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FOREST TYPES IN THE CENTRAL ROCKY MOUNTAINS
AS AFFECTED BY CLIMATE AND SOIL

By

CARLOS G. BATES, Silviculturist
Fremont Forest Experiment Station, Forest Service

CONTENTS

The method of study

Description of the localities studied

The growing season

Absolute and comparative conditions in the various forest types
Air temperatures
Wind, humidity, and evaporation

General character of the climate

Temperatures and temperature gradients
Wind, humdity , and evaporation

Winter soil temperatures

Precipitation and so-I moisture

WASHINGTON
"GOVERNMENT PRINTING OFFICE
1924

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Washington, D. C. October 6, 1924

FOREST TYPES IN THE CENTRAL ROCKY MOUNTAINS AS AFFECTED BY
CLIMATE AND SOIL.

By Carutos G. Batss,
Silviculiurist, Fremont Forest Experiment Station, Forest Service.

CONTENTS.
; Page. Page.
RMP LOUUCUOWe soso 2 0c osc ts Cae eset EL eae S 1c Recapitulationaes 02 Lia ee. eee 132
PieinepuOdOLStud y <4. Stee os cei nie - 4 General character of the climate -.--....... 132
Description of the localities studied.. ........ 7 Temperatures and temperature gradients 134
Mmheerowie seasones 2. Ae ii veth) tl. A 25 Wind, humidity ,and evaporation-...-..- 136
Absolute and comparative conditions in the Winter soil temperatures. ..........-.-.- 137
VAELOUS. LOTeSt by POSo.2- 22222 oon < 52-58 ae 27 Precipitation and soil moisture. -......-... 139
PRIUS PCLADULCS «s2)s5 oo ec tek el oe ere a = ie | WCONGIISVONS 2 52-26 oe a ee ee 141
Wind, humidity, and evaporation....... poh MATPUCALIONS: = scores Soe et ee on con Seca eee 150
Sou pompers tires. 22-0) she. es as ait 84)) sLiteraturecitediai2. 2 Ali oe 152
SHS ANDES Sale ee eee aCe ee eee 112
IBSEGUpILAOMse spe fee ste nt ek ce ine 116
Dowemoistyres. te 23 He i el 121
INTRODUCTION.

“a ae

In ‘‘Physiclogical Requirements of Rocky Mountain Trees,” (6) *
by the writer, published in the Journal of Agricultural Research
(Apr. 14, 1923), the relative physiological requirements of the
species composing the various forest types of the central Rocky
_ Mountains have been discussed. The present work on climatic and
_ soil conditions affecting the same forest types supplements that dis-
- cussion. The two papers are, indeed, so closely related that the one
_ is indispensable to the understanding of the other. ;
In the earlier paper the results of a number of laboratory studies
' were presented, in the hope that such comparisons, made under
controlled conditions equally affecting all of the species involved,
might permit judgment of the relative physiological requirements,
and thereby make it possible more definitely to decide what condi-
tions of the environment are really important in controlling distribu-
tion and what are merely variable concomitants of these controlling
conditions. If the ensuing discussion presented from the stand-
point of field observations does no more than reconcile the two

_1Parenthetical figures (italic) placed in the text refer to corresponding numbers under “Literature
- Gited’’ on page 152 of this bulletin.

73045°—24——] A

9 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

points of view and make clear what conditions of environment
respond directly to physiological requirements and thus most vitally
affect the composition of the forest, and in what terms these condi-
tions must be measured and expressed, it is felt that an advance will
have been made. But it is believed that a further progress has been
made. In these two approaches to the subject the understanding
of the relative qualities of the forest trees considered may become
clarified for the reader, and thus assist foresters in reaching a proper
understanding of the technical problems which are met almost
daily. This belief is held notwithstanding the fact that the methods
in ecological study of the environment are in some respects very
crude and in these respects still afford opportunity for much inten-
sive and fruitful work. :

A brief review of the physiological work and of the conclusions
which have already been presented is here given.

The historic concept of a difference between tree species as to ‘‘shade toler-
ance’’ or photosynthetic capacity has been borne out by observations on the
growth rate during growing seasons and the sap densities at the end of these
seasons, in which the several species were subjected to identical conditions.
The six species considered seem to be photosynthetically effective in the follow-
ing decreasing order: Engelmann spruce, Douglas fir, lodgepole pine, bristlecone
pine, yellow pine, and limber pine.

As a direct result of greater photosynthetic activity, the water use per unit of
growth is reduced; or, in other words, in relative ‘‘ water requirements’’ the most
“tolerant’’ or effective species take the lowest positions.

This.also affects the absolute requirements, as expressed by transpiration per
unit of leaf area or exposure, indirectly, through the relative evaporating rates
of solutions of different densities, so that in a general way the most active species
are absolutely the smallest users of water. lt also appears in this connection
that certain species may be much better adapted than others to resist transpira-
tion on account of the thickening of the epsdermis and the reduction of the
stomata. This places them in a lower category with respect to the absolute
transpiration rate, but does not appreciably affect their water requirements,
because such economies can be effected only through means which exclude
carbon dioxide or light from the functioning cells. Thus, Engelmann spruce,
Douglas fir, and yellow pine appear in increasing order as absolute water users;
but limber pine, bristlecone pine, and, to a lesser extent, lodgepole pine, take
lower positions than would be expected, all being ‘‘weedy’’ trees which sacrifice
growth rate for protection against great water losses.

In view of the behavior of weaker and denser solutions, it is conceived that
the species which are most effective photosynthetically may be satisfied with
the coolest environments, because, with their relatively dense cell saps a smaller
proporation of the energy of the absorbed sunlight is used in the process of evapo-
ration. In other words, a dense solution is not so rapidly cooled by its own
evaporation as a lighter solution. Hence the possessor of the dense solution
may more readily maintain leaf temperatures which will be conducive to effective
photosynthesis. That the temperature of the leaf is important in this process
can hardly be questioned. ‘Thus the relative light requirements and heat require-
ments of the several species are seen to be identical. On the basis of light re-
quirements the species would be zonated in the order already given, with spruce
occupying the situations of lowest air temperatures.

It is also conceived that, as the most effective species is least readily cooled
by the evaporation of its cell sap, it may not only require less heat for effective
growth, but may most readily become superheated to the point at which direct
protoplasmic injury results. For this reason no species may become established
in a temperature zone or in an exposed site whose temperatures exceed a certain
maximum. It must, however, be admitted that the direct evidence on this
point is extremely meager. It is also apparent that the maximum tempera-
tures encountered in nature, namely, those at the surface of insolated soils,
are so intimately connected with the dryness of that layer and with the water
supply and transpirational losses of young seedlings that evidence of direct
heat injury, apart from injury through water loss, may be very difficult to obtain.

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FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 3

At this stage it is not unimportant whether the high temperatures of the surface
soil react directly or indirectly, but it is of greater importance to recognize that
the combined effects may be the most common cause of the death of forest-tree
seedlings in the region under consideration. The present evidence is that the
action is most commonly indirect; that spruce and lodgepole pine seedlings
are most sensitive to excessive heat and drying, because their poorly developed
roots and thread-like stems do not permit them to push the necessary supplies
of water past the superheated section of the stem; that Douglas fir and yellow
pine are relatively resistant, because they are much stronger both as to length
of root and size of stem.

It would seém that the species which creates carbohydrates most readily, main-
taining the sap of greatest density and osmotic pressure, would, by reason of
the last-named quality, extract water from the soil to the lowest point, and there
is no reason to believe that this is not the case. Even in carefully controlled
pan tests, however, and more markedly in the field, drought resistance is much
more than extracting the water from the soil particles with which the roots may
be in contact. It is the ability to reach with the roots the supply of water that
very largely determines which seedlings will last the longest. Thus although

‘Engelmann spruce seedlings do resist drought remarkably well when the losses

by transpiration are at a very low rate and when slow capillary action may assist
in bringing water to the roots, yet ordinarily they succumb before Douglas fir
or yellow pine seedlings, because the latter at the outset develop roots nearly
twice as long. On the contrary, spruce trees of greater age, having developed
a great many root branches, evidently have at their command a larger proportion
of the water supply of a given soil space than either yellow pine or fir, whose
roots are then coarse and few in number. Both because lodgepole pine develops
a root as slowly as spruce, and because it apparently exerts a relatively low
osmotic pressure, its seedlings stand out prominently as the least drought-
resistant of those considered.

As a result of the various investigations upon the relative physio-

logical functioning of the species named, Engelmann spruce (Picea

engelmanni) and Douglas fir (Pseudotsuga taxifolia) may be considered
to be the most highly developed plant organisms, in the sense of effect-
ing most readily the processes on which growth is dependent; and at
the other extreme are found western yellow pine (Pinus ponderosa)
and limber pine (P. flerilis). Although further mvestigation may
tend to alter therelative positions of the four pines, it is believed that
BecHHtenee of the order first given above will not be seriously mis-
eading.

a ects from this physiological development, which is considered
most fundamental, there are two lines of adaptation which evidently
affect distribution. In the bristlecone (Pinus aristata) and limber
pines, both of which are ‘‘ weedy” or below the stature of true forest
trees, and perhaps more facultative in adapting themselves to site, the
stomata are greatly restricted in size. This inevitably reduces growth
rate by making carbon dioxide less available. Thesetrees arecertainly
resistant to ee evaporation which might result from strong wind,
but it is not believed that they are drought-resistant in the sense of
exercising a strong osmotic control over their water supplies. This
distinction is important, for, although the property of being resistant
to evaporation makes them valuable on wind-exposed sites, 1t does
not equip them for enduring the actual drought of the soil, which
results, for example, from competition between trees.

On the other hand, in the early and vigorous root development of
yellow pine and Douglas fir, as contrasted with lodgepole pine (Pinus
contorta) and spruce, there is an effort on the part of the former to
adapt themselves to the meager moisture supplies which usually obtain
in the localities where these species find their proper heat conditions.
Itis believed that the heat conditions are fundamentally controlling,

4 BULLETIN 1283, U. S. DEPARTMENT OF AGRICULTURE.

and that the moisture conditions have been met by this adaptation
as the need arose. |

As between spruce and lodgepole pine there is plainly a distinction.
The former is evidently under no stress to root vigorously, for under
the temperature conditions to which it is adapted exhaustion of the
surface moisture is a very slow process. Furthermore, according to
the present interpretation, the species is capable of coping with a
considerable degree of drought. It seems almost certain that lodge-
pole thrives best with relatively high temperatures, yet the root
development is adapted only to existence in perpetually moist sites.
The sluggish germination of lodgepole pine seeds, except when
exposed to wide temperature ranges, suggests adaptation to open,
exposed situations without other cover. *Rroid this and other bits
of evidence it may be concluded either that lodgepole pine is a recent
migrant to the central Rocky Mountains and is poorly adapted to a -
region of great atmospheric dryness, or is capable of developing only
a early stage of a succession, when competition for moisture is
acking. |

With these facts and theories as a working hypothesis, it is now
intended to examine the environmental conditions for evidence as
to the factors which must be controlling in the formation of each
forest type. |
THE METHOD OF STUDY.

The primary data to be presented in this bulletin are records of
climatic and soil conditions in different forest types. The main
object of such a presentation must be to show that differences in
climatic and soil conditions between the forest types either do or do
not exist in sufficient degree to account for the varying phenomena
of occurrences and growth.

As the special data collected by the Forest Service do not cover
periods long enough to establish the “normal” conditions of any of
the forest types (and by this is meant the average conditions for a
period of at least 20 years), and since even the average conditions for
a period of 10 years may vary considerably from ‘‘normals,”’ it is
necessary to assume that important differences between forest types
are fairly constant, and will be brought to light by the consideration
of short-period records. Such bases can not be fraught with any
serious dangers, moreover, when, as in this case, the forest types to
be compared are in the same general locality, that is, in the path of
‘the same air currents and storm centers. y considerable separa-
tion of the stations, however, especially in a rugged mountainous
region, is likely to introduce temporary variations in certain condi-
tions which are not ‘normal,’ and particularly in those conditions
which are most directly influenced by the path of storm centers.
Thus at the time of this writing it appears that the storm centers
have for some time been passing considerably to the north of the
Pikes Peak region, giving that locality unusual westerly winds and
leaving it without its usual amount of moisture. Consequently as
early as the end of May an unprecedented shortage of water exists,
tiiile scarcely 100 miles to the north unusually large accumulations
of snow are reported.

Again, the moisture factor is most variably influenced by the
restricted character of many of the summer showers; especially are

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1233, U.S. Dept. of Agricultur

Bul.

Bul. 1233, U. S. Dept. of Agriculture. PLATE II.

Fic. |.—STATION F-2, SHOWING THE FOREGROUND SOIL WELL, THERMOM-
ETER PIPES, AND EVAPOR!METER, AND IN THE BACKGROUND, TOWER ON
WHICH OBSERVATIONS WERE MADE FROM I9I0 To I9I2. JULY 7, 1918.

FIG. 2.—STATION F-6, LOOKING ACROSS THE SLOPE.

Note the sparseness of the vegetation in the foreground where it has not been disturbed.
July 7, 1918,

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FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 5

the heaviest downpours in a given locality likely to affect only very
small areas. Temperatures are generally not so directly affected by
local conditions. Thus the month of December, 1917, was an un-
usually warm month not only at the Fremont Forest Experiment
Station, but over most of the western part of the United States.
January, 1918, likewise, was generally cold to an unusual degree.
It seems safe, therefore, to compare the records of any two near-by
stations for short periods, whatever the factors under consideration,
and an increasing period will be demanded as the distance between

_ stations increases.

Fortunately the dozen or more stations located in the vicinity of
Fremont all come under the same general influences. This is true of
the entire area from the plains to the summit of Pikes Peak, with the
exception that summer rains frequently fall in one part of the area
and not in others. Winter snows may also be so localized, but usually
in conformity with altitudinal zones. It is true that two stations not
100 yards apart may on a given day have temperatures varying by 2°
F’. in one direction, and on the next day varying in the opposite direc-
tion. Such variations from a consistent relation are, however, always
small, and there is every reason to believe that the means of a single
month usually express essentially the normal temperature relation
between two stations for that month of any year.

Therefore the method of presenting results in this bulletin is to
compare each condition at any outlying station, for whatever period
observations may have been taken, with the corresponding condition
recorded during the same period at the so-called “control station.”
The latter is merely a single point, near the headquarters of the Fre-
mont Forest Experiment Station, at which all of the factors to be
considered have been measured almost continuously from 1910 to
1921, or later, with the important exception of the evaporation factor
for which satisfactory measurements were not obtained until 1916.

In the study of air temperatures, several attempts have been made
to cover the somewhat distinct sets of conditions which may exist at
any station—that is, to measure both the very local air temperatures
near the ground and those well above the ground, which may repre-
sent the mean for the locality, obtained by the mixing of the warmer
and cooler strata of air. On a breezy day, of course, this mixing is
constant and effective and even brings together the warm air near the
earth and the much cooler air of high strata in the atmosphere. On
quiet days such mixing may be very incomplete, so that even at 20

feet above the ground temperature records may reflect local rather

than general conditions. Sudden changes of temperature are then
likely to be recorded when a stronger movement of air begins.

The temperature of the air close to the ground is a condition in
which foresters are greatly interested because of its relation to seed-
lings and young growth. A number of observations have been made
at an elevation of 1 foot from the ground surface, and these will here-
after be spoken of as “ground temperatures” in contrast with tem-
peratures in the soil, on the one hand, and the temperature of mixed
air at higher levels, on the other. At the ‘‘ground” elevation the
rate of air movement is low, and, unless brisk ‘breezes are blowing, is
such as to permit the air to attain almost the temperature of the
ground and of the vegetation with which it comes in contact. An
elevation of 1 foot was adopted for ground temperatures, because it

6 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

was the lowest elevation at which maximum and minimum thermome-
ters could be conveniently operated without seriously disturbing the
snow blanket, and because it was thought adequate to represent
purely local conditions of absorption and radiation. An elevation
of 43 to 6 feet, which is very convenient, may represent a good degree
of air mixing without nullifying local character, except to modify daily
extremes. With greater elevations, which have been used in a few
instances to measure the conditions affecting the crowns of trees, it
will be seen that the local character of the temperatures partly dis-
appears.

In securing temperature records at these elevations the ordinary

method of exposing thermometers is not satisfactory where wind is
very light. Unless there is considerable wind to cause constant
change of air within a standard thermometer shelter, the temperatures
recorded are certain to reflect the heat absorption of the walls of the
shelter itself during the day, and at night the degree of heat retention
by the same walls, rather than the absorption and radiation of natural
objects. Although, because of the free air movement, this influence
is not very potent at considerable elevations above the ground, special
precautions must be taken in exposing thermometers for ground tem-
peratures. Direct insolation of the thermometers must of course be
prevented. At the risk, however, of some reflected radiation striking
the instruments, it has been found best to attach them to the north
faces of boards and to protect them from the sun above and on the
east and west, leaving them open to air currents on the north. There
is nothing below them to interfere with ground-and-air cooling at
night. Where standard shelters have been used on the ground, their
doors have been constantly open to the north except for short periods,
and usually the floors have been partly open. Although by no means
all obstacles to proper temperature recording have in this.manner
been removed, it is bateeed that the records presented contain only
occasional errors due to improper exposure.

Records of evaporation have been obtained with a variety of
instruments and methods, omitting evaporation from free-water
surfaces, but while an attempt has been made to compare the results
obtained by different methods, it must be recognized as a general
principle that such comparisons are of little value. Inasmuch
as the rate of evaporation at any time is dependent not only upon
the heat supplied by the sun and the airin contact with the evaporat-
ing surface, but also upon the rate of diffusion of the vapor as in-
fluenced by wind movement and atmospheric humidity, it is irref-
ragable that two different evaporating instruments or surfaces, with
constant variations in each of these four contributing factors, can
not maintain constant relative evaporating rates (6). Furthermore,
it is hopeless to expect that the response of any instrument or
evaporating surface will bear a constant relation to the response
of any given plant, which must be differently affected by each of
the four factors. The problem is made even more intricate by the
variation in the capacity of different plants for sunlight absorption
and for transpiration. For these reasons, the comparisons of types
are based almost wholly on records of Type 4 evaporimeters, although
some of these records were not obtained until the early part of 1921.

Methods of soil study are so far from being standardized, and the
methods used by agricultural investigators are in some respects so

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FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. y

inapplicable to mountain soils, that considerable ingenuity has been
required to develop suitable procedure for this study. The aim has
been to obtain an accurate measure of soil temperature on the one

‘hand and on the other to determine the moisture content of the

soil throughout the growing season. At the same time measure-
ments have been made of those more or less stable soil qualities
which have an influence upon the availability of soil moisture.
After considerable observation and experiment the conclusion has
been reached that soil-moisture data can be directly valuable in
such astudy as this onlyif they are reducible to terms of exterior osmotic
pressure, which is opposed to the osmotic pressure in the plant itself.
Although soil-moisture quantities have not been idedered: in exactly
these terms, in the data certain guides are found by which the rela-
tive osmotic pressures may be approximated. There remains, how-
ever, a great need for further refinement of the data and for new
methods especially applicable to the surface of the soil.

The special conditions affecting the value of the soil moisture and
temperature data will be described under these respective captions
just before the presentation of the results.

DESCRIPTION OF THE LOCALITIES STUDIED.

For the purpose of compilation the stations have been arranged
in alphabetical and numerical series, rather than according to the
forest types which they represent. The letter represents the initial
of the geographic name by which the station is hkely to be known,
and the number indicates simply the serial order under each letter in
this particular study. As the reader is not likely to hold these
descriptions in mind and may wish to refer to them frequently, the
indexing idea will prevail in their arrangement, except that the
control station at Fremont will be first described.

F-1: Control station—kElevation, 8,836 feet; aspect, S. 15° W.;
slope, 21 per cent. Whether this station represents any forest type
or will for ages remain a grassy park is very uncertain. It is situated
practically on the crest of a narrow east-west ridge, lying only a few
feet above stream channels on either side. The ridge represents
eranite in situ, however, rather than stream deposit. At the exact
spot where the instruments are located there were no tree specimens,

- but a very compact sod. Within 30 feet, on the southerly slope,

aspen, limber pine, and yellow pine seedlings were found, the aspen
particularly being of very poor development, the nearest trees not
over 10 feet high. On the northerly slopes of the same ridge yellow
pine, limber pine, and Engelmann spruce form a poorly defined
stand. Irregularities in conformation also give room for birch and
aspen on this slope.

hh the light of both topographic and vegetational evidence, the
exact spot at which the station is located must be considered to repre-
sent conditions most conducive to a growth of yellow pine or perhaps
limber pine. This classification, however, is not so important, since
the station was designed primarily to secure data for the general
atmospheric conditions of the Douglas fir zone in which it lies; and
for the study of either atmospheric conditions or soil conditions one
common point of comparison is perhaps as good as another. The
records of this station are used in all comparisons of type conditions.

8 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

To make the records as free as possible from purely local influences,
all atmospheric conditions have been observed on a tower so con-
structed that the thermgmeters, anemometers, evaporimeters, and
other apparatus could be placed about 20 feet above the ground. (See
Pl. I, fig. 1.) At this height no artificial influence acts upon any of the
instruments, and even the nearest trees are too distant to have any
appreciable effect upon wind movement or direction, although both
movement and direction are strongly influenced by the configuration of
the larger valley in which this small ridge lies. In 1917 a house was
erected about 30 feet to the northwest of the tower, with the roof
comb slightly higher than the instruments on the tower; but no effect
from this structure has been noted in any of the records, and it is not
thought that an appreciable effect can be produced by it, either
upon temperature or wind movement.

In addition to the records obtaimed on this tower, it was thought
desirable, in 1916, to measure air temperatures near the ground, as
was being done at many of the outlying stations, in order that knowl-
edge might be had as to the influence of elevation on temperatures.
These ground temperatures go under the station number 1-G to dis-
tinguish them from the records designated 1-A. The equipment of
this station, where most of the observations have been continued to
date, has been as follows:

Tower (F-14A):

Maximum and minimum thermometers, in standard shelter. Standardized
U.S. Weather Bureau instruments used, January, 1910, to April, 1920.

Air thermograph, March, 1910, to April, 1920.

Anemometer, standard, with register, February, 1910, to date.

Wind vane, with register, February, 1910, to date.

Sunshine recorder, electric themometric, with register, February, 1910, to date.

Psychrometer, January 21, 1910, to April, 1920.

Evaporimeter, inner-cell, Type 2, March 22, 1916, to January 1, 1917; inner- |
cell, Type 4, January 1, 1917, to date.
Ground (F-1G):

1-foot soil thermometer, in iron pipe, February 10, 1910, to September, 1918.

2-foot soil thermometer, in iron pipe, February 10, 1910, to June 30, 1914.

4-foot soil thermometer, in iron pipe, July 1, 1914, to date.

1-foot soil thermometer, in wooden tube, February 20, 1918, to June 10, 1920.

Maximum and minimum thermometers, on shielded board, April 1, 1916, to
February 20, 1918. .

Maximum and minimum thermometers, in standard shelter, February 20, 1918,
to September, 1918.

Air thermograph, in standard shelter, February 20, 1918, to September, 1918.

8 and 12 inch rain gages, with register, January, 1910, to date.

Soil wellin operation from June 29, 1914, to 1918. Determinations of moisture

. during growing season at depths of 1, 2, and 3 feet, and for 1917 at surface. :

No records whatever for any of the above-described equipment were
obtained during the periods from January 21 to March 20, 1914; from
December 11, 1914, to February 28, 1915; and from September 21,
1918, to May 1, 1919. For some conditions, the record obtained up
to 1918 seems adequate for establishing practically normal values;
for others, the two succeeding years introduce very different values,
and their records are therefore used.

C-1: Colorado Springs plains.—Elevation, 6,098 feet. On flat

lains about 100 feet higher than the valley of Monument Creek, _
immediately adjacent on the west. The observations here were taken
on the roof of one of the buildings on the campus of Colorado College,

ossibly 50 feet above the ground. The building is practically on the
reak between the flat plains which lie to the east and the valley of

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Bul. 1233, U. S. Dept. of Agriculture. PLATE IV.

Fic. |.—STATION F-7, PRIOR TO REMOVAL OF LOGS FROM THE GROUND,
LOOKING TOWARD UNCUT SPRUCE AND FIR BELOW AND IN THE ADJACENT
PLOT WHERE STATION F-9 1S LOCATED. JULY 7, I914.

FIG. 2.—STATION F-II, FOXPARK, WYO., AT THE SOUTH EDGE OF A THINNED
STAND OF LODGEPOLE. AUGUST Q9, I914.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 9

Monument Creek. Other buildings are not close enough to affect the
records in any way. ‘The free sweep allowed the prevailing westerly
winds probably accounts for some of the phenomenal velocities re-
corded at this station. In addition, the southward sloping creek val-
ley tends to introduce an air drainage factor in the form of northerly
breezes, which are not at all felt on the mountain slopes, and the
north and northeast winds accompanying cyclonic storms are also
not at all interfered with. Colorado Springs lies 44 miles from the
first abrupt rise of the mountains at Manitou, and 6 miles in an air
line from the Fremont Forest Experiment Station.

The equipment of this station is that of a first-class weather bureau
station, namely, maximum and minimum thermometers, thermo-
eraph, anemometer, ‘saba eauase: sunshine recorder, rain gages, and
triple register. No observations of ground or soil conditions are made.
The station has been maintained by the faculty and students of Colo-
rado College for about 35 years. The station is principally valuable
as forming a base for the wnole Pikes Peak series, which extends from
this elevation to timberline at 11,500 feet.

D-1: Black Hills western yellow pine.—Elevation,4,535feet. Valley
situation. Deadwood, 8. Dak., 1s situated in the higher and moister
portion of the Black Hills but in a valley surrounded by relatively
hich ground. It is therefore doubtful whether the station receives
ereater precipitation than the average for the Black Hills region.
The record of precipitation and temperatures has been obtained for
a great many years, under a variety of conditions, and while the
conditions surrounding the taking of the late record used in this
study have not been investigated, it is believed that in distribution
of precipitation at least, which is the more important feature, the
record will be properly indicative.

Due to artificial influences, the yellow pine forest immediatel
surrounding this station is far from representative of the Blac
Hills. However, this locality seems capable of producing the same
dense reproduction and the same excellent development of pure

ellow pine stands as any other portion of the hills, provided the soil
is favorable; but everywhere these qualities seem to be strongly
influenced by soils, and especially unfavorably by soils of limestone
origin.

F-2: Fremont south slope western yellow prne.—Elevation, 8,921 feet;
aspect, S. 35° W.; slope, 34 per cent. This station is situated but
400 feet from the control station, and as it occupies the south ex-
posure of the same main valley it is subject to the same air currents.

The forest isthe typical western yellow pine stand of semiarid regions,
consisting of not over 20 or 30 trees per acre, with a maximum height
of 60 feet. Occasional specimens of limber pine are found, but
none of them attains a height equal to that of the yellow pine. The
station is quite strongly shaded on the east, but is more open to the
south and southwest. This fact, with the fh eae aspect,
accounts for a very late maximum temperature each day, and in
part, toh the yee soil temperatures recorded in the morning. (See

a) Bact ae

_ Durmg the two-year period, March, 1910, to February, 1912, this
station was visited daily a few minutes later than the control sta-
tion. As at the control station, all observations except soil tem-
peratures, were made on a tower 20 feet above the ground. During

10 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

later years some additional records of ground temperatures and
evaporation have been secured for short periods. This station, with
the following one, furnished the main part of the data presented in
the preliminary report (3) on this study.

The full equipment is as follows:

Tower (F-2A):

Maximum and minimum thermometers, in standard shelter, March 1, 1910,
to February 28, 1912.

Air thermograph, February 1, 1911, to February 28, 1912.

Anemometer, March 1, 1910, to February 29, 1912.

Psychrometer, March 1, 1910, to February 29, 1912.

8-inch raili gage, March 1, 1910, to February 29, 1912.

Ground (F-2G):

1-foot soil thermometer, in iron pipe, February 10, 1910, to February 29, 1912.

2-foot soil thermometer, in iron pipe, February 10, 1910, to February 29, 1912.

(In addition, these points were observed in spring and fall of 1913, and in
1914 from May to July, when the 4-foot depth was substituted for the 2-foot
depth.)

1 and 4 foot soil thermometers, in iron pipes, July 9 to December 12, 1914;
March 1, 1915, to February 29, 1916; and May 11, 1917, to September, 1918.
On April 26, 1918, a wooden tube was substituted for the iron pipe at the 1-foot
depth.

Soil samples at random during growing season July, 1910, to October, 1911.

Soil well in operation during growing seasons after June 29, 1914, and exclud-
ing 1916. Regular determinations at 1, 2, and 3 feet, and in 1917 at surface.

Piche evaporimeters, as modified by Weather Bureau, May 5, 1914, to Feb-
ruary 29, 1916.

Evaporimeter, inner-cell, Type 4, February to September, 1918; May, 1919,
to April, 1920.

Maximum and minimum thermometers, on shielded board, March 2, 1915, to
February 29, 1916.

F—-3: Fremont Canyon spruce.—Elevation, 8,860 feet; aspect, N.
44° K.; slope, 54 per cent. This station, like the preceding, is only
about 400 feet from the control station. It is situated at the foot
of a steep northeasterly slope, only a few feet above one of the small
streams. (See Pl. I, fig. 2.) It is cut off from the air currents which
principally affect Stations 1 and 2 by the westward extension of the
ridge on which Station 1 is located, and which becomes considerably
higher opposite Station 3. The ground at Station 3 is so close to
the stream as to feel very markedly the cold-air stream at night,
which on the 20-foot tower is much less apparent.

This type is strictly Engelmann spruce at the station, although
as one ascends the slope to a more exposed situation the forest quickly
changes to one of Douglas fir. For this reason, in an earlier report
(3), this station was described as representing a spruce-fir type. The
predominance of spruce at the very foot of northerly slopes at this
elevation is ty ent and the purely local condition ee, here be
considered. The stand has come in since a fire which occurred about
60 years ago.

This station has had the same equipment and has been under
observation during the same periods as F-2, with a few insignificant
exceptions.

F-4: Fremont east slope pine-fir.—Elevation, 9,117 feet; aspect, N.
88° I.; slope, 27 per cent. This station is placed on a moderate east
slope, where it receives the full effect of the morning sun, which,
especially during the warmer months, is usually unobscured. The
station lies between two very shallow depressions, not over 20 feet

a

Nil he ee ele)

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. J ig 8

apart, which possibly gives the soil moisture a trifle more than the
average value for the slope as a whole. (See Pl. ILI, fig. 1.)

The site represents a somewhat common condition in the yellow
pine forests of the region, which, when not too strongly insolated,
give way to Douglas fir as a climax, in agreement with Clements’
description (10). Here it appears that the succession may have been
arrested by the fire which burned over the whole territory about 60
years ago, but on this area merely scarred the mature yellow pine
trees. These are now in a decadent condition and somewhat in-
fected by mistletoe, which, through its attacks on the yellow pine
saplings, seems to be an important factor in hastening succession.
In the tally of the trees within a radius of 50 feet of the station center,
it will be noted, the Douglas fir does not hold so prominent a place
in the younger reproduction as in the class from 2 to 5 inches in dia-
meter at breast height.

Yellow | Douglas | Limber
Class. pine. i pine.
Raman mOn tTderA aileitre. cme Na. 2 eae An ae wna Sob oa nee ein oe 20 | 6 20
SAPS ee LO MP eICh es erry nee tae Sh et Rol olan ita one eee: shi 15 | 7 5
SSPUHES a aol LOIN CN ES se ope reo en Mena aeh cena car Soka p ee 4 | 9 5
EL TEGS OVEL OH CUES = cet eee ars ee nee seers SS es Fe a aie eee te Sea iain 17 | 3 4

The site then is evidently such that the slightest disturbance might
cause a swing toward either Douglas fir or yellow pine predominance.
The occurrence of considerable aspen (Populus tremulo a and limber
pime is not considered significant. : |

All observations have been taken at or in the ground. The equip-
ment has been as follows: :

1 and 4 foot soil thermometers, in iron pipes, July 9, 1914, to December 12,
1914; March 3, 1915, to March 1, 1916; and May 11, 1917, to September, 1920.
1-foot iron pipe replaced by wooden tube, April 26,°1918.

Ground and soil-surface temperatures, May to September, 1920.

Soil well after July, 1914, with soil-moisture samples taken during the open
seasons of 1914, 1915, and 1917.

Evaporimeter, inner-cell, Type 4, February to September, 1918, May, 1919, to
September, 1920.

F-5: Fremont Canyon spruce.—Elevation, 9,044 feet; aspect, S.
20° E.; slope, 10 per cent. Conditions here are very sunilar to those
of Station F-3, except that Station F-5 isin the creek bottom, about
10 feet north of the east-west channel. There is no surface stream
here, but the underground seepage must be considerable. The soil
is alluvial and contains considerable granitic gravel.

The spruce stand forms only a narrow band along the stream chan-
nel and the base of the north slope. Aspen reaches better develop-
ment here, perhaps, than anywhere else in the vicinity. The station
is practically at the base of a spruce 18 inches in diameter by 70 feet
high. (See Pl. Ill, fig. 2.) It is strongly shaded except on the
southwest, where an artificial opening permits some light to enter.
A natural opening also permits sunlight to reach the ground for an
hour or more before noon.

The equipment and observations have been the same as for the
preceding station.

« 7 5? 7 es ~
- . - awit

12 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE, —

F-6: Fremont west slope limber pine.—Elevation, 9,060 feet;
aspect, N. 48° W.; slope, 32 per cent. Situated near the top of a
steep northwest slope, with a wide canyon to the west, this station
gets the full sweep of the desiccating winter winds from the west.
At this point the slope is less steep than below, and the type is less
completely one of limber pine, for it contains some yellow pines,
which form the main stand on the ridge just te the east. However,
at-the station the limber pine strongly -predominates. Like all
exposed westerly slopes in the Pikes Peak region, this one shows the
effect of the winter blasts. The ground is swept bare, and practi-
cally no ground cover appears even for short periods in the summer.
(See Pl. LI, fig. 2). In spite of its apparent poverty, however, the
soil at the station was found to contain an unusual amount of fine.
mineral material and humus. Pure stands of limber pine are ex-
tremely rare in the central Rockies, and for this reason the study of
the conditions which might have brought this one into being was
begun with great interest.

The equipment of Station 6 has all been placed at the ground, as
follows:

1 and 4 foot soil thermometers, July 9, 1914, to December 12, 1914; March 3,
1915, to February 29, 1916; and May 11, 1917, to September, 1920. Iron pipe
at 1 foot replaced by wooden tube April 26, 1918.

Ground and soil-surface temperatures, May to September, 1920.

Soil well, with moisture determinations for 1, 2, and 3 fest, established July,
1914, and operated during open seasons of 1914, 1915, and 1917.

Maximum and minimum thermometers, in shelter, March 15, 1915, to Febru-
ary 29, 1916.

Anemometer (elevation, 18 inches), March 5, 1915, to February 29, 19iG.

Evaporimeter, inner-cell, Type 4, February to September, 1918; May, 1919, to

September, 1920.
_ E-?: Fremont north slope spruce-fir, clear-cut.—Elevation, 9,105 feet;
aspect, N. 16° E.; slope 34 per cent. This station is situated near
the middle of a north slope whose total width is about 400 feet. It
lies 60 feet higher than the stream channel directly below, where
Station 5 is situated.

Station 7 is in the lower third of an opening about 250 feet square
which was made by clear-cutting the forest. At this level the
original stand was a dense mixture of Engelmann spruce and Douglas
fir in almost equal proportions, with occasional specimens of limber
pine and yellow pine and a few subordinate and decadent aspens.
The area was cut over in the fall of 1913, and up to the present
there is practically no cover except aspen sprouts, 3 to 5 feet high,
and herbs.

This station, together with Stations 8, 9, 14, and 15, represents
the conditions on four near-by identical acre plots from which the
timber has been cut in different ways. Because of a break in the
slope just above the middle of plot 1, two stations, 7 and 8, were -
ficught necessary to represent its soil conditions; but a single
station at the center, known as 7-8, was considered adequate for
atmospheric measurements. Thus together, Stations 7, 7-8, and 8
are the counterpart of Station 9 in the uncut plot (2) to the east,
and of Stations 14 and 15 at the centers of the two additional plots
from which only a portion of the timber was removed. These will
be described later. It is important to remember that these four
stations (or six, considered individually) represent essentially the

Bul. 1233, U.S. Dept. of Agriculture. PLATE V

Fic. |.—STATION F-12, WESTERN YELLOW PINE RIDGE TYPE AT THE FRE-
MONT FOREST EXPERIMENT STATION.

In the view are also shown limber pine and Douglas fir saplings, and in the background large
Douglas firs. July 7, 1918.

Fic. 2.—STATION F-1I3, LOOKING SOUTH, SHOWING A PORTION OF THE HIGH
RockY KNOB WHICH TERMINATES THE RIDGE ON THE EAST. MAy 2], I9I5.

PLATE VI.

1233, U.S. Dept. of Agriculture.

Bul.

‘vyI6I “€ ANNE *AVIO
HONW ONINIVLNOD THOS GALYOdSNVYL
NO “O109 ‘YSaSVY4 AO ALINIOIA AHL NI
SANid 310d59007 ALIIVNO-LSul4A—'S “SIS

‘9TGT {10G0}0Q ‘ODplA AX901 B JO AO\OYsS
OY} Ul UOIRAOTO JOYSIY B SUIeIyB ooNAdS OY} JSAM OY} OF YY) OJON

‘Vdd
SSAxXld NO SANIT YSEWIL LV ‘G|-4 NOllvis—'| ‘DI4

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 13

same original conditions, now changed by disturbance of the forest
cover. (See Pl. IV, fig. 1.)
At this station the following equipment has been used:

; 1 and 4 foot soil thermometers, in iron pipes, July 9, to December 12, 1914,
and March 3, 1915, to April, 1920, with the exception of March, 1916. 1-foot
pipe replaced by wooden tube April 26, 1918.

Soil well in use from July 13, 1914, during open seasons, to end of 1917, weekly
determinations being made of moisture at 1, 2, and 3 foot depths, and during
| 1917 at the surface.

Py Evaporimeter, Piche, July 7, 1914, to December 12, 1914, and June 8, 1915,
to October 24, 1915. Later evaporation records made at Station 7-8.

F-7-8§: Fremont north slope Douglas fir, clear-cut.—This station
hes midway between Stations 7 and 8 in a clear-cut plot. At this
elevation, which is only 16 feet higher than Station 7, the Douglas
fir in the original stand was clearly predominant, while at Station 8
it had practically no competition. This location was selected in the
center of the clear-cut plot to represent atmospheric conditions for
the plot as a whole.

The equipment has been as follows, all placed about 1 foot above
the ground:

Maximum and minimum thermometers, on shielded board, September, 1915,
to September, 1918, excepting March, 1916.

Air thermograph, on shielded board. April 1, 1916, to December 31,

Psychrometer (beginning May 1, 1916). 1917. Rotated with Stations

Anemometer, with register. 14 and 15, this station being

Evaporimeter, Type 2, 1916; Type 4, reached the first decade of each
1917. month.

Evaporimeter, Type 4, January 1, 1918, to April, 1920.

Sunshine recorder, electric thermometric, with register, during first third of
each month from July to October, 1917.

8-inch rain gage, April 6, 1916, to December 31, 1917.

F-8: Fremont north slope Douglas fir—KElevation, 9,137 feet;
aspect, N. 22° E.; slope, 26 per cent. The conditions surrounding
this station are very similar to those at Station 7, except that the
slope to the north is 8 per cent less steep, and Station 8 is at a higher
position on the slope. This difference was sufficient to introduce a
marked difference in the original forests at the two stations, the
forest at Station 8 being almost pure fir, while at Station 7 Engel-
mann spruce comprised half the stand. The possible insolation of
the soil at Station 8 is probably made greater by the nearness of
the station to the top of the ridge, with reduced opportunity to
secure seepage water and transported soil. ‘The evaporation records
of 1914 also indicate that the higher position gives Station 8 some-
what greater exposure to drying influences.

The equipment of this station has been identical with that of
Station 7, except that the Piche evaporimeters were used here only
during 1914. The conditions recorded here may properly be averaged
with those at Station 7 to obtain data representing the clear-cut
area as a whole.

F-9: Fremont north slope Douglas fir, wneut.—Klevation, 9,099
feet; aspect, N. 10° E.; slope, 33 per cent. It is seen that this
location is practically identical with that of Station 7, except that
the slope does not bear quite so much away from the north and
is protected by a virgin forest cover, while Stations 7 and 8 are in
an artificial opening of the same stand. (See Pl. IV, fig. 1.)

Os ee ee eA

ae eR Oe oe te Ee eee ee.

14 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

The forest surrounding Station F-9 may be depicted by reference
to a tabular statement for an area of 0.942 acres, at the centerof
which the station is situated. This shows a total of 1,143 trees
over 4.5 feet in height, of which 367, or 32.1 per cent, are Douglas
firs. The firs comprise 68.6 per cent of all the trees over 4 inches
in diameter at breast height, but only 18.8 per cent of those below
this size. In contrast, Engelmann spruce comprises only 22.6 per
cent of the larger trees, but 47.4 per cent of the smaller ones. This
relative status of the two species would indicate, without further
evidence, that Douglas fir is unable to hold its own under the dense
canopy of the larger trees. That this crowding out of the Douglas
fir is not due to lack of seeds, is repeatedly shown by the compara-
tively large number of fir seedlings which germinate, but soon die.
For example, in the new crop of seedlings in 1916 there were 1,183
Douglas firs and only 334 Engelmann spruces. After a few years
only about a 3:2 ratio of the two species exists among the small
seedlings. In the half-cut plots adjacent, where the canopy is not
over one-third as dense, there were practically no spruce seedlings,
but there was about the same number of firs. This may be due
either to the differences in physical conditions, or to the complete
removal of spruce seed trees from the cut-over plots, though it
would seem that considerable spruce seed might be blown in. Al-
though fir, because of its control of the canopy, may initiate a large
part of the new seedlings each year, there is little doubt that the
survivors are mainly of spruce. In other words, the conditions
being measured at Station 9 are those conducive to spruce survival.

The equipment of Station 9 has been maintained almost con-
tinuously, so that the conditions at Stations 7-8, 14, or 15 might be
compared with the conditions at this station, as a secondary control
for any period. The record follows. All atmospheric observations
have been at an elevation of 1 foot.

1 and 4 foot soil thermometers, in iron pipes, July 10, 1914, to April, 1920,
with the exception of March, 1916. 1-foot iron pipe replaced by wooden tube
April 26, 1918.

Soil well in use from July 13, 1914, to end of 1917, during open season, giving
weekly records of soil moisture at 1, 2, and 3 foot depths, and during 1917 at
the surface.

Maximum and minimum thermometers, on tree, September, 1915, to Sep-
tember, 1918, excepting March, 1916.

Air thermograph, under partial shelter, April 1, 1916, to December 31, 1917.

Psychrometer, April 23, 1916, to December 31, 1917.

Anemometer, April 23, 1916, to December 31, 1917, with register most of time.

8-inch rain gage, April 6, 1916, to December 31, 1917.

Evaporimeter, inner-cell, Type 2, April 1 to December 31, 1916.

Evaporimeter, inner-cell, Type 4, January, 1917, to April, 1920.

F-11: Foxpark Plateau lodgepole pine.—Elevation, 9,000 feet; aspect,
S.; slope, about 5 per cent. ‘The station is situated within the forest,
but ae about 50 feet from the edge of it, where it opens out abruptly
into a grassy bog or park bordering Foxpark Creek. ‘This station
is barely 10 feet above the creek level. As the cover has been con-
siderably lightened by the cutting of trees in the rahger station yard,
it is altogether probable that the summer temperatures represent
about a mean between those of the dense lodgepole forest and those
of the much warmer glades. As winter temperatures are more
largely controlled by strong winds and a deep snow blanket, they
would be little affected. (See Pl. IV, fig. 2.) The soil at this point

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 15

is a decomposed gneiss which forms a grayish, gravelly sand not
wholly devoid of clay. On account of the heavy snows which lie
until summer, and the presence of the water table at a depth of 6 to -
8 feet, the soil is usually saturated during the early part of the growing
season, but gradually dries out to a low point. This seems to be
true of a considerable area about Foxpark, though the moisture is
perhaps greater at the station than in the forest as a whole.

This station is the only one in the lodgepole pine type for which
soil data are recorded. It is located very close to Foxpark post office,
almost on the line between Colorado and Wyoming, in longitude 106°
W. The entire plateau about Foxpark bears an almost unbroken
forest of lodge Ae of very good quality, but a few high points pro-
jecting out of the plateau bear mixed stands of spruce and lodgepole.
About 5 miles to the east of Foxpark the plateau breaks off abruptly
to the plains, and on these slopes are oad. a few specimens of yellow
pine and Douglas fir.

The Weather Bureau installed maximum and minimum ther-
mometers and an 8-inch rain gage at this station in March, 1911, and
the earlier record of temperatures as used in this report is taken
directly from Weather Bureau summaries. In March, 1916, the
Forest Service supplemented this equipment with the following,
which was in use through August, 1918:

Psychrometer.

Anemometer.

Evaporimeter, inner-cell, Type 2. This was replaced by Type 4 evaporimeter,
January 15, 1917.

_ Still earlier, in August, 1914, soil thermometers had been installed, in iron
pipes, at depths of 1 and 2 feet, thelatter being lowered to 4 feet on March 15,
1916.

A soil well was prepared in July, 1916, and weekly soil samples at the surface
and at depths of 1, 2, and 3 feet have been taken during the open season since
August 1, 1916.

It is to be noted here that the observations for the Foxpark station
have been more or less irregular, owing to the frequent absence of
the observer for several days at a time. However, the air temper-
atures, which would be most affected, have now been observed for
a sufficient number of years so that the uncompensated error should
be quite small. .

-12: Fremont Ridge western yellow pine.—Elevation, 9,164 feet;
aspect, S. 20° E.; slope, 8 per cent. This point is on a broad, well-
rounded ridge that has a general east-west bearing, but very little slope
for a considerable distance east or west of the station. (See Pl. V,
fig.1.) Itis thus very evenly exposed on all sides, and at the outset
the exposure to wind from all directions was thought to account largely
for the very scrubby growth of the yellow pine.

The stand immediately surrounding the station is, for the most
part, young, having probably originated after a fire about 60 years
ago. The trees range from a height of about 20 feet down to tiny
seedlings. Yellow pine strongly predominates in numbers, but
Douglas fir and limber pine are also coming in, and these two species
likewise occur in all sizes. The yellow pine was very badly intected
with mistletoe, which was seriously retarding the growth of most
trees and gradually claiming victims. The elimination of diseased
trees during the winter of 1917-18 has markedly improved ener
ances and the general rate of growth. No cutting was done close to

16 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

Station 12. The soil at this site is extremely thin and poor. In
fact, there is so little accumulation of organic matter, and the granite
is so slightly disintegrated that it can hardly be said that there is
any soil.

The equipment of this station consisted of:

Maximum and minimum thermometers, in standard shelter, on the ground,
March 8, 1915, to February 29, 1916.

Air thermograph, same location and period.

Anemometer, 8 feet from ground, with register, same period.

Sunshine recorder, electric thermometric, March 7, 1915, to September 10,
1915.

Evaporimeter, inner-cell, Type 4, 1 foot above ground, February 16, 1918,
to September, 1920.

1 and 4 foot soil thermometers, in iron pipes, the former from May 22, 1915,
and the latter from June 19, 1915, to February 29, 1916. Both from August
1, 1917, to September, 1920. One-foot iron pipe replaced by wooden tube,
April 26, 1918.

Ground and soil-surface temperatures, May to September, 1920.

Soil well, furnishing moisture samples at 1, 2, and 3 feet, from July 7, 1915,
to end of season, and also surface samples during season of 1917.

F-18: Fremott high-ridge limber pine.—Elevation, 10,300 feet;
aspect, flat. This station presents the conditions existing on an
exposed ridge at the northeast base of Pikes Peak, where much of
the original forest was destroyed by fire many years ago. For the
most part this forest was of spruce. Limber pine is now coming in
all over the burned area, and in the more sheltered spots the spruce
followers are already appearing. At present the stand of limber

ine about Station 13 is not sufficient to furnish any protection.
rhe chief characteristic of the site, therefore, is an intense exposure to
both sun and wind. (See Pl. V, fig. 2.) It is interesting to note that
a few specimens of western yellow pine may be found in the vicinity of
this station where the exposure is less severe. Apparently they are
of the same age as the first group of limber pines gaining a foothold
after the fire. Observations at this point were at 5-day intervals.
No soil-moisture determinations were made.

This station was equipped with:

Maximum and minimum thermometers in shelter 5 feet above the ground
May 21, 1915, to October 1, 1916.

Air thermograph, same location and period.

Psychrometer.

Anemometer 7 feet above ground. In addition to dial readings at 5-day
intervals, the velocity was observed during about an hour for each period from
May 21, 1915, to March 5, 1918. Even with this aid, however, it was impossible
to determine the 5-day movements with any certainty when high velocities
prevailed.

Evaporimeter, wick Type 1, August 26, 1915, to February 11, 1916.

Evaporimeter, inner-cell, Type 2, April 1 to October 1, 1916.

Kight-inch rain gage, May 21, 1915, to October 1, 1916.

Soil thermometers, at depths of 1 and 2 feet in iron pipes, from May 21, 1915,
to October 1, 1916.

Soil thermograph, at 1 foot depth, September 1, 1915, to October 1, 1916.

F-14; Fremont north slope Douglas fir, half cut.—Elevation, 9,087
feet; aspect, N. 3° E.; hoa 35 per cent. This station and the
following are counterparts of Station 7-8 in clear-cut and of Station 9
in uncut Douglas fir. Station 14 represents conditions under a
half canopy of the taller trees of the original stand with all under-
growth removed. The character of the cutting carried out in 1913
was that known to foresters as the “shelterwood system.’ The

. ree

Bul. 1233, U.S. Dept. of Agriculture. PLATE VII.

FIG. |.—ROLLING SAND-HILL LAND, HALSEY, NEBR.

Extreme right of picture corresponds to situation of Station H-3 on southerly exposure.

FIG. 2.—WEATHER STATION AND SURROUNDINGS AT LAKE MORAINE, NEAR
PIKES PEAK, COLO. (STATION L-I). AUGUST, 1918.

Bul. 1233, U. S. Dept. of Agriculture. PLATE VIII.

Fic. |1.—STATION M-I, SHOWING IMMEDIATE SURROUNDINGS OF INSTRUMENT
SHELTER.

The soil well is just to the left of the anemometer at the foot of the oak clump. May 23, 1915.

FiG. 2.—WESTERN YELLOW PINE FOREST NEAR PAGOSA SPRINGS, COLO.,
AT THE FOOT OF AN EAST SLOPE, IN SANDSTONE. APRIL, I9I7.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 17

object of this kind of cutting is to leave at the first cut only such
trees as will be merchantable in a few years, and these for the value
of their protection to the new growth, whose development deter-
mines the time when the old trees shail all be removed. Inasmuch
as the original stand was not single-aged, though it was mainly so,
this process involved forced cutting of material not merchantable
except for fuel. All trees except those of Douglas fir were removed
in the cutting, and of the latter about 83 trees of an average diameter
of 9.7 inches and an aggregate basal area of 43 square feet were left
on the acre plot. This represents just one-fourth the basal area of
the original stand. Presumably, therefore, the crown density is not
over one-fourth of the virgin crown density nor of that now shelter-
ing Station 9.

Although a few observations of mimimum temperatures were made
at this point in the fall of 1915, the more important records do not
begin until 1916. The equipment used was as follows:

Maximum and minimum thermometers, in standard shelter, 1 foot above
ground, April 1, 1916, to December 31, 1917.

‘ Air thermograph, in shelter.
: Psychrometer.
: Anemometer, 1 foot above ground, April 1, 1916, to December 31, 1917. Ro-
with register. tated with Stations 7-8 and 15, this

vaporimeter, inner-cell, Type 2,{ station being reached the second decade
; in 1916. of each month.
: Evaporimeter, inner-cell, Type 4,
; in 1917.

Evaporimeter, Type 4, January 1, 1918, to April, 1920.
: One and 4 foot soil thermometers, in iron pipes, April 1, 1916, to April, 1920;
; 1-foot iron pipe replaced by wooden tube, April 26, 1918.
| Soil well furnishing samples for moisture determination at depths of 1, 2, and
|

3 feet, from May 23 to end of season, 1916, and also from the surface during the
whole season of 1917.

F-15: Fremont north slope Douglas fr, half cut.—"levation, 9,080
feet; aspect, N. 8° W.; slope, 28 per cent. This station is only 250
feet east of the preceding and practically on the same contour. How-
ever, it will be noticed that the slope is shehily less steep, and this
location has a very slight bearmg to the west of north. The differ-
ence in slope, giving somewhat more direct insolation at Station 15
than at Station 14, is probably a little more than counterbalanced,
for the general area in which Station 15 is located, by a somewhat

oreater “peat of cover. At the station, however, the cover is fairly
heavy on the east but relatively open to the west and southwest,
ermitting sun action in the afternoon, and this will fully account
or the slightly higher soil temperatures here than at Station 14.

The plot in the center of which Station 15 lies, like that surrounding
Station 14, has been heavily cut over, but with the object here of
leaving trees of all sizes, or in other words, of producing a “selection”
forest. Again, this cutting was forced, as the original stand was far
‘richer in mature than in immature sizes. This was especially true
of the Douglas fir, and as the cutting completely removed all other
species, the number of healthy young trees left could not be very

eat. However, the number of trees is somewhat greater than in
the “shelterwood” forest, both of large and small sizes. The princi-
al shortage is in trees 4 to 6 inches in diameter. In the stand
eft after cutting there are 110 trees of an average diameter of 10.2

73045° —24——_2

18 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

imches and 160 trees of an average diameter of 2 inches, making a
total basal area of 66 square feet, or 50 per cent more than in the
“‘shelterwood”’ plot.

The equipment and operation of this station has been the same as
of Station 14, except that the periodic use of recording instruments
fell here on the third decade of each month.

F-16: Timberline.—Elevation, about 11,500 feet; aspect, N. 10°
E.; slope, 40 to 45 per cent. This station represents conditions on
the north slope of Pikes Peak, at the upper limits of the Engelmann
spruce type, limber pine and bristlecone pine, as well as spruce, being
found in the edge of the forest. It is just a few feet outside the
timbered zone, which here ends very abruptly. (See Pl. VI, fig. 1.)
Presumably, on account of local wind exposure, the point represents
one of the lowest dips in the timberline, and was chosen for the obser-
vations because of its accessibility.

Owing to the difficulty of the trip, the station has been visited only
at 5-day intervals since its installation in October, 1916. For air
temperatures such a program necessitates more than the usual
dependence upon the thermograph record, as but one correction of
the maximum and one of the minimum is secured for each 5-day

eriod. The system has been to apply these corrections to the trace
or each day of the five, with slhght modification, and then to obtain
from the trace the maximum and minimum temperature for each
day, calculated from midnight to midnight. This, it is thought,
would have permitted no serious error over a long period had the
recording thermometers been perfectly reliable. Owing to the high
wind velocities it was found impossible to obtain a correct record of
minimum temperatures in the shelter where the other instruments
were located, and as a final resort two other minimum-registering
thermometers were placed on more substantial supports. Although
these did not fail to register properly, they did not of course register
the temperatures prevailing in the shelter. The entire temperature
record, therefore, presents a combination of values, all of which are
not representative of the same point, but doubtless represent the
locality well enough.

The thermometer shelter, anemometer, and evaporimeter were so
placed on a tower that an elevation of about 10 feet above the ground
was secured for each instrument. The periodic psychrometer read-
ings, which of course do not have great value, were likewise secured
at this elevation. The precipitation record during most of the first
year was secured by means of an 8-inch gage placed in the shelter of
apace trees 100 feet from the other equipment. It was thought that
the trees afforded no more than enough protection from wind to
insure a good snow catch. With rain and little wind, however, it
was f punt that a 12-inch gage in the open gave much higher values.
For this reason the entire precipitation record, except for August
and September, 1917, is questionable and has not been used. No
soil moisture determinations have been made at this station.

The following equipment was used:

Maximum and minimum thermometers, in standard shelter, October 1, 1916,
to February 28, 1918.

Air thermograph, in shelter, October 1, 1916, to February 28, 1918.

Psychrometer, in shelter, October 1, 1916, to February 28, 1918.

Anemometer, with register, October 1, 1916, to February 28, 1918.

Evaporimeters, inner-cell, Type 2, October 1, 1916, to December 31, 1916.

&
;
3
,
2
¥

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 19

Evaporimeters, inner-cell, Type 4, January 1, 1917, to September, 1918.
jae’ 4 foot soil thermometers, in iron pipes, October 1, 1916, to September,

Soil thermograph, October 1, 1916, to October 16, 1917.

8-inch rain gage, under shelter of trees, October 1, 1916, to February 28, 1918.
“1st heaeeapaualeaer’ rain gage, with register, August 1, 1917, to September

F-17; Frances Douglas fir.—Elevation, 9,300 feet; aspect, easterly.
This station is situated near Frances, Colo., on a steep mountain
side, in the bottom of a gulch opening to the east. The ground in
the immediate vicinity of the station is not far from level, but it
rises precipitously a few rods to the west. The situation is appar-
ently such as to give marked air-drainage effect at night, and to
cause stagnation of the air during the day. The temperature records
of this station are thought to have considerable comparative value,
because the station is at almost the same elevation as Fremont, and
similarly situated on the east slope in the Douglas fir zone.

The forest surrounding the station was at one time a fairly even
stand of comparatively small Douglas firs. It has been almost com-

letely destroyed by cutting and fires over a large area, and in this
ocality there is almost no reproduction. A little higher the granitic
and glacial soils are being occupied by lodgepole pine.

F-18: Fraser Basin lodgepole pine.—Elevation, 8,560 feet; aspect,
northerly; slope, 1 to 2 percent. This station, in north-central Colo-
rado, is situated in the broad valley of the Fraser River, which at
this point runs slightly west of north. The drainage area to the
south of Fraser comprises about 100 square miles, and the valley at
this point has widened out into a meadowlike basin of very little
slope, having a width of fully a mile between the forest-covered
slopes on either side. Although it is thus considerably removed from
the forest, there appears no reason for any great difference between
the atmospheric conditions over the station and those over the forest.
The locality is characterized by precipitation so well distributed over
the entire year that, although the month of June is usually about
the driest, the melting snow furnishes abundant moisture for the
early part of the growing season. The winter temperatures are
extremely low, but always accompanied by a good snow blanket.
Under these conditions lodgepole pine attains the best development
noted anywhere in Colorado, both as to stature of trees and density
of stands. (See Pl. VI, fig. 2.)

The character of the soil, considered in connection with the well-
distributed precipitation, probably is an important factor in this
development. The soil in the forest is of granitic origin but is so
thoroughly broken down as to contain a high percentage of clay, and
consequently to have a high water-retaining capacity.

The lodgepole forest about Fraser comes down on all slopes prac-
tically to the edge of the valley and to an elevation within 100 feet of
that of the station; to the south, where the valley is narrower, the
forest extends entirely across it. This fact might lead to the con-
clusion that the forest does not reach the station on account of the
forbidding conditions of an alluvial soil. It is also interesting to
note that there is no transition zone between the mesophytic condi-
tions of the slopes and the ‘‘desert”’ conditions of the valley. To
be sure, a very few specimens of western yellow pine may be found
on the lower hills, while somewhat more frequently a clump of

20 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

Douglas fir occupies some low, rocky point. The occurrence of
Douglas fir seems always to be the result of natural protection from
fire, and we may conclude, as in most of the lower lodgepole regions,
that Douglas fir originally controlled the situation.

H-2: Nebraska sandhills ——Because of the marked success of the
lanting of yellow pine and jack pine (Pinus banksiana) in the Ne-
raska ese ve it has been deemed advisable to observe to what

extent the conditions here differ from those of the yellow pine type
in the mountains. Since the establishment of the Halsey Nursery
in 1903, temperature and precipitation records have been secured in
a river-bottom situation (H-1). In October, 1918, an additional
primary station (H-2) was established on the hills 80 or 90 feet higher
than the river bottom, on a gentle northerly slope. A station (H-3)
for the observation of soil temperatures on a steep southeasterly
Bree a also installed only 200 to 300 feet from H-2. (See Pl. VII,
cD.

In considering the conditions shown by the records for these stations
it should be borne in mind that the locality does not produce yellow
pine naturally, and that no success has been had from direct seeding.
This may be due to the very sandy soil as much as to any atmospheric
conditions observed. The latter, apparently, are favorable for a
very high growth rate of established trees.

IL-1: Lake Moraine Basin spruce.—KElevation, 10,265 feet; aspect,
southeast; slope, about 20 per cent. This station is next to the
highest in the Pikes Peak series. It is operated by the officials of
the city water department of Colorado Springs in cooperation with
the Weather Bureau, and the record of precipitation and air tempera-
tures has been obtained for 24 years. This site is on a gentle slope
not far from the west bank of Lake Moraine, 20 feet above the water
and just outside a thick clump of aspen. (See Pl. VII, fig. 2.) The
body of water probably modifies temperature extremes to a slight
extent. The station is not protected by forest, for this consists only
of scattered specimens of limber pine, 4 to 8 feet high, which followed
a destructive fire 60*years ago. Some protection from winter wind
is furnished by the steep hill to the northwest.

I-2; Leadville flat spruce.—Elevation, 10,248 feet; aspect, south-
westerly; slope, 5 to 10 per cent. The surroundings of this station
are on the west, an almost flat bench sloping very gradually to the
Arkansas River, 3 miles away, and on the east the hicheD mountains.
The station is near the top of a ridge within the town of Leadville.
Most of the ground outside the town is clothed with a young, some-
what open growth of lodgepole pine, which apparently has replaced
the virgin forest since the first mining activity in this region dis-
turbed the natural conditions. This, however, has not been closely
investigated.

The most striking conditions of the locality are the severe wind
exposure afforded by the high elevation and the free sweep over a
broad basin to the west, and the apparent poverty of the soil, which is
composed of a gravelly sand. This soil, which is of the general char-
acter often chosen by lodgepole, may account for the openness of the
stands and the not too vigorous growth. On the other hand, open-
ness of the stands may be due to artificial factors, such as smelter
fumes, which have been fatal to some of the hill forests nearby.

*
Y

Bul. 1233, U.S. Dept. of Agriculture. PLATE IX.

Fic. |.—STATION W-Al, WAGON WHEEL GAP, IN NORTH-SLOPE DOUGLAS FIR.

,» The instrument shelter may be seen just within the timber on left (east) of rock slide. 1910.

FIG. 2.—LOCATION OF INSTRUMENT SHELTER AT STATION W-C, IN BORDER-
LAND BETWEEN THE FORESTED SLOPE AND THE FLATTER ‘‘ PARK.”

PLATE X.

Bul. 1233, U. S. Dept. of Agriculture.

1334 009°I1

"CIGI ‘8S WddW “NOILVAS139
lV ‘4-M NOILVLG 3d01S HLYON

NO LSSYO4 3ONYdS NNVWISONS AO AdAL—’SG ‘DIA

‘OIG §=“*2-ef WOTIT1S 1¥ ST OJON sity,

'dV5 IFSHM NODVM

‘

o-VW NOIWVLS LV LYVHE OL

YVTINIS AYSA ‘YI SVISNOG 3dO1S-HLNOS—"| ‘Dis

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 21

The station, of course, does not exactly represent the conditions of
_ the forest. It is thought to be advisable to use the records as show-
_ ing the conditions at the upper limit of lodgepole growth, in contrast
_ with the more nearly optimum conditions of Nast, on the western
slope of Colorado, of Foxpark, on the eastern slope in Wyoming, and
of Fraser, in an interior basin.

M-1: Foothills western yellow pine.—Elevation, 7,200 feet; aspect,
easterly; slope, 3 to 4 per cent. ‘This station is located at the Monu-
ment Nursery, 2 miles west of the railroad station of the same name,
and about 20 miles north of Colorado Springs. It may well be com-

ared with other stations in the Pikes Peak series. It represents the
oothills type of yellow pine forest, this type being characterized by
scrub oak as a secondary cover, which appears to be able to drive
yellow pine to the rocky situations, poorer soils, and the banks of
eroding gulches.

The station is scarcely more than 100 feet higher than the valley
of Monument Creek, which is devoid of forest growth. The pine
forest, however, appears on the first hills above the creek. Else-
where, under similar circumstances, the pine occurs at elevations
below 6,000 feet, along ravines and blufis; hence this station site
represents the lower limit of pine growth only as determined by
local topography and soil. The present site of the station is on a flat,
east-west ridge or bench about 20 feet higher than the parallel gulch
just tothe north. Prior to January, 1915, temperature and precipita-
tion records were obtained in this gulch, where cold-air drainage is
much more pronounced.

The station is surrounded by oak brush. (See Pl. VIII, fig. 1.) In
this brush a few yellow pine seedlings of various ages are making
headway. ‘The nearest mature pines stand 40 or 50 feet to the south
on an elevated part of the ridge and do not shade the station.

The soil is a very rocky mixture of granitic origin. In the first
foot the fragments of granite occupy only a small part of the space,
while at a depth of 2 or 3 feet the grayish sandy soil claims only about
_ 60 per cent of the space. The soil contains nearly equal parts of
_ gravel, sand, and finer material. It is evidently a transported soil
_ deposited here before the beginning of the secondary erosion, though

i als,

_ its coarse texture suggests glacial rather than water action.
The following equipment has been in use in connection with the
daily observations since January, 1915:

Maximum and minimum thermometers, in shelter, 5 feet above the ground.

Air theymograph, until January 1, 1920.

Psychrometer, April 8, 1916, to June, 1919.

Anemometer, on pole, 7 feet above the ground. This height gives some
freedom from the protection of the oak brush, most of which is 5 to 7 feet high.
7 Evaporimeters, Type 2, 5 feet above ground, April 3, 1916, to December 30,

916.

Evaporimeters, Type 4, 5 feet above ground, January 27, 1917, to date.

Soil thermometers, at depths of 1 and 4 feet, in iron pipes.

Soil well. Samples taken weekly during growing seasons from 1915 to-1918.
Sent by mail to Fremont Experiment Station, usually involving about two days
in transit before moist weight is taken.

8-inch rain gage.

N-1: Nast western slope lodgepole pine.—Elevation, 8,800 feet;
aspect, westerly. This station is situated on the Colorado Midland
Railroad between Hagermann Pass and Thomasville, and lies only

a few miles west of the highest comb of the Rockies, on a slope

:
;
2
a
:

Apu BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

dropping rapidly to the west. It represents a region that once, in —

all probability, bore a heavy forest of Douglas fir, of which only a
few relics remain. The lodgepole pine forest which now clothes the
slopes is, for the most part, young and vigorous. No particular
study has been made of this locality. The temperature data for the
station are introduced simply to show that the temperature conditions
conducive to a successful invasion by lodgepole pine are quite uniform
throughout the range of the species in this region.

P-1: Southern Colorado western yellow pine.—Elevaiion, 7,108 feet;
nearly flat river valley. Although the pine forest has been almost com-
pletely removed from the vicinity of Pagosa Springs, this station may
still represent in a fair way the temperature and moisture conditions
favorable to the optimum growth of yellow pine in southern Colorado.
(See Pl. VIII, fig. 2.) A valley forest about 20 miles from Pagosa
is typical of the forest found here. This type is distinctly not the
mesa type so characteristic of Arizona and New Mexico. It is a
hilly type in which the best development is reached only on the moister
ground along stream courses, and the strongly sloping hillsides.
The conditions of this region approach those of Arizona in the amount
of the winter precipitation, but at corresponding elevations the
Colorado type is deficient both in snow and in the volume of the
summer rains. It is probably for this reason that the higher, nearly
level ground corresponding to the mesas of Arizona bears only a
meager stand of pine.

W-A1l: Wagon Wheel Gap north slope Douglas fir.—tElevation,
9,610 feet; aspect, north; slope, 40 per cent. This station is one of a
series which has been very carefully and consistently operated since
1910, in connection with the stream-flow study conducted by the
Forest Service and the Weather Bureau. The meteorological records
are of the highest order, although it lias not been possible to reach
some of the more inaccessible stations for daily observations. The

equipment of this station, as of Stations W-D and W-G, has been.

about as complete as possible. There was, however, no sunshine
record, no recording apparatus for anemometer, nor any record of
evaporation until August, 1919.

The site represented is a steep north slope bearing in general a
Douglas fir stand badly damaged by fire some 25 years ago, and here
and there replaced by aspen. (See Pl. IX, fig. 1.) About the instru-
ment shelter there is a fairly dense stand of firs about 40 feet high,
but the anemometer and rain gages have been exposed in an opening
a few rods to the west where there is an almost bare rock-slide about
half an acre in extent. In some places the reproduction is about
equally of spruce and of fir, but in general at this elevation the spruce is
not of sufficient weight to warrant calling the site a fir-spruce type.
On the whole, the struggle between spruce and fir is in almost the
same stage as at the corresponding station at Fremont, F-9.

In every respect except that of wind velocity, the records of this
station agree so closely with those of W-B1, similarly situated on
another drainage area one mile north, that the statement of condi-
tions at the latter point would mean sheer duplication. This simi-
larity gives further assurance that the records fairly represent the
type.

The equipment of this station has been maintained essentially in
the same status since September, 1910, and is as follows:

Se ee Et ee en ee

ae Te

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 23

Maximum and minimum thermometers in shelter, 8 feet above ground.

Air thermograph, 8 feet above ground.

Hygrograph, 8 feet above ground.

Psychrometer, read daily at 9 a. m.

Anemometer, on post in opening, 41% feet above ground.

8-inch rain gage, on post in opening.

Marvin shielded gage in opening, since 1913.

1-foot soil thermometer is bulb of telethermoscope having no direct connection
with the air above. ‘This has been compared at various times with thermometers
in iron, wood, and porcelain tubes. ‘The porcelain tube is retained for frequent
checking.

4-foot soil thermometer, in iron pipe, beginning August, 1913.

Soil well, with moisture determinations weekly, at depths of 1, 2, and 3 feet,
during open seasons since 1913.

W-A2: Wagon Wheel Gap south slope Douglas fir.—This station is
directly opposite Al on the same watershed, the elevation and slope
being almost identical, but the aspect almost south. The stand is
here also predominantly one of Douglas fir, but is more open than
on the north slope. The trees have attained much larger size.
(See Pl. X, fig. 1.) Aspen is restricted to the base of the slope
below the station, or to gullies, and only bristlecone pine among
conifers competes with the fir. Specimens of the pine are here
rare, and become numerous only on the more rocky ground at the
top of the slope. Limber pine is relatively rare in the Rio Grande
region, and bristlecone pine appears largely to replace it on rocky

~ ground and wind-exposed points.

Although air temperatures Were recorded at this station during
its earlier years, the longer record covers only soil temperature and
soil moisture. The soil temperature at 1 foot has been determined
by a thermometer in a wooden tube since January, 1913, and at 4
feet by the one in an iron pipe since October, 1913.

W-C: Wagon Wheel Gap pine-fir.—Elevation, 9,360 feet; aspect,
east; slope, 30 per cent. At the initiation of the stream-flow experi-
ment, the point here mentioned was established as a control
station with whose conditions those on the two watersheds might
be compared. It hes on an east slope between them, and is not
affected by forest cover of any kind except as a few aspens form a
windscreen for the rain gages. It is, then, spoken of as a pine-fir
type only because it lies about on the line between the Douglas fir
forest on the higher slopes and the occasional yellow pines which
occur on warm, exposed breaks below. (See Pl. IX, fig. 2.) In
fact, the soil conditions here, and more markedly on the flatter
ground just below, seem to be prohibitive of coniferous growth by
reason of the presence of a considerable amount of alkali (carbonate
of soda).

The plan at this station has been to measure particularly those
conditions which are more or less general for the locality, and which
could not be conveniently recorded at ae farther from head-
quarters. The equipment has consisted of:

Maximum and minimum thermometers, in shelter, at 10 feet above ground.

Air thermograph, in shelter, at 10 feet above ground.

Psychrometer.

Anemometer, on tower, about 15 feet above ground, with register.

Wind-vane, on tower, about 18 feet above. ground, with register.

Sunshine recorder, electric thermometric, with register. _
Tipping-bucket rain gage, 3 feet above ground, with register.

24 BULLETIN 1283, U. S. DEPARTMENT OF AGRICULTURE,

Soil thermometers, 1 foot in wooden tube,* and 4 foot in iron pipe.
All records at this station except those for sunshine, rainfall, and soil tempera-
tures were discontinued about the end of 1912.

W-D: Wagon Wheel Gap burned bench spruce.—Elevation, 11,000
feet; aspect, east; slope, 5 per cent. This point is typical of much of
the best spruce land in southern Colorado and represents a high,
well-watered bench of deep, transported soil, partially protected on
the west by a higher ridge and fully open to the sun. (See Pl. XI,
fiz. 1.) The stand, evidently a very heavy one, was completely
killed, probably by the fire which swept this region 25 years ago.
The oceasional spruce seedlings which have come in do not affect
present conditions as measured. The ground is covered by a heavy
sod, mainly of sedges. Dead trees, standing and down, furnish con-
siderable protection, but can hardly affect the wind record-of the
station, which has been taken at a point 20 feet above the ground,
in order to eliminate purely local influences. |

The station has been equipped with:

Maximum and minimum thermometers, in shelter, about 8 feet above ground,
read at 6-day intervals.

Air thermograph, in shelter.

Psychrometer.

Anemometer, on pole 20 feet above ground, connected with register at
headquarters.

Eight-inch rain gage. -

Tipping-bucket rain gage, connected with register.

Marvin shielded rain and snow gage. ’

Soil thermometers, 1 foot in wooden tube,? and 4 foot in iron pipe, read every
6 days.

Soil well, for moisture determinations, at depths of 1, 2, and 3 feet, every 6
days during open season.

W-G: Wagon Wheel Gap high-ridge spruce.—This station is located
at 11,580 feet, on an east-west ridge. The soil thermometers are on
sround sloping almost inappreciably to the south. This slope is
hardly sufficient to affect any other condition measured. Although
the station is within about 500 feet of timberline, and the soil of the
ridge is very rocky, the spruce is of remarkably good quality, running
up to 70 feet in height, while even on better soils near by it rarely
attains more than 80 feet. (See Pl. X, fig. 2, and Pl. XI, fig. 2.)

An opening in the forest of about 1 acre was made in 1913, when
this station was established, in order that atmospheric conditions
and sunshine might be recorded free from the direct effect of the
forest. This record is supplemented by soil temperature and mois-
ture data from Stations E and F, located only a few hundred feet
away, at the same elevation, on northerly slopes in the heart of the

forest. Stations KL, F, and G were abandoned at the end of December, |

LOI 7.
The equipment of Station W-G consists of:

Maximum and minimum thermometers, in shelter, 8 feet above ground, read
at 6-day intervals.

Air thermograph, in shelter.

Sunshine recorder, operated only during winter season.

Tipping-bucket rain gage, operated only during summer season.

Marvin shielded rain and snow gage.

Soil thermometers, 1 foot in wooden tube,* and 4 foot in iron pipe, read every
6 days.

3 Except when snow becomes deep, when iron pipe with aerial extension is used.
§ Except when snow becomes deep.

Bul. 1233, U. S. Dept. of Agriculture. PLATE Xl.

Fic. |.—LOOKING DOWN ON STATION W-D FROM THE HILL ON THE WEST.

Note small extent of reproduction in this spruce area 25 years after fire. November 5, 1914.

Fic. 2.—STATION W-G, IN AN ARTIFICIAL OPENING OF THE HIGH SPRUCE
FOREST, RIDGE TYPE, 500 FEET BELOW TIMBERLINE. JULY 28, 1914.

the

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 25

Soil wells, representative of the high spruce type, were located at Stations E
and F, but none here. Moisture determinations every 6 days during open season.
| Summary.—In Table 1 is given a summary of the stations which
_ have been enumerated, the arrangement being in the general alti-
_ tudinal order in which the types appear. Within each group the
stations appear in the order of their absolute elevations. Station
_ F-9 has been designated as representing a transition zone between
_ fir and spruce for the reason, already stated, that the conditions
under the dense cover here, as shown by the reproduction, are more
favorable to the growth of spruce than of Douglas fir. The other
stations on the same slope, which are not so heavily shaded, and
which seem likely to reproduce to limber pine and fir, have been
classed as Douglas fir sites.

TABLE 1.—List of stations from which data are drawn.

| 2 | Character of record.1

{
|
Sta- | Tom-

Vegetative type} tion = Me Ree Eleva- | More
represented. num- _ General locality. Geographic name. one Nee iat eee aa
precipi- | data.
tation. [meteoto-
only. Sh saad
Feet.
Bigg S745 ero C-1 Pikes Peak....-.- Colorado Springs, Colo.| 6,098 |___..... x None.
Western yellow |f D-1 Black Haus: 305s Deadwood, 8S. Dak....} 4,535 AAG i tet oes None.
eS P-1 Southern Colorado| Pagosa Springs, Colo..} 7,108 ridin Ws a ce None.
estern yellow |( M-1 Pikes Peak....-... Monument, Colo. ....- T ZOOS OL s L M.&T.
pine. 1g Bae GO 22 esa see 2 Fremont Station, Colo.| 8,921 |......_. £ M.&T.
iP ep eee dot, Aes es: OV tte BY Be ee ee 9164.) 5 ee [Pe 8g M.&T.
Limber pine. .-.. OE ey eee Goes Sos 322 TA i ae: eae seer 9, (60 hice sa2 Lee M.&T.
. A We eG a ei IT ie Bi Soe pees Ul ab ae UU epee Se I S00 nee Pe od Temp.
F-1 ea Peak (con- |..... doer oes: 4a o BSB | » Seely 1g | r M.&T.
rol).
Pine-fir.. 5 ~<.--7 F4 | Pikes Peak.......}..... ae TOTS 3h 9 1A AI T0 2 | M.&T.
W-C Rio Grande Valley Wagon Wheel Gap, | 9,360 |........ | L Temp.
Colo.
F-7-82 | Pikes Peak......- Fremont Station, Colo | 9,121 |__...... (aie M.&T.
F-17 | Northern Colorado} Frances, Colo. .-.....- 9,300 L | BS SNES None.
Douglas fir. ....-. W-Al1 | Rio Grand Valiey ee Wheel Gap, | 9,610|-......- [aoe M.&T.
; Solo.
W-A2'}.-..- doer SEO FS | Ue Hie BY! 9,600} x | sah | WL &T.
F-18 | North Central | Fraser, Colo.....-..-- 8, 560 AY EERE None.
Colorado. |
Lodgepole pine..|} N-1 Western Slope,Colo.| Nast, Colo--......---.. 8, 800 1 ames (Co ees os None.
F-11 | Southern Wyoming} Foxpark, Wyo..----.-- 9, 000 besa. Yee L M.&T.
L-2 Continental Di- | Leadville, Colo......-. 10, 248 crea isvegepetied None.
vide, Colo.
Fir-spruce ......- F-9 Pikes Peak... .:.. Fremont Station, Celo.| 9,099 |_....._- ff M.&T.
iss tod baa GOURL: 4285.55 SUT dd jest. 244: SESOO Ns oo £ M.&T.
ina e aee C10 Sette Satan (PL 2 a dons 2s sage ao OM 7 al ie ae x M.&T.
Engelmann Ly et ie tote ee Lake Moraine, Colo...| 10,265 : ig iE, tet None.
spruce. W-D | Rio Grande Valley Waren Wheel Gap, | 11,000 |]........ | L M.&T.
yr solo.
W-Ges ee Ls ETFS TA ES oS i3)5 es SERED IES eS EL S80 ese o r M.&T.

_ Timberline scrub] F-16 | Pikes Peak... .... Fremont Station, Colo.| 11,500 |.....__- | x Temp.

1Jn the first two columns under this caption the letter zis used as an affirmative sign with reference to
_ the subheadings. In the third column the abbreviation “M. & T.’”? means that both soil moisture and
soil temperature data have been secured.

2 Also F-14 and F-15 under different conditions of forest cover.

THE GROWING SEASON.

In the study which attempts to detail the climatic conditions of a
region in relation to vegetation, one can hardly fail to realize the
necessity for distinguishing between those conditions which char-
peers the growing season and those which occur during the period
of rest.

re

26 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

In a study of annual or perennial herbs and of deciduous trees
it may be possible to draw a fairly sharp line between the growing
season sid the dormant period, if only on the basis of the period
each year in which the plant is in foliage or of that period which is
free from killing frosts. With conifers, however, or in fact any
evergreen vegetation, the problem is a difficult one; first, because
it is next to impossible to determine, in the field, when the period of
height or diameter accretion begins and ends; and, secondly, be-
cause it is by no means certain that evergreen plants do not lay up
carbohydrates necessary for growth whenever lght, temperature,
and moisture conditions make photosynthesis possible, in winter
as well as summer. It is fairly apparent that photosynthetic activ-
ity in conifers may occur for short periods of warm winter days.
Although we do not know the actual internal leaf temperature
necessary for photosynthesis in any plant, it is readily seen, from
the melting of the snow, for example, that the leaf temperature may
be several degrees higher than that of the air, and that the minimum
temperature for photosynthesis may be reached in bright sunlght
when the atmospheric condition seems very forbidding. The tem-
perature of light-absorbing bodies, with little wind, may well be as
much as 40° F. above that of the air. On January 10, 1918, at
2 p. m. snow was observed melting on the edge of a black shingled
roof, where there was reflected as well as direct sunlight, but which
was entirely removed from artificial heat, at an air temperature of
—6° F. (—21° C.).

From the consideration of such facts it becomes apparent that
any ‘‘growing season”’ for evergreen trees or herbs must be set
apart on a purely arbitrary basis. We may better speak of a period
of maximum or optimum growth, if it is desirable, rather than con-
vey the impression that growth is restricted to the warmer portion
of the year. In this bulletin the term ‘‘growing season”’ will be
used in this sense.

In the preliminary report (8) on this study the writers did not
attempt to divide the data between growing season and dormant
period, but presented some facts to show that, considering a soil
temperature of 41° F. (5° C.) essential to the growth of any of the
Rocky Mountain trees, there is a marked difference in the length
of the growing season as between different slopes and different
forest types. ‘The soil of a south exposure, for example, at a depth
of 1 foot was found to be above this temperature for about 220
days, and that of a north exposure for only 133 days. But while
there is no occasion for minimizing the importance of this difference
the assumption that growth is dependent upon a soil temperature
of 41° F. is not substantiated by the facts. The phenological ob-
servations of a number of years show that the first swelling of the
buds of Douglas fir may occur at soil temperatures as low as 39°
F., or as high as 50° F. So far as can be determined by this crude
method of observation, soil temperature and moisture are strongly
interdependent, and it is the molecular activity of the soil water,
as determined by its temperature and amount, and as well by the
solutes it contains, which controls the beginning of new growth.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 27

The time at which the first growth of a given tree may be noted in
successive years is so variable as to make the fixing of even a mean
date almost impossible. When the variations between different sites
and elevations and the different species on the same site are brought
into the problem it becomes so involved that its significance is likely
to be obscured.

For all of the above reasons it has been decided to consider the
“orowing season”’ as synonymous with “summer,” or the months of
June, July, and August, and the first decade * in September. The
three months stand quite apart from the remainder of the year. At
the low elevations, of course, much vegetative activity begins before
June 1; at very high elevations deep banks of snow remain until
after July 1. But at all elevations in or very close to the high moun-
tain ranges the cooling effect of a general snow covering disappears
soon after June 1, and this causes a very abrupt change in the tem-
Clay conditions. The first decade of September is included

ecause it is nearly always as warm as if not warmer than the much
cloudier decade preceding. Thus a comparison of temperatures for
these 10 decades may be quite as valuable as a comparison of mean
temperatures or temperature sums for growing seasons of variable
length, as we have practically no knowledge of what temperatures
are growing temperatures for the plants under consideration. It
may be said quite safely for the limited latitudinal range covered by
this discussion that, if the summer temperature of a given site is
found to be relatively high, the season during which it has favorable
temperatures for growth must also be relatively long. It is there-
fore thought worth while in this bulletin to give summer or “‘ growing
season’? means for all data, as well as annual means. Any greater
refinement of the distinction between periods of growth and rest
does not appear justified.

| ABSOLUTE AND COMPARATIVE CONDITIONS IN THE VARIOUS
FOREST TYPES.

AIR TEMPERATURES.

pas, og

i In taking up the study of atmospheric conditions which may
_ cause the differentiation of mountain forests into types dominated
_ by different species it is natural to turn first to the subject of air
_ temperatures for two reasons, viz:
_ (1) The decrease in temperature with an increase in altitude is
_. the most obvious change which occurs between plains and mountain
tops, and hence has the appearance, at least, of being a primary
_ cause for the zonal distribution of each species of the forest. The
_ difference in the warmth of north and south exposures is equally
perceptible and suggests a primary cause for the difference in forest
cover which usua ly characterizes opposing slopes at the same
_ elevation.
_ (2) Air temperatures are more readily measured, perhaps, than
any other conditions, and this has given rise to more records of air
temperatures than of other conditions except possibly precipitation.
This fact makes it possible to draw upon several localities besides

4 Throughout this bulletin the term ‘“‘decade”’ is used to denote a third of a month, ordinarily a period
of 10 days, but varying from 8 to 11 days when the last decade of a month is referred to,

28 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

those especially covered by the present ‘study in considering air
temperatures of the various forest types.

As already stated, the air temperature observations in this study
fall into two groups, namely, (1) those referring to conditions several
feet (44 or more) above oe eround, and hence more or less repre-
sentative of an extensive locality; and (2) those applying to condi-
tious within a foot of the ground and reflecting the local influences
of insolation and radiation. Data of the second class will be referred
to as “‘eround temperatures” and are available for only a few of the
special stations where the conditions directly affecting forest repro-
duction seemed very important.

By agreement the mean temperature for any period of 24 hours
may be computed by adding the highest and lowest temperatures in
that period and dividing the sum by 2. It is well understood that
for any single period this method is not at all likely to give a true
mean temperature. But common experience shows that over a
number of daily periods (for example, a month) the mean tempera-
ture so computed will be practically the same as the mean computed
from hourly temperatures. The latter, of course, can be had only
where a thermograph is employed, and to make use of the many
records obtained from maximum and minimum registering ther-
mometers it 1s necessary to employ the simpler method. It seems
desirable, therefore, to keep all records to the same standard, even
where hourly temperatures might be secured from thermograph
traces.

In addition to the convenience of the method the mean tempera-
ture derived from the maximum and minimum temperatures has
this virtue—that it involves no element of judgment in the computa-
tion. Without gomg into the mechanics of the thermograph, it may
simply be said that the correction to be applied to different portions
of a thermograph trace can not properly be determined by rule-of-
thumb as is usually done. The character and amount of such
corrections must rest upon the question of whether or not the observ-
able errors in the trace are due to the natural inertia of the instru-
ment or to improper adjustment of its mechanism. Hence slightly —
variable results may be obtained in reading hourly temperatures
from the thermograph trace, and this fact detracts greatly from the —
value of hourly records as ordinarily prepared.

The temperature records of the control station at Fremont have
been prepared by corapuling the maximum and minimum tempera-
tures for each midnight-to-midnight period from the thermograph -
trace as corrected from thermometer registrations. The daily range
is the difference between the highest and lowest temperature from
one midnight to the next and the daily mean the average between
the highest and lowest temperatures. To obtain records for other
stations exactly comparable with the control station as to period,
the same method has been followed where thermograph traces made
it possible. For those stations having no thermographs the method
has always been to make observations in the morning, tabulating
the minimum temperature as of the day on which recorded and the
maximuin temperature as of the day previous.

SS

a

|

— == -

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 29
Temperatures at the control station—The air-temperature record
for the control station, representing atmospheric conditions 20 feet
above the ground and in all probability mean conditions for this
elevation (8,836 feet) and tavelity, little affected by local exposure
and other circumstances, extends from January, 1910, to April, 1921,
inclusive, with the exceptions already noted. For the most part
these records were compiled and comparisons between the types
were made on the basis of data secured to the end of March, 1918.
Such comparisons seem quite adequate but to arrive more nearly
at normal absolute temperatures records secured since March, 1918,
will sometimes be used. The constant use of a thermograph and

of standardized thermometers has made this a very satisfactory

record. It has been very carefully checked over since its compila-
tion, as secured month by month, so that little error is likely to
remain in it except that inherent in the short method of computing
means.

DEGREES FAHR.
8

ee 2d FS ae ae ey,

a/c
Bike

o

/ =H F|- an Pag
rar he a hd |
/

S
Oo
N

AUG, 2

~ A |
. - 2 . . »
= Q = ro
@: 3 S x = S 8

Fig. 1.—Air temperatures at control station by 10 day periods. (F-1) Elevation 8,836 fect.

Mean air temperatures—The complete record of mean air tem-
peratures for the control station is given in Table 2, by decades
and months, for the purpose of showing the possible variation in
corresponding periods of different years, as well as the constancy of
certain conditions which are of interest. The average temperatures
are shown in Figure 1. |

Notrs.—All original records of air temperatures for decades, months, and
years are carried to two decimal places. Hence, some of the means given in
the following tables, do not agree precisely with the 1-place figures from which
they are apparently derived. In dropping hundredths to save space in these
tables, the rule is followed of throwing 0.05 to the nearest even tenth. Thus0.45
becomes 0.4 and 0.55 becomes 0.6.

BULLETIN 1283, U. S. DEPARTMENT OF AGRICULTURE,

30

TaBLE 2.—Mean air temperatures at the control station (20 feet above the ground) .}

24.9) 39.11) 55. 35

at

|

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1OIS ALIS

1See Note, p. 29.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. ol

The following points in Table 2 are worthy of note: (1) In all but
the summer months, temperatures in consecutive decades are subject
to very radical changes. Even the mean monthly temperatures
are very dissimilar in different years, and, in any one year, for ex-
ample, April may be cooler than March. Such discrepancies point
to the need for a long record to insure even approximate “‘normals”’
for any given period, but more particularly in this study to the abso-
lute necessity for comparing temperatures of different stations for
identical periods only.

(2) The first decade of June is seen to attain a temperature of
almost 50° F., and temperatures above 50° F. prevail from then
until after the first decade of September. It will later be seen that
these ten decades, except the first one, are also free from the liability of
frost. They form, then, a fairly logical as well as convenient basis
for separating the growing season from the remainder of the year,
at least at this middle elevation. In other localities the winter snow
may remain considerably after June 1, and this necessarily means a
shortening of the frost-free period for all adjacent areas.

(3) The highest temperatures occur from the middle to the end
of July. Thereafter the probability of cloudiness and rain increases,
until about September 1, when a rise in temperature may again occur
with clear weather. It will be noted thatin 5 years out of 11, this
first decade of September is markedly warmer than the last of August
Thus, while much of the temperate United States is feeling its most
oppressive heat in August, the mountain area subject to the “‘summer
rainy spell” avoids any excess at that time. The cloudy weather
depresses the minima quite as much as or possibly a little more
than the maxima, which may be reached, on many rainy days, before
the afternoon clouds gather. That the minima are not held up is
evidently due to the quick clearing away of the clouds and to the
ee eR after each shower.

t will be noted that the normally highest decade has a tempera-
ture of only 58° F. and that the highest single decade of record showed
a temperature of only 63° F. (July, 1910). Detailed examination of
the records shows only a few individual days attaining to the height
of 66° F., which, according to Baker (1), is approximately the mean
summer temperature required for the proper development of corn.
The failure of the mountains to produce agricultural crops is, on this
basis alone, quite fully explained.

(4) The winter temperatures are not extremely low. The total
range from summer to winter temperatures is, in fact, much less than
in the lowlands of the northern United States, and the winter tem-
peratures are actually higher in the mountains. It will later be
shown that they are somewhat higher in the Pikes Peak region than
elsewhere at corresponding elevations. This fact alone augurs unfay-
orably for vegetation, and especially for vegetation which retains its
foliage through the winter. When, however, it is noted that the
lowest winter temperatures are synchronous with periods of calm
(February), and the warmer periods always associated with high
westerly descending winds, the full importance of this period begins
to be apparent. The table shows that the temperatures do not
decrease in normal fashion to about February 1, but reach practically
their lowest point at the end of December, then rise during the Janu-
ary windy period, to find a second depression in the calm of February

32 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

and again an unseasonable rise with the recurring winds of March.
April brmgs much less wind and the heaviest snows; hence the low
temperatures.

(5) The autumn temperatures contain little of interest unless it
be in the somewhat sharp depressions for the second decade of Sep-
tember and the second of October, which are very often periods of
heavy, wet snows.

Range of air temperatures.—It is very doubtful whether the range
of air temperatures is of much biological significance, at least in con-
nection with plants so ‘“‘hardy”’ and so accustomed to a great variety
of conditions as the coniferous trees. For this reason it seems neces-
sary only to show the normal values, by decades, as computed up to
March, 1918. ,(Table 3). These have a value also in connection with
the table of mean temperatures in indicating the maxima and minima,
which may he directly computed by use of the two tables and which,
therefore, will be omitted.

Table 3 shows that the mean daily range at the control station is
22.3°; and, although it varies for individual decades by as much as
7° and is greatly mfluenced by cloudiness, wind, and other circum-
stances, the maxima and minima, respectively, may be approximated
if 11° are added to or subtracted from the mean temperature for any
period.

TaBLe 3.—Mean daily range of air temperatures at the control station (averages to
March, 1918).

i s bo

| Record by decades and months in degrees Fahrenheit. sg

eS =
Saf tite on,
Z Pe + : 3 a
Decade. go te oO hg 3 E &
be >] Se = oO S 3 2
a SB brah al ae 2 B 2 = E aa

= 2 me a bs ba 2 ie ~ > 3 o

(2 148) & | soydilbs oe elt sds aas Te

Ss os = < = — = <j DQ .o) Gq A =

Dane veescet oo. Sasa oe: 22.5) 24.4] 21.9} 20.9] 21.2) 23.6] 23.3) 23. 4) 23. 4) 23.2) 23.6) 22.2)......). 2...
Dee ae ae Ee: 21. 2) 22.0] 23. 2) 19. 6} 20. 8 23.7} 21. 8} 21. 4) 22.2) 23.0] 22.6) 22.5). -. to . si
3) REEDS RRR aaa Bee eS 22. 2| 19. 8) 22.0} 20. 4) 21. 8) 24. 4] 20. 9} 22.1] 22.4 23.5 20 23: iP Sse:
[Weittilyece se eS 22.0) 22.2) 22.3} 20.3) 21. 3} 23.9) 22.0] 22.3] 22.7) 23.2 23, 1 22. 6] 22.32] 22.80

1 See note, p. 29.

From the purely meteorological standpoint, the shortening of the
daily range in the stormy period of April is of some interest. Of even
sreater interest is the decrease during the windy winter months, which
does not show up in the means because here the action and reaction
are about equal. A west wind may have any one of several effects
on the temperature curve, but nearly always gives it an abnormal
shape, because the wind rarely attains its greatest force during the
warmer part of the day. Arising in the evening, after a cloudy day,
it may bring the maximum temperature at midnight. If the follow-
ing day witnesses a dying of the wind, its maximum temperature ma
be recorded at the beginning and its minimum at the end of the mid-
night to midnight period. A steady wind may hold the daily range
down to 8° or 10°, but a sudden rise or cessation will elevate it to 40°.
There are, of course, other cosmic conditions at the source of such
winds, which accentuate their effect on temperature.

Tae vear’ =

|

“FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 5 Fi

From the average values it is seen that the daily range is slightly

eater during the growing season than for the year as a whole
Froguicr, only the year 1910 shows this character in a marked degree.
If this year were eliminated, the growing season and annual means
would be almost identical. The peculiarity of the season of 1910
deserves consideration. The very high mean temperatures for that

ear, as well as the high daily ranges, it was at first conceived, might

ave been due to some condition in the exposure of thermometers
which was later altered. However, no such factor could be recalled
except that during 1910, and 1911 as well, extra observations were
taken at 1 p. m., and the opening of the shelter at that hour very fre-
quently resulted in a slight jump of the thermometers and thermo-
graph. Allowance was always made for this when the maximum at
this moment exceeded that of any other hour, which it rarely did.
On examining all the conditions for 1910, however, it is found that
that year was one of the driest on record, both as to precipitation and
atmospheric humidity, and had a very high percentage of sunshine.
As these conditions are associated with great daily ranges for short
periods, so they may be seen to account for the general character of
the year 1910, and foi the somewhat high ranges of 1917.

Absolute minimum air temperatures.—The Pikes Peak region is not
subject to extremely low temperatures in winter, even for short
periods, nor is it evident that where very great extremes do occur,
in other parts of the central Rockies,they have any but a beneficial
effect on established forests. However, a discussion of minimum
temperatures (Table 4) is of interest in connection with forest repro-
duction, especially in a locality where late germination is induced by
especially favorable conditions in July and August. Pearson (16)
concluded that young reproduction of yellow pine is extremely sus-
ceptible to frost injury, and indeed there is evidence to indicate that
lodgepole pine, spruce, and Douglas fir seedlings succumb to fall frosts
if they are not fully matured. The forester is in the habit of saying
that the seedling must be “‘lignified,”’ that is, its tissues must be
solidified, before it can withstand freezing. It would probably be
more correct to say that it is necessary for the young plant to carry
on eae and by the accumulation of soluble carbohydrates
make the sap so dense that it will not freeze with the first frost or
freeze firmly at any temperature. Lignification would naturally be
an immediate sequel to ane accumulation of building material.

In Table 4 the lowest temperature for each decade of the record
has been set down, and these temperatures have been averaged at
the foot of the table in such a way as to show the probable (not the
possible) minimum for any period of the year, in so far as a record
of this length may indicate probabilities. Because of very unusual
occurrences since March, 1918, it has seemed desirable to extend this
record to April, 1921. The data recorded since May, 1920, are for a
site where the probable minimum appears to be 1° to 2° higher than
at the control station. However, his will not affect the averages
more than 0.1° or 0.2°, and the additional record assists in eliminating
irregularities.

73045°—24—_3

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FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 35

_ The probability of freezing temperatures in the first decade of
_ June has already been mentioned in the discussion of the “ growing
_season.”’ Nevertheless, it is difficult to see how the factor of frosts
at this time can have much effect on the problem of what causes the
survival of one species and the failure of another. New seedlings
_ have rarely appeared at this time, and those from previous seasons
which have survived the winter should hardly be susceptible to
_ injury at so late a date. New growth on older trees will barely have
started by June 10, and shows no injury from the snows which may
occur at this time. Even young aspen leaves just emerging from the
bud withstand temperatures considerably below 32° F. without
apparent injury. Observation at a more advanced stage has not
_yet been possible. The great certainty of freedom from frost in the
first decade of September fully justifies extending the growing season
over this period. The second decade is equally certain to record a
thorough freezing.

The lowest temperatures of the year are almost certain to be
‘recorded in January, notwithstanding the fact that the lowest mean
“temperatures occur in February. This is made possible by the fact
that the absolute minima fall in periods of calm, when the protection
(Gf such it may be called) of the westerly winds is temporarily with-
drawn. It will later be seen that the high, rather than the low,
temperatures of January have the greater significance.

_ Classification of days according to temperature.—The computation
of mean temperatures even for so short a period as a decade tends to
hide the fluctuations and smooth over the extremes. In a biological
study it may be of interest to know, for example, that in a January
_having a mean temperature of 17.7°, whose warmest decade was 23.7°,
there might be two days having mean temperatures above 41°, and
to consider whatever possibilities this may present in the matter of
| the photosynthetic activity of trees, and also the more important
robability of damaging transpiration.

_ Therefore, to present most strikingly the temperature conditions
at the control station, Table 5 has been prepared, showing for each
month up to March, 1918, the number of days of the following
arbitrary classes, based on mean temperatures:

¥ No thawing: Mean and maximum, 32.0° F. or less.

. Freezing: Mean below 32°, and maximum above 32°.

Cold: Mean temperature, 32.1° to 41°.

Cool: Mean temperature, 41.1° to 50°.

Moderate: Mean temperature, 50.1° to 60°.
Warm: Mean temperature, 60.1° to 72.°

| temperatures as low as 0° F.

7 MS eC
e - ' +i <r ngt P| —
: e i >
36 BULLETIN 1233 U. S. DEPARTMENT OF AGRICULTURE.

TaBLE 5.—Classification of days according to air temperatures.

Record by months in degrees Fahrenheit. o
call fc
| 3 | ef: t|s4eek
Year. Class. 2 | B 4 | ati Odeo ee ae

3 he © ot » r<3) > fe) : i sj 3S

ef | Srp et) Ee | Mee be) eee es ae

a o pi _ = = = @Q >) (=) =)

— = < P=) 5 i <a |m o.4 = Ee
No thawing... il LC, STi tt ONO 0 Ce eee 6 49) 132:
Freezing. ....- 10 9 3 3 3 0 0 ) 0 1 8 13) Cl ae
old? 22 70 5) Sl 27) 2a Pope oe a og ae eee ae
1910. .....-- aS a a, Sg 11). (6), 15) 4) Uy Sa) Bhs ahs Ah ees 67/7
Moderate......|..... Re a Fc 2 6| 18 9} 19} 21) 12 ii eae 88] 53
Wheatbrnys 2-0 3) fugit eo] te B (28.7 FO) PRE er eee 42} 41
\Nothawing...| 3, 11) 3} 2 of Oo} oF of O 4 4 1m 44)...
Freezing... ..- gf ra] aah Cap 2a Sap? el ia aise 5 ea ec eee
cL 15). -6| | 43h+ D4) 78h: 0] © «OMY 20h piaegn Pee ones
1911. ...---- Cn te 7 7 Mii a) Viet) AS” a Wage = ote SES "ag 54| 16
Moderate. - 3.222312. 24a ee 12)2°° 20) 2 27 ae 24 S| ets Bae Be 100; 70
ca cr ee MPO PEPE H WYO LOGS es NL 5) Of Sal. eet ae eee 17] 16
No thawing Sie att 9 1 ¥ 0 0 0 4 3 2p Sie Sate.
Freezing. ..... 14). 17] 19) 43) 4). 24] Vol 2 c0l<o 4h aie Aah doe
Gnlae reso ss 9 3, 13] 6 2 Oo} Oo} 68 612 14) 8} 2
ee eee Dopkeercd tlt tt | Pee a kinds 3 ul li] 3] ) 3)): 10) a4 t Blak. 60) 19
Moderate: 2.) ese peo) see a Bis 1G). al) oD 9 i ees = rt) A!)
WV Arm FPL S| a es eh ah CR eerie oe! 4 Glo sc2 clas. cepee eee 10; 10
Nothawing...| 14, 14} 8} .3) OF o| oO} of i 4 2 36 62)...
Freezing... ..- 12). 1 12, a) Or a a 6: ae
Coll tn Sia: Bb ico? v8lioie)s “SOC Ol Sols Se) aerial a bees
1913. -.----- Conises ctxt Lk ba 9} 19) iil. S42 oO) a0) 1S se ee 68} 12
Moderato ii... eS) eee Re Q) 9G? 1S tol See eee 7 65
Watty (s- 25" aun es eee ee oe 30) Ol; , Ole dee ed Sees 22} 22
No thawing...| 1 5)..--.|.---- fe a0 + Oe 0 0 0 7 | ees eee ees eee
Freezing. ..... rN eas AS AS GeO rap) SO OSS Sea eee apenas
a Call? 2 os Tal So Lelie. a ge | Game Cer | andi Seer = Eo iz 1
sia Cool: 085 EP Sf ATS aa) Mae gps 2S eas ane age aes
Moderate. 82 5.|_.- [ree ele Sol. al 8) 221) 4 25h 22) Fi 28 y | ae eee reriy 102} 76
Wah ss as I oi Se eee eae 4 5 9 4 pad wl (eS isten 4 20; 20
No thawing...|.....|..... 18}. Ok. Steal Of” Saersalilt arr hess
INOOZING = ee 2 te Hoe 9 4 6 1 0 0 0 4 8 Sits 1
sae Golds CJS AS Aen PMR | mk | RO pe ae eb 4
=o Pern Goolt...cco.osda. cee pentaleesesbica9] AlOb g10lo cerh ees Bie beeches
Migderate; ta tee ee ee a 45t. 18) 20) ee ees | 82, 63
Warns .221-¢ | s/o birt hon aabts ree ie peel (oats rae 6| 6
No thawing. Wp ey Bo Bf A BO gM y Oh Ow 7} 17 54}...
Freezing.....- ia) 14) gf 74 oS OSG) Ugh a ae sae epee
1916 Ooictta 2H at, Cloee Gh care 6h. 9 Bry ON AOR x8 96 8} | 6 65, 2
Fey ee Cool. --..=--.--f 4\.....| - 24). 8] 12). SA BE GPO) ene
; Moderate... Tintin 2 ee 2 9} 15} 131 22] 45] 51.-..-1.-... 79} 57
Warr. os ie ee ee ee ee Als 07}. (ep eae es Bre eae 27, 27
Nothawing...| 11) 11; 12 5 5 0 0 0 0 1 2; 5) 52)...
Freezing...... 144 10) 13 9 8 1 0 0 0 7 5, 9 7 1
— 6 1s eg ea, 6 6 St Pa tO 0} = a Sa ie ee ee
satchel a Cool...........|-.-.f 1Pe UP) 4] 5S) 28.0 6 fp Sae Ase eee
Moderate......|-....|.--.- Meret Cea 2-13) 48) ai] 18)" epee elena 78| 61
Warn. 606. 6. tt ee ele SE Re | 6 13| 4.3 2 aerate 23) 23
No thawing... 19 6 BN oe bcpabnie tance = | emm pont See ee seca eee | oak hee
Freezing Sang gees as (1 aed a ee (ARR Bee mS oie ees ree
Gs ee 4 5) mae | Pe erate Mn Peres) eecaas DE [CT EEE ee pe
1918. ....... rane ieee ale 2} 2] aoc ee ee ME ie Pees re ae We BaLhs oes
Moderate......|..... See eres aces? eee” ee BE een eee, eee fe FS ee a
Lhe a epee Daeg Fee e Ro Lo | emt ee as ee Se ee re
No thawing...| 10.2| 10.0} 7.1| 3.2} 2.0} 0.0] 0.0) 0.0] 0.6) 2.4] 3.3] 11.9] 50.7/.....
Freezing...... 10. 9) 11.9 11.0) 6.6] 3.2} 0.4] 0.0) 0.0] 0.9| 3.7] 86] 10.7167.9 .4
bai Cold..........| 7.9] 6.0] 82/121] 7.8] 1.8] 0.0] 0.1] 2.5! 8 5111.1] 7.6] 73.6] 1.9
—hatalat) | 0): ae Oe EA 2.0, 0.4, 4.7| 7.81 10.6] 7.2] 2.6! 3.8] 7.9] 11.9] 6.9| 0.8] 66.6] 14.7
Moderate.....|.....|.-.--|e-.s-| 0.3] 7.4] 16.9] 19.0] 20.41 17.1] 4.5] O.il..... 85. 7| 64.4
Warm. ....... Pe 2 MES At ee akcaee 2.7) 9.4) 6:71, 4-0), hoe cles 20. 8| 20.6
| | | | 365. 3)1 0

1 January, 1914, based on record of Jan. 1-20 expanded.

It is seen that considerably more than half the days of the growing
season have a moderate mean temperature, and that their number
does not vary greatly from year to year. On the other hand, the
number of warm days, practically all of which occur in the growing
season, has varied in 8 years from 6 to 41 per year. Whether or not

44 —_—

———— ee NS

maximum and mimimum registering thermometers for t

-

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 37

such days are favorable to the growth of conifers, or whether, for the
Rocky Mountain species, they may not represent excessive heat,
might possibly be determined by growth studies; but nothing can be
said on the subject at present.

No less interesting 1s the fact that January and March show not
only a number of days with mean air temperatures above freezing,
but the probability of a few days when the mean will be above 41°,
the minimum scarcely below 32°, with the possibility of very effec-
tive thawing of the soil on well-insolated sites, and of some snow
melting on any site at this elevation.

Ground temperature at control station.—The temperature of the at-
mosphere at the control station has been shown in considerable detail
for the purpose of depicting prevailing conditions at a middle eleva-
tion in the mountains. In addition to this the temperatures at the
control station are to serve as a base with which all other temperature
records will be compared for their respective periods. At a number
of the compared stations in various forest types only ground tem-
peratures have been recorded, and at practically all these stations
the elevation of the thermometers is considerably less than 20 feet.
It is desirable, therefore, to know how the ground temperatures at
the control station May compare with those of the higher aerial
position.

As the control station is on a southerly exposure, though the gra-
dient is very slight, it is to be expected that the purely local tempera-
tures will be somewhat higher than the mean temperatures for the
locality, recorded 20 feet above the eround. ‘This proves to be true.
In Table 6 are given, in condensed form, the results of a comparison
of the two for two years ending March 31, 1918. In this Ege the

e ground
location were placed on a shielded board, which did not entirely pro-
tect them from reflected ight and heat, but allowed very free cooling
at night, as already described under “‘The Method of Study.” As no
thermograph was employed, the ground temperatures are not syn-
chronous with those of the tower station, but they always represent
periods about 8 hours later.

TABLE 6.— Departure of ground temperatures from temperatures 20 feet above ground
at the control station (with dissimilar exposure of instruments).

Record by months in degrees Fahrenheit. Mean
Mean. grow-
Datum. an- | ing
nual. | sea-
Jan. | Feb. | Mar. | Apr. pd aE: July.| Aug.| Sept.| Oct. | Nov.| Dec. son.
Mean temper-
ataress..:/: +0.4 |+0.5 }41.3 |+2.3 |4+1.5 |+0.9 |+0.8 /+1.1 |+0.9 |4+1.5 1 |+1.1 |+1.0 | +0.9
Mean maxima.|-+1.7 |+2.1 |+3.0 |+5.3 |4+4.5 |4+4.5 J+4.1 |+3.9 |+4.2 ]43.4 |+2.2 |+2.6 |+3.5 | +4.2
Mean minima.|— .9 |—1.2 |— .4 |— .7 |—1.4 |—2.6 |—2.4 |-1.7 |—2.4 |— .4 |-2.3 |— .5 |-1L.4 | —2.3
Absolute min-
BEA yk, es —1.8 |—3.5 |—1.8 |—3.9 |—1.2 |—2.2 |—2.8 |—1.8 |—1.0 |— .7 |—1.2 |—2.1 |—2.0 | —2.2
Daily range. ..|+2.3 |+-3.3 |+3.4 Cg +5.8 |+7.1 |+6.5 |+5.6 |+6.6 |+3.8 |44.5 |4+3.0 |449 | +6. 4

1 See note, p. 29.

It is seen that the maxima are always higher and the minima always
lower near the ground than at the 20-foot elevation. This results, of
course, in a much greater daily range, and in a slightly higher mean
temperature close to the ground. The difference seems to be greatest

+ 2 a, 35

38 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

in April and May, possibly because at that time the higher air
cooled by passing over snow fields, while locally the ground may be
bare. The daily minima, it is found, are likely to show the greatest
ee at the ground, in the coolest periods with extremely calm
nights.

‘As has been indicated, the exposure of the thermometers at the
ground was not such as to insure their recording strictly the tempera-
ture of the air itself at the time of the highest temperatures each day.
Because this was realized, at the end of the period above described,
the equipment, with thermograph added, was placed in a standard
shelter, free from direct insolation or reflected insolation, and well
ventilated through the floor to insure radiation at night. The site of
the shelter was only a few feet from the other site, but was more fully
clothed with vegetation. Since this change was made the higher
maxima near the ground have been at all times just about counter-
balanced by the lower minima, and the result has been, as before, a
higher daily range for the ground position, but essentially the same
mean temperature. The data for April, 1918, are especially interest-
ing, because during this month, in the earlier arrangement, the
greatest excess of ground heat was recorded. (See Table 7.)

TABLE 7.—Departure of ground temperatures from temperatures 20 feet above ground
at the control station (with similar exposure of instruments).

Apri, 11s, July, 1918.

+29 | 44.1

Me Tempera Gure Lt. °. . cmocco see ss once een Cae a5 Bene S Se es a a | +0. 2 +0. 2
Meaiimaninia.t<: 5b bet ed. See es eee EP EE ee eae +1.6 +2.2
1.4 SEPTATE TTT oh: ee en a oe Iie rami Soap ea py eDaMey: SSE Lh G —1.3 —1.9
MBSOLUILO MMI... ox SS Ae ee Re ee a eee eee —1.8 | —2.0
Moan Callyrange. «505 os acesce tS: AS reset ees | ee Eee, yee

1See note, p. 29.

Although these data are hardly extensive enough to invalidate
those previously presented, it is probably safe to state that in an open
situation of practically neutral aspect there is no important difference
between the mean temperatures near the ground and those 20 feet
above the surface; but that the ground location may easily show daily
ranges exceeding by 3° or 4° those of the atmosphere above. Hence,
of course, the probability of frost is much greater at the ground than
on the tower where our main temperature record has been secured.
It is sech from Table 4 that the probable mmimum for the second
decade of June, when expressed in terms of ground temperatures, may
be as low as 32° F., and for the first deals of September as low as
34° KF. [tis thus apparent that the frost-free season may actually be
10 or 15 days shorter at the ground than would be indicated by Table
4; and of course it is the seedling at the ground level that is most
sensitive tofrost. On the morning of September 7, 1918, for example,
the ground temperature was 30.2° F., and the frost was severe enough
to kill tomatoes but not other garden vegetables. The minimum
temperature 20 feet above round was 32.5°.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 89

These facts evidently do not apply to sites of decided northerly
or southerly aspect, where the amount of insolation received may |
appreciably affect the local mean temperature. (Compare Stations
3A and 3G, 2A and 2G, in Table 9.) ;

Temperatures at various elevations in the Pikes Peak region.—The ~
control station in this project, the Weather Bureau station at Colo-
rado Springs, the Forest Service station at the Monument Nursery,
the Weather Bureau cooperative station at Lake Moraine, and the
special station at timber line, operated for 17 months, comprise an

MONTHLY MEAN TEMPERATURE, DEGREES F.
5 $
()

10

JAN. FEB. MAR. APR. MAY JSUNE SLY AUG. SEP OC7- NOV. DEC.
———~

(ps)
Fic. 2—Monthly mean .emperature for different e-evations in the Pikes Peak region series.

altitudinal series (Table 8) from which a very definite idea may be
obtained of the primary cause of forest zonation, if, indeed, tempera-
ture is a Cause, as it appears to be. |

In Table 8 the mean temperature of each of these stations, by
months, is expressed by the difference between the temperature of the
station in question and that of the control station, for the period
indicated in column 4. The mean temperatures at the control sta-
tion for the period up to April, 1921, are given in the center of the
table, so that those for any other station may readily be computed
to the same basis. Im Figure 2 the absolute temperatures computed
in this manner are shown.

BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

40

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FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 4]

For the stations below the control station the greatest excess of
temperature is recorded in the spring and summer months and the
least in winter; for the two stations at higher elevations the least
deficiency is recorded in the summer months and the greatest in
winter. This may be expressed more concretely. Although the
control station is almost midway between the highest station and the
lowest station in the series, its January temperature is 1.8° higher
than the mean of the two and its July temperature is 1.6° lower.
This is probably because of the high position of the thermometers
at the control station, although it may have a broader significance
than has been detected. Figure 3 shows the January cradient as &
shghtly convex curve, but the July gradient is not concave, the
control station only being depressed.

malas eae ese (Oh Pellets dee) ale | ARS) LN
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Fic. 3—Temperature gradients in Pikes Peak region.

Bais ae

More important is the fact that the altitudinal temperature differ-
ence is least in the winter months, reaching a minimum of 11.5° in
January and a maximum of 17.9° in April. ~The latter is plainly due
to the quick disappearance of any snow that may fall at the lower
elevations, while the high elevations suffer a handicap from the snow
blanket, which is only nullified through the summer. The small
difference in winter is probably due to the tendency of cold air to
settle at lower levels, which sometimes results in an inversion of
temperatures in the coldest weather.

The January gradient amounts to 2.1° per thousand feet elevation,

the April gradient to 3.3°, and the mean for the year is 2.9°. It

should not be considered that these figures have any general value,
though they agree closely with the average of 10 Color ado watersheds
as computed by Robbins (20).

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42

| FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 43

Other temperatures in the Pikes Peak Region.—The remaining tem-
perature records for the Pikes Peak region (Table 9) are those obtained
at the special stations in the vicinity of the control station, all but
one at practically the same elevation, and designed to show the dif-
ference between types on slopes of different exposure. It is shown
that a difference of exposure may create as great a difference in local
air temperature as a difference of 1,000 or 2,000 feet in elevation;
hence, so far as air temperatures determine, almost any species may
find the proper conditions for initiation at a middle elevation.

In Table 9 the same plan has been followed as in Table 8, the records
being for a variety of periods and none covering the whole period
during which the control station has been operated. Several of the
records do not have sufficient individual value to warrant the com-
putation of means for every month. In these records one month in
each quarter has been considered, and the annual means have been
derived from these four months, a comparison being made, of course,
with the same months at the control station.

Tt is seen from Table 9 that the growing season excesses of the
outiying stations are somewhat greater than the excesses for the
whole year, where both have been computed, except at Station F-2A,
which, like the control station, represents conditions 20 feet above
the ground. These facts may account for the low summer tempera-
tures of the control station, as compared with other stations in the

- Pikes Peak series.

When the most typical pine and spruce stations shown in Table
_ 9 are compared with corresponding types at different elevations, as
_ shown in Table 8, namely, 2 with M-1, and F-3 with L-1, it is
found that the air temperatures of corresponding forest types vary
_ in the same direction, but that the local variations are not so great
as those in which a difference in elevation is involved. The pine
_ type locally may be 4.4° warmer than the control station, if the ground
— conditions for the whole year are considered, but at a lower elevation
_ the pine type is 4.8° warmer. Here the agreement is close.
One of the local spruce types is decidedly cooler than the control
_ station for the summer natiol in which record has been secured, but
_ the other is cooler only during the winter. In the first spruce type
_ there is a very marked depression of the minima, undoubtedly due
_ to cold air drainage; but in the second type the summer maxima are
so high as to lead to the belief that the poorly sheltered thermometers
received some direct insolation, the possibility of which was not sus-
pected during the observations. It is to be noted that insolation,
reaching the ground through breaks in a heavy canopy, produces
very sharp effects on account of the stagnation of the air. Because
of the factors possibly affecting the record for F—3, more significance
_ should be attached to the soil temperatures, which more clearly show
_ the general coolness of this site.

- Itis noted that the western yellow pine ridge (K-12), like the con-
trol station, is freely exposed to wind and is practically no warmer
than the control station. It will also be remembered that this site is
reproducing to Douglas fir and limber pine, as well as to yellow pine,
and that the yellow pine is in a very unhealthy condition. This
condition is equally well brought out by the soil temperatures. In
the summer, at least, lower air temperatures occur at F—4, where
the maxima are not high on account of the easterly exposure, and

44 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

where the minima are relatively low after the long period of radiation,
and possibly somewhat affected by air drainage in theslight depression.

The temperature record of the local limber pine type (F-6) is a
wholly dependable one, and is, perhaps, significant of the require-
ments of this species. It is especially noteworthy that the winter
temperatures, in apie of the fact that they represent ground condi-
tions, are slightly higher than those of the control station, and there
can be little doubt that this is due to direct exposure to west winds.
As the slope is a northwest one, the high summer temperatures may
be accounted for only by the fact that such insolation as is received
locally, coming late in the day when the general air temperatures are
already high, causes high and late maxima and hence high minima.

The record of the higher limber pine type (F-13) does not seem
to bear out the assumption that this species demands a great deal of
heat. Although the mean temperatures are always considerably
lower than for the control station, it is, however, possible to conceive
that during the summer months, when the difference is rather slight,
this site experiences a few hours of high temperatures satisfactory
to a heat-demanding tree, and that the growth of limber pine is re-
stricted to such short periods. Even so, the maxima are considerably
lower than for the control station, especially during the winter
months. The maxima for the growing season are 1.7° lower than at
the control station, the minima 1.8° higher, and the mean daily
ranges 3.5° less. For this period the station would compare favor-
ably with ground temperatures at the control station. The daily
ranges at the higher station are relatively the least during the winter
months. The decade minima are usually higher during the coldest
months when they are most affected by wind. For the growing
season they are 0.8° lower, on the average, and only very rarely
higher than at the control station.

On the whole, no plausible explanation is found for the occurrence
of limber pine in burns of the spruce zone except on the assumption
that ground temperatures during the growing season may be about
the same as at lower elevations, at least giving short periods of fay-
orable temperature. Having no competition, the limber pine is
able to survive even with a very small annual accretion.

The temperatures of the four north-slope stations that represent
different weights of cover in the Douglas fir type, are worthy of note.
The mean temperature in the fully open position is slightly in excess
of that of the control station—about as much in excess as the ground
temperatures at the control station might be expected to be. The
two stations under partial cover are almost identical with the control
station for the year as a whole; however, they are decidedly cooler in
winter and warmer in summer. ‘These high temperatures on a north
slope are to be accounted for only by air stagnation, the wind move-
ment being much less here than at the control station, even if veloci-
ties at the latter point are reduced to ground conditions. The whole-
cover station (F-—9) is appreciably cooler than the others during most
of the year.

The mean temperatures, however, do not tell the story for these
four stations. In Table 10 the extremes and daily ranges for the
control station and the four north-slope stations are shown. The
absolute figures given for the controlstation are for the longer period cor-
responding to Station 7-8, namely, October, 1915, to December, 1917.

_——s

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 45

For the most part, the variations from the control station, what-
ever the condition measured, progress rather regularly through the
four stations, that is, become greater or smaller, as the case may be,
with increase in density of the cover. The influence of cover may
then be directly measured by comparing Station 9 with 7-8. In
July, the complete canopy reduces the maxima 2.2° and raises the
minima 2.5°; in January the effect on the minima is similar, but, as
almost no January sunlight reaches the ground under the canopy,
the maxima also are reduced about 3.1°.

TasiLEe 10.—Actual temperatures at the control station and departures therefrom
on north-slope Douglas fir sites, with different weights of cover.

Record in degrees Fahrenheit.

Station. Datum. x . | al 1
January.}| April. July. October. aoe
Mean maximum.....-.... 33.86 42. 84 70.14 52. 67 49. 88
Mean minimum.......-... 11. 46 22. 09" 47.52 28.15 27.30
onal a 1915, to |) Daily range............... 22.40] 20.76| 22.62] 24.51 22. 57
” 2 Absolute minimum.......| —15.65 4.70 41.0 5.80 8. 96
Mean maximum. ......... — 1.9 +2.8 +6.3 +1.5 +2.2
Koes anor Mean minimum. .......... — 1.4 0 —1.8 —1.0 —1.0
LD AIRER Soar 2a Daily range =): 60ss22. 8.422 = ibs +2.8 +8.1 +-2.5 ease?
Absolute minimum.......| — 1.0 —3.4 —1.3 —1.7 —1.9
eee be Soe ee — 4.0 + .5 +5.0 am foi + .3
fean minimum. .........- — .3 + .7 — .6 + .2 0
¥-14, part cover......... Marly ranve sees. 2 ye — 3.7 — .2 +5.6 — .2 + .4
Absolute minimum.......} — .2 +1.2 — .6 —1.0 —.tl
peat MAKING Sess — 3.8 + .1 +4.9 —1.0 0 >
4k fean minimum. .......... + .6 +1.5 + .2 ae =
F-15, part cover......... Daily range............2.. AG es) rie hari eR eB Cae
Absolute minimum.......} + .2 +2. 4 — .2 0 + .6
vee i >-ahn bot a ee — 5.0 —1.1 +4.1 —3.3 —1.4
qt ean minimums. .2 5-..- = eaeery | + .7 + .7 +1.5 + 9
F-9, virgin stand........ Daily range.........---.-- al 1.9 gen ee Bue.
Absolute minimum. - + 3.4 + .4 — .2 +1.0 +1.2
S

1 See note, p. 29.

October is doubtless the most important month with respect to
the fate of new, poorly developed seedlings. Its temperatures are
similar to those of September, when the first frosts are to be expected.
In October the complete canopy has the effect of raising the mean
minima 2.5° and the absolute minima 2.7°. This is sufficient to
postpone freezing under the canopy for several days, the mean rate
of cooling, figuring from Siniambler to October, being 1° every 3
days. Frost may, then, “normally” be postponed a week through
the protection of the canopy. This should be of some slight benefit
to seedlings. However, it should tend to favor the less hardy
species, Douglas fir, as against the species, Engelmann spruce, which
in many of its habitats is subject to frost every month in the year.
This it does not do. The canopy tends to favor spruce, but fir and
limber pine flourish more abundantly in the open. I[t may, therefore,
be conceived that the influence of the canopy in reducing extremely
high temperatures, or mean temperatures for the year as a whole,
has more importance in selecting the successful species.

Other air temperatures at more distant points.—The mean air tem-
peratures for a number of stations in the central Rocky Mountain
region, believed to be representative of its different localities are
shown on a comparative basis in Table 11. With a few exces
the records of these stations cover the period during which the
control station has been operated, up to March, 1918, and all the
comparisons are made between identical months of the eight years,
or parts thereof.

» U. S. DEPARTMENT OF AGRICULTURE,

BULLETIN 1233

46

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\

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 47

The observations at all outlying stations, it is believed, have been
made at an elevation of about 5 feet above the ground as compared
with 20 feet for the control station, but, as has been shown, this factor
can not materially affect the mean temperature for any month. An
examination of such data as are readily available on the daily range
of air temperatures for other localities shows that a range of about
20° to 22° is characteristic throughout the region under discussion,
except in the Nebraska sandhills. The relation of maxima and
minima to means will be, therefore, almost the same as the relation
at the control station.

Air temperatures compared by forest types.—1. The yellow pine
station in southern Colorado shows less excess over the control sta-
tion than does a station 400 feet distant, and less than the Monu-
ment station at the foot of the Pikes Peak series, if the comparison
is made for the whole year. For the growing season, however, the
southern Colorado station is slightly warmer than the local yellow
pine type, but not equal to the Monument station. In both south-
ern Colorado and the Black Hills yellow pine endures much lower
wiuter temperatures than occur at any but the highest elevations in
the Pikes Peak region. The fact must not be lost sight of, however,
that these low temperatures are endured in the presence of a deep
blanket of snow which may largely prevent soil freezing.

All of the data for the western yellow pine sites may be briefly
summarized and reduced to absolute temperatures, in the order of
growing-season temperatures, as follows:

Locality.

Growing |
season. Year.

F.
PES PCS ee oe en es ne No oe 63. 60 43.39 | Black Hills.
61. 56 | 43.96 | Pikes Peak, M-1.
60. 88 41.69 | Southern Colorado.
158.65 42.23 | Fremont, south slope.
256.65 39.98 | Fremont, ridge.
Ua et eons a ee eee BEGG 2 <2 os Fremont , east slope.

1 Averaging ground and air conditions.
2 Based on May, July, and October differences.

It is then seen that the Black Hills station, in a low-lying region
where other forest species are practically excluded, has the highest
growing-season temperatures of any of the western yellow pine types,
and with this condition and fairly high rainfall the species reaches its
optimum development, at least in reproduction. Where the lowest
Hever -season temperatures of yellow pine types are found Douglas

r and limber pine also appear in considerable proportions.

Establishment of yellow pine by planting has been successfully
accomplished in the Nebraska enedhiity where the mean temperature
for the growing season is in excess of 70°, and for the year is nearly
48°. The former is about 7° in excess of the Black Hills yellow’pine
type. These conditions, however, not only have prohibited natural
extension of pine to this region, but have caused considerable losses
in plantations; furthermore, they have precluded direct seeding.
Although, of course, all the conditions which accompany these tem-
peratures must be referred to, still the more this matter is studied, the
greater is the conviction that temperatures do very largely control
the distribution of species, although the air temperatures as measured

48 BULLETIN 1283, U. S. DEPARTMENT OF AGRICULTURE.

may fall far short of showing the really critical conditions which must
ultimately be examined. In considering these Nebraska tempera-
tures, the effects of a scantily covered and strongly mineral att are
apparent in the high daily ranges, which in clear weather often go to
40° and occasionally to 50°, while averaging 27.6° for the year and
30.4° for the growing season. ‘Thus, for the growing season the mean
maxima are 20.7° in excess of those at the control station, while the
minima are only 11° in excess, the difference becoming less in late
summer. These facts, considered with those of distribution, are at
least strongly suggestive that, although yellow pine may make
vigorous growth with temperatures which are both theoretically and
actually very favorable for corn, yet the seedlings at their most
sensitive stage must have much more moderate conditions. The
relation of these high temperatures to surface drought in such a
sandy soil must not be overlooked, especially when it is noted that
seeding fails even in well-shaded situations.

2. The northern Colorado Douglas fir site (F-17) should possess
characteristics somewhat similar to those of the control station, as it
is similarly situated on the east slope of the mountain range. Its
considerably lower winter temperatures, however, indicate that it is
not so potently affected by descending winds, and that these may be
peculiarly developed in the Pikes Peak region. On the other hand,
the southern Colorado station (W—A1) is very much colder than the
control station during the winter—the locality being peculiarly free
of wind at all times—and dee a) cooler also throughout the

‘owing season, although the latter difference is not a serious one.

If the north slope open and covered stations are used to represent —

Douglas fir in the Fremont locality, the temperatures for this type, as
computed, may be summarized in the following order based on the
growing season:

Growing r ;
aan, Year. Locality.
°F. °F.
LBL CLEIG eh aire ree tl 25 WS a Mag rE 57. 50 39. 58 | Northern Colorado.

156.65 39. 59 | Fremont, cut-over.

155.78 38. 89 | Fremont, closed stand.
WOW CS ane oot gu a eee ee Ee 54.18 35. 76 | Wagon Wheel Gap, north.
SUT Gyr st eee a ee ee Se Ba ee 3.32 3. 82

1Based on April, July, and October differences.

The Douglas fir temperatures, which happen to have been taken
entirely at middle elevations, do not show so great a range as those

of the yellow pine type. In either annual or growing-season require- —

ments, the species seems to have a somewhat limited range, giving
way appreciably to spruce under the lowest temperatures here
recorded. Even if the brief record for the south-slope station at
Wagon Wheel Gap (W-2A) be considered, temperatures in excess of
those noted are not found. The south slope here shows an excess of
about 1.8° over the corresponding north-slope site, and the conditions
thus produced seem to be about the maximum that fir will bear, if the
large summer mortality among seedlings may be taken as evidence.

Further evidence of the lack of adaptive qualities in fir is found in the —
fact that in the Pikes Peak region, as observed by the writer, Douglas —

fir does not go to such high elevations as the western yellow pine
occasionally does.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 49

3. The lodgepole-pine type, wherever found, seems to be charac-
terized by very low winter temperatures and also by growing-season
temperatures considerably lower than those of the control station.
It is believed that the low winter temperatures of this type are always
accompanied by heavy snowfall, which, at least at the southern
Wyoming station, arrives so early as to prevent deep soil freezing.
These facts suggest that the most obvious requirement for reproduc-
tion of lodgepole pine is a winter climate of such low temperatures
and with such an abundance of snow that the evaporation factor is
essentially nullified. This would seem to explain the absence of
lodgepole pine from the Pikes Peak region, but not its ability to
grow there when planted. As has been shown by the physiological
studies, it is more probable that in the Rocky Mountains it is crowded
out of its normal temperature zone by demands for moisture which
can be met only at elevations where the snowfall is heavy and well
conserved throughout the winter. The mean temperatures indicated
by these data are:

Growing r :
See: Year. Locality.
eels 7 a8
RIGGS § Saceec manne oo Mee eee erst 54. 79 36.37 | Western slope.
52. 37 34.43 | Continental Divide.
50. 30 31.55 | Fraser Basin.
QU P Sis Det See cere: Kapha Se FF 7 ae eT 49. 94 32.49 | Southern Wyoming.
RD OESE AERO ae oo net wee HRT mihe oe 2 ee eS 4, 85 4, 82

It will be noted that these maxima overlap somewhat into the zone
of Douglas fir temperatures. It will also be noted later that the
temperatures enjoyed by lodgepole pine come within the range of
temperatures enjoyed by spruce.

4. The single spruce station listed in this table (W-—G) shows a
January temperature scarcely lower than the Douglas fir station
(W-—A1) of the same region, but for the remainder of the year the
high spruce type is much cooler. The annual and growing-season
means of this station are also remarkably close to those of the timber-
line station (f—-16, Table 8) of the Pikes Peak region, the two being
at almost the same elevation, and the former not more than 500 feet
below timber line for its locality. This suggests that an annual mean
of slightly less than 32° F. may be the limiting factor for all forest
growth in the central Rockies. It is suspected, however, that such
a limitation is placed upon growth only indirectly if at all.

All of the spruce temperatures may be summarized as follows:

Growing Sa95 Aaa
apaennt Year. Locality.
— + !
oF. | or. |
preset ee Te AMR Sets) 155. 93 1 38. 77 Fremont, canyon type.
52, 44 | 36. 16 | Fremont, middle zone.
Td ee ees Fremont, canyon type. ;
49.53 |: 32.13 | Wagon Wheel Gap, near timber line.
LS ae at J one i a es eae, 48. 96 | 31.63 | Fremont, timber line.
13a

meieamoeiice eet OUT etl ok 6. 97 | 7.14

1 Average of ground and air conditions at F-3.

73045°—24—_4

50 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

Summary of mean air temperatures —To bring all the preceding
data together for closer study, the average temperature of each type,
as studied, and the differences within each type, may be shown as
follows:

Mean temperatures.

Year.

Number é
Type. of Growing season.
stations.

Range. |Average.!) Range. | Average.t

aS at cae Ces. coe eee: Oe Raed. Usie? [yates 62 61 47.0
ORES os Fo ae ee ee et Ee 708 Lie oe 47.5
Wiesternsyellaow: pie! {pi Tte. ees el. Bees, SS 6 9.6 59. 2 4.0 42.2
Mitiahbr pine <2... 6 tcc i he allel eee ee | 3 6.6 55.2 5.8 38.4
YOU PIAS Hee oe ee ete ee a ee ee ee 4 see 55. 0 3.8 38.5
Lotigepolepine..2. os .4 jx eee Le ee | 4 4.8 51.8 | 4.8 33.7
5 7.0 51.6 | 7.3 :
!

Paha A SDFNCE. 5 ou 2 oa ee |

1 Ofstations listed in four tables just preceding.
CONCLUSIONS REGARDING AIR TEMPERATURES.

As has already been pointed out, from the physiological standpoint
the air temperatures per se may mean very little. It has been made
apparent by the preceding studies that the several species here con-
sidered show wide differences in photosynthetic capacity, and it is
logical to assume that the species which makes the most effective
use of sunlight will succeed best in a cool environment. On this basis
alone the upward extension of any species would be plainly limited
by competition with another species of greater shade tolerance and
correspondingly low heat requirements for effective growth. In this
consideration it seems that the mean air temperatures for the growing
season serve as well as any other criteria to explain both the alti-
tudinal zonation of the mountain forests and the sharp contrasts in
cover as between opposing slopes. In fact, as regards mean air
temperatures, it has beets shown that the same differences may be
produced locally as well by change of aspect as by great differences
in altitude.

But even if the zones and aspect types are accepted as pe ee
types primarily, and as sufficiently well duseribed and limited by the
erowing-season Means, an explanation has not been provided, in
agreement with physiological theory, for the fact that each species
has a lower altitudinal limit and an apparent maximum-temperature
limit. The mean air temperatures can of course give-only a sugges-
tion of what such limits might be, and even the maximum air tem-
peratures, it is believed, would fall so far short of depicting the really
critical conditions that it does not seem worth while to tabulate them |
for this purpose. When one considers all the facts of forest distribu-
tion, together with such phenomena as the marked success of yellow-
pine planting in the Nebraska sand hills and the vigorous growth of
the species there, the conclusion is unavoidable that the only condi-
tions worth studying, in the attempt to explain the natural limita-
tions of the forest types, are those atl have the most acute bearing
on seedlings in the infant stage. Even the matter of germination may
confidently be put aside, for year after year great numbers of seed-_
lings are germinated, but few of them survive even to the end of
the first growing season.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 51

To understand the limitations of the forest types, therefore, it
seems essential that there should be knowledge of the maximum tem-
peratures to which seedlings are subjected, and by this is meant the
maxima at the surface of the soil, where injury to seedlings is first
apparent. These will not wholly tell the story, as the fatal effects
are probably pepsuced by lack of moisture; but the maximum tem-
peratures would probably be a very fair criterion as to the degree of
drought produced if some allowance were made for the type of soil.

Considering the matter in its broader aspects, it is fairly apparent,
even before the other conditions of the forest types have been con-
sidered, that temperatures are primarily and closely controlling in
the natural distribution of each species. It is to be expected that
each would show some range of requirements, and that, as indicated
by the data, spruce and yellow pine might show a somewhat greater
range than the other species, because, occupying the edges of the
stands, they are not so closely limited by competition. It may be,
of course, purely accidental that the data do show this quality.
And in considering the range of temperatures under which any species
thrives it should be apparent that different temperatures, as they are
measured, may be physiologically alike. Thus, it may be said,
atmospheric humidity modifies the apparent temperatures for plants
as it does for aamnale The plant temperature must be relatively
higher in a moist than in a dry atmosphere. On the contrary, in the
critical connection mentioned as affecting seedlings, the greater the
humidity, no doubt, the higher the temperature that may be tolerated.

These considerations explain fully enough why the absolute tem-
peratures enjoyed may cover a wide range, and especially why the
_ temperature requirements in different regions may be markedly
different. In this connection, however, it should be remembered
that different races of the same species are now being dealt with.
The behavior of the Montana and Arizona forms of yellow pine has
been briefly described in the earlier physiological paper. The func-
tioning of the Arizona form would lead one to expect that it would
tolerate higher temperatures than the Colorado variety. Pearson (/9)
shows that the mean temperatures of the type in Arizona during the
growing season—June to September—range from 59.5° to 65.6° F.,
the highest (Williams) being a little in excess of the Black Hills
temperature, and being accompanied by a daily range of 33°, and
with less precipitation than is characteristic of the Black Hills.
The Arizona forest, however, reproduces itself with the greatest
difficulty, while in the Black Hills reproduction is very abundant.

The temperatures given by Larsen (15) for an even longer growing
season—May to September—are 61.2° for the Washington-Idaho
section, 59.8° for the intermontane region of Idaho and. Montana,
and 64.7° for eastern Montana. It is believed that the intermontane
region with the lowest temperature is the most favorable as regards
moisture. The relatively high value for the type in eastern Montana
may be determined by summer dryness, but there is a possibility, at
least, that the Chinook of that region may have some influence in
contine the type to warm sites where the danger of soil freezing is
east.

Pearson’s (19) Douglas fir type in the San Francisco Mountains,
at an elevation of 8,900 feet, has a growing-season temperature
almost identical with the average, but the annual mean is about 3°
higher than the average in Colorado.

52 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

There are, then, no surprising differences even when widely sepa-
rated types are compared. It seems evident that the temperature ©
requirements of the three principal forest trees of the region—yellow ©
pine, Douglas fir, and spruce—are in accord with their physiological
properties, but that lodgepole pine, which theoretically would grow |
much better in conditions similar to those enjoyed by yellow pine, —
is compelled to take a much cooler site because of its inability as a_
seedling to cope with drought. Its ability to tolerate the extreme
heat of fresh burns and to thrive here where competition has been —
destroyed is entirely in keeping with its physiological properties, and |
the inability to reproduce in its own shade is further evidence that the |
low temperatures are merely tolerated, not enjoyed. |

Somewhat similarly, limber pine, although sometimes ste:
almost to timber line, is plainly a tree of the open. Its occupation of
the wind-swept northwest slope, the warmest site here sti dhadl may —
be temporary, but it is believed to result from a greater resistance to
wind drying than is possessed by species which function more vigor-
ously. Thus it seems safe to say that none of the species chooses
warmer sites than, relatively speaking, its physiological conditions
dictate, and that, when driven 8 other considerations to sites rela-
tively too cool, the tenancy is only temporary.

WIND, HUMIDITY, AND EVAPORATION.

It would appear logical to follow the discussion of air tempera-
tures by that of soil temperatures, for the two must be somewhat
closely related, and their consideration in direct sequence would
facilitate fixing in mind the temperature conditions, as a whole, for
various kinds of sites. There is, however, one very potent reason
for postponing the consideration of soil temperatures until all of the
atmospheric conditions have been carefully weighed. ‘This is the
fact that the soil-temperature data which we are enabled to present
here are not, perhaps, so important as temperatures as they are for
expressing the physical status of the soil and the availabilty of its
moisture. How important it is that the soil should be frozen one or
six months is, obviously, not to be determined until we have first
considered the atmospheric conditions and measured, in a rough way,
the demand to give up moisture by transpiration which may be made
upon trees during the period of soil freezing. It is therefore desira-
ble and in fact necessary to proceed with the consideration of atmos-
pheric conditions before taking up the subject of soil factors.

WIND MOVEMENT.

Wind movement at the control station.—The topographic position of
the control station has already been described in general. The
station lies in a fairly wide valley formed by the junction of three
stream channels. It is not in the bottom of the valley, but on a low
ridge lying between two of the streams. As the anemometer is
placed on a tower 20 feet above the ground, the air movement is not
strictly that of the valley bottom, but represents a slight gradation
toward the freer movement of the higher air strata. evertheless,
the elevation of the anemometer, 50 feet or less above the lowest
adjacent ground in the valley, is inconsiderable in comparison with
the elevation of the walls of the valley. At this point these ris
some 300 feet higher on the north, at an angle of 18 degrees, and abou

: FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 53

200 feet higher on the south, at an angle slightlyless. Toward the head
of the more open of the three stream channels, in a northwesterly direc-
_ tion, the elevation is somewhat less than 200 feet, and the angle is 7 de-
grees. Down the valley the immediate negative elevation is still less.
The control station is thus appreciably affected only by air cur-
_ rents paralleling the main axis of the Rulley: namely those from the
- southeast or the northwest. Only rarely do currents from the south-
west or south reach the station through the valley having its origin
in that direction, as the station is somewhat west of the axis of that
valley. Due to this influence of local topography there is every
reason to believe that the full velocity of the prevailing winds is not
recorded. The prevailing wind direction is always southeast during
the periods when valley breezes constitute the chief element of air
circulation, that is, during the summer; and always northwest or west,
the former predominating, when mountain breezes or anticyclonic
winds dominate the atmosphere. Cyclonic winds, which are usually
of northeasterly origin, are rarely recorded as such at this station,
and may appear as east or southeasterly winds. It must, therefore,
be recognized that winds from the northeast or north, and to a less
extent those from the south and southwest, must conform to the con-
figuration of the valley to become effective at this station.

Further evidence of local influences is given in the gusty character
of winds, whenever the velocity is at all high. This is not shown by
records, since the gusts of wind are usually so short that any individual
mile recorded may be made up of several squalls and intermediate
calms. From extended observation the writer has estimated that
the momentary velocity of the wind gusts is fully twice the rate of
movement indicated by the anemometer records. In any study of
the mechanical effects of wind this discrepancy would be very impor-
tant, and only the pressure-tube anemometer or some similar con-
trivance could possibly depict the actual conditions. For the pur-
poses of this study, on the other hand, momentary velocities are of
no importance, and, it may as well be stated at this point, the mechan-
ical effects of wind in the region under study are quite negligible.
The soil is of such firm texture and the trees are so short-boled as to
preclude windfalls—except, of course, when trees have become badly
decayed—and even the breaking of limbs is extremely rare. On
one occasion in eight years, at a time when intense cold was accom-

anied by high wind, considerable snapping of twigs was noted.
ven this was confined to the especially long side branches whose
rapid, slender development subjected them to unusual exposure.

At timberline, of course, the mechanical effects of wind are always
apparent in the distortion of limbs and the one-sided development
ob trees as a whole. At lower elevations that which appears to be a
similar mechanical distortion of form has been traced by the writer (6)
to winter-killing of limbs on the most exposed portions of the trees.

With these facts in mind, the recorded wind movements at the
control station, as shown in Table 12, may be considered. As this
record is an almost perfect one, it can be quite positively stated that
apparent discrepancies between succeeding decades or corresponding
periods of different years are true to the facts. The credit belongs
to the designer of the thoroughly practical Robinson anemometer,
and to the fact that the daily i are compared with an
almost unbroken automatic record.

BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

54

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56 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

It is, perhaps, of first importance to note that the total wind move-
ment by years varies only within somewhat narrow limits. For the
seven years for which there are almost complete records the mean
variation from normal is only 3.7 per cent of the total movement,
while the Colorado Springs station shows an average variation of 5.1.
per cent during the eight years 1910 to 1917, inclusive. The varia-
tion between growing seasons is even less important. For the 11
complete growing seasons the local station shows an average varia-
tion of 2.7 per cent of the total movement, while Colorado Springs
for eight years shows an average variation of 7.5 per cent, the growing
season on the plains being thus more variable than the year as a
whole. The similarity of different growing seasons speaks for the
importance and stability of the mountain and valley breezes, as well
as for the practical immunity of the station to cyclonic influences
which express themselves during the summer in the form of southerly
winds. Apparently the least variation is to be expected in the last
decade of June, when the influence of insolation is probably the
greatest.

It is noted that the period of greatest calm, when the wind drops
to an average velocity of 4.1 miles per hour, is at the middle of August,
which also represents the climax of the summer rainy period, or at
least a period when rains seldom fail. The cloudiness at this season
reacts, of course, to retard the convectional flow of the mountain and
valley breezes, while the cumulative effects of summer temperatures
have doubtless at this time brought about the nearest approach to
equilibrium of mountain and valley radiation.

' From the middle of August to the end of the year the wind veloci-
ties gradually increase without important variation, and the maxi-
mum velocities are recorded during the second decade of January.
This must be related to the general cosmic conditions. These are
“anticyclonic” winds, blowing toward the centers of low-pressure
areas, Which during the winter almost invariably take a route across
the southern portion of the United States. Anticyclonic winds are
generally more violent than those in advance of the storm area, and
only the anticyclones are of importance at the control station, because
cyclonic winds durmg the winter season are locally opposed to pre-
eps mountain breezes, which result from higher temperatures on
the plains.

F pihor evidence of the character of the winter winds at Fremont
is found in the fact that they suffer a decline during February and are
again important in March. In other words, they reach, locally, their
greatest strength just before and just after the cyclonic storms have
reached their southernmost course, which is normally in February.
And they are probably less violent winds in February than in Janu-
ary or March, because the storm centers are farthest away in Feb-
ruary. Their more northerly origin in February might be thought
to affect their local force at the control station, but when other sta-
tions show the same feature this point loses significance.

The important biological fact is, then, that the winds coming during
the coldest part of the winter, when practically all mountain soils are
frozen, are most violent and incessant of any during the whole year,
are dry because they are anticyclonic, and are of an especially inju-

eS eee
FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 57

rious character on the eastern slope of the Rocky Mountains, because
in descending the slope they increase in temperature and in desiccat-
ing power. These winds not only fail to bring moisture with them
or to precipitate any on the eastern slope, but they sometimes melt
and always evaporate the snow which may have fallen in advance of
their occurrence. Vegetation, then, is not only exposed, but the soil
is denied the protection of a snow blanket which might retard its
freezing, and the soil-building humus of the ait iaglee season’s accu-
mulation is frequently moved from exposed slopes to easterly slopes
or canyon bottoms.

There is not the slightest doubt that this combination of conditions,
in which anticyclonic winds are the chief element, comprises a very
serious obstacle to forest growth in the Pikes Peak region, and that
its analysis in relation to forest types may be of prime importance.
Elsewhere along the eastern slope of the Rockies similar conditions
may prevail, though it is doubted whether any locality can show such
a powerful combination existing as a normal winter condition.
Further back from the plains, at higher elevations, and on the west-
ern slope of the main range the westerly winter winds combine far less
effectually for the destruction of vegetation and soils.

Considering the winter periods during which evaporation, as well as
wind movement, has been measured, it is found that evaporation is
not always rapid when wind velocities are high. When the mean
temperatures are below freezing the amount of evaporation is so
strongly influenced by the length of thawing periods that at this
season, more espe than at others, the temperature may be said
to affect the rate of evaporation more strongly than does the wind.
In order to show, therefore, that high winds do cause excessive evapo-
ration it is necessary only to show that they do not have a depressing
effect on temperatures. This has been done by comparing the wind
movements and temperatures for the 78 decades of record during
December, January, and February. It has already been shown in
Table 2 that the average mean temperature for these three months
is essentially uniform. The comparison indicates that with mean
wind velocities of 100 miles per day the mean temperature is 23.5°;
for velocities of 150 miles, 25.0°; and for velocities of 200 miles, 26.1°.
No doubt this power of wmter winds to increase the temperature
would be much more apparent if individual days were considered, as
the decade which witnesses a very strong anticyclone may also cover
the cyclone and the very low temperatures which often follow snow-
fall. It is, however, very certainly the tendency for higher tempera-
tures to accompany the higher wind velocities, even though excep-
tions to this rule are very numerous.

The important thing is, then, that sometime during each winter
wind movements occur which approximate velocities of 10 miles per
hour over a period of a decade, and that these winds are always of
westerly origin. Such velocities do not appear excessive, and,
indeed, examination of the automatic saeébe: shows that excessive
velocities seldom occur. A maximum of about 27 miles per hour is
the highest record that thas been specifically noted, and over a period
of 12 or 24 hours an average of 20 miles per hour is unusual. Even
excessive movement for a day or two at a time is not likely to induce

= 7 % 7 . 7 Ne
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= oo ee ‘he -
>

58 - BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

sufficient evaporation to cause any apparent injury to trees. It is
the continuity of these winds that bye? them dangerous, and prob
ably the decade is the shortest period that need be considered in this
connection. |

Only one additional feature of the record need be noted. Although
‘there is a general decline in the average velocities from the end of
April to August, there is a somewhat sharp rise at the end of May,
continuing into June, which is nearly always associated with lack of
precipitation. Whether the dry winds occurring at this season are
actually injurious or not depends, no doubt, primarily on the reserve
moisture of the soil. On the other hand, if the dryness occurs when
the season, so far as temperature is concerned, is well advanced, as i
1917, a day or two of even moderately high winds with a high evapor-
ating rate may have a very noticeable influence. On June 29, 1917,
wilting of herbaceous vegetation, in general, and of some new shoots
on conifers, was noted, although the soil was by no means dry, and
the recorded velocity for the day was only 6.2 miles per hour.

This late spring dry period, with moderately high winds, is especially
injurious to newly planted trees, and it may well be concluded that i
has an indirect relation to forest types in so far as it tends to preclude
the germination of seeds early in the season. It can not be said that
any difference in this respect has been noted between different sites
and exposures. When the atmosphere is lacking in moisture and is
freely circulating, even the most protected situations seem to suffer
superficial drying so quickly that germination is postponed unti
the beginning of the frequent summer rains, usually about July 15.
This fact may create special problems in comparing the species in the
Pikes Peak region, and it may be said that spring dryness has 2
bearing on the selection of the species which shall survive through the
autumn drought.

Wind movement in the various forest types—In Table 13 all the
available data on wind movements are given. As the stations having
this record are relatively few in number, it has not been thought
necessary to consider the local stations apart from those lying at a
greater distance from the control station, and hence subject to
altogether different influences. Neither is it necessary to separate
those stations at which the anemometer has been placed a consider-
able distance above the ground from those at which for a special
reason the surface circulation has been measured, although in com-
paring the results this factor should be kept in mind. Inasmuch as
the period of observation is very different for different stations, and
at most of them is not over 2 or 3 years in length, the method has
been followed throughout of giving the actual mean monthly move-
ment for each station, and the percentage relation which this bears to
the movement at the control station for the same period. These
percentages might then be related to the ‘‘average’’ movement for the
control station, as shown in Table 12, but it is doubted whether this
computation would have any additional value. As the matter of
primary interest is not wind, but rather its influence on evaporation,
the main intent at this stage of the discussion is to show relative
wind movements in a rough way only.

59

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS.

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60 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

If an examination is made first of the Colorado Springs station, —
which may perhaps give a better conception of the wind movements
of the region than even the control station, because of the free
exposure of the former, it is seen that, for the whole year, the wind
movement is much greater than that at the control station, and
greater than that of any of the mountain stations, except that at
timber line on Pikes Peak. The growing-season total bears almost
the same relation to the control station as the total for the whole
year. The Plains station has a low ratio to the control station dur-
ing the early winter months, November to January, which may mean
that the location of the control station gives winds at that time a
magnified local effect. The Plains station has relatively the highest
winds during the spring, probably because it is more susceptible to
the winds which blow from all directions toward storm centers during
the spring period. Actually, the most severe season on the Plains must
be just before the end of winter and before thawing has become general.

The Monument yellow-pine station is only 20 miles from Colorado
Springs, but being in the edge of the hills is fairly well protected from
north-south currents. This station shows distinctly mountain
characteristics, but scarcely more than half the total movement
recorded on the Plains. Part of this difference is no doubt due to
the elevation of the anemometer, which is barely above the top of
the surrounding oak-brush clumps. The relatively high values at
this station throughout the entire winter period define the degree of
exposure on the west. It is not doubted that, with a similar local
exposure of the anemometer, this station would show winter values
just about the same as the control station. The evaporation attend-
ing such winds would be augmented by the higher temperatures in
the pine belt, but their effect on vegetation would be decreased by
the shortness of the soil-freezing period.

The local yellow-pine station (F—2) shows no great variation from
the control station, such difference as exists being explicable, prob- —
ably, by the wind-breaking power of the surrounding trees, which
run up 30 to 40 feet higher than the anemometer. No doubt the
high January ratio at this station was due to the wind blowing from
certain directions so that the currents reached the anemometer un-
broken. On the whole, it would be best to assume that the ex-
posure, if the trees were not present, would correspond closely to
that of the control station.

The record at the yellow-pine ridge station (F—12) is difficult to
explain, inasmuch as both the soil of the ridge and the character of
the growth thereon are suggestive of the most extreme wind exposure. |
Indeed, a few isolated days may be found when the velocities of west —
winds recorded here exceed those at the control station. On the
whole, however, the movement is considerably less. The anemom-
eter was placed high enough to be practically free from the influence
of the surrounding trees, which are short and widely scattered. The —
deficit can be accounted for only by the position of the station on a
bench of considerable area, which does not directly contribute to and in
fact is out of the paths of the localconvectional currents, which naturally —
hug the slopes and valley bottoms. The station can not be considered —
to represent so severe a wind exposure as the control station; but, if
its longer period of soil freezing is considered, it represents, as will be
seen later, a closer approach to the limiting conditions for yellow pine.

FOREST TYPES IN. CENTRAL ROCKY MOUNTAINS, 61

The west-slope limber pine station (F—6) shows scarcely more than
half the movement recorded at the control station. The ratio is
very low during the summer months when the prevailing winds are
of easterly origin. The values of 74 to 83 per cent during some of
the winter months are high, when it is considered’ that the anemom-
eter was placed only 18 inches above the ground surface in a fairly
dense forest. As the strongest winds blow against the face of this
slope, this does not indicate any unusual velocities overhead, but it
does indicate extremely severe conditions for reproduction.

The four north-slope Douglas fir stations (F—7-8, 9, 14, and 15),
differing only in the weight of their forest cover, show clearly the
ability of the forest to check surface-air movements. The ratios of
each station to the control station are quite regular, on account of
the short periods involved, and because of the fact that during the
winter months the anemometers at an elevation of only 1 foot may
be completely stopped by accumulated snow. This rarely occurs for
more than a short period, but of course may obliterate some of the
highest wind movements. On the whole, the movement in the forest
is seen to be scarcely more than one-third of that in the open; in the
partly-cut forest it is two-thirds. The winter values are the high-
est, both absolutely and when related to the control station, because
this slope is hardly at all sheltered from northwest winds. It is not
difficult to believe that the wind that blows over the tops of the
trees is here quite as strong as at any similar elevation in the locality.

The local spruce canyon site (F—3) shows much more wind than
would be expected, even with the anemometer 20 feet above the
ground and well into the tops of the young spruce trees. The ratios
of movements here to those at the control station are perhaps more
constant than for any other station. This site is scarcely more than
400 feet from the control station, but in a narrow V-shaped canyon
bottom, which could not possibly be called “‘exposed.” The fairly
good velocities attained here show clearly the importance of the
mountain and valley breezes, of which the former particularly is
always most noticeable in acanyon bottom. This canyon, moreover,
appears to receive its due share of pressure from the stronger west
winds of winter.

The timber-line station (F—16) gives a fair impression of the move-
ments in the higher atmosphere. Pikes Peak to the south rises at
a sharp angle nearly 3,000 feet higher than this station, and the
slope is by no means rounded. ‘The site, however, does not seem to
be exempt from the influence of winter winds, although the move-
ment in the summer is relatively moderate. West and northwest
winds obtain a perfectly free sweep over all topographic features on
that side. The resultant velocities at the station, which is unpro-
tected by any timber, tax the credulity. The January ratio of 278
per cent would mean, when related to the control station averages,
a mean velocity of about 20 miles per hour for that month. The
following data have been picked from the record of 17 months:

Velocity,
. miles per hour.
Sugeest OAyS, ec. bE to0.2), 1916. ee 26. 0
Peres oye fee 260 2 Poly 31. 4
Pigsaseninele Gay, dan. 2r, 1917 2s 42.0
PCa eR ROUTE DNGV.aus 1Olg. 62 54i0 ae ek Ulu te 44.9
pecuen singe heur, Dec. 3, 1916. (9. eek . 49.0

62 BULLETIN 1283, U. S. DEPARTMENT OF AGRICULTURE,

It is seen from these special data, as well as from Table 13, that
this station, except for the four summer months, is exposed to wind
forces more than twice as strong as those at the control station.
Hence these winds are strong enough to move snow and soil and
actually blast off the exposed portions of trees as with a sand-blast.
There is no doubt in the mind of the writer that here the upper limit
of timber growth is set by the mechanical effects of the wind and
only indirectly by temperature conditions. The spruce and bristle-
cone pine trees which form the upper fringe of the forest are severely
exposed during eight months of the year, and during much of this
time they are cut off from soil moisture by the freezing of the ground.
Although low temperatures greatly modify the drying power of the
violent winds, it will be seen later that these winds are by no means
incapacitated for doing damage.

The high spruce station at Wagon Wheel Gap (W-D), though
fully exposed in an old burn, shows by no means the wind velocities
that have just.been described. Its velocities are only a fifth greater
than those at the control station, the lowest ratios occurring at
midsummer. The immunity to high wind is doubtless character-
istic of the vicinity of Wagon Wheel Gap, and not due to any special
features of the exposure, for even at an elevation corresponding to
F-16 (namely 11,500 feet) no excessive velocities have been re-
corded, and at 12,000 feet, on the highest ridge of the locality, the
evidences of injury from wind are confined to a small flat at the
very top, where the movement of snow is most easily accomplished.

Turning now to the Douglas fir station in this locality (W—A1),
protected by the walls of a basin which drains to the east, it is seen
that the annual movement corresponds closely to that of the fully
exposed north-slope station at Fremont (F—7-8), but that the winter
velocities of the former are much lower and are counterbalanced by
greater movement during the sprmg and summer. In fact, Station
W-A1 is the only one in the list which shows higher velocities in
summer than in winter, relative to the control station. Only the
winter velocities are of any import. It will later be seen that, in
combination with low temperatures but a drier atmosphere, these
hight winds have considerable power for evaporation; but during
the winter months there is less of this influence than is shown at the
control station.

Finally, a glimpse may be had of the conditions which charac-
terize the lodgepole forest of southern Wyoming (F-11). This
station is only partly exposed and receives a great deal of protection
from the forest on the north and west. Hence the relatively high
velocities of December and January bespeak a very great disturbance
of the atmosphere; and, even though the ratios decline during the
latter part of the winter, it is difficult to concede that the exposure of
trees is any less severe in this locality than at the contol Gatien
During the summer the wind movement might best be compared
with that of the yellow pine type at Monument. However, it must
be remembered that this lodgepole locality, and every other lodge-
pole locality which was examined in connection with air temperatures,
is characterized by very cold winters, and it will be seen later that, by
actual test, this practically nullifies the evaporation at Foxpark.

It should be brought to attention that the data presented in no
case refer to wind conditions at the level of the tree top, and that,

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 63

a
7
x

or the most part, direct comparisons of the sites are only approxi-
mate because of the varying elevations of the exposed anemometers.
Every such consideration would lead to the belief that the several
sites in the vicinity of Fremont would be quite closely comparable
‘as regards total wind movement 40 or 50 feet above the ground,
| and therefore that the exposure of the growing tops of the trees,
‘the portion most sensitive to desiccating influences, is not essentially
| different in different sites. The general air movement which char-
acterizes the locality, considered in connection with soil freezing, is
‘probably more important in fixing the types of forest than are the
specific velocities which have been measured. On the other hand,
high elevations doubtless experience much stronger winds than do
the middie elevations; the Plains region, shghtly stronger; the
“Wyoming locality, stronger winter winds; and the Wagon Wheel
Gap locality, winter winds of less importance, when like exposures
| are compared.

ATMOSPHERIC HUMIDITY.

_ Atmospheric humidity is doubtless of some importance in any eco-
logical study, but nowhere more so than in a study of evergreen trees,
which are influenced by evaporation and transpiration during the
winter as well as the summer. In this relation, the saturation deficit
is a much more valuable criterion than either vapor pressure or rela-
tive humidity, as the “‘deficit’’ expresses at once the capacity of the
atmosphere for additional moisture, without further reference to tem-
erature. In other words, a lack of vapor in the atmosphere, amount-
ng, for example, to 0.100 inch (this being the difference between the
ctual and possible pressures) should induce about the same rate of
evaporation whether the air temperature be 40° or 100°. The greater
leficits are, of course, likely to be encountered at high temperatures.
The correct measurement of the moisture of the atmosphere is one
the most difficult which a meteorologist 1s called upon to perform.
is almost impossible to eliminate the personal element in securing
t depression of the wet bulb of the psychrometer, for there is con-
tantly a tendency to fail to secure the greatest possible depression.
‘0 make the situation more difficult, there is no hygrograph, or auto-
hatic instrument for this purpose, which is at all precise. One is
ompelled to depend upon such psychrometer Sons as can be
uken in estimating the mean moisture for whole days or longer
Yeriods, and must never forget that even these observations are
allible. In spite of the most conscientious effort in the field, there-
we, the humidity data here presented can show only approximate
nean conditions.
‘Although the ultimate interest is in vapor deficits, as related to
vaporation, it will be worth while to examine first the absolute-
midity or vapor-pressure data, to determine whether there is any
ssential difference between adjacent sites other than that induced
y variations in air temperature. Even this is difficult to determine,
§ is realized when one notes the rapid changes in vapor pressure with
langes in temperature at a given point.. However, as vapor is dis-
ributed through the atmosphere very rapidly, local excesses or
| deficits tend to be nullified. Hence it is to te expected, at least for
|the several sites immediately adjacent to the Fremont station, that
lere will be little variation in absolute humidity.

, U. S. DEPARTMENT OF |

i

BULLETIN

64

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66 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

Vapor pressures at the control station.—In Table 14 are presented the
records of vapor pressures for the control station, by decades, up to
September, 1918. These a Rig writs have been computed from
a table prepared in 1911 by B. C. Kadel, of the Weather Bureau, for
a barometric pressure of 21.42 inches, which is the mean for Wagon
Wheel Gap, but slightly below the mean for Fremont.

It will be noted that up to June, 1914, these records pertain to
conditions prevailing at 7 a. m., while thereafter the psy ele
readings were taken at 8 a.m. This change has probably had a very
slight effect on the values, increasing the later ones by about 0.0050
inch.

As these data are only of indirect interest in the study, their de-
tailed consideration is hardly necessary. However, a few interesting
facts from the meteorological standpoint may be noted.

(1) The amount of vapor in the air varies greatly at times from
one decade to the next. This is not surprising in view of variations
in cloudiness, wind direction and velocity, and other conditions which
are related. However, it is surprising to find that different years
may vary by a margin as great as 25 per cent. In general, these
variations show little relation to mean temperatures; or, if they show
any such relation, they indicate that high vapor pressures tend to
lower temperatures, rather than that high temperatures permit high
humidity. This is more plainly true of growing seasons than of
whole years. Some of this variation may be ascribed to changes in
observers and to consequent changes in the standard of psychrometer
readings.

(2) The seasonal variation, as indicated by the averages for eight
years, or indeed by most of the individual years, keeps pace with the
rise and fall of temperature, but is to some extent modified by pre-
vailing wind directions and velocities. The highest humidities are
recorded in the latter half of July, with the highest temperatures;
but the middle of August is likewise a moist period, with low winds
and prevailing cloudiness, as previously mentioned.

As humidity readings at other stations in the same locality as the
control station have always followed those at the control station by
an interval veeyue from a few minutes to two hours, and as it is
peor to use the 8 a. m. humidity as a measure of the mean

umidity for whole days, it is necessary to determine to what extent
the humidity varies during the day. In Table 15 the data are given
for representative months from eee 1910, to February, 1912,
during which time observations were made daily at 7 a. m., 1 p.m.,
and 7 p. m.

TABLE 15.—Vapor pressures at the control station at various hours, April, 1910,
. to January, 1912.

[Average vapor pressures in inches.]

Increase over pres-
sure at 7a.m,

Hour. January.| April. July. | October. Benet
| |

Amount.| Per cent.

TiRbnatensebdasensscncunasest 0. 0573 0. 1344 0. 2844 0. 1352 0. 1528 | .ccncus-whlantus cb@en
MUM becwonurh vant cones tne chun’. - 0800 . 1594 - 3128 - 1726 | - 1812 0. 0284 19
7 - 0108 7

AM dovasbesuvccucsconsenses: .0670| =. 1393 . 3022 . 1457 . 1636
|

— $$$

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 67

Table 15 indicates that there is an increase in the vapor pressure,
corresponding to the rise in temperature from 7 a.m. to 1 p.m., and
amounting on the average to 0.0284 inch, or 0.0047 inch per hour, and
that most of the increase has been lost by 7 p.m. There is every rea-
son for supposing that the lowest vapor pressure of the day would be
recorded with or shortly following the mimimum air temperature, for,
if this is lower than the dew point, some of the atmospheric moisture
is certain to be precipitated as dew or frost. The humidity at 7 a.m.
is, then, probably less than the mean humidity for the day, as the tem-
perature at that hour is slightly below the mean temperature. The 8
a.m. humidity, however, can not miss the mean for the day by a very
wide margin.

To estimate the probable increase from 7 a. m. until 1 p. m., at
any other station, it is best to use the average figure, 0.0284 inch,
representing the whole year; for, though October and January do
not agree in the amount of this change, October, 1910, and October,
1911, were by no means uniform in this respect.

Vapor pressures at other stations near the control station.—Stations
F-2 and F-3, which are situated only a few hundred feet from the
control station, but represent very different sites, were observed
daily during the first two years of this project and only a few minutes
Jater than the control station. Hence their humidities are compara-
ble, with little or no correction for time. The variations from the
control station, by months, are shown in Table 16.

TABLE 16.—Departure of vapor pressures at two near-by stations from those at
control station, 1910-1912, in inches of mercury.

[All records obtained 20 feet above ground.]

Febra- | March. | April. | May. | June. | July.

F-2A..... Yellow pine, south | —0.0015 | —0.0021

slope. :
PagA. Canyon spruce.....- +.0045 +.0031 +.0063 +.0193 —.0036 —.0195 —.0070
re Type. August Sener October nee peep Year. Growite

CSS es en

—0.0097 | —0.0085 | —0.0158 | +0.0007 | —0.0008 | —0.0053 | —0.0082
—.0186 | +.0043 | —.0102 | —.0005| +.0021 | —.0016 —.0158

a |

F-2A..... Yellow pine, south
slope.
F-3A.....| Canyon spruce......

1Based on arithmetic means of 10 decades.

It is indicated that the atmospheric humidity over the south-slope
station is always less than at the control station, with the possible
exception of November. The same is true of the canyon site which
has developed spruce, if the year as a whole is taken; but from
December to April this site consistently shows higher humidity than
the control station. The amount of excess is generally more than
could possibly be accounted for by the difference in time between
the observations.

For the growing season at least, it is safe to say that the vapor
pressure 20 feet from the ground in the pine stand is less than at
the control station, and in the spruce stand less than in the pine.
Whether the forest growth has any direct relation to this fact may
be seriously questioned. It is believed that marked deficits during

Pe he:

On ea

?

68 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

the growing season are due simply to the fact that these sites do not
feel the effect of the diurnal valley breeze quite so early as the control
station, these breezes carrying somewhat more moisture than the
local air.

Certain data are given in Table 17 for somewhat more distant
stations covering a later period of two years, when observations at
the control station were taken at 8 a. m., and those at the outlying
stations from one-half to one and one-half hours later, the interval
being longer in winter than in summer. At all these outlying stations
the psychrometer was swung as close to the ground as possible.

TasuiE 17.—Departure of vapor pressures near the ground, in north-slope Douglas
fir type, from those at the control station, in inches of mercury.

Prepon Cover condition. | January. er March. | April. May. June. July.
W=7—8....| Clear cut.....--... —0.0036 | +0.0050 | +0.0046 | +0.0038 | +0. 0108 +0.0019 | +0.0064
F-14..... Pialt cut-4-is a —.0013 | —.0014} —.0036 | —.0032 | +.0055 | +.0112 +.0305
LIU nas ol RS Ose eee —.0027 | +.0005} +.0026| —.0028| -+.0021 | —.0007 +.0243
F-9._..._. Umnewts’:. ta berece +.0001 +.0036 | +.0013 | —.0053 | +.0087 | +.0041 +.0146

PPVCTHUG 62 toe sf ee sce | —.0019 | +.0019 | +.0012 |
‘|

SeEen Cover condition. August. rr del ite
F-7-8. ...| Clear cutci2 0.2.25: +0.0309 | +0.0214 | +0.0067 | +0.0089 | —0.0088 | +0.0073 | +0.0139
F-14._.... Halt cut: -225-- . = +.0321 +.0060 | —.0006} +.0031 |} —.OOS8L +. 0058 +. 0227
F-15 CLO es Se a ae +.0065 +.0083 —. 0091 +.0082 | —.0028 | +.0029 +.0099
1 Uncut 42a +.0210 +.0126 +.0026 +.0078 —.0014 +.0058 +.0132

PMVOEAPO. pen Soest tee +.0226} +.0121 | —.0001} +.0070} —.0053 |} +.0054 +.0149

1 Growing-season Means computed by adding three times the June, July, and August differences to the
September difference and dividing the total by 10.

The great irregularity of the relations indicated by the above data
is difficult to account for. It should be understood that humidity at
each of the first three stations was measured during different decades
of each month; that is, during one decade a month for each station.
The record for Station F-9 is complete, and therefore might be com-
pared with the average of the first three stations. However, even
this comparison does not help to clarify the results. Every effort was
made to eliminate discrepancies by using the same psychrometer at
all stations, and by daily reversal of the order of observations. Hence
the average time is essentially the same for each of the four stations.

All of the annual differences show higher humidity at these stations
than at the control station. The increase in humidity during the
morning hours is due to the rise in temperature, or to the setting up
of the mountain breeze, or to both. In either case the greatest change
is likely to occur in the early part of the day, and especially in summer
between 8and9a.m. Hence, the generally higher humidity at these
stations, as well as the great excess during the growing season, may be
wholly accounted for by the difference in time between these stations
and the control station. On the other hand, the variations between
the four stations are unaccountable, except on the ground of variations
in the amounts of sunlight at the points of observation and of the im-
possibility of entirely shielding the psychrometer from this influence.
Sunlight on the sy¢hrometer has the effect of lessening the wet-bulb
depression and hereby making the humidity appear higher than it is.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 69

On the whole, there is no indication that the humidity at these forest
stations near the ground is measurably different from that 20 feet
above the ground in the open position of the control station.

Vapor pressures in central and southern Colorado.—The control sta-
tion may now be compared with the only other station in this series
for which dependable humidity records are available (Table 18). The
stations are at similar elevations, and the record covers nearly seven
years. Thelater time of observations at Wagon Wheel Gap—9 a. m.—
should tend to bring out higher vapor pressures than at the control
station.

TaBLe 18.—Absolute and comparative humidity at W-Al (Wagon Wheel Gap
north-slope Douglas fir).

Datum. January. }February.| March. April. May. June. July.

Actual9 a.m. vapor pressure...| 0.0537 0. 0562 0. 0734 0.0995 0.1165 0.1849 0. 2792
Compared with 8 a. m. at con-

LTOL SALON ee. os Se desde See —.0066 | —.0050} —.0049} —.0133 | —.0345 | —.0314 —.0099

Septem- Novem- | Decem- | Growing

Datum. August. LS October. het nae Year. nue

0. 2605 0. 1894 0. 1149 0. 0735 0. 0506

0.1294] 0.2363

Actual 9 a.m. vapor pressure...
Compared with 8 a. m. at con-
LLOLSHAHONS— > enie—5- 1 Se - Fes

—.0101 | —.0140| —.0118 | —.0059 |} —.0118 | —.0132| —.0168

The Spalest difference between the two localities is seen to exist
during May and June, when their temperature difference is least.
This great deficit at Wagon Wheel Gap is due to the fact that the
rainless period in May or June, or both, is much more marked than
it is ites north and corresponds more closely to the conditions in
the Southwest.

During other portions of the year the deficit at Wagon Wheel Gap
is relatively uniform. It should, however, be pointed out that this
deficit may not mean muchhigher evaporation rates at Wagon Wheel
Gap, because the general temperature is here lower than at Fremont.

a examination of the different years at Wagon Wheel Gap, which
are comparable with whole years at Fremont, shows that the varia-
tions from year-to year are of almost the same magnitude as those at
Fremont, but that the two series are by no means parallel.

Saturation deficits —The question of humidity in combination with
air temperatures may now be considered. It has been stated that
the most direct expression for humidity, to bring out the relation of
this condition to evaporation and transpiration, is the vapor deficit.
In the following tabulation the vapor deficit has been computed by
deducting the mean decade vapor pressure, determined by the daily
observations at 8 a. m., from the saturation pressure, determined by
reference to the mean temperature for the decade. If applied to
single days, this method would be far from precise, because the
humidity might strongly increase or decrease ga a whole day.
It is beheved, however, that the 10 morning determinations of humid-
ity in a decade he a very close approximation to the true mean
humidity, and that the mean temperature is a sufficiently close
measure of the mean saturation pressure for the whole decade. Of
course, this method would err seriously if the temperature variations
in a decade were very great.

oa

BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

70

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_

72 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

Another method, which was practiced in compiling the records at
Fremont for a number of years, is to compute the saturation deficit
directly from each psychrometer reading, using vapor pressure and
saturation pressure tables, or a special table in which the saturation
deficits are shown directly. This method, however, ee gas less

recise than the one described above, and gives very different results,
te ace the saturation deficit varies much more widely with changes
in temperature than does the vapor pressure. Hence this method is
not to the recommended where dependence must be placed on single
daily observations at a fixed hour.

_ The saturation deficit data for the control station up to September,
1918, are given in Table 19.

It will, perhaps, be of interest to note that during the winter months
the saturation deficits are of about the same magnitude as vapor pres-
sures for the control station (Table 14) and indicate a relative humid-
ity of about 50 per cent. From April onward, through the growing
season, vapor deficits are scarcely more than two-thirds of the magni-
tude of vapor pressures and indicatt relative humidities nearer to 60
percent. This general change results from the cessation of the strong
mountain and anticyclonic winds.

At Wagon Wheel Gap (W-—A1) the winter deficits are always of less
magnitude than vapor pressures, probably because of the lack of any
direct mountain-breeze effects. They are also of less magnitude at all
other times, except in May and June, when the relative humidity is 40
or 45 per cent. Thus the dependence of vapor deficits on tempera-
tures as well as on vapor pressures is Clearly brought out.

EVAPORATION.

Eyaporation observations were begun at the control station and at
several other local stations as early as July, 1914. The instrument
then in use was the Piche, as modified by the United States Weather
Bureau ° fitted with 10-centimeter glass plates and with 9-centimeter
filter papers ® as a standard. Great difficulty is always experienced
in ppereting these instruments on a basis of daily observations, because
of the necessity for frequent adjustments, according to temperature
and dryness, to prevent overflowing or drying of the papers. As the
capacity of these instruments—40 cubic centimeters—was found to be
insufficient for a day’s evaporation at midsummer, extra fillings were
immediately necessary, and smaller papers were soon adopted. The
results were corrected to the atabdacd papers on the basis of the
exposed areas of the two kinds.

These observations, unsatisfactory as they seemed at the time, were
continued through 1914 and again taken up in April, 1915. During
the colder part of this period, to prevent injury to the glass vessels
from freezing, various mixtures of alcohol and water were used. It
is evident that the alcohol would accelerate losses, and it is equally
evident that evaporation of the alcohol would continue freely at tem-
peratures below 32°. Hence this expedient was in nowise satisfac-
tory, even though the mixtures were compared with distilled water
9 determine relative evaporation rates at temperatures above

reezing.

* While the modified Piche evaporimeter was employed by the Weather Bureau fora timein comparisons
with other instruments, its indications have always been regarded as most untrustworthy.
® Schleicher & Shull’s No. 595.

P
:

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 73

An effort on the part of the writer to overcome the difficulties due
to the small capacity and fragility of the container, by substituting a
larger metal container for the glass tubes, was also unsuccessful,
although this instrument was used for several months in 1915 at the
gintack sta thon. The experiment showed at least that the volumetric
method with evaporimeters could never be satisfactory for year long
observations, because of the very great changes in volume at the
freezing point, and for that matter with every change of temperature.
Often these volume changes were sufficient entirely to obscure the
water loss for a period of several days in cold weather.

As a result of this experience, four types of wick evaporimeters have
been devised and tested, in which evaporation losses were determined
by weighing. Type 4 has proved so satisfactory under all conditions
that an effort has been made to determine the evaporation rates for
all stations by means of this instrument. Types 2 and 4 have been
described by the present writer (5) as have also the fundamental prin-
ciples on which they are based. Type 2 was put in service in April,
1916, and gave fairly satisfactory records at several stations during
that year. It was an objectionable feature of this type that rain and
snow were not entirely excluded from the water chamber, although
in the so-called ‘“shade”’ instrument of each pair this was partly
accomplished. The records of this instrument are, therefore, -pre-
ferred to those of the “sun’’ instrument, which might absorb the
precipitation of an area about one-twentieth as great as the exposed
wick. The several instruments in use in 1916 were never standard-
ized, but this objection was in part overcome by rotation among the
several stations.

The usable evaporation record, then, begins with 1916, and a large
part of it has been secured during the years 1919, 1920, and 1921.
At one station (f-13), however, where types 2 and 4 were not used
for a whole year, it seems advisable to complete the record for 12
months by reference to the data obtained by the use of type 1 instru-
ments. This type will not be described, except to say that its evapora-
tion surface was a cotton wick, fully exposed to the air and sun, in a
horizontal plane. Like the Piche and Livingston instruments, it
gave relatively too high values for high wind velocities.

Evaporation at the control staton.—The evaporation values obtained
with instruments of type 2 in 1916 and of type 4 in 1917 to 1921 are
not closely comparable because of differences in shape and in the size
of the evaporating areas. They are given together in Table 20,
however, because the comparative data for two stations are in part
dependent on this 1916 record from the type 2 instrument.

Although the evaporation record of Type 4 instruments has been
secured only for parts of four years, and the averages by decades and
months can not be considered even as close approximations to ‘‘nor-
mals,” still this record is considered very Sable in several respects,
oe

(1) It shows the magnitude of the evaporation factor during the
winter months, due primarily to the somewhat warm and dry char-
acter of the descending anticyclonic winds. As already suggested,
these winds give the Pikes Peak region a peculiar character, and an
examination of the other evaporation records shows that no other
region studied has winter conditions so severe.

BULLETIN 1233. U. S. DEPARTMENT OF AGRICULTURE,

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FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 75

(2) It indicates the marked differences that are posite between
succeeding years and between growing seasons, the latter explaining
far more completely than the records of precipitation do, the varia-
tions in growth rates of trees from year to year. The growing seasons
of 1917 and 1920 are especially to be contrasted.

(3) It gives a basis on which, in the absence of a lorg-term record,
the evaporation may be computed for other years, in order to show
more clearly the cyclic character of growing conditions.

If the indicated annual evaporation is reduced to depths, a figure
of 28.96 centimeters or 11.4 inches, is found to be the approximate
yearly loss. As the normal precipitation is 21.1 inches, the evapora-
tion represents 54 per cent of the precipitation. At a corresponding
elevation at Wagon Wheel Gap, streamflow measurements indicate
that about 74 per cent of the total precipitation escapes as evapora-
tion from snow, soil, and vegetation. For the 20 months of observa-
tion, the measured evaporation is only 25.9 centimeters per year, or
only about 47 per cent of the normal precipitation when entirely
unaffected by trees. The instrumental measurements, therefore, are
evidently conservative. Evaporation records from open pans usu-
ally show more evaporation than precipitation in ea climates.

vaporation record extended by formula.—In a general way the rate
of evaporation from any type of evaporimeter or vessel of water is
determined by the capacity of the atmosphere for additional vapor,
which affects the rate of diffusion from the evaporating surface, and
by the rate of wind movement, which not only assists diffusion but
may be the principal source of heat for continuing evaporation. If
the evaporating surface is well heated by sunshine, however, it may
be warmer than the air; the wind will then be not a source of heat
but a means for loss. As evaporation is further complicated by
other factors, precise formule for computing it, even from free-water
surfaces, have never been devised. However, for approximate pur-
poses in this study, to obtain a better idea of the relative evapora-
tion for different years, it seems advisable to determine the ratio
between the actual evaporation of Type 4 instruments during 1917
and 1918 and the evaporation Sings may be computed from the
wind, humidity, and sunshine data combined in a manner to give the
most consistent results. The coefficient so obtained will then be
applied to the corresponding atmospheric conditions for other years,
to elaborate the evaporation record for the control station.

Admittedly, because the sunshine record does not indicate the
intensity of sunlight as determined by its angle of incidence on a
horizontal surface, the coefficient should vary according to the eleva-
tion of the sun. It has been considered sufficient to calculate .C for
each month, assuming that the variable atmospheric conditions
affecting intensity would probably be fairly well compensated in the
records of twenty months.

The formula used, after three other trials, to compute the coeffi-
cient for each decade of the 20 months is as follows:

0 E
Cle VD) SS

76 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

in which £ is the evaporation in cubic centimeters, W the wind
movement in total miles for the same period, VD the sum of the
daily saturation deficits, in inches, times 1,000; and SS the average
minutes of sunshine per ‘day, divided by 10, 000.

Conversely, to calculate the rate of évaporation when C has been

determined—*
E=CxSS(W+ VD).

As these data are lacking in precise value, it is necessary only to
state that the coefficients vary widely for corresponding periods of
the year, and that on the average they vary, according to the sun’s
elevation and other factors, from 0.477 in January to 1.336 in July.

The evaporation computed by decades for all periods of record
from TAS 1910, to December, 1916, is summarized in Tables 21
and 22, with the actual record since January, 1917. (See fig. 4.)

dod TT TT oT ae AN oe io ins oe Peer ere
ESRSSREREA SAREE AP URRASE SSE LSS eegoOTas.
Bn
TRRaREEERER SERRA ARE SNE Iae a
ap EEE LE LEE EGE CELE Ee eee SRSe ER
BPSCCO EEC eechaitteh CENCE EEE EEE

HEE PEEECEESSECEEE EGER

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ep et t) Tal rater) 1
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ee ee

HERBERSEDEL S
BBB N PRR AR

ego |_| — IRwe
Pit to tT TTT te Te te Td Tf EReESer eee
EEE EEE S

Fic. 4.—Variations in the relation between Bay: pa and evaporation for growing seasons. Control
Or

The principal value of Table 21 is that it shows the maximum
evaporation that may occur during the winter, the maximum rates at
this time being those which may cause injury to trees. On the
whole, however, this has little advantage over Table 20, for it is seen
that the maxima during the winter do not go to such great extremes,
relative to average rates, as, for example, fay do during June.

On the other hand, Table 22 bri ings out certain facts which can not

be reached from a consideration of any single climatic condition. In
the evaporation data alone it is seen that ‘the variations as between
years and growing seasons are much greater than might be expected
from ah conside ration of air temperatures or wind movements, and
that saturation deficits (Table 19) are not always fully reflected in
the evaporation quantities. In a general way, when precipitation
and atmospheric Ininidi ae are high ips gitar is low, and vice
versa; but as there are some exceptions to this rule, to measure the
moisture conditions it is really necessary to establish the ratio between

water supply (precipitation) and water loss (evaporation).

SOR Panay

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 77

TaBLE 21.—Mazimum probable evaporation rates as indicated by computed values,
March, 1910, to December, 1916; actual evaporation, January, 1917, to April,
1921.

Total evaporation per decade. |

[Cubic centimeters per 100 square centimeters horizontal surface.] Grow-
Decade: , | Year.| ing
Jan. Feb. | Mar.| Apr. | May. | June.| July.| Aug. | Sept.| Oct. | Nov.| Dec. ie
LSE eee 76.9 74.9} 110.0) 120.3) 203.0} 346.4) 284.5) 199.8) 237.7, 144. 8) 82. 6; ba 6172 se ieee
2 SNE TEES 108. 6} 54.4] 131.1) 95.6} 209. 0} 291.0) 274.9) 147.6) 129.5) 133.5) 61.3) 54.8 ....-.- watts tte’
5 eS ee § oe 61.7} 101.2) 97.8] 88.3] 291.3) 316.3) 280.2) 215.0) 121.8 153. 0) 6.24 O8i 3) 6. os ac|aeneeee
|

Month.| 199. 6| 223. 5] 315.7, 298. 7) 564. 3} 802. 8| 826. 9) 433.7] 448. 8) 358.9) 196 170. 5,4, 062. 8/2, 232. 3

TABLE 22.—Variations in annual and growing-season evaporation and their relation
to precipitation.
[Amounts for an area of 100 square centimeters, Evaporation computed by formula prior to 1917.]

Annual evap- Precipitation for Growing-season Growing-season
oration. corresponding year. evaporation. precipitation.

Year. Variation Ratio Variation | Ratio
~ _ |Amount.} from j|Amount.| toevap-|Amount.| from j|Amount.| toevap-
: average. oration. Average. | oration.

C.¢e; . | Per cent. Sak C. c. | Per cent. C. ¢:

POE ent ee nues 2 14,063 +27 4,704 1.16 2, 232 +51 2, 530 IAS
on |) AS ae a a ee Seas 3, 837 +20 5, 002 1.30 1, 804 +22 2,315 1, 28
1 OE a eee Sie 3, 446 +8 4,990 1.45 1, 528 +3 2, 76: 1.81
1088 2tisss. sus9e) 2, 806 —12 6, 740 2.40 1, 433 = 2, 828 1.97
Tht be Se ee Te areas 22,934 —8 6, 322 2.15 1,174 —21 3, 291 2. 80
1G Ses eS 32,801 —12 5, 718 2.04 1,284 —13 2,729 25t2
POUGESE See ee 3,657 +14 4, 990 1.36 1, 694 +15 2,492 1.47
1G RS I 3, 254 +2 4,374 1,34 1, 744 +18 1, 703 0.98
TIGER ates aes 43,106 —3 4,995 1.61 1,121 —24 2, 902 2. 59
rie ee ate 9d 5 2,752 —14 zat £27 1,291 —13 1, 734 1.34
WNT 313 ACN fe 2, 527 aio 5, 406 2.14 957 —35| 2, 982 3.12
Average........ 3, 198 +13| 5,183 | 1.665 1,478 +20 2,570 | 71,87

1 Year from March, 1910, to February, 1911.

2 Assuming missing periods, January to March, to have been “normal.’’
3 Year from March, 1915, to February, 1916.

4 Year from September 21, 1917, to September 20, 1918.

5 Year from May, 1919, to April, 1920.

6 + 0. 100,

7+ 0.157.

In point of actual evaporation and relative shortage of precipita-
tion, the year 1910 has probably been the most severe our records
have known, followed by 1911, both years having unusually high
temperatures and much sunshine. The whole year 1917, with
moderate evaporation and lowest precipitation (excepting 1919),
was only moderately severe. The year 1913, which was especially
deficient in sunshine, seems to have presented the most abundant
surplus of moisture.

ne may turn with more interest to the growing-season data,
and also with more confidence, because here both extremes are found
within the pend of actual evaporation measurements, and also
because no shifting of the period limits is necessary. Here it is seen
that the lowest evaporation, that for 1920, is less than 45 per cent of
that for 1910, and that the most severe growing season (1917), as
shown by the ratio of precipitation to evaporation, is not one in
which the evaporation was itself excessive. Although these years
are only partly covered by soil-moisture records, it should be noted
that the exhaustion of soil moisture was in 1917 more complete than
in any other year of which there is knowledge.

On the other hand, it should not be construed that the low evap-
oration and relatively great precipitation of 1920 constituted partic-
ularly favorable growing conditions. The size of rings formed has

78 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

not been determined, but other evidence points to a very serious
deficiency in the growth activity, with a resultant lack of vigor in the
new growth of 1921, notwithstanding an unusual abundance of
water in 1921.

It would seem that for the region and flora that are being described;
an increasing ratio of precipitation to evaporation might be considered
favorable until the ratio reaches about 2:1, after which any increase
is probably detrimental. This should be definitely determined by a
study of the width and structure of rings.

A succession of years such as 1916 and 1917 is evidently a very
serious strain on the forest, and probably this period has done more
toward weeding out mature trees in the Pikes Peak region than an
other set of conditions covered by this series of observations. This
has been particularly noted in relation to yellow pine, both in pure
stands and where the tree competes with Douglas fir. The yellow
pine suffered more than any pier species from the unusual drying
winds of January and March, 1916. Further weakening through two#”
seasons of drought no doubt gave its natural enemy, the mistletoe, a
very decided advantage, and the attacks of this parasite became
most evident in 1920 in a very marked deterioration of old trees, *
wherever mistletoe was at all abundant. As elsewhere pointed
out, this loss of yellow pine is giving Douglas fir in many places a
decided advantage. These casually observed facts are stated in
order to show the importance of unusual conditions in the life of the
forest, and the danger arising from short-period comparisons of the
conditions in the several forest types. ithout doubt this com-
parison of years, which has been attempted, demands as much con-
sideration in drawing conclusions as any direct comparison of con-
ditions in various forests which it is possible to make.

Relative evaporation rates at other stations.—Prior to 1916 the
efforts to measure evaporation continuously were largely unsuc-
cessful, and during 1916 satisfactory instruments were available for
only a few stations. In 1917 practically the same line-up was
secured with the latest type of instruments. After this trial the
number of instruments was greatly increased, and more satisfactory
records were secured through calibration of the instruments and
through their frequent shifting from one station to another. From
March to the cessation of observations in September, 1918, all the
local stations were covered, with systematic rotation of instruments
among them; hence this portion of the record is very satisfactory.
The record was further rounded out from May, 1919, to September,
1920, a special effort being made to cover the winter period at all
stations.

As there are no long records for any of the stations that were

secured with a single type of instrument, it has seemed best to ex-
nage the evaporation at an outlying station as a percentage of that
or the control station, for each month of record, regardless of the
type of instrument currently in use. When, however, the same month
has been covered during more than one year, the arithmetic means
for the control station and the outlying station have been similarly
compared. ‘The ratios for years and growing seasons are based on
the mean total amounts of evaporation for the respective periods.

In ‘Table 23 the comparative evaporation rates, so far as any
approximate basis will permit, are shown.

79

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS,

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80 BULLETIN 1233, U. S, DEPARTMENT OF AGRICULTURE,

Table 23 contains a number of surprises and brings out certain
relationships of the types and*localities studied, the knowledge of
which could hardly have been obtained through the examination of
the several conditions affecting evaporation—sunshine, humidity,
wind, and temperature. The desirability of a direct measure of
evaporation is therefore apparent, and the importance of a method
which gives the most probable weight to each of the component
factors In evaporation must not be minimized.

One of the first points to be noted is that, with the exception of
one or two of the very open situations, the winter ratios for the
outlying stations are much lower than the summer ratios to the
control station. This is partly due to the effect of cover and to the
fact that there is some slope, which may cut off insolation much more
completely in winter than in summer. A still more important
point is that when the air temperatures are below freezing there can
be practically no evaporation without sunlight to thaw the in-
strument (or, for that matter, the leaf), but, when the air is every-
where warm, insolation is by no means so important a factor. It
is well to bear in mind that under very arid conditions shading is by
no means a preventive of water losses.

The following observations may be made with regard to conditions
of the several forest types:

(1) it is readily seen that, of the yellow pine sites examined, the
most severe is by no means the planting site in the Nebraska sand-
hills, but is found at the foot of the Pikes Peak region (M—1), where the
low humidity of the mountains is combined with the relatively high
temperatures of the low elevations. The ratio for Station M-—1 is
uniformly high throughout the year, the depression in November and
December not being accounted for by any of the factors which have
been more extensively studied. On the other hand, because of the
low wind movement. and higher humidity, the Nebraska sandhills
present a far less severe condition, except during the summer when
the temperature contrast is greatest. It is possibly of some signifi-
cance that the highest relative rate occurs here about September 1.
Of the several yellow pine sites examined, that which approaches
a condition favorable for Douglas fir (F—4) is seen to be most mod-
erate in its evaporation stresses. The low values denote the influence
of shade more than anything else.

(2) The contrast between open and shaded Douglas fir sites is
brought out very clearly. Thus the four local north-slope stations
show a marked grading down from the clear-cut to the virgin stand,
the latter, for the year as a whole, having only two-fifths the evapora-
tion rate of the former. Whether or not the high evaporation rate
in the open area is a prohibitive factor for any of the species, there is
presented, at a single glance, a striking picture of the changed con-
ditions for reproductiion which necessarily follow cutting of the
parent forest. Again, in this group a surprise is met with in the high
evaporation rate for the Wagon Wheel Gap stations, where, as already
shown, the temperatures and wind movements are generally lower
than at Fremont, and the vapor pressures markedly lower. Thus
at Station W—A1, which probably receives less than half the direct
insolation received at the control station, evaporation goes appre-
clably higher during the early part of the summer but dwindles
almost to nothing in the coldest winter months. Where atmospheric

ae ee ore es

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 81

dryness plays such an important part, the contrast between a shaded
north-slope site and an open south exposure is far less marked than .
at Fremont, except for the coldest part of the year.

(3) The high spruce burn at Wagon Wheel Gap (W-D) shows,
for the growing season, the highest ratio to the control station of all
the stations studied. During the winter months its ratios are rela-
tively much higher than those for the timberline station on Pikes
Peak, due no doubt to the fact that the latter receives negligible
insolation. It has already been noted that Station W-—D shows at
all times somewhat greater wind movement than the control station.
The importance of this factor, combined with full msolation and act-
ing in the presence of low air temperatures, is plainly shown in the
not insignificant evaporation of the winter period.

The very low evaporation rates in the two local spruce stands
(F-3 and H-5) reflect the influence both of heavy shade and of the
close growth of the stand which reduces the air movement almost to
nothing. Here again, however, the potent effect of atmospheric
humidity may be observed; for, although the insolation at either
station is probably not more than 10 per cent of that received at the
control station and the wind velocity is reduced in almost as great
proportion, the evaporation is about three-tenths as great as at the
control station. It should be noted that at Station F-3, as at the
north-slope fir stations, the evaporimeter is sometimes completely
covered by snow. ‘This does not occur at Station F-5.

(4) The single lodgepole pine station (F—11) is marked by moderate
evaporation at all periods. The winter rate is not excessively low,
as the temperatures might suggest, because the wind is at this time
a very strong factor.

(5) The local limber pine type (I'-6) shows clearly the effects of
winter winds which strike the slope very squarely, the winter evap-
oration rates being among the highest notwithstanding the lack of
sunlight on this northwest slope. The higher limber pine site (F-13),
which has been depicted as presenting nearly timberline conditions,
is, so far as the available record can he depended upon, even more
severe in winter than is the actual timberline site, the former being
considerably warmer and better insolated.

(6) If the three major types are compared, for which the data are
adequate, it is seen that, with anything like normal cover, there is a
decided tendency for the evaporation rate to grade down from pine
to Douglas fir and from the latter to spruce. This decrease accom-
panies the decreasing temperatures which are characteristic of the
types. It is undoubtedly augmented by the reduction of insolation
due to increasingly dense stands in the higher zones. Although free
air movement evidently increases generally with increase in elevation,
the greater density of the stands tends to nullify any effect from this
source.

' It may, then, safely be said that the typical conditions for the
reproduction of the species are evidently less severe, as regards the
tendency toward evaporation, in the spruce than in the fir or pine
types. This statement is not in the least weakened by the fact that

in very open situations there is practically as much evaporationin *
one zone as in another, as is evidenced by comparing Station M-1

with the control station, or Station W-—B2 with Station W-D. It

73045°—24—_6

82 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

is known that in these very open situations the reproduction is
never abundant and often is not typical of the climax forest. In
the Pikes Peak region there is a strong tendency for limber pine to
invade every open site; in the Wagon Wheel Gap region bristlecone
pine is the pioneer.

Although, then, it may be seriously questioned whether the evapora-
tion stress is the controlling or limiting condition in many instances,
still there can be little doubt that this measurement, sensitive asit is
to every atmospheric condition, gives an extremely convenient and
valuable index to site conditions, and particularly as it reflects in a
simple term the general temperature and sunlight conditions.

The evaporation measure seems particularly useful in expressin
the broader differences between regions. Thus, only a confuse
picture of the Nebraska sandhills is obtained by determining that the
temperatures are higher, the humidity higher, the wind movement
lower, and the sunshine (this is said for illustrative purposes only)
less than at the control station. Orientation is, however, sinmapdieibeky
possible when it is found that all these differences are algebraically
summed up in less evaporation in Nebraska. When this result is
considered in connection with each of the individual factors, the
phenomena of tree growth become very clear.

On the other hand, the high evaporation rates which are found at
the Wagon Wheel Gap stations, in comparison with the control
station, demand an explanation. If the evaporation rate has any
direct bearing on the choice of species, how is it that unity evapora-
tion gives essentially the same result in the one locality (compare
Stations F—9 and W—A1) as three units of growing-season evaporation
in the other? It should be noted that even the highest rate recorded
in these data is probably not excessive for established vegetation;
with a reasonable water supply, the highest rate may, possibly, rep-
resent the most favorable and not the least favorable growing
conditions. As the high rate in the Wagon Wheel Gap Tooality
is induced primarily by a dry atmosphere, which represents a normal
condition, it seems probable that that region is not, in the long run,
subject to any more extreme Maxima than the Pikes Peak region,
where sunshine, humidity, and temperatures suffer greater fluctua-
tions from year to year. This can be certainly determined only by
much longer study.

It seems, therefore, that, if evaporation rates bear directly on the
composition of forest types, this can be determined in only two ways.
It may be determined by a more intensive study of evaporation
rates as they affect young seedlings, in close correlation with the
determination of the moisture supplies of those seedlings. This
points tothe same situation that has been reached in the study of air
temperatures and soil temperatures, namely that the conditions
which must be critical in the early establishment of forest trees are
ot perhaps short-term, conditions, and are not covered by
these general comparisons.

On the other hand, if evaporation is measured as in these tests
but through a longer term of years, it is probable that extreme
conditions in the various forest types, particularly for the more
widely separated localities, will be brought to light, which, although
not necessarily constituting criteria of the resistance of the species,
will give a better conception of the relative stresses which each species

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 83

is compelled to tolerate before it can come fully into possession of
the site. To attempt now to compare the extreme conditions in the
various types on the basis of records for only two or three growing
seasons, does not promise fruitful results. Even the somewhat
serious drought conditions presented in the late growing season of
1917 can not be used in this manner, as there were at that time only
a few evaporimeters in operation, and no special observations on the
mortality of seedlings weremade. Itis probable that the soil-moisture
conditions to be discussed later, are more significant in this connec-
tion than any evaporation rates could possibly be.

There is, however, in the whole region under discussion an annual
drought which is absolute, and which results from the freezing of the
soul. As the duration of this freezing is so different in the different
forest types, it has always seemed probable that this drought period
might have a very direct bearing on seedling establishment. It
may be said at this point that a consideration of the possibilities of
winter drying does not materially alter the conception of the relative
positions of the types, except in showing that typical warm yellow
pine sites are least liable to winter drying. On the other hand, the
evaporation at this season still further emphasizes the severity of any
strictly open site. The high evaporation rate which seems character-
istic of the Rio Grande region does not seem to be at all compensated
for by a warmer soil; in fact, the period of freezing there is a little
longer than at Fremont.

It has seemed desirable to discuss this evaporation problem some-
what carefully, and to accept the comparative data cautiously, be-
cause of the dual réle which evaporation plays, or, to be more specific,
because of the double function of the evaporation rate as a measure of
growing conditions. It is evident that loss of water by the plant is
rarely, if ever, advantageous per se, yet that it is inevitable if the
plant is to enjoy the benefits of light and is to be able to absorb carbon
dioxide. A good rate of evaporation in the growing season may,
therefore, indicate the optimum growing conditions, in other respects,
for the more light-demanding and heat-demanding of the plants.
Likewise, a relatively lower rate is likely to accompany the light and
temperature conditions which are best for the less demanding or more
“tolerant” plants. Therefore, although there may be little hesitancy
in declaring the facts as to evaporation to be so and so with respect.
to the several forest types, there is hesitation about saying that these
facts indicate anything as to the ability of the species to withstand
evaporation or to balance it with equalintake. This, of course, is the
point on which information is most needed, that of the drought-resist-
es relations.

t is greatly to be regretted that, because of their use of a different
atmometer, comparison can not be made with the evaporation records
of Pearson (/9), Shreve (21), Weaver (23), and others. A broader
comparison would be valuable in indicating the importance or the
probable lack of importance of absolute evaporation rates. It is to be
noted, however, that the results obtained by these investigators are
uniformly in agreement with the results described in this paper,
namely, that the evaporation rate is always highest in the more open
situations and in the most xerophytic plant types. Further, those
authors lay considerable stress on the probability that high evapora-
tion directly drives out the more mesophytic plants.

84 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

SOIL TEMPERATURES.

Before the initiation of this project it had been observed that the
surface temperature of sandy soils in Nebraska might be high enough
at midday to injure sans planted trees of yellow pine and jack pine
and that they apparently had the effect of drying the lightly protected
stems just at the root collar. It was also thought, after observation of
the vegetational difference between north and south exposures in the
Rocky Mountain region, that soil temperatures might express more
fully than any other measure the cause of this difference, which
results primarily from the amounts of insolation received. In the
preliminary report (3) on this project, soil temperatures were con-
sidered only as climatic elements expressing the possible length of the
growing seasons. There is still much doubt among ecologists as to
whether these temperatures have the greater significance in connec-
tion with climatic or with soil conditions; but it is now evident that a
complete separation of the two sets of conditions is out of the ques-
tion, and that soil temperatures form an important link between
them. That they have been looked upon as results of rather than
features of the weather conditions is indicated by the fact that the
Weather Bureau has never recorded soil temperatures on an extensive
scale except in connection with a few special studies, such as the
study of streamflow now being conducted at Wagon Wheel Gap, Colo.

The factors influencing soil temperatures, at a given time and place,
have been investigated, notably by Bouyoucos (7). The Wyoming
Agricultural Experiment Station has for a few years recorded soil |
temperatures at Laramie. A great many other records have no doubt
been obtained, but nothing comparable to the great organized mass of
air temperature records. During the last four years the Ecological
Society of America has lent its influence to the organization of a more
or less complete soil-temperature survey of the United States; and as
a great many competent observers have thus become interested in the
subject it is to be hoped that comparable data will soon be available
for different regions.

Pearson (/7), in his study of the yellow-pine forest and parks, made
comparative observations for only four months, July to October,
during which time, at a depth of 2 feet, he found the forest soil to be
5.1 degrees cooler than the open park. In February, 1913, he also
made some examinations which are of interest here. The day was
warm, and all snow had been melted by the direct and reflected light
on the south side of a pine tree, while on the north side, and in an
opening, the snow cover was from 6 to 8 inches deep. In the first
position there was no trace of frost in the soil; in the second it ex-
tended down 134 inches, and in the open park 23 inches. This single
observation, whose general import is indicated by data to be pre-
sented, is of principal interest in showing the conditions which young
seedlings must contend with before the forest is established; for
Pearson (16) has also shown that seedlings in the pine type are com-
pelled by summer heat and drought to adhere very closely to the
shaded spots. In a later report Pearson (/9) has given more com-
plete data for all of the forest types, and these will later be compared
with the data of the present investigation.

Shreve (2/), in his study of mountain vegetation, has given only
slight attention to soil temperatures. The few records of minima
which were obtained seem to have no bearing on the present study.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 85

Larsen (13), in his companion report on the types of Idaho and
vicinity, gives the results of systematic soil-temperature observations,
at depths of 4, 1, and 2 feet, extending over a period of several years.
One of the most interesting features of these data is that the soil of the
white pine type, in the vicinity of the Priest River Forest Experiment
Station, which occupies a northeast slope that is almost without insola-
tion for several months, barely freezes to a depth of one foot. This is
apparently due to the fact that early snows are retained, the winter
blanket becomes very heavy, and radiation is so checked that much
of the summer heat isretained. (See Bouyoucos (8).)

It will be shown that lodgepole pine, in its optimum environment,
is similarly protected. It appears to be of great significance that
western white pine, perhaps the nearest relative of any of the eastern
conifers attaining a good development in the Rocky Mountain
region, does so under very low atmospheric temperatures, but with
the resultant protection of a high relative humidity, and with its
moisture supply probably never completely cut off by the freezing
of the soil. A southwest slope, also in the vicinity of the Priest
River Forest Experiment Station, shows about the same soil minimum
at 12 inches; but its mean temperature for the whole “‘rest period”’
is 3.3° F. higher than that of the northeast slope, and for the entire
year 4.6° higher. Im neither place does the soil freeze to a depth
of 24 inches. This southwest Ap occupied by western yellow pine
and Douglas fir, shows a mean annual temperature at 12 inches depth
of 47.9° F., and a January temperature of 34.1°, both of which are
3.1° higher than the corresponding temperatures at Fremont (Sta-
tion F-2). The midwinter air temperatures, however, are lower
in the Idaho region.

Special conditions affecting soi temperatures in this study.—Soil
temperatures have been observed at each of the special stations for
this study, including Monumenteand Foxpark, practically during the
entire period of the operation of the stations. In addition they have
been recorded at each of the meteorological stations at Wagon
Wheel Gap, three of which are mentioned in this phase of the study.
A few scattered observations taken in connection with other pro-
jects will also be called into use. For the most part these were made
without any special preparation, that is, a thermometer was simply
inserted into a hole of the desired depth and protected from direct
= ort Aa naar These observations are mainly for a depth of 12
inches.

Prior to 1914 the three stations under observation at Fremont
were equipped with 1 and 2 foot soil thermometers. In 1914 it
was douideat that the 2-foot depth had no individual significance,
and with the installation of several new stations the new ones and
the older ones were equipped for readings at 1 and 4 feet. This
arrangement has been standard since 1914.

The stations at Wagon Wheel Gap were at first equipped with
1-foot thermometers only. During 1913, 4-foot thermometers were
added at all stations then in operation and the standard equipment
was placed at stations established in the fall of that year. The
1-foot thermometers have been in wooden tubes a greater part of
the time. The Foxpark station had i and 2 foot thermometers
from 1914 to April, 1916. The 4-foot thermometer was placed
when more extensive observations were begun: in 1916. Local Sta-
tion F-13 (limber pine) had only 1 and 2 foot thermometers

86 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

during the period of its operation, because of the impenetrable soil
encountered below a depth of 2 feet. Little use has been found for
the temperature records for a depth of 2 feet, but they willbemen-
tioned where they appear to give additional information of value.

For the most part, soil thermometers have been placed in iron

ipes. Standard 1-inch galvanized pipe has always been used, the
tat end being sealed either by a threaded cap or by welding.
The practical Aleaneaees of this arrangement over any other type
of tube, or over long-stemmed thermometers which may be read
without moving, are obvious—namely, durability, dryness, conven-
ience, and cheapness. In fact, temperatures at a depth of 4 feet
can hardly be measured in any other way. The exposed end of the
pipe is capped, the suspending cord for the thermometer being
sealed to the inside of the cap. The standard Weather Bureau
‘mercurial thermometer,’ Fahrenheit scale, with cylindrical bulb,
has been used mainly. To prevent any immediate change when the
thermometer is raised for reading, the bulb is inserted in a cork or
a vial of alcohol. The cork is a good non-conductor and serves as
a cushion when the thermometer is placed at the bottom of the

ipe. There is no doubt as to the accuracy of the readings at 4
feat when obtained in this way, since, with the bulb protected, no
change in the mercury is noted for 20 to 30 seconds, as a general
rule, while the reading may be taken in 3 to 5 seconds after removal
of the thermometer from its seat.

It is, however, greatly to be regretted that in the soil-temperature
observations at a depth of 1 foot iron pipes have been used, since
at this depth these introduce an avoidable error into the tempera-
tures recorded. ‘The technical problems encountered in measuring
soil temperatures have been discussed in detail in Research Methods
(24), but for the sake of clarity it is desirable to explain the influence
of the iron pipe on the temperatyre records here presented. The
iron pipe is a good conductor of heat, and, especially if the exposed
portion becomes insolated, may conduct heat to a depth of 1 foot
more rapidly than would the soil itself. Likewise, at night, the pipe
may carry away heat to the air which is cooler than the soil. The
soil thermometer in an iron pipe, therefore, goes through a greater
daily range than the soil itself. For the well-insolated control sta-
tion, in September, at a depth of 1 foot, the range in the iron pipe
was found to be 7.93°, as compared with 3.41° in a wooden tube,
in which the temperatures always corresponded closely to those of
the soil itself.

It is not admitted, however, that the use of iron pipes has any
appreciable influence on temperatures recorded at a depth of 4 feet;
in view of the very gradual changes in such temperatures and the
absence of appreciable daily oscillations, it is evident that the usual

morning observations give a satisfactory basis for calculating mean
temperatures. Also, this influence of the iron pipe is probably, in

the long run, completely balanced; that is, the mean temperature
in the iron pipe would be the same as in the soil proper, because
absorption and radiation by the pipe must be just about equal.
The pipe would, however, raise the temperatures in spring and sum-
mer, and lower them in fall and winter—the general cooling period.
ven this is relatively unimportant, compared with its effect on the
single daily temperature recorded. The great number of soil-tem-
perature observations in this study have been made between 8 and

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 87

10 a.m., and a few as early as7a.m. The minimum soil temperature
occurs usually several hours later than the minimum air temperature,
but for most sites it occurs some time between 8 a.m. and noon.
It is, therefore, readily seen that the tendency has been to record,
for 1-foot temperatures, the minima rather than the mean tempera-
tures for the days of record. The conductivity of the iron pipe has
an influence in making these minima lower than they actually were
in the soil, unless the insolation of the current day has proceeded
long enough to be felt at the base of the pipe, as, for example, on an
easterly exposure.

The mean temperature of the soil of a given site may be almost
constantly either higher or lower than the mean air temperature,
depending on the amount of insolation received. The air tempera-
ture in a large measure refiects the average soil temperature of all
the sites in a region of considerable extent. According to Moore
(15), however, the more or less constant difference between the tem-
peratures of sites in the same locality disappears when great depths
are considered. Indeed, it is sometimes said that at a depth of
about 50 feet the soil temperature does not change and is, at any
time, an accurate index to the mean annual air temperatures above.

At a depth of 4 feet the soil temperatures, although not subject to
rapid changes, appear to show in thir year-long values the amount
and effectiveness of the insolation received locally. It is, therefore,
believed that a comparison of the 1-foot and 4-foot temperatures for
entire years is the simplest means of determining the probable ex-
tent to which the factors mentioned above—that is, the use of iron
pipes and the prevalence of morning observations—have tended to
give values for the 1-foot depth lower than the true means.

Table 24 shows the probable mean error in the 1-foot soil tempera-
tures as determined by comparison with the 4-foot temperatures for
corresponding periods. It must be recognized that the mean cor-
rections so indicated are not equally applicable at all seasons, and
may not be absolutely right, even though relatively so for the several
stations. It should be noted also that the periods of comparison
here considered are not identical with those as which the average
temperatures for each station and depth have been computed.

TaBLE 24.—Probable mean error in 1-foot soil temperatures.
[Determined by comparison with 4-foot temperatures for corresponding periods.]

Probable Probable

depression depression
of temper- of temper-
ature in aturein
Station. pipe below Station. pipe below
the mean, the mean,
at hour of at hour of
observa- observa-
tion. tion.
|
oF: ih i
_ 7 MNES # che 902 60 Alas Sem re cp Rs MS Se 2s See See ee ae ee ne No data
Berra wars be oo ec adee MOA en ree tan. Boe tet. enc
Lie oe, Ry eee ape ee Biren esis tt PTAs | Art 2. Jel e. 0. 97
Ly ci oh Sele, OF Et Se ey eee ae ee eee ee RE oT ee ee re a ee eee 1.38
UTES UBS oS 22 oe ee ee ee Til A Re LS OR ED AS od SE ee 1.05
ee ee eR Pe 8 PE ie ce Pal ey Shes aR ee tdd 2, 0. 64
HEE Te ERED Sete Eek ene cede WE LUBNEWY wlestst oo ccsghs cc ties ee cone 0. 38
inst CoS co 9g hie Ce eae eee ee OGTT WT WHA fr RES AA eee iret 2 3. ori 23.16
Ec a OS ee eS eee See ASTOR Wesnes Seon Pore ges Nae oe a 0.05
vv Ari, BR) AEST eR ees eee 0.78 | Wee atk DA Eat bed gO CaS EEE ee 0. 34

11916-17 only. 21914-1917.

88 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

All other stations in the Pikes Peak locality should be subject to
less error than the control station, both because nearly all are better
protected from direct insolation and radiation, and because the ob-
servations have been taken at later hours, when, ordinarily, the
minimum temperature in the iron pipe will have been passed. Both
the time and the amount of insolation, however, are probably
important in determining the amount of the depression at the
observation hour.

One of the Wagon Wheel Gap stations (WA-~—1), which has a tele-
thermoscope for its 1-foot temperatures, should have no error in the
9 a. m. readings, but these may be lower than the mean temperatures.
The corresponding south-slope station (A-2), although its ther-
mometer has always been in a wooden tube, is subject to full radia-
tion and insolation all the year, but this is not received until late in
the day, which probably accounts for the low 1-foot temperatures
at 9 a. m. However, in illustration of the variability of this cor-
rection with season and time of insolation, it should be mentioned
that at midsummer the 9 a. m. temperatures were found to be about
0.9° F. above the daily means, while in the winter they must be several
degrees below the means. The data for the high spruce stations
should be fairly close to true mean temperatures, Because their read-
ings have been taken about midday. As the Foxpark and Monu-
ment stations have been regularly visited at 8 a. m. or earlier, practi-
cally minimum temperatures in the iron pipes may be expected,
except that the Monument station, like Station F—4, may receive
insolation very early. The sandhills station (H-2) has had only a
wooden tube at its 1-foot depth, and is on a northerly slope, both of
which factors would tend toward low corrections.

Seasonal soil temperatures at the control station—In Tables 25 and
26 are presented the results of soil-temperature observations at the
control station, by 10-day periods up to 1918, for the 1, 2, and 4 foot
depths. Although considerable data have been added at this and
other stations since 1918, it is not felt to be necessary to extend the
averages; moreover, the use of such data would further complicate
the problem of accounting for the 1-foot depressions as described
above, since at a number of the stations wooden tubes have been
substituted for iron pipes in this later period.

The following points are noteworthy:

(1) The mean soil temperature for 1 foot is 3.1° higher than the
mean air temperature for this station, and 6° higher if the correction
indicated by Table 24 is used. For 2 feet it is 5.4° higher and for 4
feet it is 5.7° higher, only corresponding periods being compared.

(2) In the six years that have practically complete records, the
|-foot annual mean temperature shows an average variation from
the normal of 1.07°. The first two years of observation were doubt-
less warmer than any since that time, although 1914 had a very warm
growing season, and may have been on the whole as warm as 1911.

(3) The greatest variation between corresponding decades occurs
about the Ist of June, when it amounts to an average for nine years
of 3.46°. This is on account of the effectiveness of insolation at this
time, and because of the fact that in some years the soil may be very
dry by June 1; but in other years snows may be occurring at this time.

(4) The least variation in corresponding decades is found at the
end of the growing season, the first decade of September showing an

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS.

89

TaBLeE 25.—Mean soil temperatures at the control station (F—1)—1 foot.

Year,

Decade.

Month,

1
2
3

Month,

1911

1912

baa

1913

ies

1914

1915

1916

1917

Month.

2
3

Month,

1
2
3

Month,

10, 1918

February.

26. 38/34. 01/32. 35
32. 12/33. 68/34. 02
33. 59)30. 59)35. 06

30. 79)32. 92/33. 85

. 10/30. 86

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32. 44/39. 12

31. 32)

38. 34/43. 75
37. 42)49. 78
41. 09/49. 25

38. 95/47. 65

(35. 35/43. 20
35. 72

36. 83/45. 59
39. 76/51. 05
37. 44/46. 67

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49, 45/38. 56131. 38|.....

42, 71/31. 40) 25. 37

\50. 48/38. 72/31. 05/20. 68)...../...--

49. 39/39. 17|/31. 64

39. 91/34. 99/33. 83

50. 79/45. 16/36. 65/32. 20/40. 63/55. 18

—_—_——— | | |
Oe ee ee

56. 06/48. 66/39. 11/32. 39|-... E
53. 19|44. 74/35. 79/30. 08|...
50. 09/40. 42/34. 36/28. 15

42, re 06

2 Began using wooden tube.

average variation for eight years of only 1.34°.

the variation is somewhat
cover; but the variation

From this time on,

eater, owing to the uncertainty of the snow
ecreases somewhat at the end of the winter.

(5) The eight growing seasons for which complete records at 1 foot
are available show a slightly greater variation—1.24°—than the six
whole years; but the six growing seasons corresponding to the six
whole years show practically the same variation, 1.04°.

(6) The annual oscillation of soil temperatures, as determined from
the decade means, is for 1 foot 31.4°, and for 4 feet 22.7°; but the
corresponding oscillation of air temperatures is 35.9°, this greater
oscillation of the air being due to winter depression when the soil is

more or less blanketed with snow.

The lowest average soil tempera-

tures occur in the second decade of January, while the lowest mean

90 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

TaBLE 26.—Mean soil temperatures at the control station (F-1)—2 and 4 feet.

3
AG re . = OD.
g | 8-1 Ol ees
= : al S/E(E/ a2) = [83
3 elelg/SleleiS18/81 8 le
A q/a/S/S]/4alalo|4ajala
8140. 51/44. 47/55. 25/59. 27159. 07159. 22155. 53/45. 92/38. 54)... |...
2140. 20/46. 18157. 59/60. 24/58. 28)59. 62/54. 03/43. 37/37. 04|....-|.....
(1 eet 43. 64/47. 53/60. 72/62. 01/58. 44/56. 55/47. 33/41. 44/33. 94|....-|_....
Month. 41, 25/46. 11/57. 85/60. 55/58. 59158, 46/52. 14/43. 58/36. 42146. 10159. O1
31. 21/36. 68/33. 70/40. 27/44. 74/55. 47/58. 29/60. 67156. 85/55. 37/40. 49/34. 37].....|_._..
32. 96/35. 77/35. 90/39. 58/50. 59/58. 03158. 43/61. 51158. 60/52. 45/39. 99133. 11|.....|.....
athe. 34. 80/33. 83136. 85/42. 83/50. 85/58. 41157. 33157. 07158. 04/45. 22137. 81132. 19].....|..__. |
Month, |33. 05/35. 54/35. 53/40. ces 80|57. 30|57. 99|59. 6657. 83/50. 83/39. 43/33. 19/45. 84/58, 21 |
1 129. 62/31. 27/30, 86)35. 82/41. 66/51. 94/56. 52]58, 31/59. 92/48. 83/39. 93)34. 52)... .| ; |
2 |30. 23/31. 54/31. 80/38. 10/39. 14/48. 61/58. 22158. 09154. 18147. 15138. 91/33. 05].....|..._.
A ES Eg ae 3 (31. 35/30. 57/32. 06/36. 49/50. 22151. 10/58. 03/59. 76|49. 31145. 07/36. 73/30. 10|.....|.....
Month, |30. 43/31. 14/31. 59/36. 80/43. 56|50. 55157. 61/58. 75/54. 47/46. 95138, 52132. 48/42, 74/56. 05
1 |29. 43/29. 44130. 71/36. 19144. 07/52. 09161. 51/61. 94/58. 89/48. 49140. 28135. 82)... |... _.
2 |28. 65/30. 33/31. 92137. 40146. 17/51. 12/61. 94159. 68154. 52/46. 21140. 45135. 09]...._|.._..
aes ee 3 |29. 19/30. 96/32. 02/40. 33/51. 05/55. 01/58. 07/59. 10/48. ae 5238, 11/34. 13].....|.....
Month, |29. 13/30. 19|31. 56]37. 97/47. 22152. 74 60. 43/60. 20/54. 0445. 64/39, 61/34. 98]43. 64/57. 94
4 183°R0}s 22 1B ee CT oie blue ec Oyen: eee Cee ees MRTe Petss Pac. a).
2 188 250le elle 2ct B54 OGISGFAO] no losc ctl. oe Lakes Mal eee ee
CL a ae Cp baa a ene 33. 40130. 41140, 19161. €Slccclaetiot e
Monthds.iealtecle cae 97 S744 18157282)» eet). lee) Mee |e ee eee
1 131. 02/32. 46/31. 81/37. 91142, 81153. 62/58. 69/60. 00/58. 72/52. 06/41. 66/35. S1|_....|.....
2 |31. 34/32. 07/33. 61/28. 25/45. 15/54. 35/59. 71/59. 39156. 73/49. 96/40. 68134. 57|.....|..._.
Average?........-- 3 |31. 78|31. 57/35. 04/40. 54/49. 77/57. 37/58. 86158. 59153. 15/45. 04128. 52132. 50]... ._|..__.
Month, |31. 39|32, 02/33. 54/38. 90|46. 03/55. 11/59. 08]59. 10|56. 20/48. 89 34. 27/44. 57/57. 93
(nese patina Ws FR REET ee (Cec 56. 68)56. 51/56. 04/47. 33]40. 78|.....|...- J
Os te Whe Cake, Ca Salt aans 855,98]57. 11|56. 95 53. 63/45. 37|.....]- 2222]... "
Re rears Bials is she Oh Se QAM be Ml i See a 55. 93/57. 01/56. ikon i?) pio Fae ao a
Month. |=s> cise tesa eee eee 56. 56/56. 92/56. 77/53. 40 ee eee. =
ih ee fee 32, 36/32. 97/39. 41/43. 84 54. 04/50. 65/46. 85/40. O1]...../.... .
Me Meals see 32. 22/35, 53/41. 14/46, 53 54, 16/48. 09|44. 07/38. 70)...--|...- é
1) aR a eed Be 5 ies 9 Se 32. 43/37. 68142. 88149. 89 53. 78146. 55/41. 90/36. 84]... ..|..-- a
Month, |.....|.....|32. 34/35. 39/41. 20146. 75 53. 99/48, 37/44. 27]38. 52}... 51.31
1 135. 70/33. 27/35. 66/39. 41/40. 77/48. 89 55. 98/53. 99/47. 28}39. S1}.....). 02. 3
2 135. 01/33. 22138. 15138. 83143. 94150. 72 55. 11/51. 95/44. 22/38. 06|.....1.... ;
(ea ae 3 |34. 02'34, 55/40. 63/40, 27/44. 35152, 93 53. 80/48. 35/41. 42135. 38}... .!. =.
ci 34. 88/33. 65138. 19)39. 50/43. 06/50. 85/56. 32156. 8554. 96/51. 33/44. 31/37. 67/45. 13/54. 80
| 1 [34. 00/32. 89/33. 16/33. 78/38. 13/40. 96/52. 95[56. 70/54. 00/52. 43/46. 50/41. 46|.....).. ;
2 |33, 96)33. 00/32. 92/35. 58/37. 84/46. 20/53. 86/55. 41/54. 16/51. 94/45. 36/38. 84]..-_.|.... ;
a ee | 3 |33. 09/33. 04/32. 82/37. 92140. 37/50. 09155. 66 52. 97/49. 23/42. 95/38. 96].....|.... ;
Month, |33. 66/32. 97/32. 96/35. 76/38. 84/45, 75]54, 21]55. 3753. 71)51. 14144. 94139, 73)43. 25)52. 00
1 (39. 24133. 75/36. 04/38. 39/40. 80/48. 72155. 92/56. 90/56. 09).....]/.....]....-1....-].-...
| 2 136. 59134, 23/37. 53/38. 14143. 98151. 76/55. 67/56. 35153. 59|.....|-....1-....|c2---e0e-.
Mw id cea octese. 3 |34. 65135. 05138. 39137. 53/47. 45154. 35/55. 87156, 92).....|.....|....-|-.--<|-----l---<.
Month, |36. 75/34. 29/37. 35/38, 02/44. 15/51. 61/55. 82156. 73]/.....|.....].....]..-..|..---|54.86
1 |36. 31/33. 30134. 30/36. 14/39. 78/45. 60153. 78156. 02/55. 13/53. 28/46, 99/40. 52|.....|.....
Average to July 2 135. 19/33. 48/35. 20/37. 02/41. 72/48. 80/55. 05155. 74/55. 10/51. 40/44. 76/38. 53].....|.....
10, 1918.......... 3 |33. 92/33. 96/35. 81/38, 35/43. 76/51. 82155. 71/55. 10154. 35/48. 97 Ay Se 5 CS
(Month, 135. 10/33. 55]35. 13/37. 17/41. 82/48, 74154. 87155. 60/54, 86/51, 14/44, 66/38. 65/44. 27/53. 28

1 January assumed to be normal.
2 Average (2 feet) February, 1910, to July 10, 1914,
' Record at 4 feet begins,

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 91

air temperatures are not to be expected until four decades later, an
aGededly late time, as has been explained in the discussion of air
temperatures. The highest average soil temperatures occur one
decade earlier than the highest mean air temperatures, the former
being slightly in excess of the latter, a fact which is made the more
certain by considering the low value of the soil temperatures recorded
at 8 a.m. The extremes at 4 feet occur two decades later than
those at 1 foot.

(7) At this station and at practically all others the soil temperature
rises in the first or second decade of September. This results from
the usual cessation of rains and cloudiness near the end of August.
Six years out of eight show this characteristic.

Soil temperatures evidently vary by years in about the same man-
ner and degree as air temperatures, and corresponding periods of
different years may exhibit quite different characteristics. Not-
withstanding these facts, it has been decided to accept each soil-
temperature record from the other stations at its face value, without
attempting, as was done with air temperatures, to compare it with
the control station for the same period. The reasons for this decision
may be stated as follows:

(1) Most of the records here presented cover periods of three years
or more, with the consequent opportunity for partial compensation
of variations.

(2) The failure to determine mean 1-foot temperatures more
directly makes great precision in this phase of the study impossible.

Absolute sow temperatures of the types—The records for each of
20 stations, by months, are accordingly presented in Tables 27 and
28, the period of observation for each station beng shown. Figures
5 and 6 show some typical soil-temperature relations.

Table 27 discloses that aonsidaralle differences in soil temperatures
may exist between sites which are similar as to forest cover. These
are equal to or greater than the air-temperature differences between
the same sites. Since it is apparent that soil temperatures at a
depth of a foot or more can have little direct bearing on growth, as
temperature factors, there will at first be an inclination to say that
soil temperatures are not so good criteria of the possibilities of the
site as air temperatures are, provided the latter are taken close
enough to the ground to represent those conditions most directly
affecting germination and seedling growth. As has been said, how-
ever, the tables show that mean soil temperatures, even for whole
years, do not bear a constant relation to the air temperatures of the
respective sites. When it is also considered that the latter have not
always been measured where they would best show the conditions
surrounding seedlings, and as the soil-temperature measurements do
bear a certain fairly definite relation to surface conditions, it is seen
that soil temperatures may have a special significance, at least in
indicating the degree of insolation of the site, and the maximum
temperatures which the seedlings will encounter. It is now believed
that the maximum temperatures near the surface of the soil are
of the greatest importance in the distribution of the species, and
it is for this reason that the 1-foot temperatures during the growing
season will be considered in the greatest detail.

.- of
|

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99 BULLETIN 1333, U.. Ss. bEPARTMRItT pm cane SULA URE a, at
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Fia. 5.—Average soll temperatures, depth 1 foot, by 10-day periods.

93

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FOREST TYPES IN CENTRAL ROCKY MOUNTAINS,

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FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 97

Extremely low air temperatures in winter may be made more
severe by melting snow in the vicinity, which, as it melts, tends
to absorb most of the sun’s heat. On the other hand, the snow
blanket tends to hold up the soil temperatures and to prevent
deep freezing. Thus, although the lodgepole pine type in Wyoming
(Station F-11) has a mean air temperature practically equivalent
to that of timber line at Pikes Peak, its mean soil temperature is
some 5° higher and its winter soil temperature is 11° higher. Again,
the high-altitude spruce station at Wagon Wheel Gap has practically
the same mean annual air and soil temperatures as the timber line
station on Pikes Peak; but while the latter’s January soil temperature
drops to 15.2° F. the former reaches a low point of only 26.9° F.
The difference is due to lack of snow cover at the timber line station,
and, similarly, the lack of forest cover gives this station much higher
summer soil temperatures.

In Table 29, both air temperatures and 1-foot soil temperatures
have been summarized in such a way as to bring out the contrasts
which exist at different seasons, and to show that soil temperatures
and air temperatures do not give similar measures of site conditions.
The differences between soil and air temperatures are to some extent
illustrated in Figures 7 and 8. It must, of course, be borne in mind
that the air temperatures given in this table are approximations to
the “‘normals” for the period 1910 to 1918, by comparison with
the control station, but that the soil temperatures represent shorter
periods, and somewhat different periods for different stations.

TABLE 29.—Comparative air and soil temperatures.

[Soil temperatures in degrees Fahrenheit at 1 foot, corrected for year and growing season by amounts
indicated in Table 24; for winter, by one-half of these amounts.]

Annual Growing-season Winter (Jan.—Mar.)
temperatures. temperatures. temperatures.
5 aes Forest type. |
: Differ- Differ- Differ-
Air. Soil ean Air Soil Se Air Soil enna
H-2..... Sandhills.......... 47.47 | 50.78 | +3.31 | 70.81 | 72.37 |4+ 1.56 | 29.87] 32.00) + 2.13
M-1..... Western yellow | 43.96] 46.46 | +2.50] 61.56 | 60.70 |— 0.86 | 29.43) 33.32} + 3.89
pine.

Rae wide aanras do... 42.23 | 45.76 | +3.53) 58.65] 58.16 |— 0.49] 29.10] 34.02] + 4.92
i De eee do..--.....:...| 39.98 |} 41.52 | +1.54] 56.65 | 55.58 j— 1.07 | 25.84| 29.19] + 3.35
F-4..... Pine fire s-Sttre| fa. AU 20 fee ire 54.00] 56.43 |+ 2.43 J........ 272903. Sess
F-7-8...| Fir with pine. .... 39.69 | 37.94] —1.75 | 56.68] 54.14/— 2.54] 25.70} 24.12] — 1.58
W=A2...|o05. 2 Gita tele. J. 37.54 | 43.42] +5.88] 55.98] 55.54 |— 0.44] 22.67) 30.98] + 8.31
F-15...«}. Douglas fir........ 39.58 | 37.53 | —2.05] 56.52] 51.39 |— 5.13} 25.50] 26.36] + 0.86
aE Sole ie 6 0 aan San ee rae 39.23 | 37.28] —1.95] 56.58] 50.62 |— 5.96] 25.03] 26.16] + 1.13
F-9..... Fir with spruce...| 38.77 | 37.08 | —1.69| 55.78| 48.17 |— 7.61 | 24.77] 26.97 | + 2.20
ESE EE Beg 0 (0 pea repels Paes 35.76 | 31.35} —4.41 | 54.18] 43.94 |—10.24] 20.87] 20.19| — 0.68
F-3..... Engelmannspruce.| 38.91 | 38.02 | —0.89] 55.95] 50.63 |— 5.32] 25.40] 27.92] + 2.52
{een eee Li aoe Bm Shae Ce SR BS-be pope aes 50.98 | 48.37 |— 2.61 |........ 28:60) |; Scere
Wate lo. GOS s=. Soe ee 32.13 | 32.90} +0.77] 49.53] 40.34 )— 9.19] 18.03} 27.50] + 9.47
F-16....| Timberline........ 31.63 | 33.24] +1.61} 48.96] 49.93 |+ 0.97] 17.83] 18.36] + 0.53
F-11....| Lodgepole pine....| 32.49 | 38.56] +6.07] 49.94] 49.07 |— 0.87] 18.30] 29.80] +11.50
F-6.....| Limber pine...... 41.92 | 40.32 | —1.60] 59.02} 53.40 |— 5.62] 27.50] 28.86] + 1.36
os ae ieee dQ: Se He: 37.01 | 38.34} +1.33 |] 54.39} 52.81 |— 1.58 | 23.67] 27.63 | + 3.96

In Table 29, certain relations stand out in bold relief, as, for ex-
ample, the ability of the soil of well-insolated sites to equal or even
exceed the air temperatures during the growing season, although
the north slopes and those aa vaied ty the denser stands fail to do
this by 2.5° to 10.2°. It is significant that all of the warmer sites

73045 ° —24_7

: eas ee a

98 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

further analyzed in the pre discussions of the types.
(1) The yee pine sites—M-1 at 7,200 feet elevation and F-2
1,700 feet higher—show nearly the same soil temperatures at all
seasons, although the constant difference between the two is even ©
greater than the difference in air temperatures near the ground. —
These two stations are almost in a class by themselves. However,
it is to be noted that F-1, the control station, practically measures
up to yellow pine standards, its 4-foot mean being as high as that at —
Station F—2. Stations F-12 and F-4, both representing sites in
which the yellow pine tends to give way to Douglas fir and limber
pine, show soil temperatures a little below the yellow pine standard.
That the growing-season temperatures indicated above as being
favorable to yellow pine will not fall far short of those occurring in
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encourage jen in some degree. These and other relations will be

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Fic. 7.—Relation of soil and air temperatures on opposing slopes.

any of the yellow-pine sites of this region is indicated by numerous
measurements made in the Black Hills region in 1914, when, during
June, July, and August, soil temperatures at 1 foot ranging from
56° to 65° were continually encountered.? Although measurements
are lacking, there is little doubt that winter soil temperatures are
kept up fully as high as in the Pikes Peak region, notwithstanding
much lower air temperatures, through the agency of a heavy snow
blanket in the Black Hills. Larsen (13) shows an August mean of
65° at 1 foot, for the Idaho western yellow pine site, as compared
with about 63° at Monument in July. Pearson (19) gives May to
October values ranging from 53.1° to 66.2° for a number of pine
sites in the Southwest.

In the typical western yellow-pine sites the soil temperature at 4
feet does not, for any 10-day period, go so low as 32°. At Station

7 Five measurements in three Quality I pine sites, between June 24 and July 8, showed an average tem-
perature of 57.7°, 1° lower than Station F-1 for the same period. Thirty observations in 14 Quality II

sites, June 22 to July 31, gave a mean temperature of 59.7°, 1.5° higher than the control station. Twenty-
two observations in 10 Quality III sites gave a mean of 58.9°, or 1.5° above the control station for the period
July 11 to August 3. From this it appears that the moister and better sites are appreciably cooler than
yellow-pine sites in the Pikes Peak region. This difference may be due wholly to denser standsin the
Black Hills. On the whole, the two localities are more similar than might be expected. The average

of all Black Hills observations, June 22 to August 3, was 59,2°, and for the control station 57.9°. .

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 99

F—4 the soil normally is frozen at the 4-foot depth for 5 or 6 de-
cades. The Nebraska sandhill soil has not, so far, closely approached
freezing at the 4-foot depth.

One of the reasons, therefore, for the crowding out of yellow pine
by Douglas fir and for the producing of a “‘succession”’ in the truest
sense, may be the shade of the mother canopy, which induces a pro-
longed winter drought for the small trees. In the one situation where
this is fairly represented (F—4) the condition of the parent trees, as
well as the composition of the reproduction seems to show the sever-
ity of the strain so produced. However, it should be observed that
a heavy canopy at the same time so reduces the exposure of each
tree or seedling that the immediate and radical effects of winter
drying, commonly described as “winter-killing,’’ are not observed
in the fairly dense forest.

(2) It may be well to examine next the limber pine sites, repre-
sented by F-6 and F-13. Notwithstanding the fact that the second

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Fic. 8.—Relation of soil and air temperatures with and without forest cover north slopes.

(ek kk Wet A fF A DP Se

station represents almost the upper limits of the species, its mean
temperatures are almost equivalent to those of the lower station,
and its growing-season temperatures slightly higher, because of a
bare and well-insolated soil. The relatively high values at either
station give emphasis to the argument that limber pine seedlings
thrive under intense radiation, and that the necessary heat for
growth, at elevations where the air temperatures are generally low,
is secured on burns and rocky ridges through purely local absorption
of heat by the ground. It is probable that the maxima attained in
the soil and immediately above it will fall little short of those for
yellow-pine sites. Also the winter soil temperatures of these sites
are considerably higher than the air temperatures, especially at the
higher elevation. In mean soil temperatures the sites correspond
closely to the quasi-yellow-pine sites, but in neither of the limber-
pine sites is freezing to a depth of 4 feet to be expected. This con-

- clusion for Station F-13 is reached after a consideration of the

;

:

relative winter temperatures at 1 and 2 feet.

100 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

The writer believes, however, that there are situations occupied
by limber pine in which the winter soil temperatures go considerably
below those characteristic of yellow-pine sites, permitting freezing
of the soil for long periods. The ability of limber pine to exist under
such circumstances, and with extreme wind exposure, points to a
marked difference in adaptation, between this species and yellow

ine, which may be quite independent of their apparently similar

eat requirements.’ There has come to the writer’s notice what may
be called an adaptation of outward form, by which during the winter
the needles of limber pine become very closely appressed. This

ves the limbs a club-like appearance quite different from the
‘feathery’ one of summer. Por!

It is therefore quite evident that while limber pine has heat re-
quirements similar to those of western yellow pine, which are satisfied
by the conditions resulting from the insolation of bare, exposed soils,
it has also a faculty for resisting winter-drying, which permits the
species to range almost from the lower limits of yellow pine to points
practically equivalent to timber line, not reached by the yellow pine.
This heat requirement makes it possible for the limber pine to be a
forerunner of Douglas fir or spruce on gravel slides too strongly
insolated to admit the other species, and on burns and cut-over
areas, but excludes it from a prominent place in the ultimate forest.

That limber pine is nowhere more abundant nor more widely dis-
tributed than in the Pikes Peak region is, perhaps, due to the pres-
ence of a young, denuded soil and to other soil properties which will
be discussed in the section on soil moisture.

(3) The conditions brought out by the Douglas fir sites are amply
illustrative of the soil-temperature variations which are pagsible in
different localities. These conditions give the impression that both
winter and summer soil temperatures may have a bearing on forest
composition, but that at times the conditions are not fully expressed
by the soil temperatures alone—that atmospheric conditions also
must be taken into account.

The data for Douglas fir, considered in connection with the quasi-
yellow-pine sites, on the one hand, and the conditions in Douglas
fir sites which seem to encourage spruce reproduction, on the other,
may be summarized as follows:

(a) Sites on which Douglas fir reproduction will about maintain the predomi-
nant position of this species (F-14, 15) are characterized by an annual mean
soil temperature of about 37° F., a growing-season mean of 51°, and a winter
mean of 26°, with the soil at 1 foot frozen for about 140 days.

(b) Greater insolation and radiation, secured on a similar site (F—7-8), by
complete removal of the cover, create summer soil temperatures about 3° higher,
and winter temperatures 2° lower, the freezing period being somewhat earlier.
The summer conditions undoubtedly encourage more germination of western
yellow and limber pines, but the winter conditions will probably keep. the pines
to a very subordinate position in the ultimate composition of the stands.

(c) Sites in the Pikes Peak region (F-—4 and 12) better insolated than the
last, but having their summer extremes modified by fairly heavy cover, favor
western yellow and limber pines with the fir, but appear to be progressing steadily
toward pure fir stands.

(d) A site (W—A2) having much more insolation than the last because of a
south exposure and a scant cover, scarcely warmer than the ‘‘normal Douglas
fir’’ sites in summer, but 4° warmer in winter, and not freezing to a depth of
sheep i tee NE ob fee SB tale oe he eae

8 An exception was noted early in July, 1917, when with air and soil unusually dry, limber pines in an
open stand on a southeast slope showed foliage injury exactly corresponding to winter-killing, while
yellow pines on the same site escaped without apparent injury.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 101

4 feet, is very discouraging to any kind of reproduction, but permits some bristle-

- cone pine as well as fir. The entrance of bristlecone rather than yellow pine

under these conditions may, perhaps, be taken as evidence of dissimilar soil
preferences of the two pines.

(e) Additional density of cover on a north slope (F—9) reduces the extremes
of soil temperature 1° or 2° without appreciably affecting the period of soil
freezing. This change of temperature extremes, which would be much mere
apparent if absolute maxima and minima nearer the surface could be considered,
is conducive to better germination of both spruce and fir, but results in an
undergrowth controlled by spruce.

(f) A site not unlike the last in its air-temperature conditions (W-Al) may
show much lower soil temperatures at all seasons (4° to 6°) without appreciably
changing the character of the reproduction. The two sites are most similar
during the growing season, both in absolute soil temperature and in having soil
temperatures much (8° to 10°) below the air temperatures. They vary widely

.in the severity of their soil freezing; but, when atmospheric conditions in this

period are considered, they are found to be not so unlike. (See Table 30.)

(4) The soil temperatures of the four spruce sites here considered
cover a wide range, the mean annual temperatures for the 4-foot
depth varying from about 38° on the two sites at 9,000 feet elevation
to 33° at or near timberline. The timberline station at Fremont and
the north-slope station at nearly the same elevation at Wagon Wheel
Gap show essentially the same mean temperatures; but temperatures
for the timberline station go to much greater extremes, owing to the
lack of forest cover at the observation point, and the lack of a snow
blanket in winter.

When the canyon spruce sites (F-3 and F-5) are compared with
the north-slope fir site of the same locality in which spruce repro-
duction is encouraged (F-9), it is found that the former have equal
or higher soil temperatures during the growing season and warmer
and less deeply frozen soils during the winter. Of all sites, these
canyon bottoms would be the last on which fir would be found. The
spruce may also control the lower and, perhaps, the coldest portion
of the fir slope. It is then evidently not the soil temperature
conditions which exclude fir and permit excellent reproduction and
development of the spruce. The character of the soil at the foot of
the slope and in the canyon bottom is suggestive, as something in
common for these two sites, and after soil-moisture conditions are
considered, it must be admitted that soil quality and moisture and
not soil temperature seem to be the controlling factors in this
situation.

The timberline station on Pikes Peak (F—-16), where the soil receives
full insolation, attains to summer soil temperatures almost equal to
those at the lower (canyon) limit of spruce. In view of this fact, as
well as the great intensity of sunlight at the high elevation, it is
evident that the heat conditions for a reasonable growth each season
are fully satisfied. By comparison with a similar elevation at Wagon
Wheel Gap, however, the effect of the canopy in modifying tempera-
ture extremes is plainly seen and it becomes evident that near
timberline the forest may have a tendency to choke itself by creating
soil conditions not conducive to vigorous reproduction. In other
words, at a certain point the temperature a4 not permit a perma-
nently dense stand. The result is that in thinner stands each indi-
vidual tree during the long winter is subjected to an exposure which
can not be tolerated beyond a certain point. The temperature con-
ditions at different seasons are thus all interrelated in limiting the

102 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

upward extension of the forest and in permitting the initiation of

single specimens only in sheltered nooks, where there is no room for
a self-protecting group or stand and where there is indeed insufficient
protection for the individual when it reaches above the level of rocks
or of the usual snow blanket.

A mean annual air and soil temperature of 32° may be approxi-
mately limiting to growth on fully exposed sites, although there is
little question that some excellent spruce forests exist and produce
very satisfactory wood increments where this ees is closely
approached. Low temperatures may directly preclude normal devel-
opment of stands during the growimg season, and they may react,
through the long period of soil freezing, to create a very great winter
exposure; but it becomes evident that it is the winter exposure which
directly creates a ‘‘timberline.”’

(5) The single lodgepole pine station shows mean soil temperatures
similar to those of the spruce type, but its winter temperatures are
more nearly those of the western yellow pine type.

(6) To summarize: Soil temperatures are more responsive to the
local effects of insolation than air temperatures are, and even the soil
temperatures at a depth of 1 foot are probably more representative
of thieke conditions at the surface of the ground that affect germina-
tion and the young seedling than are air temperatures as they are
usually and conveniently measured. Consequently, soil temperatures
bring out more closely than air temperatures the growing-season con-
trasts between north and south slopes, between forested and open
sites, and between the heat of bare or rocky soils and older or better-
protected soils. Every soil-temperature difference in a single locality
such as the Pikes Peak region is accompanied by a change in the
character of the forest reproduction, although in some cases it is
doubtful if the soil temperature is the controlling condition. Yellow
pine, limber pine, and bristlecone pine all adios to enjoy warm soils
and to reproduce on sites where very high temperatures are to be
expected, at least for short periods. Douglas fir, on the other hand,
reproduces well only with sufficient shade to greatly modify these
high temperatures. This is true in the Pikes Peak region even on
northerly slopes where the sun’s rays strike obliquely. The distinc-
tion between Douglas fir and Englemann spruce does not seem to be
so much a matter of temperatures which might injure young seedlings,
as of soil or light conditions which in the dense forest gradually starve
the fir. This distinction may be more logically discussed after con-
sidering the soil moisture data.

Evaporation during the period of soil freezing.—In accordance with
what has been shown in Table 29 as to the variation in the extent of
soil freezing in the different types, and with the suggestion that this
period should be measured, not in days, but in terms of the induce-
ment to evaporation, an effort is made to show, in Table 30, the total
probable evaporation for each such period.

ee ae

ee a ay ee

;

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 103

TasLe 30.—Probable total evaporation during the period in which the mean soil
temperature, for decades, at a depth of 1 foot, is below 32° F.

[The letter F in decade spaces indicates periods when soil is commonly frozen.]

Cubic centimeters per 100 square centimeters. Total

Station 7 Sale’ iat, la ah WW ae We GD me
No. Type. Decade. | May- | i= frozen
Jan. | Feb. | Mar. | Apr. Sept. Oct. | Nov. | Dec. Iperiod.
eto, Control. .........- i eae
317.7
H-2....| Sand hills......... aR ics
144.3
M-1....| Western yellow : TT ppisad MERE SEES 3!
pine. |__|
148.6
F-2.. eeclcose: Gl Ee Se ee Sees ee Sea ras
63.8
EI so.0| aktdO. asep 8225. eee

|

| 343.7
1 ae Pine-fir........... Wee h Ai
217.4
W-A2.., Douglas fir, south. Tt PHL Oe Oe sce
153.5

F-7-8... Douglas fir, north, : : ¥ s big Fe Boe
| open. Se

| et . . -
. . - .

F-9.. ..| Douglas fir. north. AT Ae

117.4

(ja 538 Re Oa teas BL PIES

194.9

F-11. ..| Lodgepole......... ene

170. 4

F-6..... Limber ine, | ge te 5

REOEENECSESONOM| Pe Atha ACs cement lets. “MPN Fr Sarees eet n et Safes a(R | ee

312. 4

F-13....| Limber pine, high oll Fae PEE
ridge.

796. 7

a a a. a

1 Relative evaporation for these months estimated at 150 per cent and 130 per cent, respectively, of that
at the control station, as no record is available.

104 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

Tas ie 30.—Probable total evaporation during the period in which the mean soil
temperature, for decades, at a depth of 1 foot, is below 32° F.—Continued.

Cubic centimeters, per 100 square centimeters. Total
a | for
Station | T : Decade. an a
No. | ai J May- frozen
| an. | Feb. | Mar. | Apr. | gen¢_ | Oct. | Nov. | Dec. |period.
i F.
2 F.
(ie: Bae Canyon spruce. ... | 3 F.
Month. 9.8
1 F.
2 F.
co aes, eae iL: Byars 3 F.|
Month.} 36.4
op me ae ae OE Sina I Vaasa oars FS
2 F.
W-D...| Highspruce, burn. 3 F.
Month.| 64.4 | 102.0

ed
_

1 F:
| 2:4 ily
F-16....| Pikes Peak, tim- 3 F.

| -berline. ei a ee

Month.| 23.8| 13.8| 40.1] 62.6.

These data are based on (1) the period, by decades, in which the
mean soil temperature is below 32°, the recorded temperature being
corrected by one-half the amount mdicated in Table 24, and each
such decade being shown in Table 30 by a letter F; (2) the relative
evaporation by months, for the particular station, as shown by Table
23; (3) the average evaporation for any decade or month at the
control station, as shown by Table 22. The result, of course, is the
merest approximation to the probable evaporation stresses of the
average season, and does not indicate at all the extreme conditions
that might be met. In a consideration of these data the fact should
be borne in mind that at most of the local stations evaporation has
been recorded 7 to 12 inches above the ground surface, but at others
the height of instruments has been 5 to-20 feet. (See Table 23.)

Table 30 is based on so many assumptions that it can not be taken
as conclusive, yet in a broad way its indications are felt to be mport-
ant and valuable. The general effect of using the mean soil tempera-
tures for several years is probably to make the period of soil freezing
appear longer than it actually is m the average individual year.
This is shown by using the actual data for the season of 1919-20,
when a good many evaporimeters were in operation. At a well
insolated station like the control the soil temperatures in a single
season are found to consist of several depressions well below the
freezing point, with intervals in which the soil moisture is very evi-
dently available. The maximum continuous stress, therefore, is
here only one-third as great as indicated by Table 30, and probably in
the most severe of winters it would not be more than one-half as
great. On the other hand, even a small amount of shade seems to
be sufficient to prevent the thawing of the soil on warm days. The
use of the mean soil temperatures, rather than maxima, of course
introduces another chance of prolonging the freezing period beyond
its actual limits.

Both the possibility of occasional thawing on well-insolated sites
(that is, maxima above 32°) to a depth of 1 foot, and the probability
that on such sites moisture nearer to the surface will be frequently

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 105

available, are seen by considering even a few original observations.
For example, in a yellow pine site at Fremont, which has a very
slight tilt to the south, and averages 1° to 2° cooler than the control
station because of shading, while the soil temperature, as recorded
at 8 a. m., was continuously below 32° from November 28, 1920, to
February 27, 1921, a period of 91 days, there was during this period
only one decade in December and one in February in which no sur-
face temperatures above 32° were recorded at 8 a. m. Nor does
this by any means speak for the highest temperatures of the day;
and even the two exceptional decades showed maximum air temper-
atures above 40°.

The conclusion is inevitable, therefore, that the evaporation stress
during periods in which soil moisture is completely nonavailable is
far less for well-insolated sites than is indicated by Table 30. On
the other hand, a continuously frozen soil, not only for the average,
but in each individual year, is a practical certainty on every north
slope and even in the canyon bottoms, which might be reached by
the sun, but where the summer moisture supply is adequate to develop
very dense stands. Table 30 may possibly. exaggerate the evapora-
tion stress by prolonging the period a little after the time when snow
melting is rapid enough to provide the surface roots with water, but
this very melting greatly retards the warming of the deeper soil.
This difference between insolated sites which generally produce open
stands and cooler sites which encourage denser stands must be further
accentuated when it is considered that the evaporation measurements
have nearly all been taken close to the ground. On a warm site the
evaporation is likely to be little, if any, higher at a considerable
elevation than near the ground. This is true not only because the
open forest permits very good air circulation at all levels, but also
because, in the winter period, radiation close to the ground may
frequently cause thawing which would not otherwise be possible, and
this will be an important element in the total evaporation. On the
contrary, it is self-evident that in a close stand, the higher the eleva-
tion in the midst of the crowns, the greater will be both the insola-
tion and the wind movement. The evaporation at the ground,
therefore, is no measure at all of the stresses to which the more exposed
crowns are subjected.

With the quantities shown by Table 30 as a guide, then, but with
the factors which influence the relative values taken into considera-
tion, the following observations may be safely made:

(1) The evaporation stresses to which seedlings may be subjected
in a dense forest, even in a region of high winter-sunshine percent-
ages, are appreciable for the total period of soil freezing, but un-
doubtedly Boetiite wholly insignificant with a snow blanket. On
the other hand, the dense forest, as it commonly is on a slope facing
the north, creates for its larger and more exposed trees a winter
drought of long duration coupled with considerable evaporation
stress. It is believed that the capacity for resisting the consequent
drying out is, perhaps, a better measure of drought resistance than
any ability which has, in the course of this study, been shown during
warmer periods, because no severe summer drought has been encoun-
tered. The winter drought, as it affects the older trees in a dense
stand, not only is of yearly occurrence, but seems to be almost inde-
ae of precipitation, except as the presence of a general snow

lanket may increase the atmospheric humidity.

106 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

The trees which tolerate or in fact thrive under such conditions
in this region are spruce and Douglas fir—the so-called ‘‘shade-
tolerant’’ species. It is too early to draw a clear line between the
two, although it is believed that the spruce is the more resistant of
the two. To these conditions yellow pine is plainly unadapted, but
limber pine fits in moderately well.

Thus, for the first few years in the life of the trees on the sites which
produce the very dense forest, the conditions may give almost equal
encouragement to several species. Although dense shade is more
favorable to the tolerant spruce, Douglas fir and even the pines may
become established because of deeper rooting and more ready adapta-
bility to surface drought. On the other hand, when the trees begin
to be exposed above the snow blanket, and the moisture supply is
no longer controlled by depth of root, the struggle becomes almost
altogether one of resistance to long-continued drying. The evidence
presented in the paper on “Physiological Requirements of Rocky
Mountain Trees,’’ (6) which tends to show the greater photosyn-
thetic activity of spruce, and which suggests that a lok osmotic
pressure is more normal in that species and, hence, less likely to be
injurious, is applicable in this connection. The species which carries
a dense sap, especially in a limited hght, not only begins to resist
evaporation earlier, but is least liable to injury should drying with
all species be carried to the same point.

It is therefore believed that the struggle which determines the
composition of the forest may continue long after the seedling stage
is passed. The evidence, which has already been used more or less,
indicates that on north slopes at middle elevations the original forest
may be largely of Douglas fir, with considerable pine; but that in
the second generation, from the sapling stage upward, spruce pre-
dominates, and must therefore form the climax.

(2) That the spruce forest is subjected to, or that spruce may
tolerate, winter evaporation stresses of great magnitude is shown by
the high spruce burn (W-D) and timberline stations. In neither
place can there be much question that the soil freezing is severe and
continuous, with no chance for even surface melting. The quanti-
ties indicated—around 400 grams of evaporation—are probably more
nearly correct than any others that can be given, and must represent
the approximate limit of resistance in plant growth. It must, how-
ever, be conceded that, after a certain amount of drying of the leaves
has occurred, the process is halted until the atmospheric conditions
become more severe (warmer or dryer, or both), so that beyond a
certain time mere continuance of the stress does not comprise an
endurance test.

The fact that both limber pine and bristlecone pine follow spruce
nearly to the most exposed of its habitats seems to indicate either that
the long exposure, as has just been depicted, is not so severe as it seems,
or that these species possess a phenomenal ability to resist trans-
ee The sum of the evidence necessitates falling back on the
velief that neither of the pines occurs where the long winter drought,
which is possible at a high elevation, is entirely unbroken. ‘This is
certainly the explanation of the conditions recorded on the high
ridge (f-13). Moreover, this record is open to serious question from
two angles.

(3) The considerably higher winter evaporation at the Wagon
Wheel Gap stations, as compared with similarly situated ones at

> we

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 107

Fremont, is not without its significance. (Compare W-A1 with
F-9, W-A2 with F-2.) It is due both to a drier atmosphere and to
greater prolongation of the soil freezing as a result of lower tempera-
tures at Wagon Wheel Gap. Although the forest conditions at the
north-slope stations are not dissimilar in the two localities, it is
believed to be true, generally speaking, that both spruce and fir are

ushed to somewhat lower levels in the Wagon Wheel Gap locality.

n the south exposures, there is at Fremont a yellow pine forest.
with conditions slightly encouraging Douglas fir, and at Wagon
Wheel Gap, a Douglas fir forest with considerable bristlecone pine
of all ages. It is believed that the generally lower temperatures at
Wagon Wheel Gap would not alone explain the complete absence of
yellow pine, but that the greater possibility of destructive winter
evaporation does explain it in part.

(4) Examination of the data for yellow pine sites, in the light of
what has been shown as to probabilities of relief through frequent
thawing, indicates that this species thrives best where the danger of
winter drying is least. There is a slight suggestion that the lower limit,
as approached at Station M-1, may be reached through an increase in
this risk; but there is insufficient data on the plains conditions to
justify this statement. It is known that the level plains freeze for
considerable periods,’ and that during such periods the evaporation
may be relatively high; but as to the severity of this combination
no estimate can as yet be made. It is evident that the Nebraska
sandhills are not extraordinarily severe in this respect, and there is no
evidence of winter killing in the established yellow pine plantations
there. Stations F-12 and F—4 both appear to represent conditions
which, asa result largely of the shade cast by the pine trees themselves,
are approaching too great severity for the reproduction of that
species.

The importance of this factor of winter evaporation with reference
to yellow pine is clearly shown by the frequency with which pine
forests in various parts of the range are injured by winter killing,
which is rarely “killing” except to the foilage, but must detract
greatly from the vitality of the trees affected and ultimately decrease
their chances for predominance. It has been seen currently that the
very lowest evaporation stress occurs on an open, smooth, southerly
slope (F-2), and under these conditions the stand is almost pure
yellow pine. It is significant that on other parts of this slope and on
other similar slopes, which are strewn oa boulders, a great deal
more injury from winter drying has been noted; and that all such
boulder-strewn areas develop a large proportion of Douglas fir, which
is less seriously injured than is the western yellow pine.

To sum up: Successively lower temperatures in the types from pute
to spruce, particularly well measured by the soil temperatures which
show the effects of insolation or its lack, lead to more continuous,
more severe, and more certain periods of soil freezing each winter.
Consequently, other conditions being at all equal, the fully exposed
trees growing on the coldest sites or in the coldest types are most
liable to a winter drying so extensive as to be injurious to the trees
and probably to affect the composition of the forest. Exactly the
reverse is likely to be true with respect to conditions affecting young
seedlings small enough to be protected by the snow blanket. Al-

9 At Laramie, Wyo., the soil temperature at a depth of 3 feet is normally below freezing for 3 months,
January to March.

108 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

though low temperatures tend to prolong the drought, they are usually
conducive to the conservation of moisture in summer, and to the
production of dense forests in which the individuals to some extent
mutually protect each other from drying. Low air temperatures and
high humidity reduce the relative drying power of the atmosphere
and limit the extent of drying of the foliage. This may be more
decisive than the duration of the drying or the possible amount of
drying as indicated by an instrument which does not progressively
increase its resistance. The physiological data, however, have
indicated that the greatest degree of desiccation occurs in spruce
in its coldest habitats.

Because of the many factors complicating the situation, both as
to the actuality of complete freezing in the soil, and as to the stress
to which the tree is being subjected, only the most direct and thor-
ough measurements of both soil and atmospheric conditions can
be entirely convincing. It is believed, however, that the factors
have been sufficiently weighed to make it safe to say that spruce in
the Rocky Mountains is subjected to winter drying of a degree and
duration which would be quickly fatal to yellow pine. The importance
of this factor with yellow pine, the writer is convinced, can not be
overdrawn. It is only to be hoped that no error has been made in
assuming that the other species, which less frequently show winter
injury, are nevertheless susceptible in a degree, and that their dis-
tribution is affected thereby. As these drought conditions, how-
ever, result indirectly from and are so much involved with tempera-
tures, it is still possible that temperatures in their growing-season
relations control more definitely the composition of the forest types.

Surface temperatures.—It has been indicated that the soil tem-
peratures at a depth of one foot bring out more fully than do the air
temperatures the contrasts between sites which result primarily from
differences in insolation and secondarily from differences in the den-
sities of the stands, and the possible consequences of a marked cool-
ing of the soil have just been depicted. ‘There must remain, how-
ever, some question as to whether the critical effects of the tempera-
ture differences are felt more keenly in the winter than im the summer.
It may, ag ee clarify the situation and at the same time offer an
explanation of certain successional changes if the statement is. re-
peated that except in the typical, open and unblanketed yellow
pine sites the effect of severe soul freezing can hardly be felt until the
seedling has developed considerable height, and is then probably felt
increasingly as the tree assumes a more exposed position. What-
ever influence may be expected from surface temperatures on the
contrary and from certain degrees of surface drying which must
accompany them, is the influence exerted upon very young seed-
lings. The experience of planting shows conclusively that trees
3 or 4 years old are often immune to what are conceived to be the
critical surface conditions.

As just suggested, extremely high surface temperatures may be
en & fatal, or the extreme dryness resulting frtith superheating
of the soil may tend to desiccate young trees, even though the roots
are adequately supplied with moisture. As the direct or indirect
influences of high temperatures are necessary concomitants, if is
probable that injuries in both ways often occur concurrently. Even
in closely oantrolied experiments, it will always be difficult to deter-
mine whether injury to seedlings is the direct result of superheating

A

a

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 109

and coagulation of the protoplasm, or results primarily from an
excessive transpiration rate induced by the high temperature.

In “ Physiological Requirements of Rocky Mountain Trees’ (6)
the results of a heating experiment are reported, which show that
seedlings of Douglas fir and yellow pine, on the one hand, are relatively
much more resistant to this form of injury than the seedlings of
Engelmann spruce and lodgepole pine. In this test superheatin
was induced by direct sunlight in a warm greenhouse, Be the hig
temperatures were usually maintained for several hours each day.
A temperature of about 130° F, as measured by a blackened ther-
mometer laid on the surface of the soil, appeared to represent the
critical point. The form of inj was invariably wilting, first
shown by the collapse of the tire oie seedling at the ground-line.
In view of the fact that the pans containing the wettest soil showed
little injury, the conclusion can hardly be avoided that in this test
excessive transpiration and to some extent desiccation of the soil
surface and of the stems at this level, were the primary causes of
injury. Therefore, yellow pine and Douglas fir seedlings withstood
injury because they are stouter and more deeply rooted than spruce
and lodgepole pine seedlings of the same age.

Illustrating the complexity of the problem that is encountered
when the effects of surface temperatures in the field must be noted,
a more recent test may be cited.*° In this test very high tempera-
tures were secured by placing the pots of seedlings in close proximity
to an electric heating coil with its radiation of long wave length.
With exposures of about 10 minutes, little or no injury was shown

-until the pots were brought close enough to the coil so that a tem-

perature of 140° F. was recorded by a thermometer at the level of
the cotyledons. Temperatures as high as 130° F. could be endured
for much longer periods without marked injury. It therefore ap-
pears that a temperature of 140° F. or slig a higher, is directly
injurious in a period too short to permit much transpiration loss.
Under these conditions the cotyledons of Douglas fir shriveled
markedly, and usually with fatal results to the entire seedling;
ellow pine seedlings were next in order, but the injury was usua
ocalized in portions of the leaves, and not so likely to be Ae £
spruce seedlings shriveled completely, like those of Douglas fir, but
much less frequently; within the range of the tests lodgepole pine
seedlings were practically immune to this heat injury.

With this view of the problem, and a realization of its intricacy
and the need for most careful and complete field records, there can
be but one object in introducing the available data on the surface
temperatures of sites in the Pikes Peak region, namely to show the
magnitude of the variations in surface temperatures, as compared
with the variations in the temperatures at a depth of 1 foot. After
considering these few data, and the facts which have been stated
above relative to the resistance of the several species to direct and
indirect heat influences, it will be apparent that while high tempera-
tures bring out a nice distinction between yellow pine and Douglas
fir, the line between fir and spruce is probably drawn on a different
basis, most probably on the basis of the moisture relations alone.

The determination of the exact temperature of the surface soil,
during insolation, is next to impossible. It is practically possible

U.S. Dept. Bul. 1263, Relative Resistance of Tree Seedlings to Excessive Heat.

110 BULLETIN 1283, U. S. DEPARTMENT OF AGRICULTURE,

only to determine the temperature of some object lying on the surface,
exposed to the same insolation as the surface and presumably ab-
sorbing somewhat the same amount of the radiant energy. In
this study the Freiz soil thermograph has been used, the bulb (1
inch in diameter by about 12 inches long) being laid up and down
the slope and being about half imbedded in the surface material.
Corrections for the thermograph readings have been secured b
exposing mercurial thermometers in the same way the thermograp
bulbs were exposed. Frequent readings have been made when both
thermograph and thermometer would be affected mainly by the soil
with which they were in contact. The temperatures, therefore, at
correction points, are considerably below the maxima secured in sun-
light, and this introduces another possible source of error when the
maxima of the thermographs are used. The observations in 1920
extended from May to September, inclusive; but, as the July tem-
peratures were greatly in excess of those for other months, only
these need be considered. (Table 31.) It is possible, with the cor-
rections obtained through the 5 months’ period, to estimate the
maxima from the thermograph traces very closely, except in the
record for Station F—5, where the instrument behaved very errati-
cally.
TaBLE 31.—Soil temperatures in July, 1920, in degrees Fahrenheit.

[Numbers in parenthesis indicate the decade or day of month in which maxima occurred.]

|

Maximum surface
temperatures.
cot ae Type.
Monthly | Highest | Highest
mean. | decade. day.
H-12-¢-__] Western. yellow pine rider. .42 0 7s eee ee 112. 57
PA! cosa: | Pine4ir-east slepens/.1 ho gsc bebo Bos i pee | 116.68
2? DARA | Limber pine northwest slope. ..............----------++---+-+- | 94. 45
1 UES pfs | ESPPEICC LCIDEASERD rene Sern re ee re a a re ee 84. 24

Even though these data do not cover a great variety of conditions,
it is possible to make certain generalizations, which at least indicate
the importance of the maximum temperatures attained by the soil
surface.

(1) The extreme temperatures which may be expected at the
surface of the soil vary, as between differently insolated sites, by
three to four times as wide a margin as do the corresponding 1-foot
soil temperatures. For example, the mean July temperature for
Station F—5 (Table 27) is 49.0°, and for Station F-12 is 58.3°, a
difference of 9.3°;. but the difference in their surface temperatures
is 28.3°. It is thus seen that the surface temperatures are needed
to give an adequate idea of the possible effects of direct insolation.

(2) In this locality it seems that an east exposure may be liable
to greater extremes of temperature than a flat or slightly southerly
exposure, simply because of the normal tendency of clouds to accu-
mulate as the day advances. Thus the month in question gave 8,336
minutes of sunshine before noon, and only 3,027 after noon, the
hours from 7 to 10 a. m. being about 88 per cent clear. Likewise,

—— Se

ee eee Se

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 111

a west exposure is frequently prevented from reaching a high surface
temperature; but, if sunshine continues all day, its temperature, as
a natural consequence, may be very high after the air has become

well warmed.
SUNSHINE.

Since the control station was established in 1910 the duration of
sunshine has been measured there by means of the standard Weather
Bureau electric-thermometric recorder. At stations in the various
forest types it has not been measured long enough to give records
of much value. Hence it is believed that, in comparing the types,
the indirect evidences of insolation may ea be more. useful,
though it is not desirable to reach any such decision without ex-
haustive trial. In short, then, the available record is only for the
control station, which gives a measure of sunshine values for the
general locality at a middle elevation, and for a similar period at
Wagon Wheel Gap. These data are valuable in the general study
of the climate, and currently in studying such other phenomena as
soil temperatures. They may prevent reaching wrong conclusions as
to the import of other factors.

Sunshine at the control station.—This record is made up wholly from
the graph written by the pen of the triple register, with almost
no interpolation, and without the additions which are termed “ twi-
light corrections,” and which are sometimes employed to compensate
for the lag in the response of the recorder—or, perhaps it might
better be said, its lack of response—when the sun is very close to
the horizon. The study has not been intensive enough to permit
the application of such corrections, although it may be roughly
estimated that they would add, on the average, 30 minutes each
day to the duration of sunshine, increasing the yearly average by
11,000 minutes or about 5 per cent of the possible. Because of the
failure to apply these corrections, the record is not strictly com-
parable with the Wagon Wheel Gap record or with Weather Bureau
records in general.

The possible duration of sunshine at the point where the recorder
has been located was determined by observations of sunrise and
sunset at that point on almost every clear day during 1910 and 1911.
By plotting these hours, the length of each day, decade, and month
was computed. The day at this location is considerably shorter than
it would be on a plain in the same latitude, as the sun sets at an
elevation of nearly 10°, and on the longest days also appears first
at a slight elevation.

The basis at Wagon Wheel Gap is the same, namely, short days
caused by a high horizon to the west, which on the average shortens
the day by more than two hours. Here, however, closer observation
has permitted twilight corrections.

In Table 32, the actual sunshine for each decade of record is
shown, together with the mean for each decade and the percentage
relation of this mean to the possible. It is desirable to call attention
to the fact that the records of the last two years have very materially
altered the averages, especially for May and June.

BULLETIN 1333, U. S. DEPARTMENT OF AGRICULTURE,

112

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114 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

TABLE 33.—Distribution, by hours, of apna in representative decades at Station
; —1.

[Average, 1910-1918, inclusive.]

Sunshine duration in minutes.

Period. For hour ending (a. m.)— For hour ending (p. m.}—
12 2
6 | 7 8 od OY a TE cae 2 3 4 5 6 | 7

First decade,

ER at le 2.5 | 28.9 | 46.7 | 53.7 | 53.8 | 53.3 | 54.8 | 51.4 | 47.7 | 43.71 18.9 |__|...
Last, June. ...... 11.6 | 42.7 | 52.1 | 52.5 | 52.9] 51.8] 51 | 48.6 | 42.5 | 38.8 | 37.8 | 35.4 | 30.3] 6.6
“oes aes 18,9 | 44.6 | 51.8 | 49.5 | 41.9 | 33.7 | 28.6 | 26.6 | 21.9 | 17.1 | 16.8]11.6] .3
Third decade, |
hi SR Be fo ed 19 | 37.6] 39 | 42.3 | 44 | 42.3 | 40.5 | 40.4 | 36 | 27.1 22.7 | 13.2 | .2

To show more clearly the immediate causes for high and low sun-
shine percentages, there are given in Table 33 and Figure 9 the dis-
tribution of sunlight by hours for representative decades.

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F 1G. 9.—Normal sunshine distribution at control to July, 1921.

The first three months of the year, when anticyclonic winds pre-
vail, are characterized by high percentages of sunshine, which reach
a climax at the middle of March. After this, as storm centers move
eastward in a more northerly path, the humidity of the air is greatly
increased, and this makes itself felt in a great deal of foggy weather
in April or May, accompanied by much snow or rain. In general, a
tendency is noted for the fog to be dissipated in the middle of the
day. The control station is at such an elevation as to be frequently
engulfed by the clouds which hang over the plains country, while
above this elevation it may be clear. Or, again, it may be clear over
the plains, but the rising air currents during the day induce the for-
mation of clouds over the mountains.

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ized again by dry weather, which reaches a culmination at the end
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FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. ‘Ti5

air movements at midsummer are influenced by the position of the
continental low-pressure area, the phenomenon of clouds is really a
local one, and those that form about Pikes Peak are the result of
daily convectional currents. Soon after noon precipitation is likely
to occur, and this so cools the ground that convection ceases; the
rest of the day is often but not regularly clear. This phenomenon is
of interest in that an easterly slope during this period may receive
almost as much insolation as a south slope. The autumn is char-
acterized by gradually decreasing cloudiness, but this is interrupted
by storm periods about the middle of each fall month. This apparent
regularity of the storm periods would probably disappear with a
longer record, although the mid-September storm is considered typi-
cal of the whole region and usually brings the first snow. ~

Comparative sunshine in central and southern Colorado.—In Table
34 the mean sunshine percentages are shown for the Fremont and
Wagon Wheel Gap stations. The record for the latter is brought up
to October, 1920.

TaBLE 34.—Sunshine at Fremont and Wagon Wheel Gap stations.

Percentages of the possible, by months.

; = Grow-
Ele- . ke be : 2
Station. va- | 5 | ie eae 8S | Year. oe
tion. | & | S| .g |; a|8#)/o/8/q a
B © jhe Oo . 7) ° oO oO son
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Kromont: (HU) >= csss-c 5 2s ty 8,856 | 69 | 71 | 72 | 60 | 58 | 62 | 61 | 55 | 57 | 62 | 64 | 66 63 59
Wagon Wheel Gap (W-C)..... 9,237 | 57 | 59 | 58 | 52 | 55 | 56 | 44 | 48 | 52 | 59 | 61 | 57 55 50

The sunshine percentage at Wagon Wheel Gap approaches that at
Fremont only in May, when the humidity is very low, and from
September to November, when clear weather is characteristic of
the Rocky Mountain region. The winter at Wagon Wheel Gap is
characterized by much less sunshine and more precipitation. The
depression is very great in July also, when the rainy season reaches its
height, which is about a month earlier than at Fremont.

The general comparison indicates that the vicinity of Pikes Peak is
one of high sunshine percentages, a condition which is of especial
importance in the winter in facilitating evaporation. As we have
seen, this influence on south exposures may be just about counter-
balanced by the action of the sunlight in thawing the soil; but where
the sunlight does not reach the ground its influence must be solely to

increase transpiration.
PRECIPITATION.

a a at the control station, by seasons.—The precipitation
record for the control station is complete for the entire period of oper-
ation of the station, beginning with January, 1910, and ending with
July 31, 1921. Although this record has been obtained as care-
fully as possible with the 8-inch standard gauge, supplemented by the
tipping-bucket gauge during the open season of mach year, it is con-
sidered to be, like all other precipitation records, only an approximate
statement of conditions. Because of this general weakness, it has not
been thought necessary to check up minor discrepancies which may

116 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

exist in the original tabulations of the data. A further reason for not
attempting any high degree of precision in the record is the realization
that the total precipitation can show only in the most general way the
amount of water available for plant growth, as this amount will
depend perhaps as much on soil properties as on water received.

TABLE 35.—Monihly and annual precipitation at the control station, in inches.

| l

| Total
Apr. | May.|June.| July.| Aug. | Sept.| Oct. | Nov. | Dee, | Total | grow-

Year. Jan, Feb. | Mar. annual| ing
season!
i ea 0.01 | 0.44 | 0.51.] 1.18 | 2.56 | 1.89 | 4.47 | 3.27 | 1.01 | 0.86 | 0.53 | 0.61 | 17.34 9.97
iti. | ar eae | -.06 | 1.56 | 1.67 | 2.20} .93 | 1.21 | 5.42 | 2.01 | 1.44] 1.45] .61 | 1.13 | 19.69 9.12
1A 2S ere 2 3 | .06} 1.12] .60| 1.56 | 2.67 | 4.04 | 4.62 | 1.82] 1.17]1.59] .17 22 | 19.64] 10.87
fotos}... 2 } .95 99°) 1504.) 2°06") 1.76 | 3791" | 2° 18°7 3292 13:32 71.06 232). Gn aodoe | 201.12
iy Cee eee bfcdbr)- eet. sia! 4, 01 | 2.40 |} 4.05 | 6.76 | 2.06} .23}1.98} .10 J.c lec}L eee... 12.95
Rept e ype y (Sa ecole 1.66 °} 3.53: 1.3: 43°] 3:09 | 25 2sh478 | 20 38cF 70} aed he AO 10. 74
1916 3 i522 It |} .40} .03 | .97 | 4.54] 1.99 | 1.83 | 4.72] 3.11] .48] .54] .63 | .40]| 19.64 9. 82
IM Lee ee | 25) 1.08 | 1.83) 1.77 | 3.77) 279 | 1.50 |-3.87 | 2.66 | 226) 2254) 618) oe 71
Tho Sy Cee ae . 96 O45 82 | S2 63a] 52" SarOs Pore ea soar oe eae lee beet eee eee eee 311.42
OV ES ie ee Pee epee His ees ck Sell Sc. kslane -49 87) |.-4.16.) 45460 182-4 Ti 294 7 eee 8
i128 oy ae eee | 74:| 1.12 39 | 1. 86,} 2.23 | 1.88 | 2.63 | 6.39 | 1.17 | 1.87] .27 73 | 21.28 | 11.74
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1 Prior to 1918 growing-season totals are obtained by using one-third the total for September; thereafter
the actual amount September 1-10 is used.

2 Based on 0.23 inch, January 1-20,

3 Based on actual fall of 1.27 inches, September 1-10.

The complete record for the control station, by months, is shown in
Table 35. In this, as in all the succeeding records, the total for the
growing season has been obtained by adding to the amounts for June,
July, and August one-third the total for September. In fact, the
greater part of the September precipitation, or atleast considerably
more than one-third, belongs to the second decade; hence this compu-
tation is only an approximation to the truth.

Perhaps the most important thing shown by this table is that nor-
mally almost one-half of the total precipitation occurs during the
growing season. In nine years the least amount in this 102-day
period has been approximately 7 and the greatest 13 inches. To the
7 inches falling in 1917 there may be added, for all practical purposes,
the 3.77 inches in May, which left the ground in a soaked condition up
to June 5, and which materially assisted in tiding over the marked
drought which followed that date. Notwithstanding this, however,
the soil moisture at the end of the growing season reached a lower point
than has been recorded in previous years, and it may safely be said
that with less than this amount in the growing season there would be
danger of drought conditions which might affect the composition of
established stands. Unfortunately there were no_ soil-moisture
measurements made in the dry summer of 1919.

In brief, then, the summer precipitation is ordinarily quite adequate
to maintain the soil of the most exposed sites at this elevation in very
good condition. The winter precipitation is very inadequate, either
to protect the soil from severe freezing or to store up moisture for
late spring use, except on north slopes and under dense stands.
Probably there is about as much water oars in the form of snow to be
melted in the spring, as the soil will hold within reach of the roots.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. Tri

Although the individual years do not vary in precipitation much
more widely than in other conditions, the months do. Thus a long
record seems necessary to establish even an approximate “normal”
precipitation for a given month. The addition of the 1918 record to
the record for the eight preceding years created two new extremes,
namely, a new high for January and a new low for May, and the
erowing-season total was the highest of record except for 1914.
Again, the year 1921, through July, created three new monthly
maxima, and the last 12 months of record show a total of 34.38 inches.

Comparative precipitation.—No serious effort has been made to
compare the precipitation in the several forest types in the vicinity
of the Fremont Station, for it has been assumed that, so far as
topography might influence the actual fall, differences would be com-
pensating over any long period. It is perfectly evident that one spot
may at one time receive from summer rains much more moisture than
another only a few hundred feet away, and such a variation may not
be compensated for a long time. However, where the difference is
marked, the excess at a given point is likely to be largely wasted in
surface run-off.

From March, 1910, to February, 1912, at-three stations within a
radius of a few hundred feet gages were maintained on towers so
situated that there was almost no interference by branches of trees,
but with varying degrees of protection from wind. Table 36 gives
the mean annual catches that were recorded.

TaBLEe 36.—Precipitation catches at three stations, 1910-1912.

Mean

t annual
Station No. Type. precipita-
; tion.
| Inches
[RA a Se i ee eS akg CODTROIRODEN. + Vee 2 eee ae. ee gp ee 19, 25
Be PAVIS EO J ALT ee Fae, Mio t3 South-slope western yellow pine................--.-------- 18. 95

123i, ER es ee EE eee a Canyon spnice-fe. 24 <p. --beel asa bese ceees co Leth ise 20.12

For the period April, 1916, to February, 1917, a gage on the ground
at Station F—9, under dense cover of fir and spruce and less than hal
a mile from the control station, gave a catch of 22.73 inches as com-
pared with 19.32 inches at the control station. The excess under
canopy was very great from certain rains on account of some spruce
branches, but with snow a deficit was generally created by these same
branches. A very different result would doubtless have been obtained
2 feet farther from the tree whose branches most directly influenced
the path of falling rain or snow.

If the amounts of snow on the ground in the different types are
compared for the late spring period when such snow may represent
an asset for the coming growing season, the greatest contrasts may be
obtained. It is not the purpose of the writer to discuss this feature
here. Moreover, because of lack of adequate data, it is not possible

to give any numerical weight to the retained precipitation of the

various forest types here studied. However, the data given in
Table 37, reproduced from the preliminary report on this proleels
indicate the importance which must be attached to this subject in
considering precipitation data.

118 BULLETIN 1283, U. S. DEPARTMENT OF AGRICULTURE,

TABLE 37.—Snow on the ground under various conditions of exposure, winter
1910-11.

[Mean snow depth, inches.]

widens | Decem-} Janu- | Febru-
be ary.

Station No. Type. “a a ary. March.| April. | May.
eae Control, open ridge.............- 0.2 0.6 0.1 3.5 0. 6°

i Oe South-slope western yellow pine - 2 Si. va 3.5 6
fa Canyon spruce}... .........=...- qe 1.5 .6 3.0 1.7

Ee vegan Openresst SlOnG. =~ = =. ~~ bas - 4.6 | 4.1 10. 4 20.0 20. 3 9.2
W-A1......| North-slope Douglas fir........- | 13.4 17.1 23.5 21.2 28. 4
W-A2......| South-slope Douglas fir.......... .6 4,2 5.0 .9 Pr
W-D....... Higher spruce bench............ 11.8; 16.9 24.6 24.5 36.3

1 The ground is the lower edge of a northeasterly slope.

_ The precipitation in various forest types within the range of this
study may now be considered, as shown in Table 38. For the control
station the complete record to the end of 1917 is used. For other
stations having an unbroken record from 1910 to i917 the same
months are eliminated as are missing from the record of the control
station. When the compared record is considerably broken, all that
isavailableisused. | ,

In the Pikes Peak series (C-1, M—1, F—1, L-1) the seasonal varia-
tions are very similar to those at the control station. The total
amount of precipitation for the year or any shorter period increases
fairly regularly with increased elevation, except that in the step from
the Plains type to the yellow pine type the increase is generally larger
than elsewhere. This, it is thought, may be due to the exposure of
the Plains station, which is nearly always subject to north and east
winds during precipitation. These winds make it more difficult to
secure a complete catch. The yellow pine type for almost the entire
winter period, moreover, shows more precipitation than the much
hi ae control station. No adequate explanation of this can be
offered.

The yellow pine types of the Black Hills and of southern Colorado
(D-1 and P-1, respectively), which support yellow pine stands of far
better development than those of the Pikes Peak region, have the
benefit of a much greater total precipitation; but this excess is almost
wholly outside the growing season. It may be said that the heavier
winter snows are of direct benefit to seedlings, and that they protect
the older trees by preventing the freezing of the soil. If, however,
the low rainfall in southern Colorado during April, May, and June is
considered, it is readily seen, as has been pointed out by Pearson (18),
that the moisture of winter snows may not hold over till summer and
hence can not directly affect the rate of growth. In the Black Hills
it probably does not hold over, but melts early at that low elevation,
but the spring precipitation is itself abundant and undoubtedly is an
important supplement to that of the growing season. Nevertheless,
in the search for conditions which may influence the growth of all
species probably none has been found which is more important than
the winter deficit of precipitation in the Pikes Peak region, when con-
sidered in connection with high winds and atmospheric dryness.

One hundred miles farther north, in a similar topographic position
(Station F-17), this winter deficit is far less apparent, though the
high precipitation for this station may possibly be due to purel
local conditions which encourage a transport of snow from the bak
hills round about.

|
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119

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS,

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-MO13 | [BIOL BACH u01}849
T®40\L *sqjuour Aq p10903y

SE) a ae roe I nee ene eee ec ne eee ea. cy-werconnctar= conl, nicwne-p=um=tanuee
[‘ZIGL PUL OTET WOOMIJOG WOT}LYS JO1}M09 OY} 4B psodeI JO Poysod oy} 10,7 }

‘sad hy ysasof pup $a1q1D907 snorspa ur ‘sayour ur “uoynprdwa1gJ— 3g ATAV],

——— ore ee ee ae a

120 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

Somewhat similar conditions are noted at the middle elevation at
Wagon Wheel Gap (Station W-Al1). Here, however, as farther south,
the marked deficit is im May and June. Although the winter precipi-
tation as a whole is adequate, the amount in November and Decem-
ber is hardly sufficient, in connection with the sudden drop in air
temperatures, to prevent deep freezing of the soil.

The spruce type in the Wagon Wheel Gap locality, the only one
which we may compare with Station L—1 in the Pikes Peak series,
does not show much more precipitation than the local station except
during the winter, and is much more liable to drought during the
summer. The Wagon Wheel Gap site produces a superior spruce
forest, but its conditions are not more conducive to abundant repro-
duction.

The lodgepole types are all inferior to the control station in total
precipitation, and only half as favorable in growing-season precipita-
tion. All, however, secure a better snow blanket in the winter, not
only because of larger snowfall, but because the snow is protected
by low air temperatures. Frequently in lodgepole sites the ground
is not exposed after the first heavy snow in September. It is seen,
then, that lodgepole receives and thrives with less precipitation
during the growing season than even yellow pine, but that, like
yellow pine, it is surrounded by conditions conducive to the main-
tenance oi an unfrozen or only slightly frozen soil during the winter.

SOIL MOISTURE.

When it is observed that losses by evaporation may be two or three
times as great from one site as from another receiving the same pre-
cipitation, it is at once apparent that the record of precipitation is
almost without value in indicating the moisture conditions under
which the several species develop. If it is at all true that the amount
of moisture available, or its degree of availability, appreciably
influences the character of the plant formation, then an attempt
should be made to measure the soil moisture as directly as possible.
This should be done notwithstanding the fact that precipitation and

vaporation rates, when considered together, constitute a valuable
basis for comparing the moisture conditions of broad regions and the
general types of their flora.

That the proper use of soil moisture data in a comparison of sites
is by no means a simple matter becomes more apparent as the behavior
of the soil solution is more exactly studied. In the “ Research
Methods in the Study of Forest Environment,” (24) a serious effort
has been made to depict the soil-moisture problem as it actually
exists. In order that the viewpoint and treatment in that publica-
tion may be understood, it is sufficient to say that the soil solution is
regarded as having osmotic properties similar to those of the plant
solution, and that the osmotic pressure of the soil solution is always
opposed to that pressure within the plant which gives rise to absorp-
tion by the plant (root) cells. If the plant pressure is increased by
ordinary water loss through transpiration, or if the soil solution is
diluted, the rate of absorption must increase. If the plant pressure
is decreased or the soil pressure increased, the flow into the plant
must be decreased, stopped, or even reversed.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 121

At least a suggestion of this osmotic relation is contained in a state-
ment by Livingston (/4) as early as 1902, and a much clearer treat-
ment of the subject has gradually evolved in numerous other papers.
Livingston says:

A strong solution will extract water from the organism, a weak one will allow
it to be absorbed. * * * Change in the water content of the protoplasm
may be directly effective by causing a change in its physical properties. For
instance, if water is extracted, the viscosity of the protoplasm must be increased.
The change in water content may result in a change in the chemical activity of
the protoplasmic solution, since chemical activity, in general, depends upon the
concentration of the solution involved. How it comes about is not known, but
a review of the literature of experiments upon animals and plants shows that
growth is very much retarded by an external solution which extracts water.
Especially is the elongation of cells retarded.

Clements (9) describes this relation in discussing the plant (par. 60)
but ignores it in the method of treating soil moisture control (par. 9).

The investigations of Bouyoucos (8) and Hoagland (/2), especially,
in more recent years, have created the conception of the soil solution
as one exerting a definite osmotic pressure. This conception has been
attained through the study of freezing points, and their correlation
with concentration of the soil extract. In the “Research Methods
in the Study of Forest Environment’ evidence was introduced to
show that the soil water exerts a definite vapor pressure.

With this ight on the subject, it must be evident that the total
quantity of water present in the soul has very little bearing on either
growth or survival. The ability of the plant to obtain water in
sufficient quantity to maintain life, or in sufficient quantity to main-
tain conditions favorable for metabolism and growth, must at all
times depend primarily on the differential between the osmotic
pressure aie by the cell contents, and that which, for the sake of
distinction, we may term the “‘antiosmotic pressure’’ of the soil water.

It is of course to be kept in mind that the osmotic pressure of the

lant cells is not stable. If atmospheric conditions create a heavy
fea by transpiration, and thereby create a great need for water in
the plant, it follows that the ability of the plant to extract the water
from the soil is automatically increased, by reason of increased
osmotic pressure in the cells. It does not follow that this increase
in pressure will immediately insure all that is needed. Hence, if
the demand is very great or arises too suddenly, there may be wilting;
and, if it 1s too long continued, there may be permanent injury.

Unfortunately the treatment of the soil solution as one exhibiting
or following the laws of osmosis is not entirely simple, because,
although its properties are primarily determined by the solutes
present, they are also affected at all stages by the solid particles
and colloidal masses of the soil, whose affinity for liquid water has
the same effect on the activity of the water molecules as has the
affinity of solids in solution. The critical point, however, is this,
that while there is still some water in the soil—sometimes 10 per cent
or more—it may be so completely and effectively held in thin films
or small aggregates by the soil solids, that the water entirely loses
its properties as a liquid. The amount of water so held, which is
completely nonavailable for plants, is very important, for, at least
broadly, this amount is indicative of the aggregate of forces which
may withhold the water from plants when the amount is larger.

122 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

There are, then, several ways in which the availability of soil -

moisture may be more or less precisely determined. The most
precise method is undoubtedly to determine the actual osmotic
pressure shown by the soil water when it is present in various amounts
or percentages. This of course implies somewhat exhaustive study
of each soil whose moisture condition in the field is a matter of
interest. In the present study a single approximate determination
of osmotic pressure is available for each of only about half the indi-
vidual soils which are of interest. This determination was made
by. the vapor-transfer method under conditions far from ideal.

Another method is that of comparing the current moisture per-
centage for each soil with that prrceni which represents the
completely nonavailable moisture. This, of course, must have been
determined by a wilting test in connection with each soil, or approxi-
mately through some such physical measurement as the moisture-
equivalent or capillarity determination. As fully explained in
‘Research Methods in the Study of Forest Environment,” the
‘“availability’’ is expressed by the ratio of the available moisture
to the whole. This gives a quantity always less than unity, which
has a certain logical value, as—

M- WC
ae
in which WM represents current moisture and WC the wilting coefficient.

Or, finally, granting that within a single soil type there may be a
fairly constant relation between the water-holding properties as
determined mechanically and the water-withholding properties in the
osmotic sense, the current moisture may be related to the moisture
equivalent of each soil, and a purely arbitrary ratio may be obtained
for purposes of comparison. This method is almost certain to be
misleading if radically different types of soil are involved in the
comparison.

Method of securing soul-morsture data.—During 1910 and 1911, when
only three forest sites were being studied at the Fremont Experiment
Station, the attempt was made to secure samples of the native soil
at frequent intervals in the usual manner, namely, by means of a
tubular soil borer. These samples were usually taken in triplicate at
points near the centers of the respective stations, and, like all other
samples considered in this study, were dried in a water-bath oven at
the temperature of boiling water, which at the Fremont Forest Experi-
ment Station is approximately 92°C. Until 1915, drying for 8 hours
was considered sdaGeaii. in view of the small size, coarseness, and
mineral character of the samples; but later this period was extended
to 24 hours, and the water in the bath was kept boiling for at least
the last 5 or 6 hours of the period. No allowance was made for
varying vapor pressures, which, being generally low, probably made
the drying as complete as though brought about by a higher tempera-
ture in a more humid locality.

This plan for sampling the native soil at the several stations was

not satisfactory, because, as has been said, the soil of the Pikes
Peak region in most situations and except for a few inches at the
surface, 1s disintegrated granite in situ, and is not only very unyield-
ing to penetration but almost entirely lacking in cohesive properties
when once loosened. Hence it required, in the first place, great
labor to sink the soil borer, and, in the second place, great ingenuity

1 ay ein

en ne ee Le ee ee

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. reas

_ to withdraw the desired sample. When the soil was very dry the

obtaining of samples at a depth of 2 feet was practically impossible,
and samples taken at a depth of 18 inches were substituted.

Even a depth of 2 feet does not seem adequate when one has
noted the deep penetration of occasional roots in this soil. There-
fore, when in 1914 this study was extended to a greater number of
stations both near Fremont and in other localities, a plan was adopted
which had been in use at Wagon Wheel Gap since 1911, made neces-
sary there by large rock fragments all through the soil. This was
the plan of ‘soil wells.”’

The preparation of a “soil well’’ consists in making an excavation
from 18 to 24 inches in diameter and 34 to 4 feet deep, the space
being then filled with the same material as has been taken out,
with the exception of the rocks or coarse gravel which interfere with
soil sampling. As material must be borrowed from some other
place to make up for the coarse material sifted out, the resultant
soil in the “well” is, of course, very different from that which pre-
viously filled the space.

The following objections may legitimately be raised against soil
sampling under these conditions:

(1) The “well” soul, weight for weight or volume for volume, will
possess a higher water-holding capacity than the native soil, on
account of the absence of the rock fragments, which are of relatively
little importance in water storage.

(2) The well soil will at the outset be of the same composition from
surface to bottom, whereas the native soil may vary greatly with
change in depth.

(3) The well soil, not being protected and held by humus accumu-
lations at the surface, wili tend, through leaching and transport of
fine material, to become coarser at the surface and of finer texture
at greater depths; hence the water-holding properties will then vary
with depth and in a direction opposite to the variation of the native
SO

(4) Roots are removed in excavating the well, and constant
sampling within such a small radius tends to cut off new roots which
may penetrate this special soil zone.

Whatever the objections to the method, it is the only method so
far found practicable for repeated sampling of rocky, mountain soils.
Considerable encouragement may be i from the assumption, well
founded on experimental fact, that when two soils, both moderately
moist, are placed in contact, there is a strong tendency for water to
move from one to the other until the capillary forces are in equilibrium.
If, then, it may be assumed that the moisture equivalents of the
native soil and well soil (ME and ME’) represent points at which the
two would be in capillary equilibrium, then the probable moisture
of the native soul (M) may be computed at any given time, from that
determined for the well soil (M’), by the formula—M=—7_M®.

But also, if it is desired to relate the current moisture of the soil
to the moisture equivalent, on the assumption that this ratio ex-
presses in some degree the availability of the water, then it is just

as satisfactory to use We 8 Fe because it has been assumed above

that the two are equal.

-

124 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

In considering soil-moisture quantities that approach the wilting
coefficient and that present conditions under which capillary move-
ment may almost aoe cease, it is preferable to assume that an
osmotic equilibrium is being approached by vapor transfers between
the well soil-and native soil. in this event, OL, the osmotic equiva-
lent, might be substituted for ME above. Unfortunately, there are
neither sufficient osmotic equivalents for well soils and native soils,
nor sufficient wilting coefficients for well soils, to make this desirable
method of computation possible in the present study.

Comparative moisture of native soils, local stations.—In Table 39
are shown the results of sampling the native soils at three points near
the Fremont station, under the conditions already described. These
are essentially the data published in the preliminary report on this
project (5), referrmg to the three stations which were operated
comparatively from March, 1910, to February, 1912.

TABLE 39.—Comparative moisture data from samples of native soil, July, 1910, to
October, 1911.

Percentages of dry-soil weight. Mean
: Mean | grow-
Depth. | Station No. | es : af est ing
1 | u- ep- cto- | nual.!| sea-
| April.} May. | June. July. gust. |temper| per. iat
|
|{F-1, control....... 8.75 | 6.87] 5.09] 498] 5.64) 4.65] 4.40) 5.77 5. 18
viet Oe oper F-2, pime----2.2.<. 2) 4.96} 4.30] 5.16] 5.78) 4.46] 3.34) 5.10 5. 02
PS, SPFUCG. ¢ 5A le teens 16.35 | 9.92] 12.31] 9.98] 10.07 | 7.55] 11.02} 10.67
F-1, control....... 6.01 | 5.31} 2.15] 4.28) 5.32] 5.56) 4.15] 468 4.08
Lor je2 nie ae 4.66| 4.19| 3.60] 4.10| 4.34] 4.08! 3.68| 4.09} 4.02
HiF-3, spruce. =| 8.46 | 4.99] 4.86] 9.04] 6.68) 2.83) 6.14 6.34
Average of | F-1, control....... 7.38 | 6.09 | 3.62] 4.63] 5.48] 5.10) 4.28] 5,23 4.63
al gal BORE RE aN | 6.19] 458] 305] 463] 5.05] 427 3.51| 460 | 4. 52
ptns.--||F-3. spruce. --_..- [p+--2+- 12.40] 7.46] 858] 9.51] 838 5.19] &58! 850
|

1 Arithmetic means of the months.
2 Growing-season means obtained by giving September quantities one-third the value of June, July, and
August quantities.

It is to be noted that the south-slope pine site (F—2) shows scarcely
less moisture than the control station, the latter being subject to no
depletion of moisture through use by trees. The spruce site (F-3)
at the foot of a north slope shows considerably more moisture than
the other stations, except at a depth of 2 feet at the end of the sea-
son. This exception would be very significant if the result were
based on more data; but, as the measurements covered only October,
1910, they can not be given much weight.

Periodic moisture at the control station.—As the data for native soils
are meager and can be compared only on their face value, in the
absence of physical data on the soils themselves, an examination may
be made a the more extensive data secured by taking samples from
the “ wells.’ .

In Table 40 are given the monthly mean moisture figures for the
control station for the greater part of five seasons. Each mean figure
is based on four or five determinations, at intervals of a week, except
where otherwise indicated by a number in parentheses, which denotes
the number of determinations. Only the data for 1914, 1915, and
1917 are used in computing the averages, as this is the basis at other
local stations.

co eke ae ae ee oF

Se ee ee Th See

a 4

———"T"

il te de eB i ee ee i |

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 125

Taste 40.— Mean soil moisture at control station as shown by samples from “ well.”’

| Percentages of dry-soil weight. | Mean
Mean) grow-
Year. Depth. | | annu-| ing
| Mar. Apr. | May.| June.! July.| Aug.| Sept.) Oct. | Nov.| Dec. al." me
Feet.
ee ee OA be 9.13 | 8.77 | 6.84 | 7.34 | 7.16 | 4.82 |7.85 | 8.65
1) ee ee SE Pa Bis PS FA IR 8.71 | 9.40 | 7.62 | 8.38 | 9.15 | 7.01 | 8.65] 8.85
ie ee ee wpe [oceece|sseee-] 8:88 | 9.82 | 8.94 | 8.56 [10.00 | 8.16 | 9.14 | 9.08
(3)
eee 9.01 | 8.10 | 7.50 | 6.84 | 8.78 | 7.91 | 8.00 | 7.04 |...... 7.90| 7.73
Se 2 |......|10.78 | 8.84] 8.31 | 8.42 | 9.14] 8.72] 8.68 | 8.31 |.._..- 8.90 | 8.63
3 ania 10.11 |10.67 | 9.41 | 9.80 | 9.86 | 9.28] 8.16 |..2.. 9.82 | 9.95
1)
1 | 4.24 | 7.88 | 7.34 | 6.68 | 8.08 | 8.82 | 6.34 | 6.33 |......]...... 7.35| 7.71
Oe ge fa al 2| 6.19 | 8.14 | 9.10 | 7.14 | 7.52 | 9.10 | 7.65 | 7.32 |....._[...2.. 7.99 | 7.89
3 | 6.06 | 9.84 [10.06 | 8.50 | 8.48 10.09 | 8.32 | 8.55 |... 22 |. 2oo. 9.12| 8.95
9-6 3) (3) | 9.49 | 6.05 | 5.48 | 5.78 | 5.29 | 2.78 | 3.61 |-..... 5.50 | 5.72
1917 ey Ce 7.96 | 8.48 | 7.24 | 6.14 | 6.81 | 7.21 | 6.09 | 5.91 |-..... 6.98 | 6.78
Be ornate aaah Bio) 5-13) 8875 eka 6.69 | 7.54 6.87 | 6.44 d oLekiuli. 52.1. S7ee
=a a 9.38 | 9.75 | 9.82 | 7.93 | 7.24 | 7.39 | 7.15 | 7.42 |......| 8 26 | 8.24
(2) | (3) | @) | @ |
So ee Fg Ue Ae wo ees a a 4.52] 4.18
aan a: RSI BTSs ST od 9 MR | A 7.54] 7.58
nD aemcbiienabiaa PALM Fee ee ETO NOY WAS | Bed5 oo. o.|o-ocbclew~c|ecee =| 8,28 | RSS
Beste 9.35 | 9.7 | 9.14 | 9.14 | o ool seas eleeeees 9.35 | 9.35
Ste] 1 Bare .49 | 6.05 | 5.48 | 5.78 | 5. 78 | 3.61 |......| 5.50 | 5.72
oh i ee emo | 1 |......| 8.48 | 8.29 | 7.37 | 7.37 | 8.12 | 7.32 | 7.14 | 6,70 |-..... 7.60 | 7.59
ratte era F B).15) | 9.78 | 8.80 | 8.18 | 8.06 | 8.41 | 7.96 | 7.98 | 7.97 |...-.. 8.39 | 8.19
eRe: eels J ate 10.34 | 9.93 |10.24 | 8.57 | 8.95 | 8.73 8.33 | 8.53 |... 9.20) 9.20

1 Omit December, 1914, and March, 1916, in computing yearly averages.
2The values for June, July, and August are added to one-third of the September value and the sum
divided by 10/3.

(1) The moisture at 1, 2, or 3 feet is remarkably uniform in differ-
ent years if the growing-season means be considered. This may be
due in part to the lack of tree growth near the ‘‘well”’ and the inability
of grasses and herbs to reach the deeper moisture.

2) The moisture is less during July and late summer than in the
s ie a early summer, but is well maintained by precipitation
alter July.

(3) The year 1917 was the driest of record, the precipitation and
especially that for the growing season being below normal.

(4) The mean figures for the soil well, for depths of 1 and 2 feet,
when reduced to terms of native soil by use of the moisture-equivalent
ratios, are of lower value than the actual figures for native soil ob-
tained during 1910-11. As the early sampling of the native soil
covered a considerable area, and as the soil is by no means uniform
over large areas, the lack of correlation may mean nothing more than
a difference in the conditions surrounding the measurements during
the two phases of the study.

Comparative moisture in wells, all types—In Table 41 is given a
summary of the well-moisture data for all stations in the study where
data have been obtained. For all local stations except F—12, F—14.+
and F-15, the data are for 1914, 1915, and 1917, beginning with July,
1914. For Monument the period is 1915 to 1918, inclusive. For
Foxpark it is August, 1916, to August, 1918. For Wagon Wheel
Gap it is (Al and A2), August, 1913, to August, 1918; (D) August,
1913, to October, 1917; and (F) July, 1915, to October, 1917. As
the unusually dry year 1917 is included in each average, it is not
thought the variations in period will introduce any serious discrep-
ancy.

126 BULLETIN 1283, U. S. DEPARTMENT OF AGRICULTURE,

TABLE 41.—Mean soil moisture in “‘ wells.’’

Station
No.

M-1....

W-A2..

.| Ridge

Type.

Control s4-02-.

South-slope

yellow pine.)

Monument yel-
low pine....

Yellow pine-fir -

yellow
DINC= soccer

South slope
Douglas fir...

F-14.._.| Fir, north,

half cut

Wat enc osc ra Ko us

WA.

.| High

Fir, Worthy,
clear cut.....

North slope
Douglas fir...

Lodgepole, |

Wyoming....

Canyon spruce.

High spruce,
| oT iy «age ae A

spruce,
TOLESU see eiec)s

West slope lim-

ber pine... ...

=

'
Q
°
.
.
.
'
.
.
’
'
———_ O_O Oe eee ee ee ee SS eee

Percentages of dry-soil weight.

Depth.

May.| June.| July.| Aug.| Sept.

py
iS
or
Jo)
a
or
—s
—_

i | 7.21 |10.30
2 | 9.42 |10. 85
3 |14. 10 112. 20
S.

SBENASSESERS

—

SESE SUSU On Sis 00 Or Oo Tot
SB

Co ST SES SOP ee GOA

Ne}

as]

ary
DONNID NAS SO y> 9 90 GK

—_
lor)
nse
ee

Co
o
j=)
OPS SMW MNO AINE S | 190 99 IK

=
> © 00 Go

SE ee ee Nr UR IRN Bien Brg oY Rein MRR MCV Ge eee Sty pS CHER aed (C29 CCMLO SHES GRC TPR Fe

11. 68
| 13. 32

OND PHS HP OgrIH

SAkUGNSRsZeSeSSosn

GOST te COON OD SRT See Shy eo oe ee

= ee
PAS DIP MH SO Pow

_

PNO MS
eS ee
SPrS wom

11. 65

rie

10. 62 111.99
11.65 |12. 51
13.05 |13. 42

1 Growing-season means obtained by giving September quantities one-third the value of June, July,
and August quantities,

‘i
4
a

a
é
4
4

FOREST TYPES IN. CENTRAL ROCKY MOUNTAINS. 127

iS
£

Surface moisture in the wells was measured for local stations only
in 1917, and at Wagon Wheel Gap not at all.

As the large mass of figures in Table 41 is difficult to grasp, the
averages by species groups are presented in Table 42.

TABLE 42.—Average ‘“‘well’’ moisture for growing season.

Depth.
Group. ) = Teas
Surface.! | 1 foot. 2 feet. 3 feet.

en cent.| Per cent.| Per cent.| Per cent.
WONtROUSCAUION Se eae See eee oe oe mee oe uses nia toa aked velee coe ceee 5.72 7.59 8.19 9. 20
our western yellow pinesitess: onc. dik oo. os see tone cee oe oe 5.13 8.75 10. 14 11,14
MEMCHE PI GUPIASOL SUVCSE i. 5 So eile a nod Gime cigs Seno cine aces 8.59 13. 33 14, 78 15.57
BOG TEES DI COS UGC San poe ele ain te iteamels Benita ocean ele 10. 18 17.90 19. 92 21. 44
GrHelodzepole Pine SILC! A>. sa septa a </< goes Se edo Seninicineisnininie 8. 84 10. 53 12. 54 15. 16
HIG UNTO PING SUC me stat eit rae eon ween otal ae laa ae a 8. 56 | 11. 80 12. 20 13. 02

1 Localstations only and season of 1917 only.

On the face of these data it is very evident that the yellow pine
sites paneer less moisture during the growing season than do the
Douglas fir sites, which, with one exception, are northerly aspects—
and likewise that the spruce soils have a still larger supply. It is
unsafe to estimate the positions of the limber pine and lodgepole pine
sites without some correction for the type of soil involved, as thee
is here no opportunity for compensating peculiarities.

In considering the high average values for the spruce sites, however,
it is worth while to mention that the group of four was made up of
two local stations and also of two at Wagon Wheel Gap whose soils
have much better water-holding properties. Thus, me example,
the mean 1-foot moisture for the local sites is only 12.5 per cent,
or almost the same as for the local Douglas fit sites, but the corre-
sponding ge for the heavier Wagon Wheel Gap soils is 23.32
per cent. It will be shown later, when wilting coefficients are con-
sidered, that this apparent difference is unreal, so far as availability
of the water is concerned.

In the yellow pine group, the relatively high moisture contents at
Stations F-4 and F-12 are worthy of note, as both of these sites
are fairly favorable for Douglas fir. Of the two, Station F-12 has
the more favorable conditions if allowance is made for the quality
of the soil.

As data are available for every site for the season of 1917, which
has previously been mentioned as the driest year in which extensive
records have been made, it is desirable to give consideration to
minimum moisture conditions, which are far more likely to give
something significant with respect to the survival of the species. In
using the 1917 data, the individual and successive moisture deter-
minations are found to be so variable that it seems unsafe to base

any conclusions on single observations. The most meager basis

would appear to be the average for a month, made up from four or
five dap fe determinations. The lowest monthly average for each
station and depth has, therefore, been accepted. In Table 43 the
results are grouped according to the plan of Table 42.

128 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

TaBLeE 43.—Minimum (month) “well”? moisture during season of 1917 and per-

centage ratio to mean growing-season moisture (in parentheses).

Depth.
. Group.
Surface. 1 foot. 2 feet. 3 feet.

Ranirel station..-.12 cb Ae acu eo: at 60 a { Sy (78) . SH Ne

Four yellow pine sites... -..-...++-+-+-+++++++2++0+ ee ee eee ee eee i Ne Se er ne

Seven Douglas fir sites. .-......---+-++++++++++++2seeees seers { Ce 1643 108 15.31

Four spruce sites. ..-----------+-+---++++ +++ 2-22eeee errr ee eeee eee { a ate ee 1a

One lodgepole pinesite.......-22-2-22-ceeeeeseteeeeeeeeeseeeees EE Tc aes ao
Pigetilimber pilibsite. 014... LOE nan eee aos f G8) |. DT sok eh ate

The data in Table 43 do not appear essentially unlike those of

Table 42 in fixing the relative positions of the species, yet the paren-
thetical figures showing the relation between these minima and the
approximately normal growing-season condition are full of signifi-
cance. It is to be noted that the degree of exhaustion as indicated
by these percentages is nearly the same for the 1, 2, and 3 foot depths
with perhaps a slight tendency toward greater exhaustion at a depth
of 2 feet than elsewhere. On the other hand, although the yellow
pine sites do not show greater exhaustion at the surface than in the
deeper soil and the north-slope fir sites show only slightly greater
drying of the surface, the spruce sites, as well as the limber pine and
lodgepole pine sites, show very marked drying out. In the spruce
sites this can not be ascribed to insolation; it must be charged in
part to the loose duff which forms the surface layer and in part to the
prevalence of roots near the surface. As the soil wells can not pos-
sibly show either of these factors fully, it is evident that the natural
surface soil in a dense spruce forest must represent a very great
degree of drought. That is, perhaps, the most striking point that
has been brought out by all of the soil-moisture determinations,
especially when one considers the importance already attached to
surface temperatures. Calculations as to the availability of the
moisture, made on the basis of moisture equivalents, indicate that
one of the two spruce sites for which the surface data are available
becomes physiologically drier than any of the others, and the second
is drier than any except the warmest yellow pine sites and the wind-
exposed limber pine site.

The attempt to reduce the data of Table 42, first, to terms of
moisture percentages for the native soil and then to terms of avail-
ability, does not materially alter the evidence of Tables 42 and 43,
en a that the yellow pine sites are the most arid and that the
spruce situations are only slightly moister than the north slopes
which bear Douglas fir. On the other hand, these calculations do
show that, owing to a very sandy soil used in the soil well, the lodge-
pole site is considerably more favorable than the original moisture
data would indicate. The approaches, through osmotic equivalents,
through the direct ratio of moisture contents to moisture equivalents
or through consideration of the wilting coefficients, all produce, as

yet

| '
a

in? .. in

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 129

between the forest types, essentially the same relations. All tend to
show that the differences between sites in availability of the moisture
are less than are indicated by the bare moisture contents.

Rather for the purpose of illustration than because of the value of
the data, calculations of the probable natural soil moisture and of the
availability of this moisture are presented in Table 44. These calcu-
lations can not be extended to the surface-moisture values, because
the native surface soils have not been physically*examined. More-

- over, such calculations would be of little value, because the surface

soils of the wells are so directly influenced by atmospheric conditions.

Probably the most valuable feature of Table 44 is the clear way in
which it shows that the high moisture percentages obtained with
the heavy soil at Station D, Washi Wheel Gap, are really on a level
with those for the local spruce stations. The slightly higher avail-
ability at Station D is easily explained by the almost complete
absence of forest. Similarly, although the calculations can not be
carried out for the two Douglas fir stations, the high wilting coeffi-
cients indicate that Station W-A1 (north exposure) must be just
about on a par with similar local stations, but that the strongly
exposed Station W-A2 is certainly much more arid.

As has been pointed out, these calculations show the lodgepole
pine site to be very moist. This may be due in part to the fact that
only a few trees surrounded the soil well, but is probably the result
of ow evaporation stresses. The limber-pine site is seen to be in a
class with the average yellow-pine site. Among the local stations
subject to practically the same atmospheric conditions it is evident
that direct insolation tends to keep the more insolated pine sites in
a drier state than the others; but the very slight difference between
fir slopes and spruce bottoms after a prolonged period of drying
indicates that each soil, through direct drying and the drying accom-
plished by the trees, tends to come into vapor-pressure equilibrium
with the atmosphere. The probabilities are, therefore, that in the
event of more prolonged drought, sufficient to permit an equilibrium
to be reached, equally insolated soils will attain the same status quite
regardless of the character and amount of vegetation thereon. This
is of the utmost importance, indicating, in the present case, that the
moisture supply is effective only in controlling the number of in-
dividuals ud can not differentiate between species, at least as be-
tween spruce and fir sites.

It is fairly evident, then, that soil moisture below the surface
layer is only an incidental factor in the distribution of the several
species. Strongly insolated sites and those exposed to the higher
temperatures and to the greater atmospheric stresses of low eleva-
tions, as might be expected, are always the driest, but their most
important feature, owing to the close connection between insolation,
surface temperatures, and surface moisture, is doubtless the very
rapid drying of the surface layer after every rain, rather than any
decided lack of moisture at greater depths.

73045 °—24——_9

130

BULLETIN 1283, U. S. DEPARTMENT OF AGRICULTURE,

TaBLE 44.—Calculated ‘minimum moisture in soils during 1917 and relative avail-

Station No.

' Type.
}

ee eee wee

ability.
Caleu- ¢
rlgim's'f 24 bg Nonavail- ee
MOISE; Ratio widen able “bility
Depth. lowest ME/ME' f oe . moisture M- C
month -| of native | hy test
(M’). SOD Th GUL» line tlt
: (M). ‘
Feet. | Per cent. Per cent. | Per cent. Z|
1 .o9 0. 853 3. 2.36 0. 229
2 3.90 - 632 2. 46 1.53 .378
3 3.47 | 647 2.25 1.68 . 253
1 4.93 | . 582 2.87 1. 16 596
2 5. 96 | - 636 3.79 2.63 - 306
3 6.89 - 631 4.35 2.67 - 386
1 9. 35 . 630 5. 89 1.99 - 662
| 2 10. 14 - 600 6. OR AS. ee eae
hasbapi 11.16 ~495 5. 52 1.97 643
1 5. 02 526 2.64 1.79 . 322
| 2 5.61 475 2. 66 1. 28 519
3 7.61 375 2. 85 1.43 498
1 TENS |e ane PRP) 2 SAR OAS . 452
2 6.40 niiaic. gec}. apes. \. 401
3 oP aan nah Stealth i te 5 fh 2445
1 9.03 - 463 4.18 1.26 - 699
2 10. 02 478 4.79 1.85 614
3 10. 09 - 486 4.90 1.24 747
1 7.38 387 2.86 1.76 «385
2 9.99 B25 3.25 1.64 495
3{ 10.80 ~ 263 2.84 1.18 ~585
1 12.38 481 5.95 1.79 700
2 12. 42 . 536 6. 66 2. 06 |. - 691
3 1B EDS: oll FW (i | SR ag ecient Sy
tl 10. 81 . 433 5. 22 2.25 569
ry er, . 589 CDF stern. bok. bie See
3 13.13 622 SF i era
{ 1 7.46 +393 2.93 1.62 . 447
2 9. 67 . 393 3. 80 - 96 . 747
| iby 9.69 . 435 4,22 1.62 616
1 AGs2QO tavck acne tga} lee or (: oe Pee ee Ss
| 2 T0020 [ees oc eee ioe (ot ace eo eee eee ee eee
3 POIGO NEES. SIR | LIAS. = 6263) fethegeced.
1 Re OD aa eects ene Co dT ad ek pad 2
SiO Figo LECT HOTT as Maa aehey
3 Bi60d ATED. fll eee 1 $45 S980 feb ae oat
1 LOLS EL RLV CW keris. OW te . 560
2 TOR ee ee P . 638
3 OBLAST SA NAG RS LD 5 APE PRES. . 649
1 6. 68 645 4.31 1.54 . 643
2 9. 40 . 535 5. 03 1,51 - 700
3 11.16 327 3.65 1,45 . 603
1 1. OC . 408 8.09 2.12 .314
2 10. 03 - 462 4.63 2.64 . 430
3 11. 56 . 405 4.68 1.98 577
1 17. 50 1. 033 18. 08 7. 56 . 582
2 19. 00 . 876 16. 64 3.59 . 784
3 20. 60 893 18. 40 4.14 to
1 13,90 joc deady ucoleve - cawae nied oooh > dab Cree
2 LOS TO | nn ac gre cc nc el eccomea dct cl octet ean sol naan nn
3 19. 303 72022). VIB Oe... 4Aneaee
1 AD '66.42 22405. UT A ee 516
2 14. FRilchGapec voa-| ek Bee kboved= eae . 638
3 15.66 160% Al lorcet eReeees . 652
1 6. 66 2.017 13. 43 4.72 649
2 7.72 2. 437 18. 81 3.65 806
3 8.11 1.743 14.14 2.99 . 789
1 8. 46 . 494 4.18 2. 44 . 416
2 9. 49 . 385 3.65 2.65 274
3 10. 87 329 3.58 1.85 483
1 5.91 520 3.07 2.31 248
2 6. 44 . 436 2. 81 93 . 669
3 7.15 - 526 3.76 76 - 798

|

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 131

At the other extreme stand the canyon and lower-slope spruce
sites, whose soil is mainly transported. ‘This soil is of finer quality
than that of the slopes, is rich in humus, which may blow into the low
anes as well as wash in, and consequently is of the greatest water-

olding capacity. This great capacity and the usually high level of
the moisture content encourage dense stands. Only spruce can
grow in the densest stands; other species are excluded not by lack of
moisture, or by too much moisture, but by lack of insolation. It
seems possible that these very sites may, after unusually prolonged
drought, prove to be the driest. The difficulty which is often expe-
rienced in starting plantations in gulches which encourage heavy vege-
tation is testimony to the extreme desiccation of such soils when rain
is withheld. In 1909, the writer definitely found the bottom situa-
tions to be the driest of all the sites in late summer in the Nebraska
sand hills.

The Douglas fir sites on north slopes are almost as moist as the
lower spruce sites, and probably do not dry out to so great a degree.
The surface conditions, especially, remain fairly favorable because
of the lack of drying insolation. On the other hand, the evidence of
the south-slope fir site at Wagon Wheel Gap, and the occurrence of fir
seedlings almost on the roots of yellow pine on south slopes, shows
convincingly that the need of this species is not for great moisture,
but rather for moderate temperature conditions.

RECAPITULATION.

GENERAL CHARACTER OF THE CLIMATE.

The mountain climatic conditions, which have been portrayed
mainly through the relatively long-term records at the Fremont
Forest Experiment Station, at an elevation of 8,836 feet on the eastern
slope of Pikes Peak, may be characterized as follows, in terms which
apply generally to the forest types (fig. 10).

1) The summer or growing-season conditions are not unfavorable
to the growth of vegetation that has very moderate heat requirements.
The mean growing-season temperature is about 55°, with the period
from June 10 to September 10 free from freezing temperatures.
Almost 50 per cent of the total precipitation occurs during the oTOW-
ing season (June 1 to September 10, as here considered) following
a brief drought in May or early June; hence the moisture of the
deeper soil stands throughout the season at a favorable point, rarely
if ever approaching the nonavailable content. This period is never
characterized by high winds, and great atmospheric dryness occurs
only when the drought period extends well into June.

(2) With the cessation of daily showers, which rapidly decrease
in number and volume after August 15, a period of clear, dry weather
occurs which rapidly depletes the moisture of the surface soil and
which may, by tha end of October, cause the death of a large propor-
tion of the new seedlings of the season, most of which will not have
germinated before July, and will therefore be very immature and
poorly rooted. In this wholesale elimination, which doubtless
makes some distinction between species, the early frosts may have
some influence, but do not appear to be so important as 5 nob

132 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

(3) The winter temperatures at middle and lower elevations in
the Pikes Peak region are rarely very low; relatively warm, strong
winds occur frequently from January to March: and evaporation
stresses are very telling, except at high elevations where the air
temperature rarely goes higher than 32° F. The precipitation also
during the winter months is very low. Clear weather and the lack
of material for evaporation result in great dryness of the atmosphere.
Another effect of the lack of snow is seen in the early and deep freez-
ing of the soil, except on well-insolated exposures. In the main,
then, trees are subj ected to conditions which induce rapid transpira-
tion, and at the same time are cut off from their moisture suppl
by the freezing of the moisture in the soil. Obviously, these condi-
tions are not a less severe strain upon the well-established tree than

ee es a ES OR PS
se a TEESE th
SS ee a Tae

Plains Yellow Pine Limber or 8.C.Pine Douglas Fir Lodgepole

2 att vl os]
E400 papa ta /| RSE RATION PERIPC Of SOL FF
ete Sh as pe

ee.)
pay
keer |
ara
Fia. 10.—Summary of the several conditions in the various forest types.

on the seedling which ri secure protection through shading or
f] ough a temporary snow blanket.

It is E believed that these winter conditions, rather than the poor
quality of the soil, delimit the growth of all forest trees in the localit ty,
and produce a scrubby stand which on some sites has apparently
reached its limit at a height of 40 feet or less.

(4) The extension of observations to other localities both north
and south of the Pikes Peak region indicates a less complete develop-
ment of most of the unfavorable features of climate. To the south,
to be sure, the spring drought is more pronounced and may possibly
destroy seedlings of the receding season. The summer rainfall is
bo more abundant, but the fall drought i is of less duration; a snow

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 133

cover is secured earlier; and the low winter air temperatures are
much less conducive to evaporation. To the north, in the lodgepole
pine type, precipitation is insufficient to balance losses from May to
August, and a pronounced drought develops toward the end of sum-
mer. However, as early snows are heavy, there is no acute problem
of soil moisture, nor are there menacing evaporation stresses in winter.
No doubt the evaporation problem is present at low elevations in
Wyoming, South Dakota, and Montana; but in the Black Hills,
where yellow pine develops excellently both as to reproduction and
growth rate, it is safe to say that both soil and atmospheric dryness
are considerably modified by an abundance of snow and by low air
temperatures. ;

TEMPERATURES AND TEMPERATURE GRADIENTS.

The study in the Pikes Peak region has shown that a fairly regular
change in air temperatures occurs with change in elevation, amount-
ing to 2.8° F. per 1,000 feet for the year as a whole, 2.9° for the
growing season, and 3.3° at its maximum in April. This applies to
what may be called ‘“‘neutral” sites at all elevations except the
highest one. Early study showed the air temperatures 20 feet above
the ground to be not essentially different for opposing slopes at the
same elevation; but near the ground southerly exposures have been
shown to be warmer than the mean for the elevation, and northerly
exposures cooler, to an extent compatible with the types which they
peapentinely encourage. These contrasts are more clearly seen in the
soul temperatures.

The Wagon Wheel Gap locality in southern Colorado shows a
temperature gradient of about 2.3° F. per 1,000 feet for the growing
season. During the winter months this temperature gradient is not
infrequently inverted, and in exceptionally cold weather there may
be an inverted gradient of 4° per 1,000 feet, the lowest station here
being in a valley affected by cold-air drainage. At a middle eleva-
tion this locality is always colder than the Pikes Peak region.

The relation of soil temperatures to air temperatures is by no
means constant for the different forest types. ‘abe the growing
season such differences as exist between air and soil temperatures
are largely the results. of local insolation, which does not affect local
air temperatures so markedly as it does the temperature of the
ground near its surface. If the year is taken as a whole, soil tem-

eratures may not reflect the air temperatures of different localities,
ovate of the marked influence on the former of a light or heavy
snow blanket.

Because summer soil temperatures agree more closely with the
type differences produced by different exposures, no less important
than those produced by differences in elevation; because soil tem-
peratures may be seen to have a more direct influence on type com-
postition at the time of initiation of seedlings; and because winter
soil temperatures appear to be indirectly related to survival, through
soil moisture, as fully as are air temperatures through evaporation,
the detailed consideration of soil temperatures has proved more
valuable in the present study than the consideration of air tempera-
tures, even though surface temperatures of the soil, which should by
all means be recorded, are largely lacking.

134 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

Response of the species to temperatures.—There is every reason to
believe that insolation exercises a direct control over the distribution
of the species; but in the total absence of direct measurements of
insolation, temperature data must be relied on to carry out this idea.
It is, of course, difficult to distinguish between the direct effects of
insolation and such indirect effects as soil drying; but the total
evidence points to the direct and the temperature effects as influencing
most strongly the selection of the species for each site, except possibly
with spruce, which is more susceptible to drying than to heat injury.

The range of either air or soil temperatures measured in any one
type of forest is, however, so small as to indicate that temperatures
during the growing season can not vary widely without causing the
elimination of the given species, or at least its domination by some
other species. It is true that on the lower margin of the mountain
forest yellow pine seems to thrive under a wide range of temperatures
in different localities. ‘This apparently greater adapatability of yel-
low pine may be due partly to the fact that, wherever it may extend
downward into warmer zones it finds no important competitors, and
therefore may exist even if its rate of growth becomies negligible.
More important, however, is the consideration that air or soil tem-
peratures as recorded probably mean almost nothing. The tem-
peratures which the young plant experiences will be most intimately
bound up with the temperature of the surface of the soil. As may
be readily proven, relative to other temperatures the surface maxima,
which are probably most important, will vary widely according to
the amount of rain and the moistness of the soil durmg the critical
summer months.

A mean growing-season soil temperature of about 58° to 62° F., at
a depth of 1 foot, permits the establishment of western yellow pine.
At 55° or 56° Douglas fir will appear with the pine. At 52° or 53° fir
will probably predominate, but there may still be some reproduction
of yellow, limber, or bristlecone pines, the species depending appar-
ently on winter evaporation stresses. At 50° we may expect almost
pure Douglas firif moisture is favorable. At 44° to 48° and perhaps
nigher, Engelmann spruce reproduction will come in under fir and
doubtless ultimately replace it. Spruce may hold complete sway
where the soil temperatures are from 40° to 50°, but probably does
not reproduce at all where this lowest temperature prevails; the stand
must be thinned out before it is rejuvenated.

Lodgepole pine, probably has temperature requirements, for satis-
factory reproduction, between those of yellow pine and fir. The
single site studied shows soil temperatures a little lower than those
for fir, and air temperatures no higher than for good spruce sites.
However, the well-known proclivity of lodgepole reproduction for
denuded sites, where the se dintion is intense, and some few measure-
ments on such sites not conducted long enough to establish averages,
show the reason for the usual failure of lodgepole to reproduce under
its own canopy and indicate the higher heat requirement of this spe-
cies. Measurements by Notestein *! at midsummer in 1912 showed
the soil temperatures at 6 inches to be at least 4° higher in an area
from which the pine had been cut and the brush burned than in the
virgin forest. In each place measurements were taken at about
12 points.

11 Notestein, F. B., forest examiner, Forest Service, ‘‘ Progress Report, Exp. R—11,”’ 1923.

_. FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 135

Limber pine sites even at high elevations show summer tempera-
tures higher than those for Douglas fir, and undoubtedly reaching
almost as great extremes as yellow pine soils.

It is not necessary for a proper explanation of the selective process
of insolation to. assume, as the present writer once did (4) that the
temperature of the environment appreciably influences the chemical
effectiveness of sunlight; but 1t is necessary to recognize that a high
temperature may contribute to the superheating of the plant tissues,
and that a dry atmosphere and wind decrease the liability by facili-
tating the cooling process of evaporation. |

The idea that protection arises from the cooling caused by transpi-
ration is by no means new. In 1902, Barnes (2) wrote:

34. Under the present organization of plants exposure of wet cell walls to the
atmosphere is indispensable for the solution of necessary gases, oxygen and
carbon dioxide, the plant being debarred from waterproofing the cell wall so
long as gas absorption is necessary. ‘Transpiration is, therefore, considered as
unavoidable, though in itself a constant menace to life and activity. Advantage
has doubtless been taken of the xylem bundles to facilitate the movement of
solutes, but there is no reason to think this essential. Transpiration also has
become a protective factor with sun plants, whose temperature is thereby kept
within reasonable bounds. (Since reading the paper the author has ascertained
that in certain points his view of transpiration coincides with those expressed by
Dr. C. E. Bessey in a paper on the function of stomata, published in Science
N. 8. 7:13-16, 1898.)

Each species may have, then, functional limits expressed in a
minimum amount of light and a certain maximum temperature.
It is evident also that the several species may exhibit differences not
only in ‘‘shade tolerance” but in light tolerance as well, which will
be especially marked when the seedling is very young and is subjected
to the reflected light from the sroued surface and to the attendant
temperatures which may prevail there. This limited light tolerance
is probably due to the direct effect of heat upon protoplasm. The
experiments reported have not, it is true, shown the species to be
sensitive to excessive heat in proportion to their shade tolerance.
Apparently Engelmann spruce is adapted to a much wider range of
light conditions than the other species, while Douglas fir is most
clearly sensitive to the heating effects of strong light, and with both
spruce and lodgepole pine, the injury to young seedlings evidently
arises from loss of water rather than, or commonly earlier than,
from superheating.

WIND, HUMIDITY, AND EVAPORATION,

. Wind velocities vary widely in different situations and localities
of the mountains, recorded velocities depending largely on the im-
mediate conditions of exposure. In general, wind velocities are much
higher in winter than in summer, and much higher at high elevations
than at low, exposures being equal, because of the fact that the higher
atmosphere is more turbulent. Beyond this, comparisons of the
sites studied are unsafe. It is entirely in agreement with the physio-
logical qualities of the species to find spruce tolerating much greater
exposure to wind, than does yellow pine.

umidity, as measured by saturation deficits, varies more widely
locally, on account of differences in air temperature and not so muc
by reason of changes in vapor pressure. A wind in descending the
slope of Pikes Peak may actually gain some moisture, but its tempera-
ture increases so much more rapidly that it gains very quickly in
saturation deficit and evaporating power.

136 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

Broadly speaking, evaporation rates are almost as high at the
highest as at the lowest elevations in summer, temperature and wind
movement being almost compensating. In the winter, however,
evaporation is almost nullified by very low temperatures in the higher
forest types (spruce and lodgepole) where the vapor deficit becomes
very small, and high wind is, therefore, noneffective. In the middle
and lower portions of the Pikes Peak region winter evaporation is
an important factor because of the frequent combination of high
temperatures with strong winds. In the Wagon Wheel Gap locality
the evaporation is relatively high for corresponding sites except in
winter when the temneratures are very viele lovee than at Fremont.

If measured near the ground the evaporation in the various forest
types during the growing season is found to be much more closely
controlled by local insolation and temperatures than by the moisture
or movement of the atmosphere, and in consequence 1t can be said
that the well-insolated sites are much more conducive to transpira-
tion losses than are the cooler ones. The soil temperatures in this
period give a fair indication of the evaporation stresses, which are
peoularly important in dissipating the soil moisture. Ecologists

ave generally considered that a high evaporation rate during the
erowing season constituted a barrier to the success on the site of the
more moisture-demanding species. This may be true in the extreme
case in which the rate of transpiration is likely to put a serious tax
upon the ability of the plant to secure moisture. In the present
study it has not been found that the moisture reservoir was seriously
depleted on any site. It has been seen that the species of greatest
moisture demands according to the results of this study succeeds on
the site which produces the greatest evaporation stresses currently
during the growing season and in the aggregate. The conclusion is
inevitable that, so long as the moisture supply is fairly good, high
evaporation can not be considered deleterious in the usual sense and
that it can not be a selective factor as between species. Rather it is
merely evidence of heat conditions which encourage the species that
is most likely to transpire freely, and which are injurious only to the
species that for yale reasons does not transpire so readily.

On the other baad, during the winter the moisture supply may not
be available; it is obviously less available at high elevations than at
low because of the much lower air temperatures; it is less available
on northerly than on southerly exposures because of the poorer inso-
lation of the former; it is less available where the ground 1s bare than
where it is covered by a snow blanket laid on early enough to prevent
freezing of the soil. In view of the last-named fact the Pikes Peak
region presents a unique situation, a situation in which the very
existence of established stands of evergreen trees is threatened almost
yearly by drought of the soil and air during the period of vegetative

dormancy. :
: WINTER SOIL TEMPERATURES.

It has been stated that soil temperatures during the growing season
supplement or may be substituted for air temperatures as measures
of the growing conditions for seedlings. Perhaps the temperature
of the soil near its surface is the best measure of the heat, and espe-
cially of the maximum temperature, to which the seedling is subjected
at a critical age, and no doubt this measurement should receive more
consideration than it has in the present study.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 137

Soil freezing.—The fact about soil temperatures which stands out
reeminently is that the soil which has a low mean temperature or a
ine summer maximum is likely to be frozen for a much longer period
than the soil which, either because of its low elevation or on account
of direct insolation, attains higher mean and maximum temperatures.
The period and extent of soil freezing may, however, be considerabl
modified by a blanket of snow. Everyone knows that an early fa
of snow may completely protect the sel from freezing, though Warm-
ing, (22) considering only one phase of the matter, says: ‘Only where
there is a covering of snow is the temperature of this (the soil’s)
surface, also its daily mean temperature, lower than that of the air.”
Evidence might be introduced to show that under a deep snow cover-
ing the soil temperature may remain stationary for weeks at, say
32°, while the air temperatures go to 0° or lower. Bouyoucos (7)
has convincing data on this subject.

The data available show that the mean temperatures of soils at a
depth of 1 foot in pure yellow pine types are likely to be below 32°
for two or three decades; ” that the continuance of this temperature
for 11 decades does not prove fatal to pine but does tend toward its
replacement; that in Douglas fir sites it varies from 7 to 17 decades;
and that a period of 15 to 19 decades may be somewhat characteristic
of the higher spruce types, where the mean soil temperature for the
year is 32° or less. Thus there are marked variations for any one
species, but it should be borne in mind that within certain limits the
conditions which produce soil freezing are likely to be concomitant
with conditions which reduce transpiration.

The gradation from spruce to pine sites in the severity of soil
freezing is really more marked than the average soil temperatures
indicate. The toning to freezing which will permit injury of the
aerial portions of the tree is better measured by the products of
periods and greatest depressions, which, as Bee computed, vary
from 3,600 at timber line to 30 in the yellow pine type. Where the
mean temperature for any decade is never very much below freezing
the continuity of the freeze is likely to be broken on any warm day
and the possibilities of such relief decrease just about as the amount
of depression below 32° increases. At the highest stations there can
be found no source of relief until almost the close of the freezing
period, when snow water may penetrate the surface soil in advance
of thawing at 1 foot.

Evaporation during sow freezing.—In a measure, the longer period
of soil freezing at higher elevations and on poorly insolated slopes
should be compensated by the decreased opportunities for water
loss by evaporation. It is evident that the important thing is not
the period of soil freezing, nor the temperature reached, but the fate
of the water stored in the stem and foliage of the tree, which must be
held during this period.

The best computations which it has been possible to make indicate
that the higher forest types are subject to the greatest total loss by
evaporation, notwithstanding the much lower daily or monthly
losses. A large part of this total loss is likely to come near the end
of the period of soil freezing, when air temperatures rise much more
rapidly than soil temperatures.

12 It is assumed that at a recorded temperature of 32° F. soils will never be frozen because of the
depression of the freezing point by solutes and surface tensions.

188 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

These computations show a maximum evaporation of about 100
units in the thrifty yellow pine forest, during the mean period of
frozen soil. Yellow pine barely exists, in mixture with limber pine
and Douglas fir, where the evaporation may be 350 units. Nearly
300 units are possible in an opening of the north-slope Douglas fir
forest, but within the forest the total stresses are less than one-third
as great. The same type at Wagon Wheel Gap shows the possibility
of about 300 units, but only half as much on a well-insolated south
slope. The spruce sites show from 60 units (low canyon type) to
400 units at timberline. The greatest loss noted oceurs on a high,
windswept ridge, where limber pine is now the only claimant of the
ground. The lodgepole pine type in southern Wyoming, although
soil temperatures are slightly below freezing for five months, has
evaporation stresses barely exceeding those of the yellow pine type.
Possibly the rapidly increasing exposure to wind at high elevations
and the increasing length of the period of soil freezing would set a
limit to the upward extension of spruce, though the limit of ability to
resist the mechanical effect of wind and snow may be reached first.
Regarding drought in alpine situations and the degree of xerophytism
which it develops, the following citation from Cowles (11) is of
interest:

The distribution of alpine plants, however, is apparently due in large degree
to edaphic conditions. The timberline in general may probably be referred to
atmospheric conditions, but the marked gaps and oscillations which usually
occur are due in a large measure to soil relations. While xerophytes increase
in the alpine parts of mountains, it is to be observed that edaphic as well as
climatic factors become more xerophytie upwards. While changes occur as one
traces one type of edaphic formation upwards, these changes are far less marked
than are those observed in passing from one edaphic formation to another.

Although soil freezing does not appear to be a generally critical
factor in relegating the species of the central Rocky Mountains to
their respective sites, it would seem to have a marked influence on the
distribution of yellow pine in view of the sensitiveness of the species
to winter drying. ‘‘ Winter killing” rarely does more than defoliate
the established tree, and a few trees of this species have become
established where the exposure must be very severe. However, the
heavy loss of seedlings of yellow pine durimg the winter, in both

lantations and natural stands, suggests that on sites which do not

old a snow blanket the fatal effects are most likely to be felt by very
young trees, both because there is less opportunity for storage of
moisture in the tree itself, and on account of the nearness of the foliage
to a reflecting surface which would greatly augment transpiration.

PRECIPITATION AND SOIL MOISTURE.

The annual precipitation at the control station is about 22 inches,
of which idurly one-half oecurs during the growing season. In
the Pikes Peak region this annual amount decreases to 14 inches
at the base of the mountains, and increases to nearly 25 inches in the
spruce zone. ‘There is, then, an increase of about 2.4 inches per
1,000 feet increase in elevation. In the Rio Grande region the pre-
cipitation seems to be somewhat less at a middle elevation, but is
fully as much at high elevations. ‘ In southern Wyoming the lodge-
pole pine type, slightly higher than the control station, has only 15.5
annually, and only one-fourth of this amount occurs during the grow-
ing season. |

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 139

Precipitation measurements are obviously unsatisfactory for the
close analysis of contiguous forest types, since these may receive
equal amounts and yet be radically at variance in their moisture
conditions. With the precipitation known, measurements of evapora-
tion near the ground should give a very good idea of the extent to
which the moisture supply is dissipated, or conserved for the uses of
vegetation. It has been shown that the higher forest types have
somewhat less evaporation than those at low elevations, for the
year as a whole, and that northerly exposures are much less rapidly
dried out by insolation, than those which receive the sun’s rays more
directly.

The measure of the moisture of the soil gives most directly a
knowledge of the supply available for the trees of any forest type.
It has been shown, with the limited number of stations at which this
measure has been taken, that the absolute amounts of moisture in
the soil, during either the average growing season, or the driest part
of an unusually dry season, are commonly greatest in the spruce sites
and the north-slope Douglas fir sites. The yellow pine, limber pine,
and lodgepole pine sites have considerably less. Only on strong
southerly exposures, however, and near the lower limit of the yellow
pine type, where good precipitation is experienced during the grow-
ing season, is the depletion of the supply very considerable, and even
in these situations ue wilting coefficients of the respective soils have
not been closely approached at any time during the period of observa-
tions. In fact, e higher moisture percentages of the spruce and
fir soils do not indicate much more favorable conditions oe crowth
than the lower percentages in the pine types, because the soils of the
latter class are always more sandy and hence such moisture as they
retain is relatively more available for plants.

Nevertheless, the differences between the types in soil moisture
content have a significant bearing. The fact that the minima
reached in the various types are so similar in physiological value is
in itself evidence that the density of the stand or number of individ-
uals per unit area tends always to adjust itself to the average amount
of moisture present, or perhaps even more closely to the absolute
“minimum amount reached during a long period of years. Therefore,
even if the several species showed no differences in their moisture
requirements or absolute drought resistance, the moisture content of
the soil would strongly influence the character of each forest type
by limiting the density wf the stand, and thereby affecting the
insolation, the evaporation, the soil temperature, and all other con-
tions of the forest floor, which in turn influence the character of the
reproduction. This only shows how closely interrelated are the
several conditions of the environment. The amount of insolation
fallne upon a site affects the conservation of the precipitation.
The conserved water, by determining the density of the stand in
turn, affects the amount of insolation, and so on, ad infmitum.

The important difference between the moisture conditions of the
spruce sites at one extreme and the pine sites at the other is not so
much in their absolute water contents, considering the entire soil
reservoir of each as in the fluctuations of their surface conditions.
The well-shaded spruce soil: may become very dry at the surface
when the atmosphere has been dry for a long time, but it does not
dry out severely between the rather frequent summer showers.

140 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

Hence spruce seeds have ample time tor germinating and are in no
haste to extend their roots deeply, often only penetrating the humus
layer and establishing contact with the mineral soil by the end of
the first summer. The rapid drying of a Well-insblated soul, on the
other hand, demands prompt germination and rooting of the seedling.
Yellow-pine seeds do not germinate more promptly than those of
the other Rocky Mountain species, but the seeds are large, the seed-
lings large and strong, and the root is often extended 3 or 4 inches
into the soil before the cotyledons appear above ground. Quite often
the roots of yellow-pine seedlings must extend to a depth of a foot
and below most of the grass and weed roots before a stable supply
of moisture is assured.

In all probability moisture becomes an important factor in limit-
ing the downward extension of the yellow-pine forest, though here
we have no proof that excessive temperatures may not as often be
the immediate cause of the death of seedlings. Moisture is probably
controlling where other vegetation is already established; heat is
probably controllmg on denuded sites.

CONCLUSIONS.

Each site in the various forest types of a mountain region such as
has been studied and reported in this bulletin by means of the several
stations presents a unique complex of all the conditions which com-
prise the atmospheric and soil Meanie Two sites may not vary in
a single one of the many conditions which are enumerated as wind,
humidity, temperature, soil moisture, etc., without having some
effect on the vegetation. In fact when the reproduction of a small
forest area is studied in detail most surprising and often unaccount-
able differences in the number, size, and species of the seedlings
erowing thereon may be noted in different parts of the area, even
though it be only a few yards in extent. It seems, therefore, almost
hopeless to attempt to define every factor which is accountable for
vegetation of a given type. The best that can be hoped for is that
the maxima or minima, or, in other words, the extreme conditions
of any nature which a given species will tolerate, may be depicted
by a study of the present kind, or by a series of such studies properly
correlated. To a limited extent this has been accomplished in the
discussion of each factor and in the recapitulation. Since it is a
common human desire to have complex matters made to appear
simple a single generalization may be indudged in, as a kind of sum-
mary, after which the exceptions may be noted and the specific
problems of each species may be defined as they are seen at this
stage.

A review of the facts that have been presented leaves little doubt
that the several tree species of the central Rocky Mountains are con-
trolled in their distribution almost wholly by the degree of insolation
of the site, the resultant temperatures, and the closely related surface
moisture conditions.

In a general way the forest zones correspond with air-temperature
zones, and the considerable differences between the air temperatures
of north and south exposures at the same elevation might explain the
corresponding differences in the forest types. However, even when
air temperatures are measured close to the ground, the critical differ-
ences between sites are not brought out. Air moves so freely from

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS, 141

one point to another that there can not be a great difference, for
example, between a sheltered and an exposed spot on the same rae ee
separated, say, by a distance of 50 feet. Yet it is obvious that the
insolation received on the one spot must have the greatest possible
influence on the critical conditions for the regeneration of forest trees.
Therefore, although air temperatures do generally outline the types
fairly well, one must conclude from certain important exceptions that
they reflect rather than define the controlling conditions.

The study has shown not only that soil temperatures at a depth of
1 foot bring out more clearly the differences in aspect, which for
denuded sites are fully as great as the differences resulting from eleva-
tion, but also that the correlation between soil temperatures and the
reproduction of the several species is closer than that between air
temperatures and species. The meager data on surface-soil tempera-
tures that are so far available indicate that the differences between
sites which encourage different species are far greater than might be
thought possible—at least three or four times as great as the differ-
ences between temperatures at a depth of 1 foot. To use again the
example taken above, the spot on a north slope which receives full
insolation for two or three hours in the middle of the day may have
surface maxima 20° to 25° higher than the nearby sheltered spot, and
a similarly exposed south slope may attain surface temperatures 30°
to 40° higher still. The surface ket pemaleess on south slopes, more-
over, occasionally attain to 150° F., a temperature which, it is
believed, is likely to be fatal to any young plant, independent of its
drying effect on either plant or soil.

he influence of direct insolation in drying the surface of the soil is
hardly less potent than its temperature effect in demanding special
adaptations of the plant. Again, direct insolation plays an important
part in keeping moisture available to both young and old trees dur-
ing the winter season, when it is likely to be completely withdrawn
through the freezing in the soil.

A frank consideration of the weak as well as the strong points in
the evidence which has been adduced in “ Physiological Requirements
of Rocky Mountain Trees” (6) and in the present bulletin, compels the
making of somewhat tentative conclusions as follows: ;

(1) Western yellow pine is not a conservative user of water, nor is
it efficient in photosynthesis. Itis deduced from this inefficiency that
it may thrive best in full sunlight and a warm atmosphere. ‘There is
certainly nothing in the evidence to suggest that it prefers a dry to a
moist soil. However, its seedlings have the habit of rooting promptly
and deeply, a habit which has doubtless been developed because of
the need of the species for a warm situation whose surface soil dries
out quickly. It may, then, be said unequivocally that yellow pine
seeks the warmest sites at middle and low elevations, because of their
warmth. With yellow pine at least, the evidence is convincing that
warmth of the soil in winter, and availablity of moisture, is a necessity
throughout the life of the tree.

At its lower limit yellow pine may come into competition with
pifion, or with grasses and other low plants whose life cycle is com-
pleted in the short period of mostfavorable moisture. Little is known
regarding pion, but it is the writer’s belief that this pine subsists on a
smaller water supply than yellow pine, and also that it is much less
sensitive than yellow pine to the high salt concentrations which are

142 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

ae in the poorly-watered and largely unleached soils of the
oothills.

(2) Douglas fir is often in close competition with yellow pine through
much of the altitudinal range of the latter. Douglas fir is undoubtedly
more “‘tolerant’’ of shade than the pine, is more effective in photosyn-
thesis, and uses less water. Douglas fir seedlings root. almost as
deeply as those of yellow pine in the first few weeks of life; but fir
seedlings are quite evidently more subject to directly injurious effects
from high temperatures, than are yellow pine seedlings of equal age.
Therefore, it is apparent that Douglas fir can tolerate almost the same
condition as yellow pine, and on a soil containing sufficient moisture
to support a moderately dense stand, Douglas fir may be the better
tree because of its greater photosynthetic capacity, and may even-
tually dominate the pine. But there is no point in the field evidence
clearer than the fact that Douglas fir seedlings will not tolerate full
sunlight on a slope where its heating effect is greatest. Therefore,
the fir can be a pioneer only on northerly aspects or in the shade of
such objects as boulders.

It would appear to be misleading to assume, in any instance, that fir
replaces pine as a direct result of favorable moisture conditions. Con- ‘
ditions may be conceived in which great moisture would so modify the
surface temperatures as to make possible fir reproduction in full and di-
rect sunlight, but under the conditions of the present study the moisture
factor is effective only through the production of canopies and shade.

(3) Engelmann spruce is even more effective in photosynthesis than
Douglas fir and uses its water more economically. The seedlings of
spruce, however, are very slow in placing their roots at a depth, seem-
ing to Cees the surface layer of the soil and to branch considerabl
in this layer. This is puzzling in view of the extreme dryness whic
is sometimes attained by a surface layer composed almost wholly of
only partly decomposed litter. It is possible, however, that the roots
do penetrate to the top of the mineral soil, with its more certain
moisture, and that the seedlings are satisfied with the small supply of
moisture imsured by this. Another puzzling fact is that spruce
seedlings appear to be less easily injured by excessive temperatures
than seedlings of Douglas fir, although when drying becomes a factor,
the reverse is true. |

From the present evidence the conclusion is plain that Engelmann
spruce pa a its cold sites and high elevations purely because of a
superior ability to grow with less direct hght and with lower tempera-
ture than are required by any of the other species. Again it is mis-
leading to speak of the direct effects of moisture as a determinant,
except in the sense that spruce seedlings are evidently not adapted to
situations in which the surface soil dries quickly. There is, however,
another soil factor which the writer believes to be very important,
and concerning which, it is hoped, convincing evidence may be
presented within a short time. ‘This is the factor of chemical com-
position of the types of soil commonly claimed so exclusively by spruce
or perhaps, to use a more familiar term, the factor of soil acidity,
although it is the present belief that spruce is equally tolerant of high
acidity and strong alkalinity. Without doubt, in some instances, it
is the chemical conditions of the soil, rather than moisture contents or
3p conditions, which draw a sharp line between spruce and Douglas

r forests.

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 143

(4) Lodgepole pine is little, if any, more efficient in sunlight than
ellow pine, Pub appears to be more conservative of its water supply.
The seedlings are as frail and shallow rooted as those of spruce, so
that they do not tolerate drying conditions. On the other hand, the
direct Efoct of heating seems to be less injurious with lodgepole pine
seedlings than with those of yellow pine, Douglas fir or spruce. An
additional factor in the life of lodgepole is the very sluggish germina-
tion of its seed except when stimulated by frequent and wide ranges
of temperature, as, for example, a daily range of 30° or more. This
characteristic appears to be the outgrowth of repeated regenerations
on open ground, where the temperatures are not modified by cover
of any sort, or perhaps are augmented by such an absorbent as char-
coal. In fact, every quality of lodgepole associates the tree with
pioneering in completely denuded ground. Even the slow rooting of
the seedlings may be seen to be adapted to a soil in which there is no
competing vegetation, and whose moisture has been made more
accessible by the destruction of the surface humus. But obviously
this slow rooting would be a very unfavorable characteristic in an
open situation winite the surface moisture was not frequently replen-
ished by showers, or where it was very effectively issatrerioil by
insolation, as in typical yellow pine sites.

Clements (10) has rendered a distinct service to students of forestry
by picturing vegetation as being constantly in a state of development
or fax as the result of changes in the site conditions and because of
‘the competition between species with different qualifications. In
somewhat the same vein, it may be best, since it has been shown that
the composition of any forest type is very much a matter of com-
petne elements, to consider now the typical situations which have

een studied at one or more stations, each in its ensemble, with the
hope of destroying any narrow or incomplete view which has been
created by considering the physical factors singly. This should be
done with the fact clearly in mind that the important differences
between sites arise almost wholly from differences in insolation.
Otherwise a very poor conception of causes will be obtained.

(1) A steep south slope at middle elevation.—The gradient is such
that at midsummer the sun’s rays fall almost normally on the surface.
The insolation may be rated as unity. As the resultant surface tem-
peratures are very high, frequently reaching 140°, and occasionally
150° or more, young succulent vegetation on the surface of the ground
might be almost instantly killed. There is an almost complete lack
of annual herbs. The rate of evaporation is very high. This rate
may be expressed as unity. The soil is almost excite in humus.
The surface dries quickly after showers, but owing to the unbroken
mineral soil, capillary action ordinarily keeps a reasonable amount of
moisture near thesurface. The soil has poor holding capacity because
of the lack of humus and also is lacking in nutrient salts. Conse-

uently, it can never support heavy vegetation. At the outset only
the sturdiest of seedlings can survive. if they start after midsummer,
they must develop rapidly both as to root and lignification, in order
to be prepared for the critical conditions of the next season. A
freely transpiring species is required, for transpiration may protect
the tender leaves from superheating as perspiration protects the
bodies of some animals. The ground will not freeze permanently af
at all; hence the seedling need not be inherently drought resistant,

144 BULLETIN 1233. U. S. DEPARTMENT OF AGRICULTURE,

that is, of high sap density. Yet deep rooting is essential in order
that any psi a supply of water may be had, and later extensive
rooting must be developed in order to insure the supply of water from
a soil of such meager capacity.

Either yellow pine or limber pine fits these conditions. The large
seed of either species is a fairly good guarantee of the vigorous initial
growth that is essential. Later, when the pines make shade, an
occasional Douglas fir may become established in the most shaded
spots. Limber pine will disappear if the stand becomes at all close,
because it can not compete with yellow pine in growth rate or ultimate
stature. Any species, with the possible exception of spruce, could be
planted here if sturdy young trees were used, for the only really
dangerous factor is excessive heat.

(2) A flat ridge.—The insolation is about 0.80 or 0.85. The highest
surface temperatures reach 120° to 130°, but they are not prohibi-
tive to tree establishment except as they induce rapid transpiration
and soil-drying. The wind exposure is greater than on the south
slope, and the evaporation is almost as high. Greater difficulty is:
experienced in forming any soil; all the humus is moved away and
there is little chance for any to be deposited. The same is true of
the finer particles of soil as the rocks break down; consequently there
is practically no soil over the rock. Very little water can be retained.
The forest must always be very open. The difficulties of initiation,
however, are not so great as on the south slope. Limber and yellow
pines predominate, but the yellow pines are lacking in vigor, being
evidently exposed to some Sam force they can not cope with.
This is probably the winter wind. The insolation is not enough to
keep the soil from freezing deeply, and the wind exposure is worse
than on any other site except a westerly slope. Some Douglas fir
seedlings appear, with a little protection, and they do not suffer
from the winter conditions.

(3) The middle of a north slope.—In heating effect the insolation
can not be rated over 0.50 at midsummer, and may dwindle almost
to zero in winter. Hence the striking characteristics of the slope
are ability to retain moisture and a long unbroken period of freezin
during the winter. As measured just above the surface on a denude
site, the insolation and evaporation may at times be nearly of unit
weight; but of course the evaporation may become negligible when
the sun is very low. The conditions for soil building are almost
ideal. On this kind of a site there is adequate heat and moisture
for the early development of any species. The retention of a snow
blanket in winter practically eliminates exposure and further encour-
ages a mixture of all the species. Only spruce may at first be pro-
hibited, and until other vegetation shades the ground and modifies
the superficial drying of the soil.

The large quantity of moisture present encourages a heavy vege-
tation of all sorts and quickly changes the sesieddees site to a thicket.
The later conditions are radically different from those which initiate
the succession. Direct light, while not entirely wanting during the
growing season, is at a premium. Only the species which are more
effective in photosynthesis can develop properly. At first Douglas
fir will predominate, simply because spruce was handicapped at the
eutset. In another generation without disturbance, the spruce will

1S

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 145

_ be se auew or more abundant. The disappearance of the yellow pine
_ and limber Lear which at the outset found the conditions favorable
_ is a slow and almost pathetic process. They do not, except in infre-
_ quent years, suffer for lack of moisture; and as their crowns rarely
appear among the dominants they do not even suffer the most severe
_ winter drying, which is very rigorous for the tallest trees on this site.

Unlike the suppression which is so common in crowded stands on

less moist sites, which takes the form of complete stunting, the

strictly shade suppression here results in putting all of the energy
of the tree into hbieht growth, the side branches being quickly
_ eliminated, and no food being left for the proper strengthening of the
stem. The result is a long, slender, supple pole, with only a tuft of
green at the top, which is invariably a little lower than the tops of
the surrounding firs. Limber pines and yellow pines which reach
this stage are usually destroyed by snow breakage.

(4) The foot of the north slope.—The steepest portion of a slope is

very commonly just below the middle. Further down, deposition
of material from the upper slope may more than keep abreast of
erosion. Hence the ground tends to flatten out, and frequently a
bench is formed somewhat above the stream bed. For a considerable
part of the year the site may be shaded by the steeper ground to the
south. It receives by transport the best of the soil formed on the
entire slope; very commonly it receives also the run-off from heavy
rains and, with some rock formations, the seepage. There is, then
a greater water supply than elsewhere, a deep, spongy, retentive soi
and little insolation to dissipate it.
_ Probably because such a site is a catch-all for seeds as well as for
soil-building material, it can not long remain denuded. Rank herbs
appear almost immediately, and tree seedlings rarely start except
with the keenest competition for light. Year after year the herbs
grow up about the seedlings, and there is at no time opportunity for
any but the most shade-tolerant trees. Even these have a very
long struggle before they eventually find “their place in the sun.”
The conditions, of course, do not change materially after a forest is
established. There will always be a dearth of light. If the stand
is very dense, in a protracted drought there may be a fairly complete
exhaustion of the moisture as deeply as the roots penetrate. The
most prohibitive condition, however, probably arises from the undis-
turbed accumulation of litter on the forest floor. After the snow has
melted in the spring, and this loose, jack-straw mass has slowly dried
out, 1t will in all probability not become wetted again until the next
snow falls. Certain greenhouse tests made by the writer have indi-
cated that the refusal of resinous needle litter to umbibe water creates
a serious problem in connection with the germination of seeds. In
consequence, a large part of the floor under spruce may be as barren
as the desert, and it is especially with such conditions that the marked
results of any soil disturbance, such as occurs through the skidding
of logs, are to be noted in succeeding reproduction.

The description here given refers in a degree to the lower portions
of all north slopes, and commonly to that portion of the stream bench
contiguous thereto. It is, of course, evident, that in so far as the
slope itself is short or moderate in gradient, it may fail to produce the
depicted conditions at its foot, and under certain circumstances
there may be no area in this position that would be prohibitive to

73045° —24——_10

146 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

Douglas fir or even to yellow pine. The Black Hills in general, being
“hilly”? rather than mountainous, encourage yellow pine on all sites;
but in the more mountainous portion, and where the erosion of sedi-
mentary formations has resulted in long and steep slopes, the Black
Hills spruce is found, as might be expected from this description.

In order to complete the picture of a valley cross-section, it is only
necessary to add that, except in narrow canyons whose wails shade a
great area, the creek bottom facing the south is likely to be of a very
different character. That area which is not shaded when the sun is
highest will immediately take on the character of the south slope, as
described in paragraph 1, with this important exception, that the
soil deposition, seepage, and run-off easier this area may receive, May
encourage a denser stand than characterizes the upper slope, and in
this denser stand it is not uncommon to find stragglers of the more
tolerant Douglas fir, and even of spruce.

(5) The east slope at middle elevations.—As has been clearly shown,
the east slope in the Pikes Peak region receivés a large proportion of
the direct insolation, for the simple reason that the mornings in
summer are generally cloudless. This means that surface tempera-
tures may be attained almost, if not quite, as high as on south
exposures, it beg understood that the maximum effects of insola-
tion are approached very quickly. This direct insolation is, however,
of less duration than on south slopes, and its drying powers are pro-
portionately less. The evaporation may be rated between 0.70 and
0.80. The result is of course a better conservation of moisture.
This is without doubt augmented by a fairly good soil, built up by
the wind deposits of material from west exposures.

The existence of direct insolation, creating excessive tempera-
tures in the morning if only for a short time, means that the pioneer
trees must be largely yellow pines. The conservation of moisture
however, permits heavier stands than can be supported on south
slopes. The result is that on easterly exposures the succession of
pine by Douglas fir is more prompt and more certain than on south
exposures.

Under certain conditions there may even be no pine stage. A good
illustration is that of an extensive east slope below a middle elevation,
which after being denuded by fire produced directly a heavy stand in
which fir is much more prominent than yellow pine. ‘This entire
slope, however, is boulder-strewn, and these boulders perform the
same function as pine trees; that is, they supply the necessary shaded
spots for fir seedlings. The boulders are much to be preferred to
pines as nurses, because they use no water. ‘This phenomenon may
occasionally be noted even on south exposures.

(6) The west slope-—The winter winds make this aspect by far the
most difficult of any at middle elevations to cover with vegetation of
any sort. A west roti does not in the aggregate receive so much
insolation as an east slope, yet on exceptionally clear summer days it
may easily become very warm, and is with certainty fitted only for
the limber pine and yellow pine. In summer the evaporation rate is
moderate, and the moisture conservation would be good were there
any soil to retain it. In winter the evaporation rate is nearly as great
as on a south exposure. Probably a more severe strain is presented
here because the soil is thawed much less frequently. ong with
the desiccating effects of the west wind, which is a factor only in

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 147

winter, must of course be mentioned its ability to remove the soil-
building materials.

The physiological studies have indicated that limber pine is as low
as yellow pine in photosynthetic capacity, and perhaps lower; yet a
more important factor in its distribution is the probable fact that it
never exposes itself to rapid water losses through the stomata. Lack-
ing the vital qualities which make our forest trees what they are, this
weed is apparently satisfied to “close up shop” whenever the condi-
tions are in the Tanke unfavorable, and thereafter it is practically
immune to desiccation by wind. It 1s for this reason that limber pine
becomes established on west slopes where yellow pine can not.
Although it is a valuable pioneer, it is intrinsically so worthless that
its tenure might well be temporary. Unfortunately, a cover of limber
pine on a westerly slope is usually scant, and neither the wind
exposure nor the insolation is very much modified thereby... So far
as the writer’s observation goes, only spruce may be expected to
supplant the limber pine, and this probably so slowly that centuries
may be consumed in the succession.

(7) Lower elevations.—The general relationships among the six
sites described above, must of course prevail at any elevation. The
fact of the matter is, however, that even when, topographically
speaking, these sites are duplicated near the foot of the mountains,
che conditions are found to be very different. At the foot of the
mountains there is less precipitation; higher temperatures prevail the
year round, tending further to dissipate this precipitation; the atmos-
phere is relatively drier at the eastern foot; and perhaps most impor-
tant, the higher air temperatures further augment insolation in pro-
ducing unbearable conditions at the surface of the ground. To a
certain extent these higher temperatures must be counterbalanced
by weakened light intensity.

The effect of less precipitation and its more rapid dissipation must
of course be to make impossible on any site such dense stands as
prevail at higher elevations. When to the greater openness of stands
is added the greater heating opportunity per unit of area, it is readily
seen why there should be, first, the entire elimination of the heat-
sensitive Englemann spruce, with Douglas fir occupying the most
sheltered sites, then the confinement of the forest to northerly
aspects, and finally its disappearance where the mountains merge into
the plains.

It is not necessary or relevant to do more than mention blue spruce
(Picea parryana), white fir (Abies concolor), or pifion pine (Pinus
edulis), which are evidently more heat-tolerant than their mountain
counterparts and succeed in slightly extending the group ranges.
These trees have little place in the economy of the forest and have
been given no careful study.

There is not much question in the writer’s mind that soil quality is
an important factor in the sudden termination of the forest at low
elevations. Not only in the Pikes Peak region but generally through-
out the Rockies, the mountains proper are composed of igneous rocks
and the foothills of sedimentaries. ‘The former produce open, more
or less gravelly, well-drained soils, the drainage being further insured
by the strong relief. The sedimentaries produce fine, compact soils;
and as the foothills are almost invariably devoid of springs and per-
manent streams it is evident that their soils are sea cy subjected to

148 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE,

leaching. Such soils are much more fertile than the mountain soils;

but if they were equally moist at all times they would still present a

more arid condition in the physiological sense. There are still two
other aspects to the matter. The fine, compact soils, when rain is
withheld, dry out more conapletely and more deeply than mountain
or sandy soils; consequently they may be not only more arid but more

liable to super-heating. Again, when the plain is reached the sta- |

bility of such soils precludes any denudation and permits the firm
establishment of perennial grasses; consequently competition alone
would preclude the establishment of tree seedlings.

When, therefore, the question is raised whether it is lack of precipi-
tation or heat which excludes the coniferous forests from the plains,

the answer may be given unequivocally that it is both, and that it —
is also alkalinity of soils and competition. It is nature’s unanimous ~
verdict. Yet it is to be noted that with the aoe exception of ©

alkalinity, none of these conditions need absolutely nhibit tree growth.
For this reason, wherever there is a reasonable amount of relief and
nature has had extreme difficulty in establishing tree seedlings,
planting may still be conducted with a high degree of success.

(8) Higher elevations—On the several sites at higher elevations
there is greater light intensity, modified in its effects by lower air
temperatures and increased precipitation and conservation. It would
seem, therefore, that the conditions were exactly the opposite of
those at low elevations. However, there is an important exception
in the fact that wind movement in the higher atmosphere is much
freer than near the general surface of the earth, and there is no site
at high elevations which is entirely free from this added wind effect.
This tends to make the evaporation rate relatively high.

Because the coolness of the air is largely, if not fully, compensated
by greater intensity of the sunlight, it is not surprising to find that
our light-demanding pines invade denuded areas, yellow pine to an
elevation of 10,000 feet and limber pine up to timberline. About
where yellow pine fails bristlecone pie may be expected to take its

lace. :

, Quite different, however, is the behavior of Douglas fir, which in
the Pikes Peak region is not seen much above 9,300 feet elevation.
Apparently there is no place for this species where conditions are so
readily made suitable for spruce by a minimum amount of shade.
It is not the intention to say that in an early stage of succession
there is no favorable place for Douglas fir but rather that in the
final stage there is no place, and having been crowded out of the last
climax forest it has been unable to retrieve its position since the fire
that occurred about 60 years ago.

At the higher elevations the completely denuded areas are invaded
by yellow pine, limber pine, and bristlecone pines; but, with the
possible exception of a few rocky ridges incapable of supporting many
trees, every site whose elevation is more than about 10,000 feet must

eventually be conquered by spruce. On much of the area no other |

conifer is found. A quick growth of aspen sprouts after a fire is
ossible on almost any site, and these form abundantly heavy shade
for spruce seedlings.
(9) About lodgepole pine sites, which do not exist in the Pikes
Peak region, it is necessary to say a word because of the close relation
of lodgepole pme in many localities to the more valuable species.

tliat

FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 149

This species, like the other pines discussed, is evidently light-demand-
ing; but, unlike the other pines which through long ages have
adapted themselves to the surface drying which results from direct
insolation, the germination of lodgepole seeds is slow, and the devel-
opment of root in the young seedling is feeble. For this reason
lodgepole commonly reproduces on fully insolated sites only where
there has been complete denudation, as by fire, and where its moisture
supply is made as certain as possible by the destruction of its com-
petitors. More frequently than not, mature lodgepole stands are so
open as to give plenty of light for germination; but as the open stand
results from limited moisture it follows that the seedlings may
develop only very slowly and may be wiped out in drought years.
The greater the moisture and the denser the lodgepole stands, the
more certain it is that spruce seedlings rather than lodgepole will

redominate in the reproduction, and that the ground can be regained
ae this more valuable tree. On the other hand, on drier ground
which probably once belonged to Douglas fir, there may be much less
opportunity for the fir to take possession primarily because of its
very inferior seeding capacity. For this reason much of the ground in
this region which is capable of supporting high-quality fir stands will
be very slow in returning to that species.

That lodgepole pine is almost completely absent from the imme-
diate territory of ee study appears to be due to a dry atmosphere,
a loose, rapid-drying surface soil, and hence less favorable condi-
tions for germination than it demands. On sites where the conditions
for germination might be favorable,.it is possible that soil freezing
may be prohibitive. The growth of lodgepole trees planted on yellow-
pine sites in the Pikes Peak region has been excellent, but establish-
ment by direct seeding has been accomplished only in a few favored,
though warm, spots.

APPLICATIONS.

The value of a more exact knowledge of the forest trees with which
we are dealing in the National Forests of the central Rocky Mountains
should be evident to anyone at all familar with the problems of
forestry. To what extent knowledge of any fact may affect human
plans and activities can never safely be prognosticated. Yet it is
safe to point out some of the benefits that may be had immediately
as a result of facts brought out by this study or previously discovered
and, perhaps, confirmed by the large amount of data here presented.
It is necessary, of course, to refrain from generalizing; yet it must
be apparent to foresters in other regions that in each forest region
there are counterparts of the species here discussed. It may, there-
fore, be helpful to understand the relations between species as they
have here been depicted.

Neither the physiological investigations nor the field data secured
in the present study mdicate any marked difference between the
species in actual drought resistance; and by this is meant ability to
extract water from the soil when the condition of physiological dry-
ness is bemg approached. If there is any such difference, it is
theoretically in favor of the species which are commonly thought of
as demanding moist sites, but which in reality demand cool sites,
namely, spruce and fir. On the other hand, a species like spruce
which is accustomed to moist, rich souls and develops a compact root

150 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

system, may be able to drain the moisture of the soil more completely,
because from the outset its roots are more finely divided, and they
more completely reach all of the moisture. The tree accustomed to
a dry soil insures its moisture supply by reaching into an extensive
area.

There has been shown to be, however, a very great difference be-
tween the species in photosynthetic efficiency, in ability to utilize
whatever of light may be available, in sensitivity to high tempera-
tures and drying which may result from direct insolation, and, finally,
in the efficiency with which water is used. The relations of the sey-
eral species to light are important silviculturally. The fact that one
species may utilize 10 per cent of the energy of incident sunlight in the
production of carbohydrates, and 90 per cent in the wasteful evapora-
tion of water, while another utilizes only 5 per cent of the energy of
incident sunlight, is of the vary greatest importance economically.
The species which nature has developed to serve as the climax of any
forest succession is the most highly developed plant organism of that
association, and in all probability is the species which may most fully
serve human needs, se leks in the sense 0 Sl erat production. - For
a great many regions, as well as for the one here discussed, this species
will probably prove to be spruce.

The most obvious application of the results of this study is that in
the reforestation of denuded areas or the afforestation of such areas
as our sand-dunes, it is possible to advance nature’s process by at
least one successional stage. or example, a denuded, strongly inso-
lated site, which is known to have borne in the past a stand of Douglas
fir, but which, from direct or indirect knowledge it is known will first
be stocked with pine, is under no necessity whatever to go through
the pine stage. The area may be immediately stocked with fir, with
very great assurance of success, because the 3-year-old trees which
may be planted are not susceptible to the heat and drought of the site
which would be fatal to young seedlings. To a certain extent, on the
same basis, a given species may be planted on a somewhat lower,
warmer, and drier site than would naturally, at the same stage, repro-
duce that species.

Not only are these things possible in reforestation, but they are
obligatory upon foresters, in order that the land may be put to the
highest use, and the moisture, almost always limited in quantity, may
be utilized by the most efficient of the organisms capable of existing
under the given conditions. Exception must of course be made
where a certain species is most desirable for its technical value.

In the management of forests for continuous production, which
always implies the securing of natural reproduction, the realization
that the nature of the reproduction secured will depend almost en-
tirely on the conditions of insolation must be very helpful in deter-
mining the weight and character of the cutting. It should be kept in
mind that, while the cutting of undesirable species may reduce the
number of seedlings of that species which will appear, it 1s no guaran-
tee whatever as to the identity of the seedlings that shall survive.
That will be determined by the light and heat conditions which result
from the cutting in the aggregate. In general, all cutting in climax
forests must tend to encourage the species of the successional order
next below the climax, and in clear cutting the result may be a rever-
sion of two or three stages. The maintenance of the climax forest may

«
FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 151

be anticipated only by very light cutting, except of course in the
upper spruce zone where there are no subclimatic trees.

n the lower part of the spruce zone, too heavy selection cutting is
almost certain to encourage Douglas fir seedlings where that is the
species next in order, and lodgepole in the region where this tree has
invaded the Douglas fir and lower spruce zones.

Much of the lodgepole forest would be replaced by spruce, and
other portions by Douglas fir, if the lodgepole were left uncut or were
thinned very bicelvth The heavier the cutting, the more certain it is
that lodgepole pine will continue to hold the ground.

Nearly all of the Douglas fir forest of the central Rockies, if cut too
heavily, is subject to invasion by western yellow pine or the worthless
limber pine. However, on northerly exposures, there is much less
danger of this, and a heavy cutting under the shelter-wood plan ap-
pears most likely to give prompt and desirable results.

As the less severe yellow-pine sites will in the natural sequence of
events go over to Douglas fir, there is no very evident reason why
Douglas fir seedlings should not continue to come in with the protec-_
tion of their parents only, if it should seem desirable to remove the
yellow pines, provided only the stand is not too greatly opened. A
much more serious problem is presented in the lower portion of the yel-
low-pine type, where the stands are always so open as to permit the
formation of sod, and where any new opening may be quickly occu-
pied by the perennial grasses. Silvicultural caution demands that
cutting be done only when there is a seed crop. A favorable year fol-
lowing can of course never be guaranteed. Evidence in the Southwest
to the contrary notwithstanding, the writer is firmly convinced that
the perpetuation of the mountainous yellow-pine forest in this region
can best be insured by the burning of the brush in piles, a measure
which, as everyone knows, eliminates the grasses for a period of sey-
eral years and gives the pine seedlings at Teast an equal opportunity
with other vegetation.

On the general subject of brush ee this study appears to have
offered only one other suggestion. It is believed that, where spruce
forests grow on moderately steep to very steep slopes, both the nature
of the ground and the relative openness of the stand prevent exces-
sively heavy accumulations of needle litter, which, an attempt has
been made to show, may afford a very unfriendly seedbed. On such
slopes there is likely to be advanced reproduction, and burning of the
brush is neither called for nor desirable. On the contrary, on flats
which usually produce the most superb spruce forests, very serious
delay of reproduction is likely to result, even after heavy cutting, un-
less some spots are laid bare. Furthermore, the accumulation of litter
should by no means be added to. Here the burning of brush is dis-
tinetly called for. There is some evidence that this may also reduce
the percentage of alpine fir (Abies lasiocarpa) seedlings, which are
more vigorous than spruce seedlings and establish more certainly in
deep litter. |
LITERATURE CITED.

(1) Baxsr, O. E., and Fincu, V. C.
1917. Geography of the World’s Agriculture. U. S. Dept. of Agr.,
Office of Farm Management.
(2) Barngss, C. R.
1902. The Significance of Transpiration. Science, n. s., v. 15, no.
377. (Abstract.)

;

152 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE.

(3) Bates, C. G., Norestern, F. B., and KEpLinGER, P.
1914. Climatic Characteristics of Forest Types in the Central Rocky
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(4)
1917. The Réle of Light in Natural and Artificial Reforestation.
Jour. of Forestry, v. 15, no. 2.
(5)
1919. A New Evaporimeter for Use in Forest Studies. U.S. Dept. of
6) Agr., Wea. Bur., Mo. Weather Rev., May.
6

1923. Physiological Requirements of Rocky Mountain Trees. U. S.
: Dept. of Agr., Jour. of Agr. Research, V. XXIV, no. 2.
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Concentration of the Soil Solution Directly in the Soil. Mich.
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(9) CLrements, F. E.
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1913. A Meteorological Study of Parks and Timbered Areas in the
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(18)
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Western Yellow-Pine Saplings in Arizona. Jour. of Forestry,
v. 16, no. 6.
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O