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Full text of "Climate and plant growth in certain vegetative associations"

A erric.- Forestry. Main Library 



UNITED STATES DEPARTMENT OF AGRICULTURE 
BULLETIN No. 700 



Contribution from the Forest Service 
HENRY S. GRAVES, Forester 



Washington, D. C. PROFESSIONAL PAPER October 14, 1918 

CLIMATE AND PLANT GROWTH IN 

CERTAIN VEGETATIVE 

ASSOCIATIONS 

By 

ARTHUR W. SAMPSON, Plant Ecologist 
Forest Service 

CONTENTS 



Page 

The Problem. . ." 1 

The Experiments . . 3 

Preparation of Plants 4 

Planting 5 

Measurement of Plants 14 

Experimental Error 14 

Measurement of Physical Factors . . .16 



Page 

Comparison of the Climatic Characteris- 
tics of the Three Plant Types ... 27 
Correlation Between Growth and En- 
vironmental Factors 41 

Summary 69 

Conclusions 71 




WASHINGTON 

GOVERNMENT PRINTING OFFICE 
291$ 



"V3 



A jrric,- Forestry. Main Library 



UNITED STATES DEPARTMENT OF AGRICULTURE 

BULLETIN No. 700 




Contribution from the Forest Service 
HENRY S. GRAVES, Forester 




Washington, D. C. PROFESSIONAL PAPER October 14, 1918 

CLIMATE AND PLANT GROWTH IN CERTAIN 
VEGETATIVE ASSOCIATIONS. 

By ARTHUR W. SAMPSON, Plant Ecologist, Forest Service. 1 



CONTENTS. 

Page. [ Page. 

The problem 1 i Comparison of the climatic characteristics of 

The experiments 3 | the three plant types 27 



Preparation of plants 4 

Planting 5 

Measurement of plants 14 

Experimental error 14 

Measurement of physical factors 16 



Correlation between growth and environ- 
mental factors .' 41 

Summary of the data obtained 69 

Conclusions ... 71 



THE PROBLEM. 

The relation of climate to the growth and development of vegeta- 
tion is of profound importance in both practical and experimental 
agriculture. It is extremely useful to know the cause of successful 
growth and establishment, or of partial success or failure, of various 
species in different plant associations and under widely contrasted 
climatic conditions. The climatic requirements of various plant types 
are largely responsible for the results obtained in the case of experi- 
mental seedings and plantings of most species. Once the adverse 
climatic factors are definitely known, failures with plants may be 
largely avoided by the judicious selection of sites or of species espe- 
cially adapted to withstand the limiting factors. Therefore, a series 
of experiments was undertaken, (a) to obtain a comparison of the 
climatic requirements of the main plant types, and (b) to determine, 
quantitatively, the relation between various environmental factors on 
the one hand and plant growth and certain other physiological func- 
tions on the other. The results obtained appear to be conclusive 
in most instances and should prove of value both in experimental and 
in practical agriculture and forestry. 

1 The author is indebted to F. Merrill Hildebrandt for material assistance in procuring 
and assembling the data presented in this paper. 

56866 18 Bull. 700 1 

610333 



2 BULLETIN* TOO/' IT.' '&. DEPARTMENT OF AGRICULTURE. 

It ife'V Rlatcer* oi cdrnmbn knowledge that the life cycle and struc- 
tural characteristics of plants are largely determined by the climatic 
conditions prevailing in the habitat, but the quantitative relations 
existing between the potent climatic factors and the vegetative activi- 
ties are not well known. Though the ecologist and plant geographer 
have shown that a given plant association may have well-defined 
geographical limits, 1 which in turn are characterized by rather dis- 
tinct complexes cf environmental (climatic) conditions, they have 
not as yet definitely determined which climatic factor, or set of fac- 
tors, is most influential in affecting distribution, growth, and physio- 
logical activities generally. 2 This is attributable to several condi- 
tions. In the first place, the relation of plant development to envi- 
ronment is exceedingly complicated and can be determined quanti- 
tatively only when the most influential physical factors are recog- 
nized, recorded, and properly interpreted. Secondly, the climatic 
factors of a given habitat, and, indeed, of different habitats, which 
have to do with the limitation of the life process, are in themselves 
more or less indefinite; they are highly complicated and variable, 
and their intensity can not always be measured fully by instruments. 
In the third place, methods have not been sufficiently advanced to 
warrant serious investigations. Owing to present lack of knowledge 
of the response of plant activities to climate, there is wide diversity 
of opinion as to how best to summarize and integrate climatic data. 
Temperature studies conducted by Livingston, 3 Lehenbauer, 4 Mer- 
riam, 5 and McLean, 6 and researches on soil humidity and " growth 
water" carried out by Briggs and Shantz, 7 Shreve, 8 Fuller, 9 and 
others have shown that climatic factors can not to advantage be 
expressed empirically. Suitable methods of integrating the potent 
climatic factors, as well as of recording growth and other plant func- 

1 Drude, O. Entwurf einer biologischen Eintheilung der Gewachse. (A. Shenk, Hand- 
buch der Botanik, III, p. 487.) 

2 As a preliminary study it would be desirable to reduce the complexes of the environ- 
mental factors to their simplest form, which, under controlled conditions, might be ac- 
complished by maintaining constant all but the factor investigated, and in this way 
determining the effectiveness of each. This being done, however, the combined influence 
would have to be integrated in order to approach conditions in nature. Further, since 
under natural conditions climatic factors vary widely, both in intensity and in dura- 
lion, such important variations must necessarily be included in the equation. 

3 Livingston, B. E. and G. J.,' Temperature coefficients in plant geography and cli- 
matology. Bot. Gaz. 56: 346-375. 1913. 

4 Lehenbauer, P. A., Growth of maize seedlings in relation to temperature. Physiol 
Res. 1 : 247-288. 1914. 

8 Merriam, C. Hart, Laws of temperature control of the geographic distribution of 
plants and animals. Natl. Geog. Mag. G : 229-238. 1894. 

'McLean, F. T., A preliminary study of climatic conditions in Maryland, as related 
to plant growth. Physiol. Ros. 2 : 129-207. 1917. 

7 Briggs, Lyman J., and Shantz, H. L., The wilting coefficient for different plants and 
its direct determination. U. S. Dept. Agr. Bui. No. 230 : 7-77. 1912. 

8 Shreve, F., Rainfall as a determinant of soil moisture. Plant World. 17 9-2<> 
1914. 

9 Fuller, George D., Evaporation and soil moisture as related to the succession of plant 
associations. Bot Gaz., 58 : 193-234, 1914, 



CLIMATE AND PLANT GROWTH. 3 

tions of comparable or " standard " plants developed under the par- 
ticular climatic conditions summarized, are essential steps in a 
successful investigation 

THE EXPERIMENTS. 

The investigations here reported were conducted in the vicinity 
of the Great Basin Forest Experiment Station, located on that part 
of the Wasatch Mountains embraced by the Manti National Forest in 
central Utah. Here, from the foothills to the highest elevations, be- 
tween altitudes of approximately 7,000 and 11,000 feet, three distinct 
plant associations (identical in this locality with vegetative types or 
life zones) occur. In the heart of each of these associations a type 
station was selected in 1913. From 1913 to 1916 the more important 
environmental factors were recorded, and, accordingly, the climatic 
characteristics of each type-zone are well known. The investigation 
of the influence of the weather upon the development of comparable 
plants, with which the present paper is chiefly concerned, was begun 
in 1915 and continued and extended in 1916. The types here recog- 
nized and their approximate altitudinal limits are as follows: 

Sagebrush-rabbit-brush association feet 5, 200- 6, 500 

Oak-brush association do 6, 500- 7, 800 

Aspen-fir association do 7, 500- 9, 500 

Spruce-fir association __ __do 9,000-11,000 

As indicated by the plants typifying the respective associations, all 
but the lowest are forested. No special investigations were con- 
ducted in the treeless type. 

The meteorological stations are located at elevations of 7,100, 
8,700, and 10,000 feet. They are all in the same canyon, and the 
distance between the lowest and the highest stations in an air line 
is approximately 5 miles. Owing to the possibility of the results 
being influenced by the presence of trees and other objects in the 
vicinity of the physical instruments and growing plants, the stations 
are all located in the open, on slopes dipping slightly to the south, 
and no vegetation is so close as to cast shadows on the instruments or 
potometers, except for a few minutes at sunrise and sunset. Also, 
the instruments and plants are placed as near together as practicable 
(each type station covering one twenty-fifth of an acre), so that the 
conditions recorded may be practically identical with those acting 
upon the plants. 

The investigations have been concerned chiefly with ( 1 ) recording 
and summarizing the meteorological data, and (2) determining the 
relation of certain potent weather factors to growth, water require- 
ment, and certain other physiological functions of standard plants 
developed under different climatic conditions. Measurements of 



4 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 

growth and certain other activities were recorded from time to time 
throughout the season. The plants used in each station were a pedi- 
greed strain of Canadian field pea (Pisum arvense) known as the 
Kaiser variety, cultivated wheat (Triticum durum) known as Ku- 
banka No. 1440, and mountain brome grass (Bromus marginatus) 
native to the Rocky Mountains. The seed was supplied by the United 
States Department of Agriculture and was of good viability. 

PREPARATION OF PLANTS. 

In 1915 seed of the plants grown in the three type stations as 
climatic " integrating instruments " was planted directly in poto- 
meters, without previous germination. In order to insure as prompt 
and uniform germination as possible, the seed, prior to planting, was 
soaked for 36 hours in water of approximately 65 F. The object 
of this was to start the plants at as nearly the zero point of growth 
and development as possible. 

Direct seeding, however, did not prove entirely satisfactory, chiefly 
because of the lack of uniformity in size and vigor of the resulting 
sprouts. In the absence of a known method of selecting seeds which 
would produce comparable plants, the seed used in obtaining standard 
plants for investigations in 1916 was first germinated and then such 
sprouts as appeared to be of the same size and vigor were selected 
for planting. The sprouts were secured by a method which was a 
modification of the methods employed by Schreiner and Skinner 1 
and other workers of the United States Bureau of Soils. The 
procedure was as follows : The seeds were disinfected for 15 minutes 
in a 1 to 500 solution of formaldehyde in water. Following this 
they were washed thoroughly and soaked for 36 hours at about 65 F., 
and then placed in a germinator consisting of a bed of sand over 
which two moist blotters were laid. The soaked seeds were placed 
between the blotters and a constant water level was maintained in the 
bed of sand, by means of a Marriotte flask, at such a point that the 
blotters were kept well moistened but not flooded. 

When the radicle was well formed the germinating seed was trans- 
ferred to a second germinator. This consisted of a circular granite- 
ware pan, 12 inches in diameter and 4 inches deep, the surface of 
which was covered with waxed (mosquito bar) netting held slightly 
above the surface of the pan by a glass rod 5 mm. in diameter, so 
bent as to form a frame. Into the pan a continuous flow of tap- 
water, the surface of which touched the netting, but never flooded it, 
was allowed to run. The radicles were inserted through the mesh, 
leaving the body of the seed partly dry. When the shoot had de- 

1 Schreiner, O., and Skinner, J. J. Some effects of a harmful organic soil constituent. 
U. S. Dept. Agr. Bur. Soils Bui. 70, 1910. 



CLIMATE AND PLANT GROWTH. 5 

veloped to a length of about 2J inches the seedlings were transferred* 
to the receptacles in which they were grown to maturity or until 
killed by frost. 

PLANTING. 

In order to insure luxuriant and healthy development of the plants, 
those observed throughout the season were grown in substantial heavy 
galvanized iron potometers. 

To protect them from injury by animals, hail, etc., the plants were 
grown under wire screen of the mesh usually used on screen doors, 
supported by light wooden frames. These screen frames decreased 
the light intensity between 40 and 50 per cent. 

Water was added to the potometers as needed, the need being de- 
termined by the weight of the cans. In no case was the soil allowed 
to dry to a point approaching closely its wilting coefficient, nor was 
it at any time flooded. In watering, the potometer was brought up 
to its original weight. The first watering was done about a month 
after planting and the second 15 days later. From then on it became 
necessary to add water about once a week in all stations and oftener 
in the drier' situations. 

THE POTOMETERS. 

The potometers were 17 inches high and 14 inches in diameter, 
and had a capacity of 90 pounds of air-dry soil, which provided a 
soil mass at all times affording ample space for the proper develop- 
ment and spread of the roots. The cans were fitted with lids of 
the same material as the cans, and five holes, f of an inch in dia- 
amter, were punched in each for the plants. (Fig. 1, top view.) 
In the center of the cover a hole L| inches in diameter was provided, 
which was used in watering and was fitted with a cork stopper and 
a capillary tube bent at right angles. 

Before placing the lid, sufficient soil was removed in the center of 
the can to make room for a granite-ware receptacle 4 inches in height 
by 5 inches in diameter, perforated centrally in the bottom and un- 
derlaid with 1^ inches of gravel, as shown in the sectional view of 
figure 1. This greatly facilited the addition of water. To add the 
water, a flask of known capacity was inverted and the water per- 
mitted gradually to percolate into the soil. 

After the lids were placed, the spaces between the rims and cans 
were closed by securely sealing them over with strips of surgeon's 
adhesive tape 2^ inches in width. The adhesive tape was then coated 
with shellac to prevent its loosening when wetted by rain. The 
method used in sealing and watering the plants was one devised by 
Briggs and Shantz, 1 modified somew^hat to suit special conditions. 

1 Briggs, Lyman J., and Shantz, H. L. The water requirements of plants. U. S. Dept. 
Agr. Bur. Plant Ind. Bui. 284:8-14. 3913. 



BULLETIN *ZOO, U. S. DEPARTMENT OF AGRICULTURE. 




Top View 




Sectional View 

FIG. 1. Potometer used in growing plants to determine their water requirements. - 



CLIMATE AND PLANT GROWTH. 7 

THE SOIL. 

The soil surface exposed by the perforations in the lids was pro- 
tected from evaporation by a thin layer of wax consisting of a mix- 
ture of 3 parts of tallow and 7 parts of beeswax, applied in a melted 
condition. 

The soil of the region is, in the main, of limestone origin, conse- 
quently soil of that type was selected. Except for the purpose of 
determining the relation of plant growth to soil fertility the soil 
used was taken from the upper 6 inches in a single situation in the 
aspen-fir type, and was uniform throughout all the potometers. In 
order to eliminate pebbles, roots, and other decomposed organic 
matter the soil was sifted through a J-inch wire mesh. Because of 
the presence of a large amount of clay the native soil is not so porous 
as was desired, and for this reason sand was mixed with it in the 
proportion of 5 parts of soil to 1 part of sand. The soil used was 
rich in humus, 5 samples averaging 12 per cent by weight after mix- 
ing with the sand. The addition of the sand reduced the wilting 
coefficient somewhat, the average being approximately 15 per cent. 

After the soil was thoroughly mixed in the air-dry state, water 
was sprinkled over it until it had a " fresh " consistency ; that is, 
the particles adhered in a lump when squeezed in the hand. Soil 
samples taken from each batch of soil after mixing and watering 
were found to average 31 per cent humidity, the variation being from 
28 to 34 per cent. The moist soil was moderately tamped in the 
potometers, so as to prevent breaking of the roots by cracking and 
settling of the soil when .drying. The weight of the moist soil in the 
potometers averaged 135 pounds. 

EFFECT OF SOIL FERTILITY ON WATER REQUIREMENTS AND GROWTH. 

While it has long been known that the development of the plant 
and the amount of water required for the production of a unit of dry 
matter may vary widely according to the texture and fertility of the 
soil, 1 it was deemed advisable to determine the difference in water 
requirements and growth of plants developed in soils of the same 
origin and texture, but differing appreciably in organic matter. The 
two soils investigated were of limestone- origin and formed within 50 
yards of each other in the spruce-fir type at about 10,000 feet eleva- 
tion. They may be briefly described as follows: 

(1) Infertile clay loam. The soil was well disintegrated, but owing 
to the destruction of most of the ground cover erosion and washing 
had diminished the humus content and to some extent the soluble 
salts. 

1 Sachs. J., Bericht ubor die physiologische Th-itigskcit an dor Versuchsstation in 
Tharandt. Landwirthschaftlichon Versuchsstationen. Vol. 1 : 235. 1859. 



8 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 

(2) Fertile clay loam. This was of the same general texture as the 
less" fertile soil, but owing to the presence of an adequate ground 
cover the soil had not been subject to washing and erosion. It ap- 
peared to be more mellow than the " infertile" clay loam and was 

darker in color. . 

The more important chemical properties at the time of the begin- 
ning of the experiment were as follows : 









Thos- 






Soil. 


Lime 
(CaO). 


Potash 
(K 2 0). 


phoric 
acid 


Total 
nitrogen . 


Loss by 
ignition. 


. 






(P 8 6 ). 









Per cent. 


Percent. 


Percent. 


Per cent. 


Per cent. 


. , 


1.23 


1.53 


0.22 


0.156 


6.64 




1.49 


1.30 


.33 


.488 


14.65 

















^ 


1 




(V 

, 






VD 








S3 






1 


I 
I 


to 










<? 










Q 












7 
6 

4- 
J 
2 


























fe ^ 




g 






IV) 

\ 






J 


Q 










cv 


1 






\ 









K 






\ 




1 




\ 






\ \ 




\ 




\ 


; 


^ ecty &ro/??e. M/fteaf W/?eaf hearts 
Water requjr&menfs per us?/'tofdr/ weJah-F 
mam Jn-f&rTi/e. ^o// 



czzza rerf/'/e. so/7 
^" r'&r cesiT d/f fere nee. 

FIG. 2. Relative water requirements per unit dry weight for peas, native brome grass, 
and wheat grown in infertile and in fertile soils of the same type. 

The chief difference chemically was in the total nitrogen content. 
Also there was a wide difference in the humus content as determined 
by incineration. The difference in the latter was largely responsible 
for the contrast in the wilting coefficients of the soils, this factor 



CLIMATE AND PLANT GROWTH. 9 

being 19.3 per cent in the fertile loam soil and 15.C per cent in the 
infertile loam. 

After sifting and preparing the soils for the reception of the plants 
according to the procedure previously described, two hermetically 
sealed batteries of each soil type were planted with sprouts of wheat 
of about equal leaf area and thrift, two with Canadian field peas, and 
one each with mountain brome grass. Thus 10 plants each of wheat 
and peas, and 5 of brome grass were grown in each soil type. The 
potometers were placed in the meteorological station of the aspen-fir 
type, where the plants were grown until inclement weather set in in 
the autumn. 

The water requirements for the production of a unit of dry matter 
of field peas, mountain brome grass, wheat, and wheat heads in the 
two soils are shown in Table 1, and graphically in figure 2. 



TABLE 1.- 



-Water requirement in grams of peas, brome grass, wheat, and wheat 
heads per gram of dry matter. 



soil. 


Peas. 


Brome 

grass. 


Wheat. 


Wheat 
heads. 


Infertile soil . 


841 


1,339 


472 


1,370 


Fertile soil 


467 


1 110 


343 


407 


Per cent difference 


80.3 


20.6 


37.6 


236.6 













In all cases notably more water was required for the production of 
a unit of dry matter in the infertile loam soil than in the fertile loam. 
The difference was greater in the peas than in the brome grass or the 
wheat. The brome grass was less influenced than either wheat or 
peas. The greatest difference occurred in the production of wheat 
heads, there being a requirement of 237 per cent more in the infertile 
than in the fertile soil. Under natural conditions brome grass grows 
in soils of relatively low fertility, and the species succeeds in com- 
pleting its life cycle in soils similar to the infertile soil here experi- 
mented with. 

In summing up the total water used by the plants grown in the two 
selected soils it was found that- a great deal more was consumed by 
the plants grown in the fertile than in the infertile soil, despite the 
fact that much more water was required by the plants grown in the 
infertile soil per unit of dry matter. 

It is noteworthy that a wide variation exists between the water 
requirement per unit of dry matter of brome grass and that of peas 
and wheat, even when the plants are grown in the same sort of soil. 
Thus in the case of the fertile soil brome grass uses more than twice 
as much water as the other two species, while in the infertile soil the 
ratio is practically the same. 1 This wide difference in water require- 



1 For a review of literature bearing on the subject, see Briggs, L. J., and SLantz; II. L. 
The Water Requirements of Plants, II. A Review of the Literature. IL S. Dept. of 
Agr. B. P. I. Bull. 285. 1913. 



10 



BULLETIN 100, U. S. DEPARTMENT OF AGRICULTURE. 



ment led to an examination of the character and extent of the root 
systems of the species in question. The examinations showed that the 
extent of root system varied widely, as is shown in Table 2, and fig- 
ure 3. 

TABLE 2. Relation of aerial and subterranean parts of peas, wheat, and l)rome 
grass, and comparative water requirements of aerial growth and of aerial 
and root development combined. 













Water 










Water 


require- 


Plant. 


Weight 
of roots. 


Weight 
of tops. 


Ratio of 
roots to 
tops. 


require- 
ment per 
gram of 
dry 


ment per 
gram of 
dry 
matter 










matter. 


including 












roots. 




Grams. 


Grams. 


Percent. 


Grams. 


Grams. 


Peas 


7.00 


231.31 


3.03 


368 


358 


"Wheat 


8.70 


264.92 


3.3 


358 


279 


Brome grass 


12.82 


60.11 


21.3 


516 


413 















Table 2 show r s that the dry weight of the roots of peas was approxi- 
mately 3 per cent of the dry weight of the tops, and the roots of wheat 
about 3.3 per cent ; in the case of the brome grass the roots weighed 
21.4 per cent as much as the tops. Hence the ratio of roots to tops 
in the case of brome grass was about 1 : 5, while in wheat and peas 
the roots showed a ratio of root to top by weight of about 1 : 30. In 
other words, brome grass had about six times as much root in com- 
parison to the top as the other two species. 

From these figures it would seem that the determination of the 
water requirement of the plant, on the basis of the dry weight of the 
aerial growth, is not necessarily an index to the ability of the plant 
to grow successfully in dry situations. To determine the moisture- 
absorbing power of a species account must also be taken of the depth 
and spread of the root system, as the volume of soil through which 
the roots penetrate is of the utmost importance in determining not 
only the amount of water available to the plant but the amount re- 
quired by the tops. A plant may- have a high water requirement 
when it is calculated on the basis of the weight of the tops, but at the 
same time it may be possessed of a root system great enough to supply 
the water necessary to the tops through its increased power to absorb. 
When the total water transpired by the plant is charged to the dry 
weight of the plant as a whole that is, both aerial and subterranean 
parts the water requirement data per unit of dry matter are quite 
different from those calculated on the aerial basis, as is shown in 
figure 3. 

Since two factors, (1) water requirement, or expenditure, and (2) 
water gathering, or accumulation, are involved in the development of 
vegetation, further investigations may prove that the determination 



CLIMATE AND PLANT GROWTH. 



11 



36S 
358 
3.0 



279 



S/ 



PEAS 



WHEAT 



BftOME 



Water requirement per unit of dry 

rnatter of aeria/ parts- 
Water requirement of a eria/ and 

subterranean parts combined 
Proportion of roots to tops, perceni- 

FIG. 3. Water requirement of aerial and subterranean parts of peas, wheat, arid brome 
grass, compared with the ratio of roots to aerial portion. 



12 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 

of water requirement on the basis of the plant as a Avhole may afford 
a more reliable index of successful groAvth in relation to drought than 
taking into account only the aerial portion of the plant. Most of 
the work on water requirement has been done in connection with 
cultivated plants, the root systems of which are small as compared 
with certain native species Avhich may be classed as conservative users 
of water. The more dissimilar the root systems of species compared 
the less reliable the water requirement data will be unless the roots 
as well as the tops are taken into account. 

The appreciably greater amount of Avater used by the plants grown 
in the fertile soil over those grown in the infertile soil is accounted 
for by the fact that the plants greAV much more luxuriantly in the 
richer soil ; hence the transpiration Avas much greater, and at the end 
of the season much more dry matter had been produced on the fertile 
than on the infertile soil. Exact data as to the vegetative develop- 
ment and the total water requirements of the species grown in the two 
soils are shown in Table 3. 

TABLE 3. Summary of vegetative growth and water requirements of peas, brome 

grass, and ivheat. 



Data determined. 


Peas. 


Native brome grass. 


Wheat. 


Infertile 
soil. 


Fertile 
soil. 


Infertile 
soil. 


Fertile 
soil. 


Infertile 
soil. 


Fertile 
soil. 


Number of leaves 


42 
791 
0.79 
667 

841 


112 
2,634 
6.55 
3,051 

467 


35 
2,902 
0.41 
553 

1,367 


84 
5,218 
0.85 
944 

1,110 


22 
4,474 
5.52 
2,516 

472 


47 
10, OSO 
12.09 
3,820 

343 


Leaf length (mm ) 




Water used per plant (grams) 


Water requirement per unit dry matter 





The graphical representation (fig. 4) of Table 3 shows remarkable 
contrast in the vegetative growth and total water requirement of the 
plants developed in the two soils. The number of leaves produced 
by field peas, for example, in the infertile soil as compared with that 
of the fertile soil is as 1 to 2.7 ; the leaf length, 1 to 3.3 ; the total dry 
weight produced, 1 to 8.3 ; and the water used per plant, 1 to 4.6. 
Similar contrasts are shown in the case of the other two species. 
The ratio in the water requirement per unit of dry matter, on the 
other hand, is reversed in the case of each species, as has previously 
been shown. 

The above data show clearly the importance of exercising the 
greatest care in the selection and subsequent treatment of soils for 
the study of comparative growth of standard plants as a means of 
integrating climate. While soils obtained within a limited space 
and at the same depth, and having uniform appearance in color, 
texture, and other essentials, may be similar in many respects, they 



CLIMATE AND PLANT GROWTH. 



13 



< 

Co 



^\ 





46? 



/OOQO 



30 



84 



85 



4, 



^ Numb&r of /eaves 

Avt.rOyf.Je.af /enqth in mm.f/ncas* offtos th* sftm /eng*A /s 

tjrjr W/ghf * 

Water used pir plant 

Wate.r rejuirernenr per unit dry 

of vegetative growth and water requirement of peas, native brome 
grass, and wheat in infertile and in fertile soils, 



14 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE, 

may differ appreciably in their crop-producing potentialities. Thor- 
ough mixing of the soil, therefore, regardless of the care with which 
it has been selected, can not be overemphasized as a means of avoiding 
outstanding errors from this source in experimentation. 

MEASUREMENT OF PLANTS. 

At each of the type stations measurements of growth were made 
on the following number of specimens : 20 of wheat, 10 of peas, and 5 
of native brome grass. The lid of each potometer contained five 
evenly spaced perforations (fig. 1). In the case of wheat, 10 seed- 
lings were planted in each pot, 2 in each perforation ; while in the 
case of peas and brome grass, each pot contained only 5 plants. 

Throughout the growing season both leaves and stems of the plants 
were measured at regular intervals. The object of the measurements 
was to obtain data as to the relation of the environment to (1) the 
tendency of the plants to elongate their stems and (2) the tendency 
of the plants to expand their leaves. Measurements of the stems 
furnished direct data as to the rate of elongation of the plants. In 
obtaining leaf expansion, however, indices had to be used instead 
of actual figures on leaf area. Hence in the case of the grasses the 
leaf expansion was obtained by recording the length of the leaves; 
for, as will be shown in the calculation of the experimental error, 
leaf length is proportional to leaf area. In the case of the peas, an 
index of leaf area was obtained by recording the number of leaflets, 
as they were found to be of rather constant average size and w r ere 
considered as units of area. 

Since the seedlings were all of uniform size and inconsiderable in 
comparison to the subsequent growth of the plant, the measurements 
were considered as beginning at zero. During the first half of the 
growing season all the plants were measured at 10-day intervals. 
Owing to the number of plants grown and their luxuriant develop- 
ment, it became impossible in the first week in August to remeasure 
all of the plants at 10-day intervals; so from then on the measure- 
ments were made once a month. 

At the end of the growing season in each type the plants were cut 
at the junction of the stem and the lid of the potometer, and the 
measurement again recorded. In addition, the dry weight and the ash 
content were determined. In the case of plants grown in the aspen- 
fir association, the soil w y as washed away from the roots and the dry 
weight of the latter obtained. 

EXPERIMENTAL ERROR. 

In determining the rates of growth and other physiological activi- 
ties for a given species some variations are sure to be found in indi- 



CLIMATE AND PLANT GROWTH. 



15 



vidual specimens or in plants grown in a single battery. These vari- 
ations may be due to such features as slight differences in the fertility 
of the soil, but mainly they are accounted for by the natural tendency 
of individual plants to vary. In order to eliminate such individual 
variations, it is necessary to average the results secured from a large 
number of plants. In the present experiment the number of plants 
was sufficiently large to render the probable error of the average 
measurements of any battery at a given station much less than the 
difference in measurements between the plants of this battery and 
those of the corresponding batteries of the other stations. This fact 
is brought out in Table 4. 

TABLE 4. Comparison between average error of the plant measurements at a 
given type station and the difference in experimental results of the respective 
type stations. 1 













Differ- 












ence 






riant 


Error 


Per cent 


between 


Plant. 


Type station. 


measure- 


in 


average 


measure- 






ments. 


average. 


error. 


ments 












at type 












stations. 






Mm. 


Mm. 




Mm. 


Brome grass 2 


Oak-brush 


12 569 


1 358 


10 80 






Aspen-fir 


19 103 


2 708 


14 12 


6 534 




Spruce-fir 


5 064 


818 


16 13 


14'rioq 














Wheat 2 


Oak-brush . . 


2 420 


141 


5 82 






Aspen-fir 


4,296 


290 


6 75 


1 876 




Spruce-fir 


3 359 


136 


4 Q4 


'937 


Peas 3 


Oak-brush 


155 


g 


3 85 






Aspen-fir.-. 


127 


4 


3 10 


28 




Spruce-fir . 


57 


2 


3 50 


70 















1 The formula used in Tables 4 and 5 in deriving the average error of the mean is: the summation of all the 
variations from the mean, regardless of sign, divided by the number of cases times the square root of the 
number of cases. 

2 Average leaf length per plant. 

3 Average stem length per plant. 

Table 4 shows the average leaf length (which is taken to repre- 
sent leaf area) of typical specimens of mountain brome grass and 
wheat, and the average stem length of specimens of field peas grown 
in the respective type stations. From these data are computed the 
variations from the mean, the percentage of average error, and the 
difference between the measurements obtained at the type stations. 

The greatest experimental error occurs in the case of mountain 
brome grass. This is accounted for by the fact that only 5 specimens 
of brome grass were grown in each type station, while in the case of 
the peas and wheat 10 and 20 specimens, respectively, were grown at 
each station. In each instance, however, the experimental error due 
to individual variation w y ithin a type is much less than the difference 
between two groups of different types. 



16 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



Another source of error is the use of indices of leaf area instead 
of actual leaf area. Thus when the total length of the leaves of a 
wheat plant is used for comparative purposes, it is assumed that the 
total leaf length is proportional to the total area of the plant. The 
leaves of grasses are approximately triangular in shape, and their 
actual area may therefore be determined by multiplying the length 
of the leaf by the width and taking half of the product thus ob- 
tained. In order to ascertain whether the leaf length was a reliable 
index of leaf area, the leaf area was obtained in the manner de- 
scribed above for a number of cases and the results decided by the 
corresponding leaf lengths. Table 5 gives the leaf length, calculated 
area, the ratio of these two, the average error in millimeters, and 
the per cent determined by means of the formula given in the foot- 
note to Table 4. 



TABLE 5.- 



-Relation between leaf area and le,af length in wheat and mountain 
brome grass. 



Specimen. 


Total leaf 
length. 


Total leaf 
area. 


Leaf area. 
Leaf length. 


Variation 
from mean. 


Per cent 
variation 
from mean. 


Wheat: 
1 


Mm. 
1,386 


Sq. mm. 
3, 298. 00 


2. 3795 


Mm. 
U.0690 


2.8 


2 


1,147 


2, 806. 25 


2.4465 


.0020 


.8 


3 


1,127 


2, 526. 25 


2. 2415 


.2070 


8.4 


4.. 


1,590 


3,970.75 


2.4973 


.0488 


1.96 


5 


1,374 


3, 679. 25 


2. 6777 


.2292 


9.41 














Total 


6,624 


16, 280. 50 


12. 2425 


.5560 




Mean 






2.4485 


0500 


2.45 














Brome grass: 
1 


1,771 


4, 766. 50 


2.6910 


.2570 


8.71 


2. 


1,279 


3,334.00 


2.6060 


.3420 


11.2 


3 


1,696 


5, 397. 50 


3 1820 


2340 


8 61 


4. .. 


571 


1,837.25 


3.2170 


.2690 


9.13 


5 


1,440 


4, 389. 00 


3.0470 


0990 


3 35 














Total 


6,757 


19, 724. 25 


14. 7430 


1 1930 




Mp.an 






2 9480 


1070 


3 63 















The fact that the ratio of leaf area to leaf length is nearly con- 
stant shows that the length furnishes a reliable index of area. The 
average error of the ratio for wheat, using 5 plants with about 25 
leaves in all, is 2|- per cent ; while in the brome grass, using 5 plants 
with about" the same number of leaves as the wheat, the error is 
about 3J per cent. 

MEASUREMENT OF PHYSICAL FACTORS. 

Each of the three type stations was equipped, in the main, with 
automatic instruments. Air temperature, precipitation, evaporation, 
relative humidity, sunshine, and barometric pressure were recorded 
continuously at each station. In addition, a continuous record of the 



CLIMATE AND PLANT GROWTH. 



17 



wind movement was obtained at the two upper stations. The read- 
ings of all instruments were recorded daily at 8.30 a. ni. and 4.30 p. m. 
Because of the fact that certain important weather factors may be 
measured by various instruments, it is possible to get a number of 
different sets of values for the climatic factor, depending on the kind 
of instrument used. Where it is desired to compare physiological 
activities of plants with weather factors for short intervals, such as 
a few days, a single day, or a fractional part thereof, the kind of 



S /6 /7 /S /9 2O 2X 22 23 24 ZS 26 27 28 29 3O 3 




AUGUST 1916 

White sphere 

Free wafer surface 

Average daily relative a/ r humidity 

FIG. 5. Evaporation from white sphere and from free water surface compared with 
average daily relative air humidity. 

instrument used is often a matter of important consideration. The 
bearing which the choice of instruments has on the results obtained 
from the measurement of two of the climatic factors, evaporation 
and sunshine, is described in detail on pp. 18-24. 

AIR TEMPERATURE. 

The temperature was measured automatically by carefully adjusted 
thermographs calibrated with standardized maximum and minimum 
568G6 18 Bull. 700 2 



18 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



mercury thermometers and exposed in shelters of the Weather 
Bureau pattern 4J feet from the ground. 



PRECIPITATION. 



The precipitation record was obtained by means of automatic 
tipping bucket rain gauges, the data from which were harmonized 




/ 23^,56 769 



27 28 29 3O 3/ 



AUGUST 1916 

White sphere 

Free wafer surface 

> Daily m ean temperature 

FIG. 6. Evaporation from white sphere and from free water surface compared with daily 

mean temperature. 

with standard rain gauges. In this way reliable data were obtained 
as to the amount and rapidity of the rainfall. 

EVAPORATION. 

The evaporation was recorded in two ways: (1) By means of the 
standardized porous cylindrical and spherical atmometers, and (2) 
by means of a free water surface. 

In the case of the porous spheres, both the black or radio cup and 
the white cup were used at each station. The spherical, as well as the 
white cylinder cups, were fitted with rain-correcting apparatus and 



CLIMATE AND PLANT GROWTH. 



19 



the mountings were self-contained as devised by Shive. 1 Distilled 
water was used, but in order further to insure accurate and compar- 
able readings, the spheres, after a month of exposure, were replaced 
by restandardized spheres. 

In measuring the evaporation from a free water surface a gal- 
vanized-iron evaporating pan, 10 by 36 inches, of the Weather Bureau 
pattern, was used. The evaporation from this free water surface 




/ 1 3 



/ 22 23 Z4 2S Z6 27 20 29 3O 3/ 



AUGUST I9I& 

White sphere 

Free water surface 

-Average daily wind velocity 

FIG. 7. Evaporation from white sphere and from free water surface compared with 
average daily wind velocity. 

was recorded by means of a hook gauge, reading in hundredth^ of 
an inch. 

In order that the evaporation records obtained from the two types 
of instruments 'used might be compared directly, the porous spheres 
and the free water surface were placed at the same height, namely, 
2J feet above the ground. 

1 Shive, John W.,. An improved nonabsorbing porous cup atmometer. Plant World, 
vol. 18, No. 2 : 7-10. 1915. 



20 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



Probably the variations in the measurements made with different 
instruments are greater in the case of evaporation than in the 
case of any other factor. Because of the lack of information as to 
the accuracy of certain available instruments in the measurement 
of evaporation, two different sets the free water surface pan of the 
Weather Bureau pattern and the porous cup atmometer were used 
in obtaining evaporation indices for comparison with plant activi- 
ties, and it will be helpful in future field studies to compare the 
results. 

As it was desirable to correlate evaporation and physiological 
activities for short periods, the evaporation values obtained from 
the readings of the spherical atmometer were first compared with those 
obtained from the free water surface and the data from each were 
then compared with the factors which chiefly determine the evapo- 
ration rate, namely, air humidity, wind movement, and air tempera- 
ture. By this means it should be possible to show which of the two 
instruments is responding best to the conditions controlling evapora- 
tion. The records as obtained in the aspen-fir type for August were 
selected for this purpose. The data are presented in Table 6, and 
as a matter of convenience in comparing, they are summarized 
graphically in figures 5, 6, and 7. 



TABLE 6. Daily evaporation from spherical atmometer and from free water 
surface, with corresponding relative humidity, temperature, and wind 
velocity. 



Date. 


Spherical 
atmometer. 


Free water 
surface. 


Average 
daily 
relative 
humidity. 


Daily 
mean 
tempera- 
ture. 


Average 
daily wind 
velocity. 


1916. 
Aur. 1 


cc, 
18 8 


Inches. 
21 


Per cent. 
50 


e F. 

65 9 


5 9 


2 


15.0 


19 


63 


65 6 


6 5 


3. ... 


12 1 


18 


69 


64 4 


3 9 


4 


13 6 


11 


78 


60 1 


44 


5 


6 8 


26 


70 


58 4 


3 7 


6. 


15 3 


28 


56 


64 4 


40 


7 


17 8 


16 


56 


fifi 4. 


fi Q 


8 


29 9 


21 


53 


67 6 


4 4 


9... 


28 8 


11 


27 


fi4 A 


4 A 


10 


2 7 5 


22 








11 


36 7 


25 


28 


67 1 


4 A 


12.. 


15 3 


24 








13 


9 8 


24 








14 


17 


18 


54 


64 6 




15... 


18 9 


22 








16 


11 6 










17 


24 5 


20 


42 


co 9 




18 


33 2 


01 








19 


24 2 










20 


19 2 


20 


46 






21 


25 ^ 










22 . 


27 4 










23..: 


28 2 


20 


Ofi 


AO A 




24 

25... 


27.9 
26 4 


.20 


36 


65.4 


3.7 


26 


oo Q 










27. . . 


28 5 








3.8 


28... 


25 7 










29 


10 










30... 


16 








2.9 


31.. 


17 7 























CLIMATE AND PLANT GROWTH. 21 

The evaporation from the white spherical atmometer and the free 
water surface and the average daily relative humidity are shown in 
figure 5. In order to determine which of the evaporation curves is 
the most reliable for the periods under consideration, so far as rela- 
tive humidity is concerned, note was taken of the number of cases in 
which the evaporation curves slope in a direction opposite to the 
corresponding humidity curve. As the relative humidity decreases, 
evaporation, other things being equal, would increase; hence it would 
be expected that the graph of instrumental evaporation values would 
show an opposite slope direction to the graph of relative humidity. 
For the graph representing the free w r ater surface evaporation the 
slope is opposite on 19 days out of the 30, or in 63 per cent of the 
cases. The graph of evaporation from the spherical atmometer, on 
the other hand, slopes opposite to the relative humidity graph in 73 
per cent of the total number of cases. In deriving these percentages, 
in the case of both evaporation graphs, it was deemed advisable to 
consider slopes as opposite in those cases in which they came very 
near being so, as well as when they were actually opposite. Since the 
evaporation curve for the spherical atmometer shows more cases of 
slope opposite to the relative humidity curve than does the evapora- 
tion curve for the free water surface, it may be considered that the 
atmometer is somewhat more reliable than the free water surface in 
determining evaporation for daily periods, in so far as evaporation 
is determined by relative humidity. 

A comparison of the evaporation values obtained from the two 
instruments with the daily mean temperature is presented in figure 6. 
In this instance it would, of course, be expected that the evapora- 
tion curves would show agreement in slope with the curve represent- 
ing the mean temperature. Examination of the graphs shows for 
the atmometer 67 per cent agreement (20 periods out of 30) with 
the temperature curve; and for the free water surface only 47 per 
cent (14 periods out of 30). If only slight disagreements between 
evaporation and temperature are considered, the per cent of agree- 
ment in the case of the atmometer record is even greater than that 
from the free water surface. It is interesting to note that the evap- 
oration record obtained from the free w r ater surface commonly lags 
about one day behind that of the temperature ; that is to say, if the 
evaporation from the free water surface is compared with the tem- 
perature for the preceding period there is a much closer agreement 
than when the comparison is made for the same day. In order to 
obtain an evaporation record which is comparable with the trans- 
piration of the plant for short periods, the instrument with which the 
evaporation is measured should respond quickly to temperature 
changes in a manner similar to the transpiration of the plant itself, 



22 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



so that for this, purpose the atmometer is superior to the free water 
surface. 

Figure 7 shows the relation of evaporation, as obtained by the two 
methods, to wind movements. Here, again, agreement between the 
two graphs consists in a slope in the same direction, since high wind 
velocity accelerates evaporation rate and low wind velocity retards 
it. This figure shows an agreement of 11 periods in the case of 
evaporation from the free water surface with the average daily wind 
movement, or a percentage agreement of 37. The evaporation from 
the atmometer, on the other hand, shows for the same time an agree- 
ment of 18 periods, or a percentage agreement of 60. 

From the above comparisons it is evident that the evaporation rec- 
ord obtained by means of the spherical atmometer agrees more closely 
with the relative humidity, the temperature, and the wind velocity, 
and is a more reliable index when short periods are to be considered 
than that obtained from the free water surface. When periods ex- 
tending over several days or longer are considered, however, either 
instrument may be used with good results. 

Because of the advantage of the rain-correcting device used in con- 
nection with the porous cup atmometer, this instrument was espe- 
cially suited to the experiments here presented, and accordingly has 
been used throughout. Of the three types of porous cup atmometers 
available for field use, namely, the white sphere, .the black sphere, 
and the white cylinder, all were operated in each type station through- 
out the season. The sum of the daily means of each set of cups and 
the difference in evaporation between the black and white sphere are 
presented in Table 7. 

TABLE 7. Summary of evaporation from white cylindrical, wliite spherical, and 
black spherical atmometers, and of difference between white and black 
spherical atmometers during the period of experimentation. 











Difference 


Type. 


White 
sphere. 


Black 
sphere. 


White 
cylinder. 


between 
black and 
white 










. spheres. 




cc. 


cc. 


cc. 


cc. 


Oak-brush 


3 956 3 


5 475 


3 545 4 


1 518 7 


Aspen-fir 


2780 3 


4 025 4 


2 490 8 


1 245 1 


Spruce-fir . . 


4 251 3 


5' 530 2 


3' 711 7 


1 278 9 













The figures given in Table 7 are platted in figure 8. It is interest- 
ing to find that the graphs representing the three evaporating instru- 
ments here used are all practically parallel. This parallelism also 
holds for shorter periods, as is shown in figure 9. It would appear, 
therefore, that one might select any one of these instruments to ascer- 
tain the evaporation. However, because of the desire to use evap- 



CLIMATE AND PLANT GROWTH. 



23 



6000 



5000 



4-OOO 



3000 



2OOO 



/OOO 



<3 

I 



39S6.3 




X X 






\ 



\ 






2.490.3 



Oak-brush 



Aspen 

'fc sphere 
cA- sphere 



'S30.Z. 



37//. 7 



270.9 



Difference. beTfve.e.n white, and 
black 



FIG. 8. Summation of evaporation from white cylinder, white sphere, and black sphere 
and of differences between white and black spheres.- 



24 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



orating instruments which might also furnish data on sunshine 
intensity, the evaporation data front the white and black spherical 
atmometers have been used in connection with the plant studies. 



IS 
14 

13 
12 
II 
10 
9 
8 
7 
6 
5 
4 
3 
2 
1 


0*> 


4227.6 




\ 

\ 
\ 




\ 
\ 
\ 
\ 






X S 

\ / 

\ 


3512.5 




\ 




2953.8 


X 






\ \ / ~ 

v v ' S 
\ 


2604.3 




\ 2285. 6 \ 
\ 


s r 

s S 






\ 

s 


s 






\ 


/' 




1273.4 










^^ 


^_^. 


966.9 




703^^ 


^^E 












52 Days 70 D 


vys 65 L 


ays 


k- Brush Type Aspen 
Wh 
. Bl* 


- Fir Spruce - Fir 
ite Sphere 
ck Sphere 
ference 




n;f 



FIG. 9. Summation of evaporation from black and white spheres and the difference 
between them. Record made during growth studies. Started late. 

BAROMETRIC PRESSURE. 

The barometric pressure was measured by aneroid barometers. The 
instruments were standardized from time to time at the central sta- 
tion, where the elevation and pressure were known. 



CLIMATE AND PLANT GROWTH. 
WIND VELOCITY. 



25 



The wind velocity was measured automatically by anemometers 
of the Weather Bureau pattern located about 15 feet above the 
ground. 

SUNSHINE, 

The records of sunshine obtained were for both duration and 
intensity. 

The duration of sunshine was recorded automatically by means 
of the Marvin recorder used bv the Weather Bureau. 



































































































































\ 
















... 


















































\ 


> % 








/ 
/ 

/ 




c 












/ 


\ 


\ 


































N 


N 


> 


-- 








\ 








I 






\ 
1 
% 


,'' 


S 


\ 
1 
t 


















































1 














- 










/ 


\^ 












\ 










/ 




\ 

s 




I 


1 


- 




/ 

/ 


>^ 












1 






; 






t 












\ 






/ 

/ 








\ 


t 
I 


I 

i 




t 
t 






I 
I 




1 

1 


\ 




i 








/ 
/ 


\ 
\ 


i 
i 














\ 
N 


/ 
/ 










\ 


i 


i 




\ 
1 






\ 
1 




1 

1 




\ 




/ 







/ 




* V 














\ 














\ 
\ 


/ 


^S 








\ 
I 


/ 
/ 








i 

\ ; 


i 

i 





.' / 
f / 
























































\. 

\ 


/ 


V 

\ 


/ 
/ 




























































V 


/ 








/23*.T678 9 / 


/ 


/ /Z /3 / 


4. /S S6 77 /0 /3 20 2/ 22 23 2-* 2f 26 27 ZB Z9 SO 3 



19/6 

.. Difference between h/dck and 

white spheres 
- Duration of sunshine 
Potential suns/t/'ne /n hours 

FIG. 10. Difference between black and white spheres in evaporation as compared with 
hours of sunshine recorded by black bulb sunshine recorder. 

Data on sunshine intensity were obtained by noting the difference 
in the evaporation between the radio-atmometer and the ordinary 
white porous cup atmometers. 1 The radio-atmometer used was a 
black sphere of the same size as the more common white form. 
Owing to its color it absorbs considerable of the radiant energy fall- 
ing upon it, functioning in this regard much the same as foliage of 
ordinary plants. The white porous cup, on the other hand, absorbs 
comparatively little radiant energy and is therefore not appreciably 
affected by increased light intensity. Hence, while by night the 

1 Livingston, B. E. A radio-atmometer for comparing light intensity. Plant World, 
14, No. 4: 96-99. 1911. 



26 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



evaporation from the radio-atmometer and from the white porous 
atmometer of the same size and form is practically identical, by day 
the black cup, on account of its higher temperature, shows a greater 
fate of evaporation. Therefore the difference in the evaporation 
from the radio or black atmometer and the white atmometer affords 
a rough estimate of varying intensities of sunlight for the different 
periods and type stations. The actual variation intensity, of course, 
bears an important relation to transpiration and photosynthesis. 

Only the duration of sunshine is measured by the Marvin sunshine 
instrument of the Weather Bureau pattern. Unless the investigator 
is working in the vicinity of an experiment station or similar base, 
the Marvin sunshine recorder can not, of course, be operated. The 
black and white porous cup atmometers, on the other hand, may be 
set up wherever desired. Accordingly, it is worth while to compare 
the sunshine records obtained from these two instruments. The data 
are given in Table 8 and are shown graphically in figure 10. 

TABLE 8. Comparison of sunshine records obtained from a sunshine recorder 
of the Weather Bureau pattern, and the difference in evaporation between 
atmometers with black and white spheres. Readings taken in August, 1916. 



Date. 


Differ- 
ence 
between 
black 
and 
white 
spheres. 


Dura- 
tion of 
sunshine 
(Marvin 
sunshine 
recorder). 


Possible 
sunshine. 


Date. 


Differ- 
ence 
between 
black 
and 
white 
spheres. 


Dura- 
tion of 
sunshine 
(Marvin 
sunshine 
recorder). 


Possible 
sunshine. 


1916. 
Aug. 1 


cc. 
19.6 


Hours. 
12.9 


Hours. 
12.9 


1916. 
Aug. 17 


cc. 
16.4 


Hours. 
9.9 


Hours. 
12.0 


g 2 " 


19.6 


12 9 


12.9 


18 


18.1 


11.4 


11.9 


3 


17 6 


12 8 


12 9 


19 


17.1 


10.6 


11 9 


4.... . 


18.1 


12 9 


12.9 


20 


14.0 


3.6 


11.9 


5 


17 1 


9 4 


12 9 


21 


14.8 


4.9 


11.9 


6 


14.9 


7.4 


12.3 


22 


15.9 


10.5 


11.9 


7... 


13.7 


5 6 


12 3 


23. . 


13.0 


7.7 


11.8 


8 


14 5 


7 3 


12 3 


24 


13 3 


5 2 


11 8 


9 


14 6 


9 8 


12 2 


25 


3 


2 3 


11 8 


10 


18.7 


12 1 


12 2 


26 


8.8 


4.4 


11 7 


11 


18 6 


12 


12 2 


27 


3 6 


2 


11 7 


12 


16.2 


9.8 


12 2 


28 ... 


12.2 


6.8 


11.6 


13 


13.7 


6 3 


12 1 


29 


13 9 


9.7 


11.6 


14 


6 1 


3 6 


12 1 


30 


12 8 


7 8 


11 6 


15 


10 9 


4 8 


12 1 


31 


6 6 


7 2 


11 6 


16 


10 7 


5 7 


12 1 



























In following the slope of the curves as shown in figure 10, an agree- 
ment of 67 per cent (20 periods out of 30) is found, and the disa- 
greements in practically all cases are relatively slight. The most 
conspicuous differences in ordinate values occur for days when the 
sky is partly cloudy and the sunshine more or less intermittent. In 
such instances there is almost invariably a much greater fluctuation 
in the values of the atmometric sunlight index than in the values of 
sunshine duration as recorded by the Marvin recorder. This may be 
accounted for by the fact that the evaporation from the black surface 
atmometer responds more quickly to fluctuation in sunshine than 



CLIMATE AND PLANT GROWTH. 



27 



does the mercury column of the sunshine recorder of the Weather 
Bureau type. 

The data seem to warrant the statement that the use of atmometers 
in obtaining sunshine duration affords quite as reliable a record as 
does the more costly Marvin sunshine recorder. Of course, the 
impossibility of operating atmometers when the temperature drops 
below freezing makes them of value only during the growing sea- 
son. Where it is desired merely to obtain a summary of the sunshine 
record, it is necessary to read the instruments only about twice per 
month, whereas the Marvin recorder must be read daily. 



&f 

30 
40 
30 
20 
m 


















'Z 


"~^>*^ 




















^ 


*<.> 


^ 
















// 


/ 


""^^ 


*v^^ 
















/// 


r^ + 

/ 


^^.. 


Sy 


\ 










s 





'/ ; 


S 




\ 


\ \ 

\\ \ 








/ 


' 


/ ^ 


/ / 








'.v 


v 






/ 
/ 


S 

/ . 


^ 


/' 








V A\ 


N 
\ 




J 


/ ^~ 


f / 


^ 










\p 


X N 


x ^ 


/ . 


^^ 


/ 







TYPE 
-Sage Brush-Rabbit . 
Oak-Brush 
A spen Fir 
-Spruce-Fir 




\ 


X 


*. "*, 

*^ 


* // 





/ 




^rush 




x^ 


*v 














V, % 


/ 



















OCT. NOV. DEC. JAN FEB. MAR. APR. MAY JUN. JUL. AUG. SER 
FIG. 11. Monthly mean temperatures October, 1915-October, 1916. 

COMPARISON OF THE CLIMATIC CHARACTERISTICS OF THE 
THREE PLANT TYPES. 

TEMPERATURE. 

For purposes of comparison there is probably no better way of 
showing differences in temperature in the type stations than to give 
the monthly mean temperature for each station. This is shown 
graphically in figure 11. Throughout the year the mean monthly 
temperature is appreciably lowest in the spruce-fir and highest 
in the sagebrush-rabbit-brush type. In general, the slopes of the 
mean-temperature curves of all the climatic types are similar, and 
this is especially true for the main growing season, from June to 
September, inclusive. 

In the monthly range in temperature for the respective types there 
are even greater contrasts than in the daily means. The range in the 
monthly temperatures is shown in figure 12. These temperatures 
differ most notably from those given in figure 11, representing the 
monthly means, in (1) the similarity in vertical form and proximity 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



\ 



CLIMATE AND PLANT GROWTH. 



29 



of the curves in the case of the oak-brush and the aspen-fir types, and 
(2) the decreased range or flattening of the curve in the spruce-fir 
type. 

It is not possible by a review of the monthly mean temperatures 
to form a mental picture of the relative growing and nongrowing 



5U 

+S 

ki 

* 24 

kj 
Q 
lc 20 
O 




" 











































1 




- 






1 




6 / 




"" /^? 

5" 

40 


I~ 






1 




I li 






1 




i 11 || 






I 




r^g U-^ ^ 


MM 




1 




JUNE JULY AUG. SEPT. SUMMARY 
- (JULY ~r SEPT) 



TYPE 
EH OAK- BRUSH 
Ml ASPEN- FIR 
ESI SPRUCE-FIR 

FIG. 13. Summation of temperatures above and below 40 F., 1916. 

temperatures ; that is, the temperatures above or below 40 F. These 
are summarized in Table 9 and graphically represented in figure 13. 

TABLE 9. Summary of temperatures above and below 40 F. 



Month. 


Temperature segregations. 


Oak-brush 
type. 


Aspen-fir 
type. 


Spruce-fir 
type. 




above 


F 
11,400 


"F. 
9,347 


F. 
5,090 


July . 


below 
above '. 


350 
16,919 


767 
13,773 


1,399 
8,843 


August... 


below 
above 


15 
14, 929 


42 
10,357 


73 

7,874 


September 


below 
above 



10,222 


74 

6,838 


103 
3,505 




below 


640 


1,034 


1, 786 


Total 


fabove 


53, 470 


40,315 


25, 312 




\below 


1,005 


1,917 


3,361 



30 



BULLETIN 700, TJ. S. DEPARTMENT OF AGRICULTURE. 



The smallest number of heat units above 40 F. and the largest 
number below 40 F., as shown in figure 4, occur in June and Sep- 
tember, near the beginning and ending of growth in the two lower 
types represented. 1 One of the most significant facts brought out 
in Table 9 is the absence of temperatures below 40 F. for August, 



INCHES OF PRECIPITATION 

O^lu-fc<> <* x 


















1 


1 


\ 


\ \\ \ 



1916 1916 

RABBIT BRUSH 




1916 1915 1916 

OAH-BRUSH AS PEN -FIR 





























il, 


Ipt 

||,2 


ISit^fS 


1915 1916 
SPRUCE-FIR 




MONTHLY AND. ANNUAL PRECIPITATION 



RABBIT BRUSH 
OAK-BRUSH 
ASPEN -FIR 
SPFHJCE-FII? 

MONTHLY PRECIPITATION DURING GROWING SEASON 1916 
FIG. 14. Precipitation record, 1915-1916. 

in th oak-brush type. All types considered, July and August are 
the most favorable months for growth, in so far as it is determined 
by temperature. 

1 Growth begins in 'the oak-brush type approximately June 1. In the aspen-fir type 
the leaves begin to unfold about two weeks later. In the spruce-fir type growth begins 
between 3 and 4 weeks later than in the oak-brush type. 



: 



CLIMATE AND PLANT GROWTH. 31 

On the basis of the beginning of growth and the occurrence of 
killing frosts the growing periods in days for the respective types are 
approximately as follows : 

Oak-brush type 120 

Aspen-fir type 105 

Spruce-fir type : TO 

PRECIPITATION. 

The monthly precipitation from October, 1915, to September, 1916, 
is summarized in figure 14. 

In view of the higher temperature, the longer growing season, and 
the higher evaporation in the oak-brush type, it is significant that 
the annual precipitation is less in that type than in any of the more 
elevated types in which plant studies were conducted. The annual 
averages of precipitation of the types, including the untimbered 
type below the oak-brush, as recorded from 1914 to 1916, inclusive, 
are: 

Inches. 

Sagebrush-rabbit-brush type 11. 15 

Oak-brush type 13.25 

Aspen-fir '. 27. 18 

Spruce-fir 25. 40 

During the growing period in 1916 the aspen-fir type, as in the 
case of the three-year average, received the heaviest precipitation, 
nearly the same amount, however, being recorded in the type imme- 
diately above. 

EVAPORATION. 

Monthly evaporation and precipitation are represented in the same 
figure (fig. 15). Owing to the occurrence of freezing temperatures 
in June, particularly in the two higher types, unbroken evaporation 
records were obtained only from July to September, inclusive. 

Figure 15, based upon the records of the porous cup atmometer 
(see fig. 16), show T s that the highest evaporation occurs each month 
in the oak-brush type. In the spruce-fir type the evaporation is 
nearly as great as in the lowest association, while in the aspen-fir 
type it is much less than in either of the others. Table 7 and figure 8 
also indicate that the evaporation in the oak-brush and spruce-fir 
types is much greater than in the aspen-fir type. 

In the case of growth studies begun in the stations at a later period, 
the summation of evaporation is quite as contrasted, as is shown in 
figure 9. While the records in this series of experiments cover a 
shorter period than those in the original study, being 52, 70, and 65 
days in the oak-brush, aspen-fir, and spruce-fir types, respectively, 



32 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 

evaporation was notably greater in the lowest type, least in the cen- 
tral, and intermediate in the highest. 

The high evaporation in the oak-brush type is clue chiefly to high 
temperatures and low relative humidity. In the spruce-fir type the 
high evaporation is accounted for chiefly by the high wind velocity, 
as will be shown later. Owing to the aspen-fir type being inter- 
mediate in elevation between the other two, and having a notably 
heavier stand of vegetation, especially tree growth, the factors in- 
fluencing evaporation are in no instance extreme. 



/4oo 
< /2OO 

O 1000 

\ 

* 800 

600 

? 



Q -4OO 

I 
^ 
-zoo 

o 


Odk-brush ~fype 


As pen -fir type 


Spruce-fir type 


C> x r\> (o -k Oi &> N 
f?d/n fa /I -inches 


































































































[ 


1 [ 
















"i r 






1 




f 


1 






' [ 


l 






r 


i 




July dug. Sept. 


July dug. Sept. 


July Aug. Sept 




Evaporation __ Rain -fa /I [~~ \ 



FIG. 15. Monthly evaporation from spherical atmometcrs and corresponding precipitation 

in type stations, 1916. 

WIND VELOCITY. 

Largely because of the physiographical features, the velocity of 
the wind is notably greater in the spruce-fir type than in the lower 
associations. The comparative intensity of this factor may be ap- 
preciated readily by summing the daily wind movement by monthly 
periods. Since the wind velocity during the growing season is prob- 
ably an influential factor in the development of the vegetation, 
data are presented in Table 10 showing the wind movement during 
the growing seasons of 1915 and 1916. 



CLIMATE AND PLANT GROWTH. 



33 



TABLE 10. Monthly wind movement in the spruce- fir and in the aspen- fir type 

station. 



Month. 


Year. 


Aspen-fir. 


Spruce-fir. 


June 


/1915. 


Miles. 
3,081 


Miles. 
6,501 


July 


\1916. 
U915. 


3.020 
3,055 


7, 119 
6,807 




\1916. 
/1915. 


3,697 
3,339 


5,505 
4,836 


September 


\1916. 
/1915. 


3,198 
3,008 


5,116 
7,632 




\1916. 


3,080 


6,873 


Total 


(1915. 


12,483 


25, 776 




\1916. 


12,995 


24, 613 




FIG. 16. View of atmometers and evaporating pan used in measuring the evaporating 

power of the air. 

The above figures show that the wind movement during the 
growing seasons of 1915 and 1916 was greater by approximately 100 
per cent in the heart of the spruce-fir type than in the aspen-fir asso- 
ciation 1,300 feet lower. In summarizing the wind movement by 
10-day periods the velocity is found to exceed by 200 per cent that in 
the aspen-fir type for certain periods. Obviously, the gales over the 
elevated, sparsely vegetated plateaus have a profound effect on the 
evaporation and to some extent at least on the transpiration rate of 
the vegetation. 

56866 18 Bull. 700 3 



34 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 

SUNSHINE. 

In recording the sunshine it was deemed advisable to note both the 
possible. and the actual duration, since both are important to the 
development of vegetation. These factors are graphically shown 
in figure 17 for the aspen-fir type during the seasons of growth in 
mAnd 1916. 

In 1915 the greatest actual sunshine occurred in July, while in 
1916 it was recorded in June. Owing partly to the advancement in 
the season but chiefly to the topographic features adjacent to the 
meteorological stations, there is a gradual decline in the potential 
sunshine duration throughout the growing season. The potential 
and actual sunshine durations, on the basis of three seasons' records, 

ASPEN-FIR TYPE 



ki*00 

I 



300 



200 



100 



1915 



1916 



JUNE JULY AUG. SEPT. JUNE JULY AUG. SEPT. 

ACTUAL I i POTENTIAL 

FIG. 17. Actual and potential sunshine during growing season in the aspen-fir type, 1915. 

are found to be practically identical in the three associations studied ; 
consequently, no attempt is made to correlate sunshine duration with 
the plant activities. 

BAROMETRIC PRESSURE. 

So far no direct fundamental relations have been established 
between barometric pressure and the development of the plant. 1 

The relation of high and low pressure to local rainstorms and high 
winds was observed in the aspen-fir association throughout the grow- 
ing season of 1916, and the results are shown in figure 18. 

Practically always when the pressure dropped appreciably below 
normal a change followed in the weather. While the amount of 
precipitation and the movement of the wind are not necessarily pro- 

l Zon, Raphael, Meteorological observations in connection with botanical geography, 
agriculture, and forestry, Monthly Weather B-eview, April, 48; 217-23, 1914* 



CLIMATE AND PLANT GROWTH. 



35 



/NCHES OF PRESSURE 



^ *\i 

ci o 



< )K v/ A: 





FIG. 18. Barometric pressure and its relation to storms, 



36 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 

portional to the pressure below normal, figure 18 indicates that 
local observations of pressure are of some value in forecasting 
changes in the weather conditions where the daily weather map is 
not available. As stated, however, the relation of pressure to vege- 
tative activities can best be expressed by the summation of certain 
other factors correlated with* pressure and known to exert a direct 
influence on the development of the plant. 

SUMMARY OF CLIMATIC COMPARISONS. 

The climajtic characteristics of the plant-type zone may be summed 
up as follows: The mean annual temperature is highest in the least 
elevated type zone and decreases gradually with the increase in 
altitude until, in the spruce-fir association, the season of growth 
covers a period of only TO days. In the oak-brush type zone the 
growing season is approximately 120 days. Precipitation, on the 
other hand, is normally only about half as heavy in the oak-brush 
type as in the type zones above. In general, however, the precipi- 
tation is somewhat heavier in the aspen-fir than in the spruce-fir 
type zone. The precipitation is rather uniformly distributed 
throughout the year. The evaporation is highest in the oak-brush 
type, where the greatest heat units and least rainfall are recorded. 
The evaporation factor is nearly as intensive in the spruce-fir type, 
however, while in the aspen-fir association it is only about half as 
great. The strong evaporation in the spruce-fir type is accounted 
for by the high wind velocity, which often exceeds 40 miles per hour 
for several hours in succession. The seasonal wind movement in the 
spruce-fir type is approximately 100 per cent greater than in the 
associations below. The possible and actual sunshine are found to 
be practically identical in the respective types. The barometric pres- 
sure, of course, varies with the elevation, but the seasonal fluctua- 
tions in a given locality are slight and insignificant so far as con- 
cerns any direct effect on the vegetation. 

TEMPERATURE SUMMATIONS. 

Owing to the mass of climatic data compiled, it was necessary to 
simplify them by summarizing 1 on different bases. 

The temperature factor in the respective stations for the periods 
during which the plants were under observation was summarized in 
three ways: (1) By physiological temperature coefficients as de- 
veloped by Lehenbauer 2 and later applied by Livingston; 3 (2) the 

1 The literature relative to methods of comparative summations of climate has been 
reviewed by Abbe, Cleveland, First report on the relation between climate and crops. 
U. S. Weather Bureau Bull. 36, 1905. 

2 Lehenbauer, P. A. Growth of maize seedlings in relation to temperature. Physiol. 
Res. 1:247-288. 1914. 

8 Livingston, Burton " E. Physiological temperature indices for the study of plant 
growth in relation to climatic conditions, Physiol. Res. 1 : 399-420. 1916. 



CLIMATE AND PLANT GROWTH. 37 

sum of .the positive or effective temperatures, that is, the sum of the 
means above 40 F., as originally proposed by Merriam, 1 and (3) the 
sum of the daily mean temperatures. For comparison with plant 
growth in this study, the sum of the temperature efficiencies for the 
growth periods has been used instead of the average temperature 
efficiency. This was done for the reason that the plant measurements 
represent total growth for the respective periods. 

Physiological temperature coefficients are based upon data obtained 
by Lehenbauer in the study of the elongation of the shoots of maize 
sprouts when exposed to practically constant temperature for 12-hour 
periods. These 12-hour exposures were made degree by degree at 
temperatures ranging from the minimum at which growth takes 
place, through the optimum, and on to the maximum temperature at 
which growth is possible. Varying increments of elongation 
naturally took place according to the temperature to which the 
sprouts were exposed; and these growth rates were platted against 
the temperature used, giving a curve showing the relation between 
temperature and the rate of growth of the plant. The lengths of *the 
ordinates of this growth curve furnish a series of numbers which rep- 
resent the efficiency of the various temperatures in promoting the 
growth of maize. The application of the physiological temperature 
coefficient to any plant other than the one used by Lehenbauer is 
based on the assumption that the general relation of growth and 
temperature is the same as for the maize! Whether or not the physio- 
logical temperature indices obtained under controlled conditions will 
apply to field plants where the temperatures fluctuate widely can not 
be stated. It may be presumed for the present, however, that they 
will more closely account for physiological responses of field plants 
than will direct temperature summations. 

Since these indices are based on physical and chemical processes 
taking place within the plant, temperatures at which -no appreciable 
activities take place are at once eliminated; at the same time the 
efficiency of the temperature up and down the thermometer scale 
receives the proper weight. 

In applying this method of temperature summation the daily 
mean temperatures were first obtained from the hourly corrected 
thermometer readings for the period during which the plants in the 
type stations were grown. The corresponding physiological indices 
were then substituted for the daily means and these indices summed 
for the period in question. 

By positive or effective temperatures is meant the number of de- 
grees of temperature above the minimum at which growth can take 

1 Merriam, C. Hart. Life zones and crop zones in the United States. U. S. Department 
of Agriculture Bull. 10 : 55-73. 1898. 



38 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



place. On the basis of many plants studied this minimum may be 
placed at approximately 40 F. Hence in the periodic and seasonal 
temperature summations, the daily mean temperature less 40, the un- 
effective growing temperature, were added. These summations of 
effective temperatures were made, in the case of each of the type sta- 
tions, for the period m toto during which the plants were grown, as 
well as for shorter periods. This method involves a slight error, since 
in a few instances during the growing season the mean dropped be- 
low 40 ; but the error thus introduced is so small as to be quite 
negligible. 

The sum of the daily mean temperatures was obtained from the 
hourly corrected thermograph readings and added according to defi- 
nite periods. These summations are presented chiefly for purposes 
of comparison with the other two methods of summation described. 

The temperature summations by the three methods are given in 
Tpble 11, in Section A, of the table for the plants that were started 
June 13 and grown until killing frosts arrested their activities, and 
in Section B, for those started several weeks later and grown until 
inclement weather set in. 

TABLE 11. Temperature summations, in degrees Fahrenheit, for period of 
growth of potometered plants in type stations. 



SECTION 



Type. 


Duration 
of period. 


Sum of 
daily mean. 


Sum of 
positive 
tempera- 
ture. 


Sum of 
physio- 
logical tem- 
perature 
efficiency. 


Oak-brush 


Days. 
81 


F. 
5 034 


D'J. 

1 789 


Index. 
2 473 7 


Aspen-fir 


95 


5 445 


1*404 


1 560 6 


Spruce-fir > 


91 


4 631 


'991 


'730 5 













SECTION B. 



Oak-brush 


59 


4 330 


1 528 


1 938 2 


Aspen-fir. . . 


70 


3 932 


1 132 


1 025 8 


Spruce-fir 


65 






' 48fi* 1 













1 Section A of the table has reference to fig. 19, and Section B to fig. 20. 

It should be pointed out that the temperature summations in the 
case of the oak-brush type are for a period of 81 days, which marks 
approximately the time required for the maturity of the plants. The 
summations in the aspen-fir and spruce-fir associations are for 95 
and 91 days, respectively. Owing to the relatively low temperatures 
the plants in the two latter types did not reach maturity, killing 
frosts having occurred early in September. From the temperature 
summations in figure 19, therefore, it should be understood that the 



CLIMATE AND PLANT GROWTH. 



39 



/3 



-2 



24737 



\ 



\ 



-4 



-2 



QlDtys 






\ 



N 



SIDays 



Oak-Brush Type 



Spruce Fir 



Aspen Fir 

- Temperature above 4OF 

--- Phys/o/oc//cd/ Temper-dfune eff/c/ency 

--- Sum ofc/&//y /Dean 

FIG. 19. Temperature efficiency summations for period of growth of plants used in main 

experiment. 



40 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



data given for each station represent slightly different numbers of 
days. In figure 19, as well as in certain other graphs, the curves are 
comparable in each case as to direction of slope; but it should be 



/s 
14 
/3 

/^ 
U 

10 
9 
8 
7 

S 
4 
3 
2 
1 



isze 


\ 




\ 




\ 




\ , 






\ 


II3Z 

X 






\ 


I938.Z 


\ 
x 

x 


\ 


4-330 


N 
X 

S ~~"~ _ 


\ 




\ s 3 93 a 

X 
X 








N 

\ 
X 

\ 


^ ^ 


685 

3285 




N 

\ 

10 as. 8^ 










^, 
\ 








rr^ 








>v^ 


466. / 










81 Days 70 t 


lays 65 L 


lays 


Oak-Brush Type Aspef 


i-Fir Spruce-Fir 

e 40F 
ns 

ipra-f-iire F-F-firioni*\/ 


Sum Of Daily Mea 
. Phvs/olortical Tpmr 



PIG. 20. Temperature efficiency summations for period of growth of special experiments. 

understood that the lengths of the ordinates are not in. all cases di- 
rectly comparable ; the vertical scales employed are merely convenient 
ones and are quite arbitrary. 



CLIMATE AND PLANT GROWTH. 



41 



It is a noteworthy fact that the summed physiological temperature 
coefficients and the sum of the positive temperatures; that is, those 
above 40 F., bear practically the same relation to each other in the 
respective type stations. This has also been observed to hold true, 
in general, for shorter periods (fig. 20). Neither of these summa- 
tions, however, agrees with the sum of the daily temperatures. As 
will be shown elsewhere, both the physiological temperature coeffi- 
cients and the sum of the positive temperatures show some relation to 
growth and other plant activities. This does not appear to hold true 
of the summation of the daily mean temperature. Because of the 
corresponding slopes of the graph in figure 19 between the physio- 
logical temperature summation and the sum of the positive tempera- 
tures, either may be used for comparison with the plant-growth data 
in the case of the batteries observed for the period in question. 

CORRELATION BETWEEN GROWTH AND ENVIRONMENTAL 

FACTORS. 

RELATIVE DEVELOPMENT OF THE PLANTS IN THE TYPE STATIONS, AND THE 
CORRESPONDING WATER REQUIREMENTS. 

A summation of the data obtained for the development and water 
requirements in the different type stations of wheat, peas, and brome 
grass (based on dry weight of tops) is given in Table 12. These 
figures represent the activities of the plants for 81 days in the oak- 
brush type, 95 days in the aspen-fir type, and 91 days in the spruce- 
fir type. The temperature indices and evaporation summaries for 
the respective periods are given in figures 19 and 8. 

TABLE 12. Summation of growth and water requirements of plants developed 

in the type stations. 



Type. 


Plant. 


Average 
stem height 
of peas and 
leaf length 
of wheat 
and brome 
grass. 


Number 
of leaves. 


Water 
require- 
ment 
per unit 
dry 
matter. 


Oak-brush 


Wheat 


Mm. 
3,990 


28 


Grams. 
626 




Peas 


4,781 


206 


779 




Brome grass 


15,980 


125 


803 


Aspen-fir 


Wheat 


8,560 


53 


288 




Peas 


11,863 


398 


368 




Brome grass 


22,290 


144 


516 


Spruce-fir 


Wheat 


5,280 


26 


300 




Peas 


5,584 


166 


345 




Brome grass 


8,114 


81 


756 













The values given in Table 12 are platted in figures 21, 22, and 23, 
The most striking features brought out in the graphs are (1) the 
greater vegetative development, including number of leaves, leaf 



42 



BULLETIN 700, IT. S. DEPARTMENT OF AGRICULTURE. 



length, and stem height, in the aspen-fir association, and (2) the rela- 
tively high water requirement for the production of a unit of dry 
matter in the oak-brush type. 

In the case of peas, the number of leaves produced in the aspen-fir 
type, as compared with the oak-brush and spruce-fir types, respec- 
tively, is approximately in the ratio of 4, 2, and 1.7. The leaf length 
of wheat shows a ratio of about 2, 1, and 1.3 in favor of the aspen-fir 



626 



26 



5280 
26 



3990 



\ 



s 



300 



88 



Oak-brush dspen Spruce-fir 

L eaf length 

A/umber of /eaves 

Water requirement per 

un/f dry matter 

FIG. 21. Water requirements and vegetative growth of wheat in the three climatic types. 

association. In the case of the brome grass practically the same 
relations exist. 

In each instance the water requirement per unit of dry matter is 
the highest in the oak-brush type. The fact that the plants were 
grown for a longer period in the highest and middle stations would 
naturally imply that they used more total water, but not necessarily 
that they had a higher water requirement per unit of dry weight. A 
comparison shows that in the case of wheat and peas the water 
requirements are very nearly the same in the central and in the 



CLIMATE AND PLANT GROWTH. 



43 



highest types, while brome grass shows a greater demand for water 
in the spruce-fir type than in the aspen-fir type. All three species ex- 
hibit a markedly interesting relation of development to water re- 
quirement, namely, that the lowest water requirement for the 
production of a unit of dry matter is invariably associated with the 
most luxuriant growth. Further, figure 8 shows that the evapora- 



/o 





\ 



\ 



V 



\ 



\ ^ 

\ 
\ 

\ 



5564 



\ 



66 



dspen 



Spruce-F/r 



stem 
--- 1/1/3 fer require/Den t per 

un/'f dry weight 
1 Number of/eai/es 

FIG. 22. Water requirements and vegetative growth of Canada field peas in the three 

climatic types. 

tion curve, corresponding to the period of growth of the plants, 
slants in the opposite direction from those of the development of 
the plants as platted in figures 21, 22, and 23. In the oak-brush type, 
where the water requirement is highest, evaporation is most intensive. 
The data on the relative development of the plants in the type 
stations and the corresponding water requirements are especially 
important, since they represent vegetative activities throughout the 



44 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 

season. Hence these data appear to throw some light on the causes 
of failure or success of experimental trials with plants in the types 
represented. 



/4 
/3 
/2 



22290 



\ 



\ 



/S980 



\ 



61 






756 






Oa/c 

Leaf/engfh 

A/ umber of /eaves 

Water requirement per 

unit- dry matter 

FIG. 23. Water requirements and vegetative growth of brome grass in the three climatic 

types. 

THE EFFECT ON PLANT GROWTH OF DIFFERENCES IN THE AMOUNT OF HEAT 
AVAILABLE IN THE THREE TYPES. 

EFFECT OF TEMPERATURE AS INDICATED BY DIFFERENCES IN WATER REQUIREMENTS 
BASED UPON STAGE OF DEVELOPMENT AND CONDITION OF CERTAIN AERIAL PARTS. 

The total effective heat units and length of growing season in the 
types studied are such that only in the lowest type do the plants reach 



CLIMATE AND PLANT GROWTH. 



45 



full maturity. 1 In the case of Experimental Series No. 1, in which 
the standard plants were started simultaneously in all types as soon 
as the temperature favored growth in the highest type, wheat heads 
filled well in the aspen-fir type but killing frosts occurred before the 
caryopsis hardened thoroughly; in the type above, growth was ar- 
rested when the heads were still in a developmental stage. Kecords 
of these plant specimens, supplemented by those of the late planted 
batteries (Experimental Series No. 2) afforded data as to the relative 
water requirements of plants in different stages of development. 
Because of the difference in age of the plants in the early planted 
and later planted batteries, the two sets of specimens were really sub- 
jected to different environmental conditions, for it is well known 
that the same weather factors do not affect plants in different stages 
of development in the same way. For this reason the water require- 
ments of the two sets of plants may not be entirely comparable. 

For the purpose of comparisons between the water requirements 
of the entire tops of wheat and brome-grass specimens, heads in- 
cluded, and tops without the heads, the dry matter of the specimens 
with and without heads was recorded and the water requirements of 
each determined. The results are summarized in Table 13. 

TABLE 13. Relation of water requirements of wheat and brome grass, to effective 
temperatures in climatic types. 

WHEAT (EXPERIMENTAL SERIES No. 1). 





Water requirement 
per unit dry weight. 


Per cent of 
difference 




Types. 




between 
water 
require- 
ments 
without 
heads and 


Tempera- 
ture sum- 
mation 
above 
40 F. 


Without 
heads. 


Including 
heads. 








with 










heads. 






Grams. 


Grams. 




Degrees. 


Oak-brush 


857 


626 


37 


1 789 


Aspen-flr 


358 


288 


24 


1 404 


Spruce-fir . . . 


355 


300 


18 


991 





WHEAT (EXPERIMENTAL SERIES NO. 2). 



Oak-brush. 


600 


504 


19 


1.528 


Aspen-fir 


407 


354 


15 


1 132 


Spruce-fir 


391 


391 


00 


685 













BROME GRASS (EXPERIMENTAL SERIES No. 1). 



Oak-brush 


1.303 


803 


62 


1,789 


Aspen-fir . . 


736 


516 


43 


1,404 


Spruce-fir 


853 


756 


14 


991 













The above values, platted in figures 24, 25, and 26, exhibit a 
gradual falling off from the lowest to the highest station in the ratio 

1 Owing to the early maturing qualities of mountain brome grass, this species more 
nearly reached maturity in all types than did the cultivated plants. 



46 BULLETIN 700, TJ. S. DEPARTMENT OF AGRICULTURE. 



/e 



/7 



/& 



/S 



/.3 



/2 



/O 






404 



\ 



/a 



s 



S 



991 



\ 



JOO 



Brush 

Temperature above 4O* 

Wafer r?es]uir&menr without heads expre.sse.af 

in percent of wate.r requirement with heads 
^ Wheat w/th Heads \ Wat&r ^ perpJanr 

WheCLt without Heads j 

fio. 24. Water requirement of wheat based on weight of plant including heads, compared 
& water requirement; based on. weight without heads. (Experimental Series No, 1.) 



CLIMATE AND PLANT GROWTH. 



47 



1526 



1132 



665 



600 
'0 




354 



Oak - Brush Type A spen - Fir Spruce - Fir 

Wheat Including Heads\ Water Used 

Wheat Without Heads ] Per Plant 

Water Requirement without heads expressed in 

percent of water requirement with heads 

" Te.mper3tu.re, above. 4-0 f. 

FIG. 25. Water requirement of wheat based on weight of plant including heads, compared 
with water requirement based on weight, without heads, (Experimental Series No, 2.) 



48 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



to 

/9 
/8 
17 
/6 
/5 
W 
/3 
12 
II 
10 
9 
8 
7 
6 
5 
4 
3 
l 



/76i 



1303 



809 



\ 

73 i 



5/6" 



SSI 



653 



7S6 



Oak Brush - Type A spen - Fir Spruce - Fir 

Temperature Above 40 

\W.R. Without Heads Expressed 
\tn Percent Of W.R. With Heads 

- Brome Grass.- Heads 1 Water Used 

Brome Grass + Heads J Per Plant 

FIG. 26. Water requirement of brome grass based on dry matter of plant including 
as compared with water requirement hased on dry matter without heads. 



CLIMATE AND PLANT GROWTH. 49 

of the water requirements of wheat and brome grass without heads 
to those with heads. The difference is rather pronounced. In the 
case of wheat in the oak-brush type (fig. 24) it is 37 per cent, in the 
aspen-fir association 24 per cent, and in the spruce-fir type 18 per 
cent. In the case of less mature wheat specimens (fig. 25) the dif- 
ferences between water requirement of the plants without heads and 
those plants with heads is 19 per cent, 15 per cent, and zero ; for brome 
grass (fig. 26) it is 62 per cent, 43 per cent, and 14 per cent. 

The differences in the water requirement figures serve to show one 
of the responses of the plants to the different amounts of heat avail- 
able in the three associations. In the oak-brush type, where the num- 
ber of heat units is greatest, the plants are matured or nearly so, and 
a large proportion of the total dry matter of the plants is deposited 
in the seed heads. At the middle and upper stations, where the 
summed seasonal temperature efficiency is lower, the plants are less 
mature, and a correspondingly lower proportion of the total dry 
matter of the plants is deposited in the heads. This difference in the 
stage of maturity would seem, then, to account for the difference in 
the water requirements of the plant with heads and without heads, 
and the difference itself affords an approximate measure of the rela- 
tive development and maturity of the plants in the different types. 

It is noteworthy that in the figures representing the ratio of the 
water requirements of the plants based on (1) the tops, including 
heads and (2) the tops without heads (figs. 24, 25, and 26) the curves 
in each case fall, from the lowest to the highest type, in a manner 
roughly proportional to the fall in the temperature summations. 
This agreement in slope shows that the plants mature more slowly 
as the number of effective temperature units decreases. 

EFFECT OF TEMPERATURE AS INDICATED BY PERIOD REQUIRED FOR PRODUCTION OF 

FLOWERS. 

Additional data showing the relation of the development of the 
plant to temperature were obtained by noting the number of days 
required for the first appearance of flowers in the species grown in 
the type stations. In each instance temperature summations and aver- 
age mean temperatures were recorded for each period, the results of 
which are summarized in Table 14. 

56866 18 Bull. 700 4 



50 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



TABLE 14. Periods required for the production of flowers in the vegetative types 
and temperature summations and average mean temperatures for the respec- 
tive periods. 



Type station. 


Peas. 


Wheat. 


Brome grass. 


Days 
for 
appear- 
nace of 
blos- 
som. 


Temperature. 


Days 
for 
appear- 
ance of 
heads. 


Temperature. 


Days 
for 
appear- 
ance of 
heads. 


Temperature. 


Above 
40 F. 


Average 
daily 
mean. 


Above 
40 F. 


Average 
daily 
mean. 


Above 
40 F. 


Average 
daily 
mean. 


Oak-brush 


43 
64 

88 


F. 

1,007 
988 
980 


F. 
64 
56 
51 


40 
52 
71 


F. 

915 

785 
847 


F. 
69.2 
55.2 
52.0 


39 
52 
63 


F. 

785 
685 
660 


F. 
60 

58 
52 


Aspen-fir 


Spruce-fir 





Figures 27, 28, and 29 platted from Table 14 show a rather pro- 
nounced parallelism in the different figures in the trend of the curves 
from the lowest to the highest station representing the number of 
days required for the flowering of the species, on the one hand, and 
in the curves representing the number of heat units up to time of the 
production of flowers, on the other. Provided no other factor 
was operative in holding back growth in the case of the plants in 
question it would appear that temperature was the controlling factor 
in this instance. In the case of peas, flowers appeared 21 days earlier 
in the oak-brush type than in the aspen-fir type and 45 days earlier 
than in the spruce-fir type, the period between planting and flower- 
ing in the spruce-fir type being more than twice as long as the corre- 
sponding period in the oak-brush type. Wheat spikes appeared in 
the lowest type in 40 days; while in the central and highest types 
they began to show 12 and 31 days later, respectively. In the case 
of mountain brome grass, panicles showed in 39 days in the lowest 
type; but in the central station they did not begin to show until 13 
days later and in the spruce-fir type 24 days later. One of the most 
interesting facts brought out in these observations is that in spite of 
the fewer days required for flowering in the oak-brush type a great 
many more flowers were produced than in the other types. In the 
case of wheat, for example, 40, per cent more heads appeared in the 
oak-brush than in the aspen-fir type, and over 100 per cent more than 
in the highest type. 

The fact that there is very little slope in the effective temperature 
summation curves and in the average daily mean temperature curves 
in figures 27, 28, and 29 shows that practically the same number of 
heat units were required in each type for the production of flowers. 
On the physiological basis of temperature summation for the entire 
season, as has previously been pointed out, there were notably more 
heat units in the lowest type. The lowest temperature efficiency was 



CLIMATE AND PLANT GROWTH. 



51 



recorded in the spruce-fir type, while in the central type the physio- 
logical temperature efficiency was intermediate. Since a habitat with 
low -growing temperatures requires a greater number of days for the 



10 








X 


86 


17 


/ 




/o 


'X 

x 
X 

.X 








x 










x 

X 

x 

X 




11 

10 
9 
8 

7 
6 

5 

* 

c 




X 
X 


-^ Days 






X 
X 






/007 


X 
X 

X 
X 
X 

X 

X 




~980 





X 














64 


: 








rac> 


" ~~ ~ " " "*- . 


51.0 










^ak- Brush Type Aspen -Fir Spruce -Fir 
_ Davs For Ab/oearance Of Heads 



Temperature Above 40 F. 

' Average Daily Mean Temperature 

FIG. 27. Relation of temperature to time of first appearance of blossoms in peas. 

plant to reach a given stage of maturity than a warmer situation, it 
is evident that the physiological temperature indices would be in in- 
verse ratio to the time required to bring the plant to a given stage of 
maturity. 



52 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



The difference in the time for the plants to reach approximately 
the same stage of maturity in the type stations may account, in part 
at least, for the difference in the character and composition of the 
vegetation in the respective types as well as for failures to establish 
exotic and indigenous species adapted to types of higher effective tem- 
peratures. In the spruce-fir type only those species which can com- 



16 
IS 
14 
/3 

II 
10 
9 
8 
7 
6 
S 



71 



52 



9/5 



64-7 




785 



69.2 



5Z.O 



Oak-Brush Type 



As pen -Fir Spruce -Fir 

------- Days For Appearance Of Heads 

--- Temperature. Above 40 /\ 
---- Averac/e Dally Mean Temperature 

PIG. 28. Relation of temperature to time of first appearance of heads of wheat. 

plete their development to maturity in minimum time, provided, of 
course, that their perpetuation is dependent wholly or primarily on 
seed, are conspicuously in evidence and of economic importance. 
This tendency toward early maturity is evident, for example, in the 
case of mountain brome grass, less clays beirg required in all type 
stations for its flower production than for that of peas and wheat. 



CLIMATE 'AND PLANT GROWTH. 



53 



EFFECT OF TEMPERATURE AS INDICATED BY WATER REQUIREMENT PER UNIT OF 

LEAF AREA. 

The water requirements per unit of area of the chief food manufac- 
turing agents of plants the leaves may be used as an index of the 



63 



99 



660 



60 



f* 



Oak-brush 



Aspen 



Spruce-fir 



for appearance of heads 

Temperature above 4O 

Average c/ai/y mean temperature 

PIG. 29. Relation of temperature to time of first appearance of heads of brome grass. 

efficiency of the leaves as users of water. The leaf length is used in- 
stead of actual leaf area in the data given below, since, as has been 
previously shown, it is proportional to the actual area. 



54 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



TABLE 15. Water requirements per millimeter of leaf length of wheat and 
1)rome grass in type stations. 







Per cent 
difference 


Water requirements 
per millimeter leaf 




Type. 


Temper- 
ature 
above 


in water 
require- 
ments 
between 


length. 


Physio- 
logical 
temper- 
ature 










wheat and 
brome 


Wheat. 


Brome 
grass. 


efficiency. 






grass. 










Degrees. 




Grams. 


Grams. 


Index. 


Oak-brush 


2,002 


312 


1.000 


0.321 


2, 706. 2 


Aspen-fir 


1 404 


195 


526 


273 


1,560 5 


Spruce-fir 


991 


127 


.407 


.319 


730.5 





Wheat uses nearly twice as much water per millimeter leaf length 
in the oak-brush type as in the aspen-fir type, and more than twice 
as much as in the spruce-fir type (fig. 30). In other words, water 
appears to be used most conservatively by a unit of wheat-leaf area 
in the type showing the lowest physiological temperature efficiency 
and temperature summation above 40 F., and most extravagantly 
in the type of highest temperature efficiency. Hence the curve rep- 
resenting the water requirement of wheat and the temperature 
summation curves fall from the type lowest in elevation to that of 
highest altitude in the same general way. 

In the case of brome grass the water requirement of the leaves is 
found to be practically the same in all types in spite of the difference 
in the climatic conditions and in the stage of development of the 
plants. The reason for this dissimilarity between the two species 
is not entirely clear, but it may be related to the fact that mountain 
brome grass does not naturally inhabit the oak-brush type, though, 
indeed, the specimens observed appeared to develop normally. 

In all instances a given leaf area of brome grass has a lower water 
requirement than wheat. Notwithstanding this fact, however, the 
water requirement per unit of dry matter for the plant as a whole, 
as previously shown, is greater for brome grass than for wheat. 
This is largely accounted for by the fact that the aerial part of 
mountain brome grass consists essentially of leaf blades, while a 
large proportion of the aerial dry matter of wheat is made up of 
stems and heads, the transpiration from which is low as compared 
with leaf surface. This relation between the water requirements of 
leaves of the two species is further shown by the curve representing 
the water requirement per millimeter of leaf length of wheat ex- 
pressed as a percentage of the water requirement per millimeter of 
leaf length of brome grass. These percentages, which are 312 in the 
oak-brush type, 195 in the central station, and 127 in the spruce-fir 
tvpe, indicate that wheat becomes relatively more efficient in the 
use of water as compared with brome grass as the temperature falls. 



CLIMATE AND PLANT GROWTH. 



55 



Furthermore, the curve representing these percentages bears an 
intimate agreement with both of the temperature summation curves. 
This increased conservatism in the use of water in wheat leaves may, 



2002 



\ 



X \ 



\ 



\ 



\ 



\ 



\ 



\ 



Aspen ^Spruce Fir 

Temperature aboise: 4-O* 

Hfafer repu/remerrte per rum. Leaf Lenqfr?( Wheat) 

Ptysto/oqicaj Temperature Efficiency 

Water requirements per mm. Leaf LerHfth[Brome-qrass J 

FIG. 30. Water requirements per unit (1 mm.) of leaf length of brome grass and wheat. 

in a way, account for the high yielding qualities of wheat, other 
conditions remaining the same, in regions where the summers are 
relatively cool. 



56 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



EFFECT OF EVAPORATION AND TEMPERATURE ON THE PRODUCTION OF DRY 
MATTER PER UNIT OF LEAF 



Quite as significant as the difference in water requirements per 
unit of leaf area in the respective types is the effect of climatic con- 
ditions on the efficiency of the leaves as manufacturing agents. This 
has been calculated for wheat and brome grass, the same specimens 
being employed as were used in deriving the water requirement data. 
The summations are given in Table 16 and in figure 31. 

TABLE 16. Dry matter produced per millimeter of leaf length of wheat and 
l)rome grass in the type stations. 



Type. 


Dry weight. 


Per cent 
difference 
in dry 
weight of 
wheat and 
brome 
grass. 


Evapora- 
tion 
summa- 
tion. 


Physio- 
logical 
temper- 
ture 
efficiency. 


Wheat. 


Brome 
grass. 


Oak-brush 


Grams. 
0. 00161 
.00182 
.00136 


Grams. 
0.00040 
.00054 
.00042 


403 

338 
324 


cc. 
4,550.0 
2.780.3 
4,251.3 


2, 706. 2 
1, 560. 5 
730.5 


Aspen-fir 


Spruce-fir 





Both curves in figure 31 representing the dry matter per unit of 
leaf area show a maximum for the central type, the greatest con- 
cavity upward occurring in the curve for wheat. This species also 
shows a slightly greater production in the oak-brush type than in 
the spruce-fir type. In brome grass the reverse occurs, but in neither 
instance is the difference particularly marked. 

It is significant that the curve representing the production of dry 
matter is opposite in slope to the curve showing the evaporating 
power of the air. The data indicate that evaporation decreases the 
rate at which the leaves manufacture food material, and the simi- 
larity in the production of dry matter in the case of both species in 
the three types may thus be accounted for by corresponding simi- 
larities in the evaporation conditions. 

Another interesting parallelism is derived by dividing the dry 
matter per unit of leaf length produced by wheat by the quantity 
produced by brome grass. In this instance the curve is seen to fall 
from the lowest to the highest type in the same general direction 
as the physiological temperature efficiency curve. This apparent 
correlation between temperature and the efficiency of the leaves 
as manufacturing agents is of value, of course, only if it may be 
assumed that the physiological index affords a reliable expression 
of the relation between the temperature and the plants here dealt 
with. 



CLIMATE AND PLANT GROWTH. 



57 



21 



/9 



17 



/6 



14- 



13 



12 



10 



&O3 






\ 

\ 






\ 

\ 
\ ^ 


..00/82 
\. 




^^"^\ 


\ 




,oo/ei 


^ 


^ - -^ - __ 


324-T& 


43SO.O 




\ 






\ 


\. 


4-25f.& 


s 706.2 


\ 
\. >x 


A 
/ 


.OO /36 




\ S 


/ 






\ \ 


/ 
/ 






\ \. 


f 

/ 






\ x 


780.3 






\ 








1 


T./56O.& 








\ 








. OO05+ >w 

^ ^ X 




.OOO40 


pf ,?^-' 


--V. 


. OOO+2 






x 


7JO.S 



























Oak Brush 



Aspen 



Dry H/e/qht- (Brorrre-qra&s } 
Physio/oqica/ Temperature. 
Efficiency 

FIG. 31. Dry matter produced by one millimeter of leaf length by brome grass and wheat 

(heads included). 



58 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



EFFECT OF EVAPORATION AND TEMPERATURE ON THE GROWTH OF THE PLANT 

AS A WHOLE. 

It has been shown that the efficiency of the leaves, both as users of 
water and as producers of material, is much influenced by climatic 
conditions. The true significance of this fact, however, can be fully 
understood only when the development of the plant as a whole, 
including stem height, total seasonal and periodic leaf expansion, and 
similar activities, is correlated with the controlling climatic factors., 

Temperature and evaporation are, as the preceding discussion has 
shown, undoubtedly the limiting factors in the locality in which this 
investigation was conducted, and hence it is the chief aim to show 
physiological activities in relation to these factors. 

EFFECT OF EVAPOEATION AND TEMPERATURE ON THE GROWTH OF WHEAT. 

Since more measurements were taken in the case of wheat than of 
the other species, wheat is here selected to show correlations between 
its development and temperature and evaporation. In this connec- 
tion it should be stated that the correlations obtained between cli- 
matic factors and growth of wheat hold generally for the other 
species employed. 

Four sets of measurements of wheat and the daily temperature and 
evaporation obtained for the period of growth concerned were 
recorded in each type station simultaneously. The data are summa- 
rized in Table 17 and platted in figure 32. 

TABLE 17. Growth of wheat as related to evaporation and temperature in type 

stations. 

















Type. 


Evapora- 
tion. 


Temper- 
ature 
above 
40 F. 


Average 
stem 
length. 


Dry 

weight 
per plant. 


Leaf 
length. 


Water 
used per 
plant. 


















cc. 




Mm. 


Grams. 


Mm. 


Grams. 


Oak-brush 


39 563 


1 789 


1 100 


6 33 


3 938 


3 949 


Aspen-fir 


27 803 


1 404 


1 018 


15 61 


8 560 


4*499 


Spruce-fir 


42 513 


991 


830 


7 19 


5 221 


2 147 

















In figure 32 the direction of slope of the graphs representing total 
leaf length and average dry weight per plant is similar, a pronounced 
convexity upward occurring in the aspen-fir association. These meas- 
urements, then, are in inverse proportion to the evaporation. On the 
other hand, no apparent correlation exists between the curves repre- 
senting the average stem length and evaporation. Since the curve of 
evaporation bears no distinct correlation to the temperature summa- 
tion curve, it would appear that the height growth, or elongation of 
the plant, is determined more by the temperature than by the evapo- 
ration, the factor which apparently determines elongation and ex- 



CLIMATE AND PLANT GROWTH. 



59 



/s 

/7 

/e 
/ 

/4 

39663 
/3 

/2 
// 
/O 
9 

e 

7 
6.33 

e 

s 

3949 -. 
4 

3 
2 

/ 


/7fl9 


y^ 

"X 

*x 


\ 

v 


\ 

V / 


k/^"/ 


y.. 


/^O4\ j 


4ZS/3 


( / 


x \ 

"v\ / 




x / 


\ 7 

\V X 




'too '*/ 


NX 

A x 




~~y~^. H 


x \ x 

/o/s / \ V 


99/ 


/ 2 


x ^*^ \ 

^7803 "^. \ 




/ 


**, ^-V 


830 


/ 


\ 


7/5 


/ 


X 
X 
X 




/ 
/ 

/ 


X 
V 


527.1 


7 


^439 




3938 


*^ 
% 
^^^ 






v x 

\ 


2/47 















Osk-br-ush 



Aspen 



Sprue e-F/'r* 



Evaporation 

Temperature a bo ve 4-O ~ 

/ll/ersge stem fengfh 

Dry weight per p/ant 

L eaf length 

l/Vafer used per plant 

PIG. 32. Comparison between climatic factors and the growth of wheat. 



60 BULLETIN 700, TJ. S. DEPARTMENT OF AGRICULTURE. 

pansion of the leaf. The curve representing the water used per plant, 
on the other hand, is more or less intermediate in slope between the 
temperature and evaporation summation curves, and it is indicated 
that the water used per plant is determined both by temperature and 
evaporation. 

The above figures are based on average measurements of 20 speci- 
mens in each type, but the data may not be adequate to justify the 
statement that leaf elongation and expansion of plants in general 
are locally, and under similar conditions, controlled more by evapo- 
ration than by temperature. Where the evaporation is especially 
high owing chiefly to factors other than high-wind movement, how- 
ever, as in the oak-brush type, the data appear to warrant the con- 
clusion that evaporation is the limiting factor in leaf expansion and 
consequently in the production of dry matter and other physio- 
logical activities of economic importance.. This conclusion is fur-- 
ther substantiated by the data presented in figures 30 and 31, show- 
ing on the one hand relatively high water requirement and on the 
other a correspondingly low production of dry matter in a unit of 
leaf area in the oak-brush type. The correlation between high evapo- 
ration and low production of dry matter may be explained either by 
the lack of proper turgor in the leaf cells during the long diurnal 
periods of high transpiration, or by the fact that egression of water 
molecules from the stomata and cells adjacent thereto is so great as 
to prevent free ingression of carbon dioxide essential to photosyn- 
thesis. 

From the lower border of the aspen-fir type (about 8,000 feet eleva- 
tion) throughout this association and in the less exposed sites of the 
spruce-fir type temperature and evaporation may exert 'equal effect 
on the plant. 

EFFECT OF EVAPORATION AND TEMPERATURE ON SEASONAL MARCH OF GROWTH RATES 
OF WHEAT AND BROME GRASS. 

While a measure of the relation of climate to the development of 
vegetation may be integrated by summarizing the climatic data 
and recording the dry matter produced by comparable plants during 
the entire growing season, the relation may best be known through 
concrete comparisons made at more or less regular intervals through- 
out the season. This is especially true of the more elevated regions, 
where weather within a season is subject to wide variation. Even if 
the relations between plant growth and weather were known, how- 
ever, the factors affecting growth vary in a more or less unpredictable 
manner, so that the yield of a given crop could not be correctly 
judged much in advance of actual harvest. 

If the assumption that leaf-expansion rate is retarded by evapora- 
tion is correct, the graph of evaporation platted period by period 



CLIMATE AND PLANT GROWTH. 



61 



for the season at any given station should show an opposite slope to 
the corresponding graph of leaf-expansion rate. In order to deter- 
mine whether such a relation exists, the rate of leaf expansion was 
calculated for the periods for which plant measurements were made, 
as was also the evaporation rate. 

The values of seasonal march of leaf -growth rate given in Table 
18, and graphically shown in figures 33, 34, and 35, represent average 
daily increase in length per leaf for the various culture periods. 
This quantity was obtained for each period by dividing the total leaf 








28.0 
SO. 9 



4.7 



* 



'9.4 






JT 



\ 



-50 



\ 



GROWING SEASON' 



Phys/o/og/cd/ fempera+ure 
efficiency 



Brome- grass 
--- Wheat 

FIG. 33. Periodic relation between leaf expansion, evaporation, and temperature, oak- 

brush type. 

length produced up to the time of measurement by the total number 
of leaves, thus giving the average length per leaf. The increase in 
the average length per leaf of the plants from period to period was 
then determined by subtracting from each of the average leaf-length 
values the corresponding value of the preceding period. Since the 
periods for which measurements were taken varied somewhat in time, 
these increases, in order to make them comparable, were reduced to 
daily rates by dividing each increase in average leaf length by the 
number of days in which the increase took place. The physiological 



62 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



temperature index and the evaporation rate were also expressed as 
average daily rates in order to make these climatic factors com- 
parable to the plant measurements. The data were recorded prac- 





2./0 




, 


, /9Q ^ ^s~ 
vX*^ %s ,x 




4OO 








S*( a/ X / 91 ^^ 




/3J 

72 


1_w^'''" >< N ^- 2 v. - ::t:: ^ 


27. 




6 *" "/V ^ / - if ' 






-07~' 




* Gf?0 \A///VG SEAS OA/ ^ 



Phys/o/og/ca/ Temperature 

efficiency 

Wheat 

Brome-grass 

Evaporation 

FIG. 34. Relation between increments of leaf expansion, evaporation rate, and tempera- 
ture, aspen-fir type. 

tically simultaneously at about 10-day intervals in each association. 
The first measurements here recorded are for June 22 10 days after 
establishing the.potometers. 

TABLE 18. Effect of evaporation and temperature on leaf expansion of wheat 
and bronie grass in type stations at about 10-day intervals, beginning June 
22, through growing season. 



Association. 


Brome 
grass. 


Wheat. 


Physio- 
logica'l tem- 
perature 
efficiency. 


Evapora- 
tion 
summation. 


Oak-brush .... 


Mm. 

o 


Mm. 

4 7 


19 4 


cc. 
59 6 






1.5 


40 6 


66 8 


Aspen-fir 


.50 
2.10 
2.50 
.41 

72 


1.9 
2.1 
.2 
.0 

5 5 


36.9 
36.2 
27.7 
28.0 

13 1 


56.7 
45.4 
37.4 
50.9 

40 






2.5 


21 


21 


Spruce-fir 


.64 
1.48 
1.90 
2.10 

2.00 


3-1 
1.2 

.7 
1.6 

1.90 


27.7 
19.5 
15.6 
10.6 

5.8 


42.1 
24.5 
24.0 
27.0 

50 




1.10 
1.90 
2.30 
6.50 
6.10 


.30 

2.40 
2.80 
2.10 

.85 


9.8 
12.5 
10.8 
7.4 
5.7 


83.1 
59-6 
38.8 
30.0 
49.0 



CLIMATE AND PLANT GROWTH. 



63 



6.10 



63 '' 



A. 



2.0 



501 







\ 



49.0 




5.7 



FIG. 



< GROWING SEASON * 

~~~ Physiological Temperature efficiency 

Wheat 

Brome -grass 

Evaporation 

35. Relation between increments of leaf expansion rate and temperature, spruce-fir 

type. 



64 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 

The curves (figs. 33, 34, and 35) representing leaf expansion, in 
some instances at least, follow the temperature curves, but in gen- 
eral slope in opposite direction to the curves portraying the evapora- 
tion rate. Exception to the latter occurs in the case of wheat in the 
aspen-fir type ; but data already presented have shown that evapora- 
tion is not a limiting factor for this plant in this type. Also it will 
be seen that growth toward the end of the season does not in all cases 
bear an inverse relation to evaporation. As an explanation of this 
fact it should be stated that the leaf growth is rapidly declining at 
that time, because of the approach of maturity of the plants. On 
the other hand, there appears to be no consistent correlation between 
growth increments and temperatures. The agreements that do occur 
also show a direct correlation between growth and evaporation, and 
hence it may be concluded that the temperature and growth relations 
recorded are more or less incidental. 

Considering the graphs more in detail, in the case of figure 33 the 
leaf increment curves of both species in practically all instances have 
concavities opposite in direction to those of the curve representing 
the evaporation. It should be stated that no growth data were ob- 
tained for brome grass in the second period of measurements, and 
therefore the slope of the line at that point is not in agreement with 
the growth curve for wheat, nor is it in opposition to the slope of 
the evaporation curve. In the case of wheat, the leaf expansion 
curve and the evaporation values platted for the first three periods 
show inverse relation. Between the fourth and fifth periods there is 
a slight disagreement in these values, but between the fifth and sixth 
periods the leaf measurement and evaporation curves again show 
opposite trend. The leaf increment curve for brome grass, on the 
other hand, is in inverse proportion to the evaporation in all in- 
stances. 

As would be expected, where evaporation is unusually high, as in 
the oak-brush type, temperature in general is high, and these factors 
usually run more or less parallel. This being the case, it is hardly 
to be expected that the growth increments, would follow the physio- 
logical temperature indices. As is well known, the growth rate in- 
creases with increase in temperature up to the optimum requirement 
of a given species. In the oak-brush type there is reason to believe 
that the temperature not uncommonly exceeds the optimum require- 
ments of the species observed. The lack of correlation between 
growth rate and temperature in the oak-brush type, then, would 
seem to strengthen the evidence that the evaporation in that asso- 
ciation is the determining factor in the rate of elongation as well as 
ultimate expansion of the leaf. 

In figure 34, in which similar data are given for the aspen-fir 
type, wheat, except in one instance follows the evaporation curve, 



CLIMATE AND PLANT GROWTH. 65 

hence to some extent that of the temperature; while in the case of 
brome grass the leaf expansion curve slopes opposite to the evapora- 
tion curve. In the central type, the evaporation being less than in 
the lowest and the highest types (fig. 8) , in no instance has proved a 
limiting factor for the type in question so far as concerns the physio- 
logical activities of wheat. In the case of brome grass, however, the 
contrary is true. The reason for the difference in the response of 
these species is not obvious. 

In the spruce-fir type figure 35 shows that the leaf increment 
values in the case of brome grass are in inverse proportion in every 
instance to those of evaporation. 1 Hence, between the first and third 
periods, when the highest rate of evaporation occurs, is recorded the 
lowest rate of leaf expansion for the entire period ; and between the 
fourth and sixth periods, which marks the lowest rate of evapora- 
tion, by far the highest growth rate is recorded. The rates of leaf 
expansion in wheat likewise show inverse relation to evaporation, 
though less pronounced than the leaf-expansion rates of brome grass, 
with the exception of the next to the last period. This disagreement, 
however, is explained by the fact that the wheat specimens were 
approaching maturity and the growth rate was, therefore, begin- 
ning to decline. 

As in previous instances, the curve showing the growth rate of 
wheat in the spruce-fir type seems to follow, in a general way, the 
daily temperature curve, but this is evidently more or less inci- 
dental. In order to determine these relations more definitely, aver- 
ages of daily leaf increment of wheat and of temperature and evapo- 
ration were computed for the season as a whole, the results of which 
are platted in figure 36. From these curves it is evident that leaf 
increment of wheat is in inverse proportion to evaporation, no obvi- 
ous relation to temperature being shown. This relation is also found 
to hold in the case of brome grass. 

To sum up the facts regarding the relation of leaf expansion to 
evaporation and temperature : The daily rate of growth of the species 
studied, as well as the total leaf surface produced, varies inversely 
as the evaporation, except in the case of the daily rate for wheat at 
the middle station. Evaporation in the aspen-fir type is lower than 
in the types immediately above and below. As a limiting factor the 
evaporation may be declared transitional in a sense that is, it 
may determine growth rate periodically or seasonally in one species, 
but not distinctly so in another. Since temperature and evaporation 
are admittedly more or -less interrelated, it is difficult to separate 

1 As previously shown, the high evaporation in the spruce-fir type is chiefly accounted 
for by high wind movement. 

56866 18 Bull. 700 5 



66 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 

them; consequently, their individual effect on the activities of the 
plant can not be definitely declared in all instances. 



/2 
/O 

S 
7 

S 

4 

3 
2. 


3-/..S, 






\ 




\ 




\ 1 


^.4-3 




^\ X 
X s 


^-^ 




x' \ 


*<S; 




/'' \ 


^ 

/8.O 


/.a 7 


773 


' 


\ 








\ 








\ 




S23 


(^ 


\ ; 


*/r 




^^ 




8.3 




^ 


^ 








23.7 





















Oak-bru^h 



/Ispert-fi'r 




FIG. 36. Average daily increment in leaf expansion of wheat throughout season compared 
with evaporation and temperature in type stations. 

EFFECT OF TEMPERATURE AND EVAPORATION ON WATER REQUIREMENTS OF PLANTS. 

Since the rate of growth and production of dry matter by plants 
appear to be controlled chiefly by evaporation and temperature, so 



CLIMATE AND PLANT GROWTH. 



67 



far as physical factors in the locality here dealt with are concerned, 
it appeared pertinent to determine whether or not the water require- 
ment is also correlated with the factors mentioned. The value of data 
showing the relative water requirement of different plant species has 
been demonstrated by numerous investigators. The climatic factors 
that chiefly affect the rate of dissipation of water through transpira- 
tion, on the other hand, have until very recently received relatively 
little attention, although such researches would appear to be of pro- 
found economic importance. If it were known, for instance, that in 
a region of limited rainfall the evaporation was largely responsible 
for the high transpirational demand and consequently the high water 
requirement of a given plant, habitats might be selected where vegeta- 
tion, natural barriers, or other features would afford protection 
against excessive evaporation. Likewise, if temperature were the 
factor determining the water requirement, cool north and east slopes, 
or possibly partially shaded sites might be selected at least at lower 
elevations and failure of crop production thus avoided. 

In order to determine the relation of water requirement of these 
plants to evaporation and temperature in the type stations, the water 
used per unit of dry matter by wheat, peas, and brome grass, through 
practically the entire growing season, was divided by the evaporation 
for the corresponding period. Tabular and graphic presentation of 
the results follows. 

TABLE 19. Effect of temperature and evaporation on water requirements of 
plants grown in type stations. 



Type. 


Species. 


Physio- 
logical tern- 


w aim'- 1 ty 
quirement 


evapora- 
tion for 




W. R. 






efficiency. 


per unit 
dry matter. 


period of 
growth. 




E. 


Oak-brush 


Wheat 


| 


Grams.. 
{ 626 


c c. 




158 




Peas 


[ 2 473 7 


J 779 


3 956 3 




197 





Brome grass 




803 






203 


Aspen-fir 


Wheat 




1288 






104 




Peas 


I 1,560.3 


368 


2 780 3 




132 




Brome grass 




516 






186 


Spruce-fir 


Wheat . . 


| 


{300 






071 




Peas 


> 730 5 


3 '5 


4 251 3 




081 




Brome grass 


j 


756 






178 

















As has been shown in previous graphs, the plrysiological tempera- 
ture efficiency is highest in the oak-brush type, the curve dropping 
in practically a straight line to the central and highest types. The 
quotients of the values derived by dividing the water requirement 
per unit weight of dry matter for the respective types by the evapo- 
ration for each period when platted (fig. 37) are likewise 



68 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



found to be highest in the oak-brush type, intermediate in the aspen- 
fir type, and lowest in the spruce-fir type, the curves in the case of 
each species following in a general way the physiological tempera- 
ture indices. 



/3 



2473.7 



<0 

9 
8 
7 
6 



/58 



^Os 



730.S 

7t 



Oak-brush 



Aspen-rir 

Physiological temperature 

e fficiency 
Brome -grass 



Spruce-Fir 



FIG. 37. Relation of the quotient of water requirement and evaporation C^ r - *M 

temperature. 



to 



On the basis of the agreements shown, it may be concluded that 
the water requirement of these plants is determined largely by 
evaporation and temperature. Hence it is evident that in localities 
of limited rainfall, high evaporation, and high temperature values, 
agricultural pursuits, even where the most drought-resistant species 



CLIMATE AND PLANT GROWTH. 69 

may be economically employed, should be confined to soils of high 
water-holding capacity and subject to minimum run-off, so that the 
soil may return to the plant a high percentage of the rainfall. 

SUMMARY. 

The data reported, pertaining as they do (A) to the climatic 
characteristics of three distinct vegetative associations, (B) to com- 
parative instrumentation and methods of summarizing and express- 
ing climatic factors, and (C) to correlations between environmental 
factors and plant growth and other physiological activities, may best 
be summarized under three heads. 

A. CLIMATIC CHARACTERISTICS OF THE PLANT ASSOCIATIONS. 

1. The mean annual temperature increases gradually from the 
highest to the lowest type, and this results in the longest growing 
season in the lowest type and a gradual decrease in the period of 
growth with increase in elevation. Thus from the time of the begin- 
ning of growth to the occurrence of killing frosts there are about 
120 days in the oak-brush type, 105 in the aspen-fir type, and TO 
in the spruce-fir type. 

2. The normal annual precipitation is greatest in the aspen-fir 
association but is only slightly heavier in this association than in the 
spruce-fir. Less than half as much precipitation is recorded in the 
sagebrush- rabbit-brush as in the aspen-fir association; and in the 
oak-brush type it is only slightly greater than in the sagebrush- 
rabbit-brush type. The precipitation is rather uniformly distributed 
throughout the year. 

3. Of the three associations^rlticaiiy studied^ the evaporation dur- 
ing the main growing season is greatest in the oak-brush type: but 
owing to high wind velocity in the spruce-fir type the evaporation 
is nearly as great as in the oak-brush type. In the aspen-fir type 
the evaporation factor is notably less than in the types immediately 
above and below. This is accounted for largely by the lack of high 
wind velocity, which is due to the luxuriant vegetation and to topo- 
graphic features. 

4. The wind movement is greater by about 100 per cent in the 
spruce-fir association than in the types immediately below. Not un- 
commonly the velocity of the wind exceeds 40 miles per hour for sev- 
eral hours in succession. In the lower types the velocity averages 
slightly less than half that recorded in the spruce-fir type. 

5. Sunshine duration and intensity are practically the same in all 
types studied. 

6. There is considerable difference in barometric pressure in the 
respective types, but the daily seasonal fluctuations within a station 
are slight. 



70 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 

B. COMPARATIVE INSTRUMENTATION AND METHODS OF SUMMARIZING EXPRES- 

SIVE CLIMATIC FACTORS. 

1. Temperature summations on a physiological basis according to 
the Lehenbauer plan have shown much promise in correlating air 
temperature with physiological plant activities. The summation of 
the effective temperature, namely, the temperature above 40 F., as 
proposed by Merriam, also appears to have much promise. This 
method in general compares favorably with temperature summations 
made on the physiological (Lehenbauer) basis. 

2. Summations of average daily mean and seasonal mean tempera- 
tures appear to have little value in showing correlations between 
the factor in question and physiological activities in plants. 

3. The evaporation for short periods, such as a part of a day or a 
fractional part of a week, for example, when compared with relative 
humidity, temperature, and wind velocity, can be obtained more 
accurately by means of the porous cup atmometer than by the free 
water surface evaporimeter of the Weather Bureau pattern. For 
periods of a week or longer either instrument will serve. 

4. In recording sunshine as related to plant activities, both dura- 
tion and intensity should be considered. Such records can be ob- 
tained approximately by noting the difference in evaporation between 
similarly exposed black and white porous cup atmometers.. These 
instruments appear to have some advantages over the Marvin sun- 
shine recorder, which furnishes a record only of sunshine duration. 

C. CORRELATIONS BETWEEN ENVIRONMENTAL FACTORS AND PLANT GROWTH. 

1. Lack of uniformity in the fertility and; of course, in the tex- 
ture of the soil in which the plants are grown may cause consider- 
able variation in their water requirements and in the total dry mat- 
ter produced. Soil collected within a restricted habitat often varies 
considerably in productivity, and unless thoroughly mixed may 
become an important source of error in experimentation. 

2. The total effective heat units and length of growing season in 
the three types studied are such that only in the lowest association 
do crops like wheat and peas reach full maturity. Hence, farmers 
should not attempt locally to grow the ordinary agricultural crops, 
such as cereals, above an elevation of about 8,000 feet. The eleva- 
tion at which there are normally sufficient heat units to develop and 
mature cereal crops in general varies, of course, with the latitude 
and longitude. 

3. The rate of maturity of the plants decreases directly as the 
effective heat units decrease, as is the case in passing from the lowest 
to the highest type. This decrease in the rate of maturity of the 
plant in the type stations may be shown, up to a certain point, at 



CLIMATE AND PLANT GROWTH. 71 

least, by the difference between the water requirements of the aerial 
parts of the plants, including the fruit or seeds (such as the heads 
of wheat) , and of the aerial portion without the seeds, the specimens 
with the best developed seeds, of course, having the highest water 
requirements. 

-L. The water requirement for the production of a unit weight of 
dry matter is greatest in the oak-brush type, lowest in the aspen-fir 
type, and intermediate in the spruce-fir type. These relations coin- 
cide with the relative intensities of the evaporation. 

5. In the case of all species employed, the total, and, indeed, the 
average leaf length and total dry weight produced are notably great- 
est in the aspen-fir association, these activities being rather similar 
in the spruce-fir and oak-brush types. The decreased production in 
leaf length and the production of dry matter in the respective types 
are in direct proportion to the evaporation. 

6. The elongation of the stem is greatest in the oak-brush type, in- 
termediate in the central type, and least in the aspen-fir type. Thus 
stem elongation appears to be determined largely by temperature and 
seems to be little influenced by the intensity of. the evaporation. 

7. The efficiency of the leaves per unit area as manufacturing 
agents that is, in the production of dry matter, appears to vary 
inversely with the evaporation, though, indeed, temperature appears 
to be one of the important factors. The largest amount of dry mat- 
ter per unit of leaf area is produced in the aspen-fir type and the 
least in the oak-brush type, while in the spruce-fir type, where the 
evaporation is only slightly less intensive than in the oak-brush 
type, the dry matter produced is only slightly greater than in the 
oak-brush type. 

CONCLUSIONS. 

From the study here reported, it may be concluded that in this 
locality Kubanka wheat and Canadian field peas, and doubtless 
other agricultural crops, can not be grown profitably at elevations 
exceeding about 8,000 feet because of the lack of sufficient heat. As 
has been shown by the crop production of the region, enough heat 
units were produced in the seasons studied up to an altitude of about 
8.000 feet, which includes most of the oak-brush type, to mature 
wheat, peas, and certain other crops. The amount of precipitation 
received at an elevation of 8,000 feet and lower, however, was below 
the requirements of crop production, indicating that the lands must 
either be irrigated or the moisture conserved by thorough summer 
fallowing. The native forage crop produced in the oak-brush type, 
on the other hand, is fairly luxuriant, and if properly utilized will 



72 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 

continue to be of high value in the pasturing of live stock. On the 
more favorable sites from the oak-brush up to and including the 
spruce-fir association, lands which have been overgrazed and are not 
fully stocked with vegetation may be increased in forage production 
by the seeding of suitable plants, preferably native species. 1 

Since evaporation is apparently the chief factor limiting growth 
and development of plants in the oak-brush and spruce-fir types, 
the extension of agriculture and forestry should be limited to lands 
protected from excessive evaporation. This may be done by select- 
ing sites that are more or less protected by native vegetation and 
natural obstacles. Failures in experimental plantings, in most in- 
stances, have occurred on wind-swept lands where the soil moisture 
becomes deficient early in the season. In the selection of species, 
either of herbaceous or of woody plants, only the most drought re- 
sistant sorts should be used. Failures in the case of the planting of 
suitable timber species in the central (aspen-fir) type will probably 
seldom be caused by adverse climatic conditions. Failures in this 
type may generally be traced to the employment of unsuitable stock, 
or to bad workmanship, wrong season of planting, or other pre- 
ventable causes. 



1 Sampson, Arthur W. Natural revegetation of range lands based upon growth re- 
quirements and life history of the vegetation. Journ. of Agr. Research, Vol. Ill, No. 2, 
1914. 



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