Apr. 14,1923 
Physiological Requirements 0f Rocky Mountain Trees 115 
and crowding out of yellow and limber pines by fir or spruce, the similar 
elimination of lodgepole when spruce seriously competes with it. The 
water requirements may explain certain things which we have habitu¬ 
ally ascribed to presence of or lack of shade “tolerance.” But it does 
not necessarily mean that yellow pine, for example, might not resist 
transpiration and survive under rather rigorous conditions where no 
question of relative growth rate was involved. 
Resistance to Transpiration 
To obtain a better idea of relative resistance to transpiration we should 
consider the water losses, under equal conditions, as related to plant mass, 
leaf area, or leaf exposure. In Table IV the data are summarized on 
each of these bases, but the species are arranged in the order indicated 
by the relative transpiration per unit of leaf exposure. 
Tabi^^ IV .—Water losses per unit of leaf area and plant mass 
Seasonal water loss (grams). 
Species. 
Per square centimeter of leaf. 
Per gram 
Surface. 
Exposure. 
mean green 
weight. 
Yellow pine. 
4.98 
2. 80 
34 -S ( 3 -) 
23.6 (0 
21. 2 (0. 7] 
18. 0 (2. 81 
160 
Lodgepole. 
128 
no 
Bristlecone... 
2.03 
1. 83 
2. 26 
Limber pine. 
iiy 
81 
I'?0 
Douglas fir. 
16. 8 (0. 3 
15-8 ( 3 - 0 ; 
Bnglemann spruce.. 
I. 99 
II5 
On the basis of the transpiration per unit of leaf exposure (which is 
believed to be the safest basis we have) or per unit of mass, the order of 
arrangement is essentially the same as in Table III. It is, perhaps, sig¬ 
nificant that the four important forest trees, yellow pine, lodgepole, 
Douglas fir, and spruce, appear in the same order as in the preceding table, 
while limber pine and bristlecone pine have moved to positions just below 
lodgepole. Taking the data at face value, let us consider for a moment 
what these qualities of limber and bristlecone pine must mean. In the 
first place, it has been seen that these species, which are admittedly very 
adaptive “weed” trees, use considerable water without making much 
growth. In the second place, we see that relative to their leaf area or 
whole mass they use very little. In other words, they are in some way 
adapted to protect themselves from water loss, but along with that adap¬ 
tation, perhaps as a result of it, they have very meager ability for making 
use of light in photosynthesis. It is readily seen how this may happen. 
If either of these trees, accustomed to growing on bare sites where there 
is no competition, has adapted itself through reduction in the number and 
size of its stomata, moisture loss is reduced and the ingress of carbon 
dioxid is likewise reduced. If, again, as the writer has sometimes noted, 
the needles are closely appressed for the purpose of mutual protection, 
then moisture is saved at the expense of tihe full insolation of each leaf. 
The same might be the result of thickened epidermis or heavy palisade 
tissue. It thus appears that almost any adaptation for the conservation 
