's domestic animals coupled with seasons of un- 
favorable climate, and to rebuild ranges to a higher level of pro- 
ductivity will require an understanding of the special conditions 
of local areas as well as the broad principles for the region. Our 
public lands in the West, most of which are grazed, have been di- 
vided, for research and administration, in a fashion that suggests 
a natural application of the above. Several grazing regions are 
designated, which correspond to the major differences in the 
grassland and scrub formations. These in turn are divided into 
several districts, which represent local variations in dominants and 
environment. Application of the general principles is possible for 
regional administration and management, but local application 
must be modified in terms of detailed local studies. 

AGRICULTURE 

If a crop is planted and grown successfully, it follows that the 
methods applied, within the general region, to the particular field 
and for that season, were ecologically correct since cultivated 
crops are as subject to ecological laws as are plants growing nat- 
urally. Study of the ecology of cultivated plants has progressed 
rapidly in recent years. It includes crop ecology, which is applied 
ecology in the ordinary sense, and ecological crop geography, 
which considers the effects of both physiological and economic 
factors on production and distribution of crop plants. 143 With 
this addition of "social" factors to the physical and physiological 
ones, the already complex environment becomes still more so, and 
the crop ecologist must integrate his observations and conclusions 
with additional fields. This phase of ecology is, as a whole, beyond 
our consideration here, but it is appropriate to emphasize that 
ecological principles are becoming a part of our way of thinking. 
They should undoubtedly be given even greater attention in these 
days of a planned economy, which affects us all. 

Crop Ecology.— The cultivated plant is as subject to ecological 
law as a native one, and, consequently, there is as much ecology 
to be studied in a corn, tobacco, or cotton field as there is in a 



APPLIED ECOLOGY 327 



forest. To be sure, largely by trial and error, the farmer has learned 
to grow crops so that they give a reasonable return for his labor. 
But, on the whole, this has been done at the expense of the soil as 
a natural resource. The natural fertility of most of our soils is 
largely depleted, erosion has ruined thousands of acres and re- 
duced the productivity of many more, and water tables have been 
lowered to such an extent that crops in areas with rainfall suffi- 
cient for hardwood forest are suffering during dry spells as much 
as they would in grassland climate. Thanks to increased knowledge 
of fertilizers, the development of productive hybrid strains of 
various crop plants, and modern mechanized methods, our yields 
have steadily increased, but this cannot proceed indefinitely, espe- 
cially since much of the increase in yield has resulted in further 
depletion of the soil. 

To counteract the inevitable downward trend of productivity, 
soil conservation and erosion control are receiving greater atten- 
tion. Increased knowledge of crop ecology is imperative so that 
the highest yielding species will be grown under the proper con- 
ditions of cultivation and on the right sites. If possible, yields must 
be maintained at high levels at the same time that soils are im- 
proved rather than being depleted. The ecology of weeds, pests, 
and diseases must be studied so that the depredations of these prod- 
ucts of cultivation may be held in check effectively. These things 
are not being neglected by agronomists and horticulturists, but 
there are special contributions that can be made if the investigator 
has the ecological point of view. 

Land Use.— It has been customary to clear all workable land for 
agriculture, permit plowland to revert to pasture only when it 
becomes unprofitable, and permit pasture in turn to revert to for- 
est only under the same conditions. It may be desirable to reverse 
this procedure completely. Perhaps the soundest ecological ap- 
portionment of the landscape would be represented by a minimum 
of carefully selected, skillfully operated plowland with a max- 
imum of natural vegetation. Where this natural vegetation consists 
of grassland, regulated pasture is an aspect of its normal develop- 
ment; where it consists of forest, it should be scrupulously pro- 
tected against grazing, and whatever pastures are required should 
be handled with the same measure of skill that has been suggested 



328 THE STUDY OF PLANT COMMUNITIES ■ Chapter XII 




FlG. 174. Once-fertile farm land that has been unnecessarily destroyed by 
surface erosion and gullying because of lack of concern (note that straight- 
row cultivation still prevails) and lack of understanding. Perhaps this area 
should have been put into forest long since. If it had been, it would still be 
valuable.— U. 5. Soil Conservation Service. 

for the plowland. 223 

Maintaining stands of natural vegetation provides areas for eco- 
logical comparison and diagnosis, insures that soil is being rebuilt 
and retained, provides organic matter, insures a regulation of mois- 
ture conditions that man cannot duplicate, and provides food and 
shelter for wildlife, which may be significant in reducing crop 
pests. 

The planning of such land use should be, in so far as possible, 
based upon ecological principles as related to soil, topography, 
exposure, and drainage in terms of the climate and cultivated crops 
it will support. Special land-use problems arise on hilly land, which 
need not necessarily be unproductive. Ecological studies of hill- 
culture 172 are showing how some such lands may be used to grow 
orchards, vineyards, pasture, and other crops without depletion 
or erosion of the soil.* Where streams occur, it has been shown 

*Much of the following discussion of applied ecologv in agriculture is 
adapted from an unpublished report by the Committee on Applied Ecology 
of the Ecological Society of America, 1944. 



APPLIED ECOLOGY 



329 



that artificial fishponds can be a profitable investment. The eco- 
logical problems to be solved for such ponds include sizes and 
depths for different climates, drainage, amount of available water 
and necessary aeration, rate of silting under different conditions, 
fish food relations, kinds and amounts of fertilizer necessary, kinds 
of fish, and rate of stocking. Marshes might be retained and im- 




FlG. 175. A half-acre farm pond in West Virginia of the type being wide- 
ly installed for food production and recreation.— U. S. Soil Conservation 
Service. 



proved for muskrat production, but, again, the practical problems, 
largely ecological, have not been sufficiently explored. Stream 
margins create other land-use problems. Usually they are grazed, 
and, as a result, they erode. The species that would appear under 
protection should be known, as well as the most desirable species 
for checking erosion. In many sections, planted hedges and field 
border plantings are being recommended on the unproductive 
margins of fields to reduce erosion and provide cover for wildlife. 
The ecology of the planted species must be known as well as its 
effects on the crop beside it. Also the ecology of the insects, birds, 
and mammals of these margins must be known. Are they desirable, 
beneficial, or are they harmful to desirable species? 

Land Management.— The operations by which land is prepared 
for crops, their planting, harvest, and use are known as land man- 
agement. For greatest efficiency, good land management must 
parallel good land use. These are arts but, today, arts requiring all 



330 THE STUDY OF PLANT COMMUNITIES ■ Chapter XII 




FlG. 176. This eroded stream bank in Wisconsin was graded, layed with 
willow poles, and planted with a few willow sprouts. Only two seasons were 
required to produce the growth shown in the second picture where under- 
cutting is effectively stopped and shelter is provided for wildlife.— U. S. Soil 
Conservation Service. 



APPLIED ECOLOGY 



331 





FlG. 177. Waste field-margins such as the fourteen-foot strip (1) aban- 
doned because of root competition and erosion can be made useful. (2) 
Lespedeza bicolor (tall) and L. sericea planted in strips are holding the mar- 
gin stable and producing food and cover for small game.— U. S. Soil Con- 
servation Service. 



the help of science possible. 226 A farm planted year after year to 
wheat or cotton does not, even with fertilizer, conform to the 
balances that occur in nature. Well-managed fields may seem to 



332 THE STUDY OF PLANT COMMUNITIES ■ Chapter XII 

approach a condition of balance as a result of rests with rotation 
pasture, the use of legumes, and the addition of fertilizer. Yields 
may be high and sustained, soil may not erode, and all appear to 
be at its best. 

In terms of natural vegetation, however, our modern methods 
of land management may be questioned. Cultivation produces 




FlG. 178. A simple illustration of improper management. The amount of 
runoff on this slope means leaching and erosion. Certainly the rows should 
not have been put in up and down the hill, and perhaps, without terraces, 
clean cultivation should be ruled out on this field. — t/. S. Soil Conservation 
Service. 



conditions similar to those in early stages of succession, conditions 
that in nature would be temporary and soon change in the direc- 
tion of climax. We must have crops, but, if climax vegetation 
utilizes natural conditions most effectively— and that seems reason- 
able—are our methods of cultivation the best we can use for ob- 
taining our crops? Is our method of deep plowing, with destruc- 
tion of soil structure, best under all conditions? Should all crops 



APPLIED ECOLOGY 



333 




FlG. 179. Deciduous forest farm wood lots, pastured (above) and not pas- 
tured (below), which illustrate the effects of browsing and trampling on 
reproduction and general forest condition.— U. S. Forest Service. 

be cultivated clean and all organic matter be turned into the soil? 
Might mixed crops producing a complete cover as in nature not 
be more desirable? Perhaps we have gone too far in producing 



334 the study of plant communities ■ Chapter XII 



unnatural conditions. The artificial environment of cultivation re- 
sults in soil erosion, a modified soil flora and fauna, and changes in 
water relations. Also we have more diseases of crop plants and 
more insect pests than ever before. 

These are ecological problems. Intelligent land use minimizes 
some of them. Practices like contour plowing, terracing, and strip 
cropping are moves in the direction of reducing them. But when 
the ecology of crop plants is studied further, especially in terms 
of natural vegetation, some of our methods of use and management 
may require revision. 

Pasture Problems.— Above, it was suggested that the same atten- 
tion to management should be given to pastures as to plowed land. 
This would be a reversal of the usual point of view since pastures 
are, more often than not, largely on the poorest land and are given 
little or no attention. With the steady expansion of dairying, espe- 
cially into sections of the country where adequate pastures do not 




FlG. 180 (1). An Indiana field after fall plowing showing severe erosion. 
Picture taken when it was decided to retire field to permanent pasture with 
contour furrows. 



APPLIED ECOLOGY 



335 




Fig. 180 (2). The next year, after gully-control work, this excellent 
planted pasture had taken over, the soil was stabilized and the field saved for 
long-continued usefulness.— Both photos by U. S. Soil Conservation Service. 

produce themselves, the need for pasture ecology increases. The 
necessity for seeding is now widely accepted. Many species have 
been tested for palatability, yield, food value, and soil-building 
properties. Growing pastures is still, however, largely a hit-or- 
miss affair that requires much more study. Regional pasture ecol- 
ogy has not progressed as far as range ecology. There is much yet 
to be learned, tested, and put into practice. The implementation 
of such a program will be difficult in many sections where pastures 
are not generally recognized as a crop to be managed like any 
other. 

An illustration of the misconceptions regarding pasture is the 
common practice of including the farm wood lot in the pastured 
area although it provides little more than browsing, which sup- 
plements feed during off seasons. To the ecologist, it is obvious 
that this is at the expense of seedlings and ground cover and that 
it will result in stand deterioration. 160 Silviculturists have shown 



336 THE STUDY OF PLANT COMMUNITIES ■ Chapter XII 

that properly managed wood lots can yield as great a return as 
any average farm acreage, but the wood lot pasture persists. A 
study of maple groves in Ohio 225 showed that in three years the 
elimination of grazing resulted in an increased yield of maple 
syrup, worth more than twice what the rental for pasture would 
have been. At the same time, the condition of the stand was no- 
ticeably improved. As more such information is accumulated 83 it 
is to be hoped that its application will follow. 

Regional pasture studies must be continued so that both species 
and their culture can be recommended with confidence for cli- 
mate, soils, and land management policies as they occur. To obtain 
such results, it would appear that ecological methods should be the 
most promising. 

Insect Problems.— The relationship between land-management 
practices and insect populations is inadequately known. 116 
Whether insect pests will increase or decrease with strip-cropping 
or particular crop rotations cannot be said with certainty. Prob- 
ably more complex are the relationships of insect populations to 
the birds and mammals that will appear in response to such con- 
servation practices as cover crops, hedges, and field border plant- 
ings. Whenever the acreage of a cultivated species is increased 
extensively in an area or a new species is introduced for special 
purposes such as erosion control, insects may appear with it or 
abruptly increase in numbers to pest proportions. Such relation- 
ships and innumerable others need more study. The possibilities 
for applied insect ecology in agriculture and forestry are almost 
unlimited. 

Rodent Problems.— Especially for range lands, ecological knowl- 
edge of rodents is still inadequate. In spite of this, rodent control 
has been attempted in these areas for years. More should be known 
of the effects upon rodent populations of kinds and degree of 
grazing as well as what effects the various rodent-control measures 
have on the condition of the range. With the latter, it should be 
possible to say what percentage of a rodent population can be 
destroyed by a control measure, how long before the surviving 
population will return to normal, and to what extent species move 
in from untreated areas. Complicating the above problems is the 
usually cyclical fluctuation of most rodent populations and the 



APPLIED ECOLOGY 337 



obvious desirability for adjusting control to these natural fluctua- 
tions. Other suggested ecological problems are the relationship of 
rodents to reseeding, succession, and climax in range land, and 
their numbers and effects upon orchards when managed with 
cover crops. 

Weeds.— The occurrence of weeds as a result of land use and 
their control by cultural practices have received far less attention 
than control by direct, aggressive means. Yet cultural control or 
control as a result of good land management is likely to be the 
most permanent and least costly. Certainly the weed problem has 
not been reduced by centuries of cultivation, mowing, and burn- 
ing. Even modern ''hormone" sprays are no panacea. 208 If progress 
is to be made, the autecology of the principal weed species must 
be studied in detail. If, then, the effects of various types of land 
use and management upon the occurrence of specific weed species 
is learned, there is a reasonable possibility that ecological controls 
could be recommended that would reduce the weed problem, 
under certain situations at least. 

CONSERVATION 

The problems of conservation are extremely diverse, including 
as they do such things as soil and soil water, wildlife of all kinds, 
and aesthetic considerations. All that we have discussed of applied 
ecology could be classified under the general heading of conserva- 
tion. The field is so broad as to require specialists of all kinds in 
its management, but this, of all fields, requires training to see each 
problem in the light of others. Nowhere can the ecological point 
of view be more effectively applied. 250 

To illustrate the limited effectiveness of specialization without 
ecological appreciation, witness such operations as have been 
known to take place almost simultaneously on public lands : a 
road crew cutting a grade in a clay bank so as permanently to roil 
a trout stream that another crew is improving with dams and 
shelters; a silvicultural crew felling wolf trees and border shrub- 
bery necessary for game food; a roadside cleanup crew burning 
all fallen oak fuel available for fireplaces that are being built by a 
recreation crew; a planting crew setting out pines in the only 
open fields available to deer and partridge; and a fire-line crew 



338 THE STUDY OF PLANT COMMUNITIES ' Chapter XII 

cutting and burning all hollow snags on a wildlife refuge. 153 Such 
conflicting activities have not been uncommon in the name of 
conservation. Some government agencies have spent millions for 
flooding marshes and improving them for wildlife while other 
agencies were attempting to drain marshes of questionable agri- 
cultural value. Great dams have been built for reclamation pur- 




FlG. 181. The deposition of silt and sand behind a dam in this fashion de- 
feats its purpose of water storage and reduces the efficiency as a source of 
hydroelectric power.— U. S. Soil Conservation Service. 

poses, but the watersheds above them have been ignored. 227 With 
continued lumbering and grazing, the reservoirs are silting in so 
rapidly that the usefulness of the dams promises to be short-lived. 
To assure integration of such activities may not require a "declara- 
tion of interdependence" 250 but certainly the recognition of the 
interdependence of biological phenomena is necessary. This end 
will certainly be served if those responsible are ecologically trained 
or have an ecological point of view regardless of their special in- 
terests. 

Soil Conservation.— The recognition of soil conservation as a 
national problem is of recent origin. The Soil Conservation Service 
was made a permanent bureau of the U. S. Department of Agri- 
culture in 1935 athough it originated as the Soil Erosion Service 
in the Department of the Interior in 1933. Since then great prog- 



APPLIED ECOLOGY 339 



ress has been made in educating the public to the need for a con- 
tinuous program of conservation, and soil conservation as a science 
has developed rapidly. The scope of the field and the problems 
involved have been admirably summarized in various publica- 
tions. 18 ' 136 

Early publicity by soil conservationists was essentially a plea to 
save our irreplaceable land, a great deal of which was already per- 
manently lost and much of which is in the process of being ruined. 
More recently, the emphasis has been upon rebuilding lands that 
have deteriorated. The modern philosophy considers soils, like 
forests, to be natural resources that are renewable and, therefore, 
subject to management that will give a sustained yield over an 
indefinite period of time. 174 Such a program is, of course, as justifi- 
able as the original, which aimed primarily at erosion control. It 
indicates that the conservation program has been successful and is 
maturing. 

Soil conservation is, therefore, more than erosion control. It 
also involves the retention of water, especially on slopes, and its 
utilization to best advantage. At the same time, it aims to maintain 
or increase soil fertility and productivity. Thus soil conservation 
is merely the practice of agriculture in the best possible way, and 
we have already suggested how the ecological approach to such 
problems is most likely to be successful. 

Not all the various measures successfully introduced for erosion 
control and soil building are applicable everywhere but must be 
adjusted in terms of soil types and climate. However, certain gen- 
eralizations can be made which have wide application and whose 
special use or desirability often must be determined by a knowl- 
edge of local ecology. Vegetative- cover is the most effective means 
of checking erosion. This raises questions as to what cover is de- 
sirable or possible under different conditions, where it should be 
permanent, and when it should be of native vegetation. These 
problems are related to strip-cropping, gully control, cover crops, 
and decisions to cultivate hilly land, put it into pasture, or plant it 
to forest. It is now assumed that the control of erosion will pay 
dividends only when proper crop rotations and fertilizing prac- 
tices are followed. The interrelationships must be known for every 
crop and region. 



340 THE STUDY OF PLANT COMMUNITIES ' Chapter XII 

Much advance has been made in cultural practice. Contour 
plowing, in which cultivation follows lines of equal elevation, is 
becoming steadily more common. In many areas, strip-cropping 
is an additional control, in which clean-cultivated crops are 
planted between strips of cover crops, such as legumes, which 
retard runoff and hold soil. A further necessity on contoured 




FlG. 182. Aerial view showing strip-cropping of terraces that follow con- 
tours. Erosion is checked, much water is retained, and what runs off is di- 
rected to a sodded runaway channel. Such elaborate operations may require 
co-operation of several landowners. In this instance, two farms are involved. 
— U. S. Soil Conservation Service. 



slopes may be terraces, which are ridges so placed that they catch 
and hold water in a channel behind themselves and thus check 
runoff and cause water to soak in. In special instances, deep fur- 
rows are maintained (listing) in which water and snow are held 
and crops are planted in the bottom of these troughs. Basin listing 
is done on some soils with special machinery that shapes these 
troughs with cross dams at regular intervals further to reduce 
runoff. It has been shown that wind erosion can be reduced by 



APPLIED ECOLOGY 341 



"stubble mulching" in which subsurface tillage keeps old organic 
debris on the surface. Windbreaks of various kinds are known to 
be effective also. 

All these are examples of modern practices that are proving 
effective under special conditions. They are not by any means 
new, since they have been reported in various forms far back in 
history. It is their application in the light of modern knowledge 
that marks advance. The more complete the knowledge of all fac- 
tors involved— crop, soil, climate— the greater the success of their 
application in the future. The research programs continue, and the 
kinds of investigations in progress are invariably ecological in 
nature. Here is a list of a few of the projects being studied for a 
single district : 172 

1. The effect of contouring corn, soybeans, and oats on 
soil and water conservation 

2. The effect of divergence of rows from the contour on 
losses of soil and water 

3. Cultural practices and methods of handling crop resi- 
dues in relation to soil and water conservation and crop 
yield 

4. The effect of cover crops on the conservation of soil 
and water and on crop yield 

5. Investigations of soil moisture content under different 
crops, cropping systems, and mechanical conservation 
practices 

6. Effect of crops and organic matter treatment on the 
movement of water through the soil profile 

Other studies include effects of cropping systems, crop rota- 
tions, handling of crop residues, and management in terms of run- 
off, yield, and soil properties. 

Some special problems of soil conservation still requiring a 
great deal of study are related to drainage of water-logged land 
and swamps, irrigation of lands with insufficient water, clearing 
of toxic salts from irrigated land and other lands not previously 
cultivated. 

Water Supply-— The conditions necessitating soil erosion con- 
trol and the prevention of runoff of surface water are commonly 



342 THE STUDY OF PLANT COMMUNITIES * Chapter XII 

reflected in the general water supply. In many agricultural areas 
with adequate rainfall, there are water problems that did not exist 
at the time of settlement. Where once streams and springs were 
abundant and flowed continuously, now they are intermittent, 
and summer water supplies are often low. In Ohio, the water table, 
as evident in well depths, is from fifteen to fifty feet lower than 




FlG. 183. The type of dam and spillway being installed primarily for 
water conservation. When full, this reservoir extends fifteen miles upstream 
over an area of 10,000 acres. The flow from the dam can be controlled, there- 
by providing constant flow during dry periods and reducing danger of flood- 
ing with high water.— U. S. Soil Conservation Service. 



originally. Floods appear to be more frequent and are certainly 
more destructive than before. On the credit side, there are now no 
malaria problems related to undrained swamps or typhoid epi- 
demics resulting from improper city water supplies. 223 The adverse 
conditions result partially from the removal of natural vegetation 
for agriculture. As much water falls today as before, but more of it 
runs off rapidly. Thus summer drought and spring floods are par- 
tially explainable. 

There are other contributing factors. Roads, so important to the 
farmer for transportation, likewise serve to drain off water from 
his fields. This has been especially bad in the mid-western states 
where all roads were originally laid out in an east-west, north- 



APPLIED ECOLOGY 







Ep||b& .,,vv...;.:: 



i*r*f*^ 






- 
- 






Fig. 184. A power-dam lake at the edge of a town in Minnesota as it 
appeared in 1926 when it was extensively used for fishing and recreation. By 
1936 excessive silting had left only a small channel. Watersheds above the 
dam were improperly handled; timber was removed, slopes were cultivated 
and few precautions were taken to prevent erosion.-L/. 5. Soil Conservation 
Service. 



344 THE STUDY OF PLANT COMMUNITIES • Chapter XII 

south grid pattern of blocks, which disregarded topography and 
provided a powerful system of artificial drainage. Also great drain- 
age projects were instituted in the earlier days of agriculture, and 
these, too, served to speed the removal of water. 

The trend in concern over surface water proceeded from drain- 
age projects to those dealing with flood control. Such concern is 
still with us, and necessarily so, because of the destructiveness of 
floods to both property and land, but, a new trend is now apparent 
in the attempts to conserve, retain, and store water so that it may 
be available when needed, so that water tables may be held at 
higher levels, and so that flood waters may be controlled. Dams 
and reservoirs are being constructed and watersheds are being 
protected. 

A recent factor in the lowering of water tables is the great in- 
crease of use of water in industry and the rapid increase of air- 
conditioning. Much of the water used for the latter is wasted 
because it is not used for any other purpose. The lowering of the 
water table by using water for this purpose has caused much con- 
cern in large cities. Various legislation is aimed at controlling the 
use of this natural resource. Most large users drill their own wells, 
but this practice is being limited. In some cities, it is required that 
the water must be forced back into the earth at the levels from 
which it is drawn. 

Our water supply is a natural resource just as are the others we 
have discussed. When its availability is reduced, it affects agricul- 
ture, industry, fish and game, recreation, and perhaps home use. 
The trend is already in the direction of its conservation. Probably 
it will go further. Ecological problems of many kinds will arise in 
connection with control of water in streams and reservoirs, and 
the effects upon water table levels. It is a part of all the applied 
ecology we have discussed. 

Another facet of the problem of water supply is its pollution by 
industrial waste and sewage. Here again, there are innumerable 
problems of an ecological nature. Their solution often requires the 
co-operation of engineers, chemists, bacteriologists, and limnol- 
ogists. As always, when such specialists are drawn together, their 
success is greatest when they see their own fields in relation to the 
whole. This is the ecological approach. 



APPLIED ECOLOGY 345 



Wildlife.— Like soil, water, and forests, our wildlife constitutes a 
renewable natural resource, which, consequently, can be restored 
or maintained even while it is used, if the use is a wise one. All of 
these renewable resources are so intimately related that a program 
for the conservation of one must necessarily consider the others 
as well. This ecological point of view is fully appreciated by lead- 
ers in wildlife management. It is also realized that, when man be- 
comes the dominant organism, the management of soil, water, 
forests, and grassland is inevitable— and wildlife, too, if it is to be 
preserved. 

If wildlife management is to be successful, man must know the 
ecology of the species involved, whether they are fish, birds, or 
game animals. Life cycles must be known, as must breeding habits, 
food habits, and food chains, migration routes, preferred habitats, 
diseases, predators, population trends, and the carrying capacities 
of given habitats. Such complete information is not yet available. 
"In its present state, wildlife management is an effort to apply to 
urgent problems the ecological and biological data that are now 
available, always with the consciousness that existing tools, meth- 
ods, and processes may have to be discarded as new and better in- 
formation becomes available!' 100 Ecological knowledge is still woe- 
fully incomplete for most of our wildlife, although information 
accumulates steadily. As it accumulates, programs of management 
increase in effectiveness. 

The range of ecological problems related to wildlife manage- 
ment is tremendous. The complexity of management can perhaps 
be suggested by indicating some of the kinds of things that must 
be taken into consideration. It would seem that, if food and cover 
are provided for an organism, its needs should be satisfied. But, for 
many species, the feeding habits are inadequately known. Cover 
can be provided for some species but, under present conditions, 
frequently only in localized areas. If that is true, it is not uncom- 
mon for food problems to become complicated during the winter 
months when the species tends to become concentrated on these 
restricted areas. A population that is reasonable in summer may 
become excessive in winter and result in death by starvation for 
many individuals. Encouraging the increase of one species may be 
detrimental to another one; consequently, individual species must 



3 46 THE STUDY OF PLANT COMMUNITIES * Chapter XII 

be studied in relation to others. In this connection, predation must 
be considered from an ecological standpoint. 

Species whose numbers have declined to extremely low levels 
may be propagated under controlled conditions and then released, 
but the cost is often excessive. Others may be taken from areas of 
overpopulation and transported elsewhere to start a new popula- 
tion. Such activities have sometimes been successful but in other 
instances have failed because of factors that were not known or 
understood. The ecology of the species and of the region must be 
known. If it is known, there is a reasonable possibility that the 
species can be encouraged to increase naturally at much less ex- 
pense and trouble. The problems related to overpopulations of 
protected species are no less complicated, the ideal being a condi- 
tion in which natural propagation produces a constant popula- 
tion supportable by the environment and perhaps an excess suffi- 
cient to permit a reasonable take by the sportsman. 

When it is realized that such problems and many more are in 
the process of solution for big game, birds of all kinds, fur ani- 
mals, fish, and other wildlife, it should be apparent that there is 
much basic ecological work to be done that has possibilities of 
application. The mistakes that have been made in wildlife manage- 
ment have undoubtedly resulted more often from inadequate eco- 
logical information rather than from lack of appreciation of how 
such knowledge could be applied if it were available. Wildlife 
management is applied ecology, and it will progress as basic eco- 
logical knowledge becomes available and is integrated by wildlife 
ecologists. 

Game refuges provide a safeguard against lack of knowledge 
and provide the opportunity for acquiring needed information. 
Particularly, they insure that scarce or disappearing species do not 
become extinct as some have in the past, for here they are pro- 
tected and given every encouragement to increase. Usually such 
refuges do not result in the restoration of a vanishing population. 
They do, however, insure a continuous breeding stock from which 
restoration may be made, and they give excellent opportunity for 
the study of the species involved under relatively undisturbed ' 
conditions or under available conditions. 101 A few such refuges 
are still in near primitive condition and thus can provide much of 



APPLIED ECOLOGY 



347 



the biological knowledge of habitat, vegetation, and wildlife that 
must be learned to manage other refuges and ultimately the gen- 
eral program of wildlife conservation. Other refuges provide the 
testing grounds for management procedures as knowledge accu- 
mulates. 



/.>- r+T* 



*&m 



, ** < ***r 





BfeWu ■■'■ ^■P^ 




-*^ ., 








^^''~ — * 31 




xrCwjr,-^- •■ 2SB& 



:.«*? 




FlG. 185. An unsightly, eroding road cut in Illinois and its stable appear- 
ance three years after planting with trees that blend with topography and 
native vegetation.— (7. 5. So/7 Conservation Service. 



348 THE STUDY OF PLANT COMMUNITIES * Chapter XII 



LANDSCAPING 
The planning and planting of vegetation for home beautifica- 
tion or in public parks or gardens involves aesthetic considerations 
but likewise should be backed by an appreciation of the ecology 
of the species involved. If plantings are not made in terms of the 
requirements of the species used, they cannot be successful. Soil 




FlG. 186. On such road-building projects erosion control must be given 
serious and prompt attention. These great fills have been stabilized by me- 
chanical means and have likewise been planted. If aesthetic considerations 
have entered into the stabilization program, they are not yet apparent— U. S. 
Forest Service. 

texture and structure must be considered as they affect water 
relations. Slope and exposure modify drainage and temperature 
just as they do in natural environments. Tolerance of shade, light, 
or extremes of temperature cannot be ignored when planning 
artificial combinations of species. Some species must be planted in 
moist places, some require full sunlight, some need to be partially 
shaded. Competition and all the other factors affecting natural 
communities operate among planted species as well. The same 
factors that limit the ranges of natural communities operate to 
limit the usable materials of landscape design for different sections 
of the country. Landscaping is, therefore, most successful when 
based upon ecological principles. 



APPLIED ECOLOGY 



349 



Natural landscaping is a recent development resulting from 
man's modern engineering activities, which drastically change to- 
pography, drainage, and vegetation when he constructs modern 
highways, dams, and airports. Great exposures of subsoil in cuts 
and fills require cover and replanting not only for aesthetic rea- 
sons but also to check erosion and slumping. It is to be expected 
that engineers should give first consideration to the efficiency of 
installation and use of a project under construction, but, when this 





FIG. 187. The old and the modern manner of handling a road cut. Note 
the gradual back slope, seeded surface, and shallow, sodded runoff channel, 
all designed to check erosion.— U. S. Soil Conservation Service. 



350 THE STUDY OF PLANT COMMUNITIES * Chapter XII 

has been the only concern, after effects on drainage and erosion 
have frequently created serious problems. Not only has natural 
beauty been destroyed unnecessarily at times, but extensive ex- 
panses of bare soil, in fills and cuts, have been left for nature to 
recover and stabilize. The re-establishment of natural vegetation is 
often impossible before erosion and slumping cause disruption of 
drainage, road blocks, and similar difficulties. Consequently, stabil- 
ization must be provided for through artificial means and by seed- 
ing and planting. The problem is intensified by the infertility of 
the subsoil, upon which few things will grow. Although the first 
concern should be stabilization, there should be consideration of 
succession and the possibility of harmonizing the developing vege- 
tation with that of the surrounding terrain. 

In addition to large cuts and fills along mountain highways, 
there are problems of maintaining road shoulders, ditches, and 
spillways. Certainly not all is known about the best species for 
such purposes under all conditions. Also the natural beauty de- 
stroyed by a new right-of-way need not be permanently lost. 
With a minimum of management it would seem that native species 
could be encouraged to provide cover and beauty, especially along 
the new express highways, which are increasing in number. It 
does not seem impossible that ecological knowledge applied in 
advance could prevent some erosion and drainage problems and 
save some of the destruction of natural vegetation. Certainly road- 
side ecology is worth considering both practically and aesthetic- 
ally. 

PLANT INDICATORS 

Elsewhere we have emphasized that plant communities give a 
better indication of the nature of environment than we can obtain 
by measurements of individual factors. The character and make- 
up of vegetation is an expression of the integrated effects of all 
factors operating in a habitat. When the relationships involved are 
well known, the vegetation becomes an indicator that can be in- 
terpreted or, in some instances, read like an instrument. 

The practical use of plants as indicators is nothing new, for 
Pliny 135 wrote of selecting soil for wheatland by the natural vege- 
tation it supported. More recently, in the settling of North Amer- 
ica the pioneers used the principle widely in selecting their lands 



APPLIED ECOLOGY 351 



for agricultural purposes. With increasing knowledge, their selec- 
tions became more effective as is indicated today by lands that 
have been abandoned and that have remained so. In any agricul- 
tural region, an experienced farmer knows the characteristics of 
soils and habitats supporting local peculiarities of vegetation, or 
often only a single indicator species. 

Such practices and beliefs are usually the result of trial and 
error experiences, as well they must be, until the responses of a 
crop plant are tested under the conditions indicated by native 
vegetation. The knowledge has often been acquired after costly 
experience. If the requirements of an introduced plant are known 
and the characteristics of the habitats of native species are studied, 
the guessing may be reduced. Selection of native species as indica- 
tors of local conditions and fitting the ecological requirements of 
appropriate cultivated plants to these conditions involves ecologi- 
cal methods and thinking. Actually this is not easily accomplished, 
because of our still limited knowledge of the ecology of both na- 
tive and cultivated plants. It suggests the possibilities of the indica- 
tor method, however, in an applied field. 

The scope of possible uses of indicators involves much of the 
entire field of ecology, which necessarily limits the discussion 
here. Clements' 57 exhaustive treatment explores most of the possi- 
bilities of their application, and many of these we have considered 
in other connections. Consequently, only certain practical aspects, 
in which they have been successfully applied or might be further 
expanded will be discussed. The available source material has been 
brought together, and a review is available on the modern status 
of the concept and its application. 212 

It may sometimes be difficult to recognize or select indicator 
species. Those with restricted distributions and those tolerating 
only narrow ranges of habitat conditions should be most useful. 
Such plants should show responses to. minor habitat differences. 
Thus it follows that similar local conditions in different climatic 
areas would probably support different indicators. Also the same 
species might not always be indicative of the same things through- 
out its range. Differences in geological or cultural history might 
make it necessary to interpret the significance of an indicator since 
it need not always be the same. It is rather generally agreed that 



352 THE STUDY OF PLANT COMMUNITIES * Chapter XII 



a group of species or a whole community is more reliable as an 
indicator than a single species and that dominants, especially of 
the climax, 57 or at least characteristic species 34 are more useful in- 
dicators than lesser species. Above all, application of the method 
cannot be successful without judgment, good sense, and interpre- 
tation in terms of each situation. 

Agricultural Indicators.— That crop centers and types of agri- 
culture are correlated with climate and climax vegetation is obvi- 
ous. The agricultural areas of North America follow a pattern 
very similar to that of a map of natural vegetation. 229 The north- 
eastern conifer region suggests general agriculture at the lower 
altitudes and latitudes where the land is level and soil is deep. In 
the transition from boreal to deciduous forest, white pine-red 
pine-jack pine forests are on sandy soils, which are, in general, 
undesirable for agriculture, while the northern hardwoods-hem- 
lock forest indicates the best soils for cultivation. The range of the 
deciduous forest formation marks the best agricultural region of 
the east with the greatest diversity of crops. Away from the south- 
ern Appalachian and Ohio Valley center, as the associations be- 
come less complex and oak and hickory become relatively more 
important, so also does agriculture become more specialized. 

On the prairie, both tall and mixed grasses indicate fertile and 
productive land for cereals, hay, and fodder. Likewise, the natural 
grass cover provides valuable grazing facilities. The short grass 
area indicates productive soil whose cultivated crops are limited 
by moisture. The most favorable sections can be dry-farmed, but 
otherwise irrigation is necessary for cultivation. As a result, the 
land is most widely used for grazing. 

Vegetation indicating general land use has been given more at- 
tention in the western United States than elsewhere. 230 Subalpine 
vegetation indicates a growing season too short for cultivated 
crops, steep slopes, and poor agricultural soil. The montane zone 
also has a short season with cool weather but permits some culti- 
vation if the land is not too rough. Pinon-juniper in the woodland 
zone indicates productive soil if irrigation is possible, but chap- 
arral indicates inferior agricultural land under almost any circum- 
stances. 

Plant indicators of land use in the arid regions of the West are 



APPLIED ECOLOGY 



353 



rather well known because of several intensive studies in different 
areas. Irrigation is necessary everywhere except on the best soils 
in the sagebrush areas of the northern portion of the Great Basin. 
Elsewhere, in addition to the need for irrigation, native species in- 
dicate other necessities or precautions. 228 The tabulation on page 
354 although specifically applicable only to the Sonoran Desert 
region of Arizona and southeastern California, illustrates the prin- 
ciples involved. 




FlG. 188. These productive fields and orchards in Hurrican Valley, Utah, 
irrigated from the big ditch at left, are bordered on all sides by sagebrush 
desert. Knowledge of natural vegetation and soil gained from such projects 
makes possible confident statements of probable success or failure when 
others are to be established— U. S. Forest Service. 

These generalizations indicate how natural vegetation may be 
useful in determining regional land use. It is the details of local 
conditions as indicated by native species that need more study. 
If the equivalent cultivated and native species were known for 
different soils, sites, and exposures, it would be possible to state 
with confidence which fields should be cultivated and which 
should be put to pasture or wood lot, as well as which crops 
should be grown in a particular field. The more complete such 
knowledge is, the more effectively land can be used, and the more 
certainly land values can be fixed for sale and taxation. 



354 the study OF plant communities • Chapter XII 



TABLE 11— Potentialities of Lands for Crop Production as Indicated by the 
Principal Plant Communities of the Southwestern Desert (after Sampson 212 ). 



Vegetation 



Creosote bush 



Desert sage 

Mesquite and 
chamiso 

Chamiso 

Mesquite thicket. 

Seep weed 



Saltbush and 
arrowweed . 
Pickleweed . . . 

Saltgrass 

Yucca-cactus . 



Giant cactus- 
paloverde . . . 



Predominant species 



Larrea divaricata 



Atriplex polycarpa 



Prosopis glandulosa 
Atriplex canescens 

Atriplex canescens . 
Prosopis glandulosa 

Dondia intermedia . 



Probable success 
under irrigation 



Atriplex lentiformis 

Pluchea sericea 

Allenrolfea occidentalis 

Distichlis stricta 

Yucca mohavensis 
Ferocactus acanthodes 
Oppuntia bigelovii . . . 

Carnegia gigantea 
Cercidium torreyanum 



Successful where na- 
tive cover is luxuri- 
ant; of doubtful suc- 
cess on lands of rock 
outcrop or with rock 
layers or hardpan 
Successful where 
native cover is lux- 
uriant; of low value 
on hardpan soil 
Partly successful; 
special crops on 
level tracts 
Successful 
Successful when 
salts leached out 
Not successful ; 
much abandoned 
farm land on this 
cover. Successful 
when salts leached 
out 

Successful when 
drained 

Successful when 
drained and leached 
Successful when 
drained and leached 
Partly successful; 
land usually too 
steep or soil too 
rocky 

Successful when 
drained 



APPLIED ECOLOGY 



355 



Land evaluation on an ecological basis has been made use of at 
various times, and a simple illustration will serve to indicate the 
possibilities. Not long ago the construction of dams for water 
control in the upper Mississippi River necessitated legal action to 
fix the value of much lowland that would be flooded when the 
project was completed. One of the basic questions involved the 
establishment of criteria for determining which acreages were 
cultivatable and which were not. It was possible to show by means 
of the natural vegetation, regardless of whether the land had or 
had not been cultivated, which areas were only rarely flooded 
and, therefore, desirable agriculturally, which flooded frequently, 
and which were always too wet for cultivation. Once this was 
worked out it could be applied generally throughout the area. 
The information was used effectively for establishing equitable 
land values in several court proceedings. 

Range and Pasture Indicators.— The use of plants as indicators 
is basic to range management. 248 A knowledge of the important 
indicator plants and the application of their meaning to handling 
of grazing land has become fundamental to successful manage- 
ment. Plant indicators are used to judge the condition of the range 
and particularly to recognize signs of deterioration or improve- 




FlG. 189. Death of shrubs and a browse line in a pasture as indicators of 
too heavy grazing by cattle— U. S. Forest Service. 



356 THE STUDY OF PLANT COMMUNITIES * Chapter XII 



ment under certain usages. They are used to determine the kind, 
degree, and time of grazing, and for determining the grazing 
capacity of a range. When the plants present are considered in 
conjunction with soil conditions and the climax, the previous use 
of the range can be interpreted and its potential usefulness under 
proper management can be predicted. 






FlG. 190. Winter range (Atriplex nuttallii) in Colorado, so badly over- 
grazed that there is practically no vegetation left and gullying is serious on 
all the slopes. Such depletion is obvious to anyone, but recognition of the 
onset of these conditions should be possible for those who know the indica- 
tors.— U. S. Forest Service. 



Misuse of range lands is obvious in late stages, but it is difficult 
to recognize when it first begins and should be corrected. Among 
the indicators that must be watched for are thinning of cover and 
a lowered vitality of the principal species, replacement of good 
forage plants by inferior ones, close grazing of species that ordi- 



APPLIED ECOLOGY 357 



narily would not be preferred, and, with this, accelerated ero- 
sion. 249 It is also highly desirable that the slow successional changes 
in species composition resulting from grazing under a certain 
system be recognized. Usually if these are in the direction of 
climax, they are advantageous. If they show an increase of forbs 
or of unpalatable species, management practices must be corrected 
before the trend becomes serious. 

In each grazing region, the significant indicators must be known 
and interpreted. Often selected species can be used and checked 
upon to simplify evaluations. Likewise, restricted areas, selected 
on the basis of experience, may be used for observation as repre- 
sentative of the general conditions on a range as a whole. 

Range management is obviously applied ecology in which indi- 
cators play an important part. The more completely the ecology 
of the species and communities is known under grazing condi- 
tions, the more readily their responses can be interpreted and the 
more effective management practices can be. 

Forest Site Indicators.— In forestry, as in agriculture, the indica- 
tor significance of one group of plants must be interpreted and 
applied to an entirely different group of plants. Since forest indi- 
cators are commonly herbs or shrubs, there is often some diffi- 
culty in translating their meaning to apply to trees. In the broad- 
est sense, forest indicators are site indicators, but rarely do they 
suggest more than a portion of the several factors that contribute 
to site. Physical or chemical characteristics of soil, moisture rela- 
tionships, aeration, or erosion may be indicated by some species. 
With these and others the probable development of a particular 
stand can be interpreted. Still others may indicate the past history 
of vegetation on the site or the probable successional trend to be 
anticipated in the future. 

It is fundamental to indicator interpretation that the succes- 
sional trends of a region be thoroughly understood for every type 
of habitat. Only when an indicator is considered in relation to the 
stage of succession concerned can its meaning be at all clear. 

The use of subordinate or dependent species as indicators of 
site quality has been attempted under various conditions since 
Cajander 51 set up such a system for classifying forest types in 
Finland. This system assumes that, since communities of similar 



3 58 THE STUDY OF PLANT COMMUNITIES * Chapter XII 

structure occupy similar sites, it is possible to judge a site and the 
nature of the dominants from the ground cover alone. Thus recog- 
nition of the herbs, mosses, and lichens on the forest floor with an 
estimate of their relative proportions might suffice for evaluation 
of the stand and the quality of the site on which it grows. 

Perhaps the most comprehensive attempt to apply the method 
in North America was made in the Adirondack Mountain area. 121 
Elsewhere smaller areas with fewer communities have been studied. 
Although special phases of the method have proved useful in cer- 
tain situations, the method as a whole has found limited applica- 
tion. Although herbs undoubtedly affect the dominants by modi- 
fying soil structure and water relations, and likewise through 
competition with seedlings of dominant species, there are argu- 
ments against the validity of information based on herbs alone, 
particularly since they derive water and nutrients from different 
soil horizons than do the dominant trees. It is, therefore, suggested 
that all the lesser woody vegetation should also be included. There 
is evidence that the same herbaceous species predominate on more 
than one soil type, and, therefore, their significance is questioned. 
Often the indicator types are of limited extent, and several may be 
present within a single stand. Interpretation, then, becomes diffi- 
cult in terms of management. Undoubtedly, the foresters' not 
uncommon lack of familiarity with lesser vegetation and frequent 
inclination to ignore it entirely have been factors in limiting the 
testing and application of the method in American forests. 

Because the subordinate vegetation changes after lumbering or 
fire and because height of trees, the commonest criterion of site, 
cannot then be known, it is desirable that some relatively simple 
means of evaluation of site be available that can be applied at any 
time. T S. Coile 65 has approached this problem through physical 
measurement of the soil, as others have attempted before, and, 
after extensive investigation, has found that the site index can be 
accurately determined if only the depth of the A horizon and the 
soil type are known. Using the xylene equivalent (determined like 
moisture equivalent) of the B horizon (known for the soil type) 
and the depth of the A horizon, a positive statement of site quality 
can be made whether the land is in forest, cultivated, or abandoned 
and regardless of slope or exposure. This would seem to be the 



APPLIED ECOLOGY 359 



most promising approach to recognition of site quality. Once 
these two factors are known for the soils of an area, they can be 
recorded like a soils map, which then becomes a map of site index 
to be interpreted for management purposes. 

Innumerable indicators, other than site indicators, are used in 
forestry. Relicts are particularly useful, and successional indi- 
cators are applied regularly. Special instances have been suggested 
elsewhere. It is appropriate to emphasize that indicator applica- 
tions are invariably successful when the ecology of the region and 
the species is known. 

HUMAN ECOLOGY 

Perhaps this final section seems out of place in a textbook 
intended as an introduction to plant ecology. Undoubtedly, its 
subject should not be looked upon as an application of ecology in 
the sense of the preceding paragraphs of this chapter. The intent 
is to emphasize that all organisms are related to their environments 
and, consequently, to each other and that, therefore, they will be 
best undestood when studied from an ecological point of view. 

Considering the youthfulness of the science of ecology, it has 
contributed much to our understanding of plants and animals at 
the same time that its methods have won approval and even adop- 
tion in other fields to the benefit of all. Although we still have 
plant ecologists and animal ecologists, and probably will continue 
to have such specialists, there has been a steady increase in the 
appreciation of interrelationships among plants and animals. 61 

Furthermore, there is a growing realization that man is like- 
wise subject to ecological laws. This is completely reasonable 
since man, like other organisms, is basically dependent upon his 
environment and is likewise a factor in that environment. With 
man's increasing dominance, it is desirable that these relationships 
be better understood. How better can one approach that under- 
standing than through studies of the structure of the communities 
in which man dominates, their origins and successional develop- 
ment, and the controlling factors involved. This is human ecology. 

This is not a new idea, but it has not been widely recognized 
or accepted. There is much evidence that it is gaining recognition. 
There is an increasing appreciation of the concepts and values of 
ecology among the public in general as evidenced by the not 



360 THE STUDY OF PLANT COMMUNITIES * Chapter XII 

uncommon use of the term in popular magazines and even occa- 
sionally in newspapers. This represents one phase of progress. 
The other is indicated by the use of the term and ecological 
methods by scholars and investigators in fields ordinarily not 
thought of as ecological. Anthropologists have undoubtedly led 
the way in adapting ecological methods to their problems and 
have, consequently, influenced others to try similar applications 
in different fields. Although the social ecology of animals has been 
given much attention, there have been only a few advocates of 
ecological methods in the analysis of man's social behavior. 2 How- 
ever, human ecology is gaining increasing recognition among 
sociologists under the pioneering influence of a few of their 
number 177, 178 who have thought in terms of social ecology for 
many years. As a part of the interpretation of man's activities 
and responses, it follows that certain phases of psychological 
action must likewise be given consideration in human ecology. 
Also, if human communities are to be studied as a whole, econom- 
ics, too, becomes susceptible to ecological interpretation. These 
things make it apparent that human ecology is a comprehensive 
subject but one with promise of substantial returns for its study. 

Some ideas of human ecology as expressed by a sociologist 177 
seem particularly pertinent here. The scope of human ecology is 
so great that it must have a synoptic view of plant, animal, and 
human communities since all are interrelated and governed by the 
same principles involved in competition, symbiosis, succession, 
balance, and optimal population. Approached in this fashion, the 
laws, processes, and structure of human population are seen to be 
subservient to the more comprehensive laws of ecology since the 
latter are the determiners of regional economic and social types. 
When the arrangement and spatial adaptations of populations are 
considered, such ecological processes as aggregation, mobility, 
specialization, distance, and succession are excellent bases of 
evaluation. They permit the establishment of ecological indices 
for the measurement of types and trends of social mobility, dis- 
stance, dominance, and change. 

Finally, let us return to a phase of the discussion that has been 
touched upon earlier in several connections. No science can be 
completely justified for itself alone since science is supported by 



APPLIED ECOLOGY 361 



society. It is hoped that, in this last chapter, enough practical 
aspects of ecology have been suggested to show its wide appli- 
cability. Furthermore, the aim has been to show that its application 
is necessary if man is to continue to enjoy the full benefits of his 
environment upon which he is dependent, in which he is a factor, 
and over which he is a dominant. We have suggested that people 
with a wide variety of interests have concerned themselves with 
the general subject of human ecology. Among plant ecologists, 
Dr. Paul B. Sears is outstanding for his efforts in behalf of applied 
ecology and; particularly, human ecology. As a conclusion to this 
section it is, therefore, entirely proper that we quote one of his 
chapter headings from "Life and Environment" 220 which reads, 
"The social function of ecology is to provide a scientific basis 
whereby man may shape the environment and his relations to it, 
as he expresses himself in and through his culture patterns!' 

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for Crop Production in the Great Plains Area. 
L. A. Stoddart and A. D. Smith. Range Management. 
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Nebraska and Kansas. Ecology, 24: 
421-434, 1943. 

268. , and Clements, F. E. Plant Ecology. 

New York: McGraw-Hill Book Co., 
1938 (2nd ed.). 601 pp. 

269. Wells, B. W. Plant communities of 
the coastal plain of North Carolina 
and their successional relations. 
Ecology, 9: 230-242, 1928. 

270. . Salt spray: an important factor 

in coastal ecology. Bull. Torr. Bot. 
CI., 65: 485-492, 1938. 



271. 



— . A new forest climax: the salt 
spray climax of Smith Island, North 
Carolina. Bull. Torr. Bot. CI., 66: 
629-634, 1939. 
— , and Shunk, I. V. The vegetation 



272. - 

and habitat factors of the coarser 
sands of the North Carolina coastal 
plain. Ecol. Monog., 1: 465-521, 1931. 

273. Went, F. W. The dependence of cer- 

tain annual plants on shrubs in Cali- 
fornia deserts. Bull. Torr. Bot. CI., 
69: 100-114, 1942. 

274. Wodehouse, R. P. Pollen Grains, Their 

Structure, Identification and Significance 
in Science and Medicine. New York: 
McGraw-Hill Book Co., 1935. 574 
pp. 

275. Wolfenbarger, D. O. Dispersion of 

small organisms. Distance disper- 
sion rates of bacteria, spores, seeds, 
pollen, and insects; incidence rates 
of diseases and injuries. Am. Midi. 
Nat., 35: 1-152, 1946. 

276. Woodbury, A. M. Distribution of 

pigmy conifers in Utah and North- 
eastern Arizona. Ecology, 28: 113-126, 
1947. 



'. 1 






Index 



Page numbers in bold face type indicate illustrative material. 



Abelia, 142 

affected by length of day, 142 
Abies a?nabilis, 276 

balsamea, 63, 240, 241, 243, 245, 248 

concolor, 70, 264, 272, 274 

fraseri, 244, 245 

grandis, 277, 279 

lasiocarpa, 244, 261, 276 

magnified, 70, 270, 271, 274 

nobilis, 276 
Abstract communities, 69 
Abundance, 56-58 
Abundance scale, 58 
Acer glabrwn, 70 

rubrum, 63, 249 

saccharum, 63, 246, 247, 248, 249, 
253, 255 
Acid and alkaline soils, 178-179 

see Alkalinity, pH 
Adaptation and survival, 30 
Adaptations, aeration 

aquatic plants, 175-176, 217-218 

emergent plants, 175-176 

lacunar tissue, 175-176 

pneumatophores, 176 

submerged leaves, 176 
Ad eno stoma jasciculatum, 139 
Aeration, 174 

and leaf structure, 138-139 

decreases with depth (soil), 174 

toxicity, 182 
Aes cuius califomica, 275 

octandra, 248, 249 
Agave, 290 
Agricultural indicators, 352-355 

crop centers and natural vegeta- 
tion, 352 

land evaluation, 355 

land use, 352-353-354 
Agriculture, 326-337 

crop ecology, 326-327 

land management, 329-332-333 



land use, 328-329 

pasture problems, 333-336 

pests, 336-337 
Agropyron, 268 

repens, 292 

smithii, 297 

spicatum, 297 
Agrostis alba, 292 
Ao horizon, 154 
Air capacity of soil, 175 
Alkalinity, of soil, 178-179 

calciphiles, 183 

causes, 183 

pH, 178-179 

plant relationships, 183 
Alluvial soils, 149-150 

texture of, 150 
Alnus incana, 242 
Alpine soil, 145 
Alpine tundra, 236, 239-240 

location, altitudes, 239 
Alpine vegetation, 145, 236 

Krumviholz, 145 
Altitudinal zones, 124, 133 

in Utah, 125 
Amelanchier spp., 269 
Ammonification, 196-197 
Andromeda polifolia, 239, 242 
Andropogon, 218, 219, 221 

furcatus, 292, 295 

littoral is, 51, 52 

saccharoides, 293 

scoparius, 292, 295 

tener, 293 

ternarius, 293 
Anemometers, cup and Biram, 99 
Animals • 

as dependents, 26 

as factors 

dissemination, 198-199 
grazing and browsing, 201-202 
in soil, 200 
man, 202-210 



371 



372 



THE STUDY OF PLANT COMMUNITIES 



Animals, as factors— Continued 
pollination, 198 
soil organisms, 200 
as influents, 26 
Animals, of soil, 200-201 
macrofauna, 201 
microfauna, 200 
Applied ecology, 315-361 
agriculture, 326-337 
and secondary succession, 216 
conservation, 337-347 
forestry, 316-321 
human ecology, 359-361 
landscaping, 347-350 
plant indicators, 350-359 
range management, 321-326 
Aquatic plants 
aeration, 175-176 
characteristics, 217 
emergent, 217-218 
floating leaved, 217-218 
lacunar tissue, 175 
Aralia nudicaulis, 241 
Arbutus menziesii, 281 
Arctic tundra, 238-239 

climax, 239 
Arctostaphylos spp., 274, 275 
glauca, 282 
tomentosa, 139, 282 
Aristida longiseta, 297 

stricta, 253, 256 
Arte?nisia spp., 268, 286 
tridentata, 284, 285 
spine sc ens, 285 
Artificial forest types, 317-319-320 
Asclepias mexicana, 199 
Aspect dominance, 67, 68 

in grassland, 297-298 
Association, 225-226 

individual, 45 
Aster, 221 

acuminatus, 241 
Asymmetric growth and wind, 101- 

102-103 
Atmometers, 85-86 

description and operation, 85-86 
indicators of light, 85 
Atmosphere 
capacity to hold moisture, 78 
gaseous content, 75-76 
of the soil, 174-177 
variations in composition, 76 
water content, 77 
Atmospheric moisture, condensation 



causes, 87 

clouds, 87-88 

cooling of air masses, 87 

fog, 87, 88, 89 

precipitation, 88-95 
Atmospheric moisture 

and evaporation, 78, 79 

and vegetation, 95-97 

dew point, 86 

measurements, 81-82 

plant distribution, 96 

precipitation of, 77-78 

relationship to temperature, 77 

saturation, 77 

terminology of, 78-79 
Atmospheric pressure, 97-98 

relation to wind, 97-98 

varying with temperature, 97 
Atriplex spp., 286 

confertifolia, 285 

nuttallii, 356 
Autecology, 17 

and physiology, 17 

in the field, 20 
Auxins, 135-137 

and differential growth, 137 

formation of, 135-136 

relation to size, shape, 136 
Available water, 170-171 

and root growth, 68, 164 

capillary rise, 164 

degrees of availability, 171 

in different soils, 171 

soil solution, 171 

soil temperature, 171 

B 

Bacteria, soil 
nitrates, 196-198 
nitrogen fixing, 196 
nodule bacteria, 195-196 
succession of bacteria, 197 

Balance of population, 207-209 
biological control, 208 
destruction of predators, 208 
introduced species, 207-209 

Basal area, 59 
determination, 62 
in phytographs, 62, 63 
relation to dominance, 62 

Base exchange, 179-182 

Batodendron arboreum, 259 

Beech-maple association, 249-250 

Betula lutea, 63, 245, 250 



INDEX 



373 



Betula latea— Continued 

nigra, 255 

papyrifera, 63, 240, 245, 269 
Bidens frondosa, 199 
Biological factors, 187-210 

animals, 198-210 

competition, 188-190 

plants, 187-199 
Biological balance, disturbance by 

man, 209-210 
Bisects, 50, 54 
Black Hills vegetation, 269 
Bluffs 

moisture-temperature and expo- 
sure, 31 
Bog 

development, 216 

drainage, cultivation, 209 

floating type, 216 

forest, 216, 311 

succession, 216 
Boreal forest formation, 240-245 

Appalachian extension, 244-245 

climax, 240-241 

range and climate, 240 

successions, 241-244 

transitions, 243-244 
Bouteloua, 268 

curtipendula, 292 

gracilis, 292, 293, 295, 296 

hirsuta, 292, 293, 295 
Broad sclerophyll formation, 280-283 

broad sclerophyll forest, 280, 281 

chaparral, 280, 281, 282 

fires, 282-283 

ranges, distribution, climate, 280- 
281 
Bromus tectorum, 227, 297 
Browse line, 201, 208-209, 355 
Buffalo, as a factor, 202 
Bulbilis dactyloides, 293, 295, 296 
Buried forest, 112, 114 



Calamovilfa longifolia, 293 

Calcification (soil), 156-157 

Calciphiles, 183-184 

Calcium compounds, and soil, 183 
tolerance to, 183 

Calliergon giganteum, 303 

Capillary capacity, 168 

Carbon dioxide 
content of air, 75 
relation to soil depth, 174 



Carex capillaris, 239 

nardina, 239 

rupestris, 239 
Carnegiea gigantea, 288 
Carnivorous plants, 257 
Gary a alba, 254 

cordiformis, 254 

laciniosa, 254 

ovata, 254 
Cassiope tetragona, 239 
Castanea dentata, 251-252 
Castajiopsis chrysophylla, 140, 281 
Ceanothns, 268, 274, 275 

cuneatus, 282 
Celtis spp., 255, 269 
Cejichrus pauciflorus, 199 
Cercidium microphyllwn, 288 
Cercis canade?isis, 25 
Cerococarpus, 268 

betidaeformis, 282 

ledifolius, 269 

parviflorus, 269 
Chamaecy paris lawsoniana, 279 

thyoides, 257, 258 
Chamaedaphne calyculata, 242 
Chaparral, 28, 275, 280, 281-282-283 

and fire, 282-283 

growth form, 28 

leaf structure, 139 
Characteristic species, 72 

indicator significance, 73 
Chestnut blight, 190, 252 
Chiogenes hispidula, 241 
Chlorophyll and light, 135-136 
Chrysopsis breweri, 272 
Chrysothamnus piiberulus, 286 
Circle of illumination, 116, 117, 118 
Classification of communities 

basis of life, form, 20 

static and dynamic viewpoints, 1 7 
Classification of vegetation tvpes 

associations, 225 

faciations, 225 

formations, 225 

lociations, 225 
Cladonia leporina, 218 
Climate 

and climax, 160, 224 

and soils, 157-161 

and vegetation, 15, 160 

kinds of plants, 28 
Climatic factors 

air, 71-115 

control of growth form, 31-32 



374 



THE STUDY OF PLANT COMMUNITIES 



Climatic factors— Continued 
insolation, 116-118 
precipitation, 88-97 
radiant energy, 116-143 
temperature, 118-128 
Climax 

an indicator of climate, 224 
basic concept, 226 
characteristics, 223-224 
monoclimax interpretation, 226-229 
polyclimax interpretation, 226-229 
present distribution of, 234-235- 

299 
relation to climate, 223-224 
relationships of successional trends, 

223-224 
stability, 224 
types, 226-229 

uniformity and variations, 224-226 
variations related to time, 224 
Climax communities 
present distribution, 236-299 
shifts with time, 301-314 
Climax, distribution of, 234, 235-299 

controlling factors, 234 
Climax formations of North America 
listed, 237 
map, 235 
Climax regions, 225 
formations listed, 225 
uniformity of life form, 224-225 
Climax regions of North America 

by formations, 236-299 
Climax, study of 

criteria for recognition, 230 
procedure in local study, 230-233 
sampling, 232-233 
use of quantitative data, 231-232- 
233 
Climax, types of 
disclimax, 227 
edaphic, 226 
physiographic, 226 
postclimax and preclimax, 227-229 
subclimax, 226 
Climaxes of past, reconstruction, 301- 
314 
dendrochronology, 308-310 
paleo-ecology, 301-304 
pollen analysis, 304-307 
relict method, 310-314 
Climographs, 98 
Clintonia borealis, 241 



Clouds 
causes, 87-88 
classification, 88 
effect on temperature, 125 
source of precipitation, 88 
Coefficient of community, 74 
Cold air drainage, 98, 124 
Cold front (air masses), 87 
Coleogyne ramosissima, 286 
Colloids 
and exchangeable bases, 180 
and soil characteristics, 153 
and soil water, 162 
Colluvial soils, 150 

talus, 151 
Community 
abstract, 21, 69-74 
analysis a necessity, 34 
basic vegetational unit, 21 
classification by life form, 20 
concrete, 21 
definition, 21 
description justified, 33 
first recognized as basis of study, 

16 
fixing the concept of, 33-34 
illustrated, 18, 19 
its nature, 21 
recognition, 21 
size, 21 

synthetic analysis, 69-74 
Communities (layer or strata) 
of the forest floor, 23, 24 
synusia, 25 
Community disturbance 

drainage, fire, irrigation, 203 
Community dynamics, 211-314 
methods of study, 229-233 
plant succession, 211-233 
present distribution of climaxes, 

234-299 
shifts of climaxes with time, 300- 
314 
Community structure 
Quantitative characters, 56-63 
cover and space, 61 
density, 57 
frequency, 57 
numbers of individuals, 56 
Qualitative characters, 64-69 
dispersion, 64 
periodicity, 65 
sociabilitv, 64 
stratification, 65 



INDEX 



375 



Community structure, qualitative 

characters— Continued 
vitality, 64 
Compass plants, 137 
Competition, 21-24, 188-190 

and dependent species, 25 

and soil moisture, 30 

causes of, 22 

direct (physical), 188 

intensity of, 22 

introduction of new species, 189- 
190 

through physiological require- 
ments, 188 

tree seedlings, 25 
Conopholis americana, 27 
Constance, 71-72 

diagram, 71, 72 
Conservation, 337-347 

soil, 338-341 

water supply, 341-344 

wildlife, 345-347 
Coptis trifolia, 241 
Cormts canadensis, 241 

florida, 254 
vernal aspect, 25 
Cover, 61-62 

and temperature, 125-127 

by strata, 62 

classes, 62, 66 

estimation, 61-62 

in grassland studies, 62 

measurement, 61-62 

square foot density, 62 
Coverage classes, 66 
Cover-stratification diagrams, 66 
Cowa?iia, 268 
Crop ecology, 326-327 
Cuscuta, 191 
Cy penis, 175 
Cypress swamp, 31, 176 
Cyrilla racemiflora, 257 

D 

Dalea, 288 

Dasylirion longisshnum, 290 
Death Valley, 287-288 
Deciduous forest (beech-maple), 18 
Deciduous forest formation, 245-259 
beech-maple association, 18, 249- 

250 
hemlock-hardwoods association, 

250-251 
maple-basswood association, 249- 
250 



mixed mesophytic association, 245, 
247-249 

oak-chestnut association, 251-252 

oak-hickory association, 252-255 

range, climate, topography, 245- 
256 
Decomposition and available nitro- 
gen, 197 
Deer, 26, 201, 208-209 
Dendrochronology, 308-310 

applications, 308 

correlations with climate, 309-310 

methods, 308, 309 

sunspot activity, 310 
Density, 58, 59 

in phytographs, 63, 231 

applied in succession, 231-232 
Dependence 

animals, 26 

community, 23 

Conopholis americana, 27 

epiphytes, 26 

kinds of organisms, 26 

Monotropa imiflora, 27 

parasites, 26 

saprohytes, 26 
Desert formations, 283-289 

areas, 283 

Desert Scrub, 286, 287-289 

extent, climates, conditions, 283 

Sagebrush, 284, 285-286 
Desert Scrub formation, 286, 287-289 

Chihuahua desert, 289 

Mojave desert, 287-288 

Sonoran desert, 288, 289 
Dew point, 78 
Dionaea muscipida, 257 
Disclimax, 226 

Rromus tectorum, 227 

Opuntia, 227 
Dispersion, 64 
Disseminules, 199 

animal transported, 198-200 

transporting devices, 199-200 

wind transported, 108-109 
Distichlis spicata, 185 
Distribution of vegetation 

and temperature zones, 15, 324 

causes, 16, 324-325 

correlation with single factors, 15, 
324 
Dominance, 23 

aspect, 65 

criteria of, 25 



376 



THE STUDY OF PLANT COMMUNITIES 



Dominance— Continued 

relation to basal area, 61 

relation to cover, 61 

seasonal, 65 
Dormancy and photoperiodism, 142- 

143 
Drainage, artificial, 209 
Dryas octopetala, 239 
Dryopteris dilatata, 241 
Dunes (see Sand dunes) 
Dust storm, 109 



Earthworms, 200 
Ecological training, 14 
Ecology 

applied, 315-361 

approaches to the subject, 16 

breadth of the field, 14 

definition, 11 

human, 13, 359-361 

objectives, 12, 13 

practical considerations, 315-361 

scope, 13 

static and dynamic viewpoints, 17 

subject matter, 11, 13 
Edaphic factors, 144-174 
Eichornia, 206-207 
Elymus condensatus, 297 
Elyna bellardii, 239 
Empetrum nigrum, 239 
Environment 

a complex of factors, 16, 75 

and life, 12 

and physiological processes, 12 

climatic factors, 75-143 

components, 13 

defined, 13 

factors, 13 
Ephedra spp., 287 
Epilobium latifolium, 239 
Epiphytes, 26, 193 

latitudinal distribution, 193 

Spanish moss, 28, 193 

specificity, 193 

throughout plant kingdom, 193 
Equinoxes, 116 
Eriophorum spp., 238 
Erodium cicutarium, 199 
Erosion, 327 

control, 333, 334-335 
Euphorbia, ipecaciianhae, 257 

polygonifolia, 52 
Eurotia lanata, 286 



Evaporating power of the air, 85 
Evaporation 

and transpiration, 82-83 
measurement 
atmometer, 85, 86 
evaporimeters, 85 
open tank method, 85 
precipitation ratio, 96, 97 
Evernia vulpina, 272 
Exchangeable bases, 179-182 
Exclosures, 42, 312 
types and uses, 43 
Exclusives (fidelity), 72 
Exposure and insolation, 133 



Faciation, 225 

Factors, of the environment, 13 

air, 76-113 

and plant distribution, 15 

biological, 187-210 

climatic, 75-143 

exchangeable bases, 179-182 

insolation, 116-143 

organisms, 187-210 

physiographic, 144-187 

soil, 144-161 

soil acidity, 178-179 

soil atmosphere, 174-177 

soil water, 161-174 

temperature, 118-127 

topography, 185-187 

wind, 97-115 
Fagus grandifolia, 246, 248, 249-250, 

253 
Fairy rings, 182 
Fallugia paradoxa, 269 
Festuca idahoensis, 297 
Fidelity, 72-73 

and constance, 73 

characteristic species, 72-73 

classes, 72 
Field capacity, soil, 168 
Field margin, plantings, 331 
Fimbristylis casta?iea, 52 
Fire 

and pine savannah, 254, 256 

as a factor, 215, 266-267 

controlled burning, 205 

effects, 203-204-205, 226-227, 282- 
283 
Fish ponds, 329 
Fixation of nitrogen, 196-197 
Flourensia, 297 



INDEX 



377 



Fog, 87 

causes, 87 

coastal and inland, 87, 88, 89 

relation to vegetation, 87 
Food chains, 12 
Foothills forest 

Rockies, 267-269 

Sierra Nevada, 274, 275-276 
Forbs, 298 

Forest site indicators, 357-359 
Forest types, artificial, 317-319-320 
Formations, 225 

criteria for recognition, 229 
Foiiquieria splendens, 289 
Franseria dumosa, 288 
Fraxinus spp., 255 

americana, 63, 250 

caroliniana, 258 

profunda, 258 
Frequency, 58, 59 

and size of quadrats, 59, 60 

classes, 59 

classes and homogeneity, 61 

diagrams, 61, 71 

in oak-hickory forest, 231-232 

used in phytographs, 63, 231 

Kenoyer's normal, 61 

meaning of classes, 60 

Raunkiaer's, law of, 60 

Raunkiaer's normal, 61 
Frost injury 

abelia, 142 

and hardening, 142 

desiccation, 118 
Frost penetration of soil, 127 

under snow, litter, 127 
Fungi, as factors, 26 

of the soil, 194-198 

parasites, saprophytes, 194-198 



Gaultheria shallon, 278 

Gay ophy turn ramosiss'nnum, 272 

Germination 

and aeration, 174 

and temperature, 127 

growth inhibiting substances, 182 
Glacial soils, 151-152 
Glaze, 90 

damage, 90, 91 
Gordonia lasianthus, 257 
Grassland formation, 290-298 

aspect dominance, 297, 298 



extent, transitions, general climate, 

290-291 
mixed grass prairie, 293, 294-296 
other grassland climax, 297 
short grass plains, 295, 296 
tall grass prairie, 291-292-294 

Grassland precipitation, 95 

Grassy balds, 18 

Grayia spinosa, 286 

Great Salt Lake, saline vegetation, 
185 

Gregariousness, 64 
Grimmia laevigata, 218 
Growth form 

indicator of climate, 28 

controlled by climate, 28-29, 31-32 

H 

Habitat, 30 

hydric, 216-217 

local variations, 30-31 

mesic, hydric, xeric, 216-223 
Halophytes, 184 

xeromorphism, 184 
Hammock vegetation, 256-258 
Hardening and frost injury, 142 

Abelia, 142 
Hechtia, 290 
Hemlock-hardwoods association, 

250-251 
Heteromeles arbutifolia, 282 
Heterotheca subaxillaris, 51, 52 
Hydrogen ion concentration, 178- 
179 

and acidity, 178 

pH, 178-179 
History of plant ecology, 15 
Holoparasites, 191 
Homogeneity of vegetation, 61, 64 
Hudsonia, 252 
Human ecology, 13, 359-361 
"Humidity (see also Relative Hu- 
midity) 

absolute, 78 

relative, 78 
Humus, 154 

mull and mor, 154 
Hydrarch succession, 215, 216, 217 
Hydrophytes, 137-139, 215-217 
Hygrometer, 81, 82 
Hygroscopic coefficient, 167 
Hygrothermograph, 82 
Hyoscyamus niger, 141 
Hyperdispersion, 64 



378 



THE STUDY OF PLANT COMMUNITIES 



Hypodispersion, 64 
Hypnum crista-castrensis, 23 



Ilex glabra, 257 

vomit oria, 259 
Indifferents (fidelity), 72 
Infiltration, 163 

on forested and bare land, 94 
Influents (animals), 26 
Inhibition of growth, 182-183 
crop rotation, 182 
decomposition products, 182 
experimental evidence, 182 
fairy rings, 182 
seedlings, 182 
toxic excretions, 182 
Insolation, 116-118 
exposure, 133 
equinoxes, 116 
greatest total, 117 
heat, 116 

maximum effectiveness, 124 
position of the earth, 117 
seasonal, 116-117 
solstices, 116-117 
variations 

absorption, 116 
angle of incidence, 116 
daily, 116, 117 
latitudinal, 116, 117 
seasonal, 116-117 
Interglacial plant remains, 302-303 
Introduced species, 207-209, 318 
effects of, 206, 207-208 
Elodea, 206 
gypsy moth, 207 
mongoose, 208 
muskrat, 206-207 
prickly pear, 207 
rabbits, 207 
sparrows, starlings, 206 
water hyacinth, 206-207 
Irrigation, 353 

J 

Jatropha stimulosa, 257 

J uncus, 175 

Juniperus cembroides, 267 

monosperma, 267 

occidentalis, 267, 273 

pachyphloea, 267 

scopulorum, 267-268 

utahensis, 267 



K 

Kochia vestita, 286 
Koeleria cristata, 292, 293, 295 
Krummholz, 101, 102, 145, 263 
Knees, of cypress, 176 



Lactuca scariola, leaf position, 137 
Land management, 329-332, 333 
Landscaping, 347-350 

ecological relations, 348 

natural, 347 

road building, 347, 348, 349, 350 

Land surveys 

reconstruction original vegetation, 
53 
Land use and ecology, 328-329 
fish ponds, 329 

hedges and field margins, 329, 331 
hillculture, 328 
pasture, plowland, forest, 328 
stream margins, 330 
Larix laricina, 216, 241, 303 

occidentalis, 279 
Lacunar tissue, 175 
Larrea, 297 

tridentata, 288, 289 
Laterite and laterization, 156 
Leaching 

and soil acidity, 178 
solubility of soil constituents, 145 
Leaf arrangement, 136-137 
Leaf exposure, 137 

profile position, 137 
Leaf fall and photoperiod, 143 
Leaf structure 

affected by water and aeration, 

138-139 
in mesic habitats, 1 39 
in sun and shade, 137-140 
Ledum groenlandicum, 242 

palustre, 239 
Lemaireocereus schottii, 288 
Length of day, 141-143 
and hardening 
Abelia, 142 
evergreens, 142 
effects on plants, 141-143 
Leptilon canadense, 51, 52, 221 
Libocedrus decurre?is, 274 
Lichens 

epiphvtes, 193 

in rock succession, 218, 219-220 



INDEX 



379 



Life forms, 18, 19 

as basis for classification, 20 
Light, 129-143 

effect on size, form, 136 

chlorophyll production, 139 

effect on elongation, 136 

flowering, fruiting, reproduction, 
140 

in forest stands, 130, 132, 133 

interception, 132 

leaf exposure, 137 

leaf orientation, 136-137 

leaf structure, 137-139-140 

movement and position of chloro- 
plasts, 135-136, 139 

self pruning, 137 

shade tolerance, 133, 134 

source of energy, green plants, 129 

sun and shade leaves, 137-139 
Light and leaf pattern, 135-136 

mosaics, rosettes, 135-136 
Light and physiological responses 

chlorophyll production, 135-136 

opening, closing of stomata, 135- 
136 

photosynthesis, 134-135 
Light measurement, 129-132 

atmometers, 85, 132 

cautions and limitations, 131 

photoelectric cell, 129-131 

photometer, 130-131 

radiometer, 131-132 
Light penetration, water, 217-218 
Light quality, atmospheric 

absorption, 132 

diffusion, 132 
Light requirements, 132 

quality and intensity, 129 

vary for species, 129 
Light variations, 132-133 

biological importance, 132 

daily and seasonal, 132 

with latitude, 132 

with slope, 132, 133 
L (litter) layer, 143 
Liqiiidambar styraciflua, 254 
Limnology, 15 
Line transects, 54 
Liriodendron tulip if era, 255 
Lithocarpus densiflora, 278, 281 
Litter 

as an insulator, 127 

Ao horizon, 154 

differential decomposition, 154 

L layer, 154 



Lociation, 225 

Loess, 110, 112-113, 149 

Lonicera japonica, 189 

in competition, 189 
Long day plants, 141 

M 

Magnolia acuminata, 248 

virginiana, 257 
Maianthemum canadense, 241 
Man, a dominant, 315-316 

responsibilities, 316 

must recognize biological laws, 316 
Man, a factor, 202-210 

a dominant, 203 

cities, highways, 203 

cultivation, 203 

disturbance of biological balance, 
209-210 

fire, 203-204-205 

introduction of species, 206 

lumbering, 203 

modification of environment, 210 
Maple-basswood association, 249-250 

day and night temperatures, 127 
Maritime forest, 258 
Mean temperatures 

annual, 122 

daily, 121 

desert vegetation, 122 

maximum and minimum, 122 

usefulness of, 122 
Mechanical analysis, of soils, 152 
Mesophytic leaf structure, 139 
Mesophytism, 223 
Minimal area, 45 
/Mistletoe, 191 
Mixed grass prairie, 294-296 
Mixed mesophytic forest association, 

247-249 
Moisture and leaf structure, 138 
Moisture equivalent, 169 
Mojave desert, 287-288 
Monardella odoratissima, 272 
Monoclimax versus polvclimax, 226- 

229 
Monotropa uniflora, 27 
Montane forest 

Rockies, 263-267 

Sierra Nevada, 272, 273, 274 
Mor, 154 

Mulching and soil water, 165-166 
Mull, 154 
Mutual relationships 

competition, 21 



380 



THE STUDY OF PLANT COMMUNITIES 



Mutual relationships— Continued 

energy cycle, 12 

food, 12 

to environment, 27 
Mycorhiza, 26, 194-195 

and alkaline conditions, 195 

ectotrophic, 194-195 

endotrophic, 194-195 

of orchids, 195 
Myrica calif ornica, 281 

cerifera, 52, 257, 259 
Myriophyllum, 175 

N 
Natural resources 

can be conserved and used, 316 

communities and environments, 
315 

forests, 316-321 

range, 321-326 

soil, 326-336 

water supplies, 341-344 

wildlife, 345 
Natural thinning, 24 
Neocalliergon integrifolium, 303 
Nitrate fixation, 196-197 

algae, 197 

bacteria, 196-197 
Nitrogen in soil 

fixation, 196-197 

product of decomposition, 197 
Nodules 

legumes, 196 

nitrogen fixation, 196 

on leaves. 196 

soil fertility, 196 
Nolina, 290 
Nyssa aquatic a, 258 

biflora, 258 

sylvatica, 254 

O 

Oak-chestnut association, 251-252 
Oak-hickory association, 222, 252- 
255 
fire and swamp subclimaxes, 255- 
259 
Oak-hickory forest, 222 

day and night temperatures, 127 
stratification, 25 
vernal aspect, 25 
Oak-mountain mahogany climax, 

268-269 
Oenothera hiimifiisa, 51, 52 
Ohieya, 288 



Opuntia, 268, 288, 297 

arborescens, 67 

inerviis, 207 
Organisms 

mutual relationships, 12 

reactions on environment, 212-213 
Original vegetation, land surveys, 53 
Overgrazing, 42, 355, 356 
Overstocking, forest, 24 
Oxalis montana, 241 
Oxydendrum arboreum, 254 
Oxyria digyna, 239 



Pacific conifer forest, 276, 277, 278- 
280 

montane zone, 276-280 

northern part, 279 

range, climate, altitudes, 276 

southern part, 279 

subalpine zone, 276 
Paleo-ecology 

fossil evidence, 302 

interglacial relicts, 302, 303 

methods, 301-302 

stratification of peat, 303-304 
Panicum virgatwn, 292, 293 
Pantograph, in use, 39 
Papaver spp., 239 
Parasites, 26 

beetles, borers, 26 

chestnut blight, 190 

community structure, 190 

dodder (Cuscuta), 191 

Dutch elm disease, 190 

moths, 26 

witches brooms, 191-192 
Pasture indicators, 355-357 
Pasture problems, 333-336 

planting, 334-335 

woodlots, 333 
Peat bogs, 258-259 

development, 216 
Peat deposits, 302-303 

drained and cultivated, 209 

pollen analysis, 304-307 
Peat sampler, 304 
Pedalfer, 157-158 
Pedicularis semibarbata, 272 
Pedocal, 156, 157-159 
Percolation under litter, 127 
Periodicity 

aspect dominance, 65 

in deciduous forest, 67 

leaf fall, 68, 143 



INDEX 



381 



Periodicity— Continued 

length of day, 68 

of growth, 67-68 

seasonal dominance, 65 
Persea borbonia, 257 

pubescens, 257 
pH, 178-179 

and microorganisms, 179 

and plant responses, 179 

determination, 179 
Phenology, 65 

Phoradendron flavescens, 191 
Photometer, 129-131 

solarization, 131 

uses and limitations, 130-131 
Photoperiodism 

abscission layers and leaf fall, 68, 
143 

applied aspects, 141-143 

ecological significance, 143 

effect on Abelia, 142 

failure to become dormant, 143 

greenhouse uses, 143 

longday and shortday plants, 141 

necessary light intensity, 141 

seasonal phenomena, 141 

vegetative and reproductive activ- 
ity, 141 
Photosynthesis 

and temperature, 128 

relations to light, 134-135 

Vant Hoff's law, 128 
Photosynthetic efficiency 

light, 129 

species differences, 129 
Phytographs, 62, 63 

in climax studies, 231 

oak-hickory, 231 
Phytometers, transpiration, 83 
Phytosociology 

basic problems, 55 

development of, 55 

in successional studies, 233 

objectives, 55-74 
Picea engehnanni, 145, 261 

glauca, 240, 241, 243, 261, 269, 303 

mariana, 216, 241, 303, 311 

rubens, 63, 244 

sitchensis, 279 
Pifion-juniper climax, 267-268 
Pinus albicaulis, 272 

aristata, 263, 264 

attejinata, 274 

balfouriana, 273 

banksiana, 241, 242, 248 



caribaea, 254 

contorta, 70, 262, 265, 271, 272, 
274 

echi?iata, 252, 255 

flexilis, 145, 265, 273 

jeffreyi, 274, 276 

la??ibertiana, 272, 274 

latifolia, 266 

leiophylla, 266 

mo?iticola, 70, 271, 279 

muricata, 274 

murrayana, 194 
mycorhiza, 194 

palustris, 253 

ponderosa, 265, 266, 268, 269, 272, 
313 

var. arizonica, 266 
var. scopulorwn, 265 

resinosa, 243, 251, 318, 319 

rigid a, 252 
var. serotina, 257 

sabiniana, 275 

strobiformis, 264 

strobus, 243, 249, 250, 318, 319 

taeda, 24, 255, 319 
in succession, 222 

virginiana, 252, 255 
Pioneer plants, 219-220 

hydrarch succession, 216-217 

xerarch succession, 218 
Pirola picta,. 272 
Plants as factors 

competition, 188-190 

epiphytes, 193 

parasites, 190-192, 193 

soil flora, 197-198 

symbioses, 193-194 
Plant geography 

descriptive, 234-299 

floristic, 15 

historical development, 15, 211- 
212, 234 

of North America, 234-299 
Plant indicators, 350-359 

agricultural, 352-355 

forest site, 357-359 

nature and use, 350-352 

range and pasture, 355-357 
Plant nutrients, 179-182 

effect on distribution, 180 
Plant sociology, 13, 55-74, 233 
Plant succession, 211-233 
Platanus occidentalism 255 
Playas, 286 
Pneumatophores, 176 



382 



THE STUDY OF PLANT COMMUNITIES 



Poa, 268 

pratejisis, 292, 293 
Pocosins, 257 
Podsolization, 155-156 
Pollen, wind-borne, 107-108 

amounts, 107-108 

characteristics, 108 

distances, 107-108 
Pollen analysis, 304-307 

correlation with climate, 306-307 

methods, 305-306 

peat sampler, 304 

pollen diagrams, 305, 306, 307 

theory, 304-305 
Pollination 

animals, 198 

devices, 190 

wind, 107-108 
Polyclimax versus monoclimax, 226- 

229 
Polygonella polygama, 257 
Polystichum spp., 278 
Population balance, 207-209 
Populus acuminata, 266 

angustifolia, 266 

sargentii, 266 

tremuloides, 243, 244, 262, 266 
mycorhiza, 194 
Porcupine, damage, 202 
Postclimax and preclimax 

in altitudinal zonation, 227-228 

in latitudinal zonation, 227-228 

relicts, 228 
Postglacial vegetation, 301-314 

progressive changes, 305, 306, 307 

reconstruction of (see Pollen an- 
alvsis) 
Prairie "peninsula," 293-294 
Precipitation 

and base exchange, 181 

and runoff, 93-94 

average annual, for U.S., 96 

causes, 88-89 

effectiveness, 93, 163 

evaporation ratio, 96, 97 

forms of, 89-90 

interception by vegetation, 93-94 

measurement, 93-95 

seasonal distribution, 93 

seasonal variation, 96, 97 

source of, 88 

records, 95-97 

polygonal diagrams, 95, 96 
seasonal, 95-96 
Preclimax, 227-229 



Predators, destruction of, 208 
Preferents (fidelity), 72 
Presence, 69-71 

diagram, 71 

scale of, 69 

tabulation, 70 
Primary succession, 213-214 

hydrarch, 216-217 

xerarch, 218-219-221 
Profile diagrams 

topographic, 50 

vegetational, 54 
Prosopis chilensis, 288, 289 

juliflora, 289 
Primus serotina, 249 
Pseudotsuga, 277, 278, 279 

mucronata, 194 
mycorhiza, 194 

taxifolia, 262, 264 
Psychotria punctata, 196 
Psychrometer, 81 
Pulpwood forest, Maine 

composition by phytographs, 63 
Purshia, 268 

tride?itata, 269, 286 



Quadrats, 36-51 
kinds, 36-43 

chart, 36, 37, 38 

experimental, 41 

list-count, 36 

permanent, 36, 40 
mapping, 37, 38 

by pantograph, 39 
marking, 36, 37, 39, 40, 41 
photographing, 38, 39 
methods 

distribution, 49, 50 

in stratified vegetation, 48 
nested, 48 

random versus systematic, 49, 50 
relation of shape to efficiency, 44 
shape, 43-44 

size and number, 44-49, 46, 48 
spacing, 51 
Qualitative sociological characters, 

64-68 
dispersion, 64 
periodicity, 65 
stratification, 65 
sociability, 64 
vitality, 64 
Quantitative sociological characters 
abundance, 56-57 



INDEX 



383 



Quantitative sociological characters 
—Continued 

cover and space, 61-62 

density, 57 

frequency, 57, 59-61 
Quantitative studies 

of climax, 231 

of succession, 232-233 

phytographs, 231 
Quercas alba, 245, 248, 253 

agrifolia, 140, 281 

borealis, 249, 254 

catesbaei, 137, 255, 256 

vertical leaf position, 137-138 

chrysolepis, 275, 281 

cinerea, 255, 256 

coccinea, 252 

douglasii, 275 

dumosa, 275, 282 

durata, 140 

emoryi, 269 

fendleri, 269 

gambellii, 269 

gunnisoni, 269 

imbricaria, 254 

lyrata, 253 

macrocarpa, 252, 254, 269 

margaretta, 256 

marilandica, 254, 256 

montana, 252 

phellos, 253 

prinus, 253 

stellata, 254 

undulata, 269 

velutina, 254 

virginiana, 28, 258, 259 

wislizeni, 275, 281 

R 

Rabbit damage, 202 
Radiant energv, 116-143 

light, 129-143 

source for earth, 116 

temperature, 118-128 

visible spectrum, 116 
Radiometer, 131-132 
Rainfall (see Precipitation) 
Rain gauge, 93-95 
Range depletion, 322, 323 

indicators, 355-357 

management, 321-326 
ecological principles, 325 
ecological studies, 323-324 
indicators, 355-357 

objectives, 321 



results, 322, 323, 324 

recovery, 42, 322, 324 
Reaction of organisms, 212-213 
Relict method, 310-314 
Relicts, 310, 311,312,313 

ecological usefulness, 311-314 

factors in survival, 312-313 

interglacial, 302-303 

postclimax, 311, 313 
Reproduction and temperature, 128 
Reseeding range, 324 
Respiration and temperature, 128 
Rhamnus, 27 A 

californica, 275 
Rhododendron, 19 

californicimij 278 

lapponicwn, 239 
Rhus, 220 

copallina, 219 

triloba, 269 
Ribes viscosissimum, 272 
Rock succession, 218, 219-221 

mat formation, 218-220 
Rocky Mountain Forest complex, 
259-269 

Black Hills, 269 

climaxes 
Douglas fir, 263-265 
Engelman spruce— subalpine fir, 

261-263 
oak-mountain mahogany, 268-269 
pinon-juniper, 267-268 
ponderosa pine, 265, 266-267 

extent, 259-260 

zones and climaxes listed, 260 
Root distribution 

aeration, 174 

mapping, 147 
Rumex pulcher, 199 
Runoff 

and frozen soil, 126 

on forested and bare land, 94 



Sagebrush, 126, 267-268 
Sagebrush formation, 284-287 

extent, conditions, 284-285 

vegetation, 285-287 
Salic ornia spp., 286 
Saline soils 

bordering oceans, 184 

in deserts, 184-185 

physiological drought, 184 

vegetation, 286 

water absorption, 184 



384 



THE STUDY OF PLANT COMMUNITIES 



Salt spray, effects, 102, 103 

distribution of vegetation, 51, 52 
Salt tolerance, 184-185 

zonation, 185 
Sample plots, 35 
Sampling, ecological, 35-54 
efficiency, 45-47 
random versus systematic, 49, 50 
Sand dunes, 111-115, 149-150 
as plant habitats, 150 
blowouts, 150 
moisture conditions, 171 
stabilization, 111, 113, 114, 115 
Sandhills, Nebraska, 296 
Saprophytes, 26-27 
Indian pipe, 27 
squaw root, 27 
Sarcobatus vermiculatus, 185, 286 
Savannah, 253, 254 
and fire, 254, 256 
Sclerophyll, anatomical characteris- 
tics, 140 
Seasonal 
aspect, 65, 67 
dominance, 65 
Secondary succession, 214-215, 221- 
225 
after cultivation, 221-222 
after fire, 215, 254 
old fields, 221-222 
rate, 215 
Selaginella acanthonota, 257 
Selectives (fidelity), 72 
Self pruning, 137 
Sequoia gigantea, 273, 274 

sempervirens, 278, 279 
Shade 

and seed production, 140 
leaves 
characteristics, 137-139 
where found, 137 
plants, 132 
tolerance 

and photosvnthetic efficiency, 

134 
practical considerations, 134 
relation to succession, 134 
water versus light, 134, 135 
Sierra Nevada forest complex, 269- 
276 
east slope, 275-276 
foothills (woodland) zone, 274, 

275-276 
montane zone, 274 



range, climate, altitudes, 269-271 
subalpine zone, 270, 271-274 
Silvics and ecology, 317-321 
artificial forest types, 317-319-320 
continuous production, 320 
plant sucession, 317, 320 
pure and mixed stands, 317 
virgin and climax forests, 321 
Short grass plains, 295, 296-297, 312 
Sisymbrium altissimum, 206 
Sleet, 90 
Slope 
exposure, 124, 125, 133 
relation to light and temperature, 
133 
Smilax laurifolia, 257 
Snow 
as an insulator, 127 
effects on water supply, 68, 69 
in subalpine areas, 91-92 
measurement of fall, 94-95 
Sierra Nevada, 92, 271 
source of soil moisture, 90-91 
water content, 95 
Sociability, gregariousness, disper- 
sion, 64 
Sociological analysis 
objectives and procedure, 74 
summary of concepts, 73 
Sociological data, application, 56 
Soil, 144-161 
acidity, 178-179 

and soil organisms, 178 
decreases with depth, 1 78 
H ion concentration, 178 
relation to precipitation, 1 78 
acid or alkaline reaction, 145-146 
aeration 

poorer with depth, 1 74 

root growth and distribution, 

174-175 
type of vegetation, 175 
air analysis, 177 
air capacity, 175, 177 
aggregation and alkalinity, 183 
alkali, 184 
alkalinity, 183 
animals, 200-201 
atmosphere, 76-77, 148, 174-177 
and root growth, 77 
and soil organisms, 77 
changes with depth, 174 
determination of volume and 

composition, 177 
relation to water content, 177 



INDEX 



385 



Soil, atmosphere— Continued 

respiration, 174 

winter and summer, 174 
base exchange, 179-182 
capacity, 180 
classification 

by mature profiles, 155 

climatic, 155, 157-158, 160 

zonal, 157, 160 
conservation, 338-341 

strip cropping and terraces, 340 
defined, 144 
development, 147 

and vegetation, 159-161 

biological activity, 146 

moisture, 159 

temperature relationships, 159 
flora 

organisms free in soil, 197-198 

symbiotic fungi and bacteria, 
194-196 
formation, 144-147 

agents, 144 

carbonation, 145 

oxidation and hydration, 145 

processes, 144 
horizons, 146, 147 

and root distribution, 147 
litter, 153 

major components, 148 
organic content, and microorgan- 
isms, 146, 153-154 
origin, 153-154 

fermentation (F) layer, 154 

humus (H) layer, 154 

litter (L) layer, 153 
organisms 

and acidity, 178 

growth inhibiting substances, 182 
origin 

cumulose soils, 148-149 

residual soils, 148 
sedentary soils, 148 

transported soils, 152 
plant relationships, 148 
point cones, 173-174 
porosity, 177 
profile, 146, 147-148 

and weathering, 147 

development of, 147 

processes of development 
calcification, 156-157 
laterization, 156-157 
podsolization, 155-156 
salinity, 184-185 



sampler, 167 
samples, in place, 167 
solution, 171 
source of nutrients, 153 
structure, 153 

aggregation, 153 

determines air capacity, 175 

effect of colloids, 153 

shrinkage on drying, 153 

single grain, 153 
temperature, 122, 123 

daily and seasonal lag, 122 

extremes at surface, 119 

forest litter, 127 

germination, 126 

modified by cover, 126 

relation to atmosphere, 122 

seedling survival, 126 

water content, 171 

water relations, 126, 171 
texture, 152-153 

a basis of classification, 1 52 

and naming of soils, 153 

and shrinkage, 153 

mechanical analysis, 152 
total pore volume, 177 
transported 

alluvial, 149-150 

by ice, 151 

colluvial, 150 

colluvial cones, 150 

dunes, 111-115, 149-150 

glacial moraine, 151 

loess, 112-113, 149 
types and climate, 157-159 
types, climatic, 155, 157 
variations, local, 148-154 

in origin, 148-154 

structure, 148 

texture, 148 
variations, regional, 154-161 

relation to climate, 154-155 
water, 161-174 

and competition, 30 

and temperature, 126 

availability to plants, 170-171 

classification, 161-162 

constants, 166-171 

capillary capacity, 168 
capillary potential, 164 
field capacity, 164, 168 
hygroscopic coefficient, 167 
maximum water holding 

capacity, 168 
moisture equivalent, 169 



386 



THE STUDY OF PLANT COMMUNITIES 



Soil, water constants— Continued 

permanent wilting percentage, 

169-170 
readily available water, 1 70 
wilting coefficient, 169 
evaporation loss, 165-166 
infiltration, 163 
loss by transpiration, 166 
measurements, 172-174 
content, 172 

electrometric methods, 173 
expression of, 172 
forces with which held, 172 
of variations, 172 
physical forces, 173 
sampling and weighing, 1 72 
soil point cones, 173 
tensiometers, 173 
weight versus volume basis, 
172 
movement, 163-165 
origin, 162-163 
weathering, 144 
well, 146, 147-148 
Solstices, 116-117 
Sonoran desert, 287, 288 
Sorbus americana, 63 
Sorghastrimi nutans, 292, 293 
Space (occupied), 61-62 
clipping and weighing, 62 
estimation of volume, 62 
relation to basal area, 62 
Spartina pectinata, 292 
Species : area curve, 45, 47 
Spectrum, 118 
Sporobolus cryptandrus, 293, 295 

heterolepis, 292, 293 
Stand, 21,22,23, 45 
Stipa, 268 

eomata, 293, 295, 297 
leucotricba, 293 
pulchra, 297 
spartea, 292, 293, 295 
Stipulicida setae ea, 257 
Stomatal activity, 135-136 
response to drought, 136 
response to light, 135 
Strangers (fidelity), 72 
Stratification, 22, 25, 66 
and dependent species, 23 
and dominance, 23 
and layer communities, 25 
and sampling, 65 
causes, 22 
diagrams, 65 



dependence, 26 

light, 132 

sampling methods, 48 

studied with bisects, 54 

subordinate species, 23 

synusia, 25 
Stratification-cover diagrams, 65, 66 
Stream margins, 330 
Street lights 

photoperiod, 142 

winter killing of trees, 142 
Subalpine forest 

Rockies, 261-263 

Sierra Nevada, 270, 271-274 
Subclimax, 226 
Subordinate species, 23 
Succession 

after fire, 242 

causes, 212-214 

concept, 212 

historical background, 211-212 

in old fields, 221-222 

kinds, 214-216 

primary and secondary, 213-214 

quantitative studies, 231, 232, 233 

rate, 221-223 

stabilization and climax, 223-224 
Successional diagram, 213 
Sun leaves 

characteristics, 137-139 

where found, 137 
Sunspot activity 

climate and tree growth, 310 
Swamp 

forests, 258-259 

succession, 217 

vegetation, 217 
Symbiosis, 193-196 

mycorhiza, 194-195 

nodules, 195-196 
Sy?nphoricarpos spp., 269 

rotundifolius, 272 
Synecology, defined, 17 
Synthetic characteristics, 69-74 

coefficient of community, 74 

Constance, 71 

fidelity, 72 

presence, 69, 70, 71 
Synusia, 25 

forest floor, 23-24 

T 

Taiga, 240 

Tall grass prairie, 291-292-294 

Talus, 150-151 



INDEX 



387 



Taraxacum, 199 
Taxodium distichum, 176, 257 
buttresses, 176 
knees, 176 
Temperature, 118-128 

and atmospheric pressure, 97 
and cover, 125-127 
and germination, 127 
and growth, 128 
and reproduction, 128 
and water relations, 128 
and weathering, 144 
extremes, 125 
forest versus open, 125-126 
general plant relationships, 118-119 
hardwood forest, 127 
means, 121-122 
widest fluctuations, 144 
Temperature adjustments 
alpine and arctic plants, 119 
hardening, 119 
seasonal, 119 
Temperature and physiological proc- 
esses, 127-128 
Temperature measurement 
instruments, 119, 120, 121 
maximum and minimum ther- 
mometers, 120, 121 
procedures, soil and air, 119 
thermograph, 119-120 
thermometers, 119, 120, 121 
Temperature ranges 
for germination, 127 
for species, 118, 119 
photosynthesis, 128 
water content of protoplasm, 118 
Temperature records, 121-122 
annual mean, 122 
computing means, 121-122 
continuous, 119-120, 121 
mean, 121 

relation of mean to duration, 121 
value of extremes, 122 
Temperature tolerances 
conifers, 118 
desert plants, 118 
hardening, 119 
optimum, maximum, minimum, 

127 
seeds, and spores, 118 
Temperature variations 

altitudinal range of species, 124 
clouds or fog, 125 
differ with insolation, 122 
exposure, 124-125, 133 



follow insolation, 122 

lag, 122 

large bodies of water, 122-123 

slope, north and south, 124, 126 

133 
soil, 122 
soil surface, 119 
valleys and ridges, 98, 124 
Temperature zones, 15, 122 
in mountains, 124 
latitudinal, 122, 124 
Merriam's, 15, 324 
disrupted 
by cold air drainage, 124 
by lakes, 123 
by mountains, 124 
by slope and exposure, 124 
Tensiometer, 173 
Thermograph, 119, 120 

soil-air, 120 
Thermometers 

maximum and minimum, 120, 121 
standard, 119 
Thinning 

artificial (of forest stands), 24 
natural, 24 
Thuja occidentalism 242, 251 

plicata, 277, 279 
Tillandsia usneioides, 28, 193 
Tilia a?nerica?2a, and spp., 247, 248., 

249, 250 
Topography as a factor, 185-187 
effects are indirect, 185-186 
local and regional, 186-187 
Tortula pagorimi, 193 
Toxicity 
aeration, 182 

carbon dioxide in soil, 174 
experimental evidence, 182 
high acidity, 179 
soil solution, 171 
Transects 
data, 52 

early land surveys, 53 
mapping, 51 
sizes, 52 

transition zones, 52 
uses, 51 

variations in methods, 53 
zonation, 52 
Transitions, 30, 224 

forest and grassland, 29, 31 
Transpiration, measurement of, 82-83 
cobalt chloride method, 83 
phytometers, 82, 83 



388 



THE STUDY OF PLANT COMMUNITIES 



Tree-ring studies, 308-310 
Trenched plots and shade tolerance, 

134, 135 
Tropical formations, 298-299 

factors, 298 

growth forms, 298 

listed, 299 

rain forest, 298 
Tsuga canadensis, 248, 249, 250 

heterophylla, 277, 279 

mertensiana, 70, 261, 272, 276, 279 
Tundra formation, 236-240 

alpine tundra, 236, 239 

arctic tundra, 238 

U 

Ulmus, 255, 269 
a?nericana, 249 
Umbellularia calif ornica, 281 
Uniola paniculata, 51, 52 

V 

Vaccinium spp., 242, 278 
Vapor pressure deficit, 78-80 

application, 79 

nomogram, 83 

relation to relative humidity, 80 

significance, 79 
Vegetational analysis 

basis for other work, 34 

methods, 33-54 

objectives, 55-74 

quantitative data a necessity, 34 

sampling, 35 
Vegetational changes, historical, 301- 
314 

climatic parallels, 301-302 

modern evidences, 301 

relations to glaciation, 301 
Vegetation girdles 

aquatic, 216 

rock succession, 219-220 
Vegetation type 

and climate, 29 

local variations, 29, 30 

variation of species, 30 
Vegetation types, North America, 

235 
Vegetation zones, disrupted 

by angle of slope, 124, 133 

by exposure, 124, 125 
Vegetation zones, Rockies, 260 

extent, 260 

factors involved, 259 

foothills (woodland), 267-269 



montane, 263-267 

subalpine, 261-263 

tundra, 239-240 
Viburnum alnifolium, 241 

cassinoides, 241 
Virgin forest, need for study, 321 
Vitality, 64 

classes, 65 

indicator significance, 65 

W 

Warm front (air masses), 87 
Water 

capillary, 162 
gravitational, 161 
hygroscopic, 162 
of the atmosphere, 77-97 
of the soil, 161-174 
solvent in soil formation, 145 
Water absorption and movement 

modified by temperature, 128 
Water balance and temperature, 128 
Water holding capacity, 168 
■ Water conservation, 341 -342-343-344 
pollution, 344 
trends, 344 
Water supply, and snow, 68 
Water table 

and evaporation from soil, 165-166 
hydrarch succession, 219 
Weathering 
biological, 144 
chemical, 144 
hydration, 145 
oxidation, 145 
physical, 144 
soil acids, 145 
Wildlife conservation, 345-347 
ecological problems, 345-346 
management, 345 
refuges, 346 
Wilting coefficient, 169 
Wilting percentage, 169-170 
Wind, 97-115 
and atmospheric pressure, 97-98 
anemometers, 99 
daily and seasonal variation, 98 
effects on plants, 99-105 
general pattern, 97-98 
Krummholz, 101, 102 
measurement, 99 
physiological-anatomical effects, 

99-100 
salt spray, 102-103 
Wind and soil, 109-115 



INDEX 



89 



Wind and soil— Continued 

blowouts, 111, 112 

buried forest, 112, 114 

during droughts, 110, 112 

loess, 110, 112-113 

sand dunes, 111, 113 
Wind, coastal, night and day breezes, 

98 
Wind, in mountains 

cold air drainage, 98 

valley breezes, 98 
Wind, physical effects 

flagform, 106 

windthrow, 104, 105 
Wind, transportation 

of disseminules, 108-112 

of pollen, 107-108 
Witches brooms, 191-192 



X 

Xanthium canadense, 199 

Xerarch succession, 213-218-219-221 

arctic, 238-239 
Xeric habitats and leaf structure, 139 
Xeromorphism 

in halophytes, 184 

transpiration, 184 



Yucca 290 

brevifolia, 287, 288 



Zephyranthes atamasco, 68 
Zones of vegetation (see Vegeta- 
tion Zones)