126 
Journal of Agricultural Research 
Vol. XXIV, No. t 
i 
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Fig. 2 .—General relationship between water requirements and growth rates for all species in 1917 and 1920. 
Table X .—Water requirements and growth in igiy and ig20 
1917 
Species. 
Num¬ 
ber of 
trees. 
Water 
require¬ 
ment. 
Growth 
percent¬ 
age. 
Timber pine. 
I 
Gw- 
T. TAA 
31.0 
46.6 
32.4 
31-3 
64. 9 
75*2 
Lodgepole pine. 
2 
•‘^44 
954 
555 
I, no 
684 
525 
Yellow pine. 
2 
Bristlecone pine. 
2 
Douglas fir. 
2 
Engelmann spruce. 
Scotch pine. 
2 
Siberian larch. 
1920 
Aver¬ 
Aver¬ 
Num¬ 
ber of 
trees. 
Water 
require¬ 
ment. 
Growth 
percent¬ 
age. 
age 
water 
require¬ 
ment. 
age 
growTth 
percent¬ 
age.® 
I 
Gm- 
773 
7-5 
Gw- 
95S 
19. 2 
6 
73 ° 
27. 8 
786 
32. 5 
8 
S 7 I 
25. 6 
768 
27. 0 
2 
376 
14.7 
743 
23.0 
3 
600 
II. 0 
' 634 
32.6 
9 
441 
66.4 
456 
68.0 
2 
436 
24. 6 
436 
24. 6 
2 
29s 
S 3-5 
295 
S3. 5 
® Dry accretion related to dry weight at beginning of season. 
At least two important points are gained by the combination of the 
1920 with the 1917 data. The position of limber pine, as the least 
effective user of water, is more nearly established, and lodgepole pine is 
brought into this class, where our empiric estimates would place it, as 
we shall see later. 
But the main reason for presenting the growth data in Table X is to 
explain the positions of the species as given in Table IX. It is seen that 
Engelmann spruce maintained in 1920 almost as high a growth rate as 
in 1917, and this explains its appearance as a relatively extravagant 
user of water on the leaf-exposure basis. Conversely, limber pine and 
Douglas fir both appear as conservative users of water in 1920, evi¬ 
dently because they were relatively inactive. 
