Apr. 14, 1923 
Physiological Requirements of Rocky Mountain Trees 12 j 
In combining, then, the data for the two years on transpiration per 
unit of leaf exposure, we shall not lose sight of the fact that the posi¬ 
tions of Douglas fir and limber pine, at least, and to a lesser extent that 
of bristlecone pine, are determined by relative inactivity in 1920, and 
that they really belong higher in the scale than here shown. There are 
possible means of making allowance for this influence of growth activity 
on the total transpiration, but they are so purely arbitrary that we 
hesitate to use them. 
Tabi,^ XI.— Mean resistance to transpiration, igiy and ig20 {growth data repeated to 
explain relative positions) 
Species. 
Mean 
transpi¬ 
ration per 
square 
centi¬ 
meter leaf 
exposure. 
Mean 
growth 
percent¬ 
ages. 
Lodgepole pine. 
Gjn* 
17-35 
14. 96 
14 - 15 
!■?. 10 
32-5 
27 0 
Yellow pine. 
I/imber pine. 
10. 2 
Bristlecone pine. 
xy. * 
23. 0 
68. 0 
Kngelmann spruce. 
II. 97 
9 - S° 
Douglas fir. 
12. 6 
That these relations of the species are not fixed and might easily be 
modified by additional data or consideration of different climatic varieties 
is fairly apparent from the divergence of the individual results. On the 
same basis as Table XI, the three lots of yellow pine studied in 1920, 
each of which maintained a healthy condition and performed, relatively, 
much as it had been performing in the nursery, gave the following results: 
Variety. 
Transpira¬ 
tion per 
square 
centimeter 
leaf expo¬ 
sure. 
Growth 
percentage. 
Harney, S. Dak. 
Gm> 
8.3 
9 - 5 
14. 2 
II. 7 
19 * 5 
27. 0 
Bitterroot, Mont. 
Tusayan, Ariz. 
There is every reason to believe these results were normal and express 
something of varietal difference. It is seen that the high growth rate 
of the Arizona form was accomplished with the extravagant use of water. 
Whether this is generally true of the more southern forms our data are 
inadequate to determine. Rather similar differences occur with lodge- 
pole pine, but here the Wyoming form is the most extravagant and rapid 
growing. 
