Apr. 14,19*3 
Physiological Requiremerds 0f Rocky Mountain Trees 157 
This zonation holds, definitely, however, only for Engelmann spruce, 
Douglas fir, and yellow pine, which w^e have shown to be so equally 
developed as forest dominants that the fundamental physiological differ¬ 
ences control all their relations. With the three more or less weedy pines 
there are, plainly, adaptations which are equally effective or more effective 
in controlling distribution. It is significant of the importance of high 
temperatures as absolute limitations that these three species are all found 
in higher and cooler zones than their physiological conditions necessitate. 
Supplementing physiological characters, we may have stomatal reduc¬ 
tion, thickened epidermis, or clustered leaves, all tending to reduce the 
absolute transpiration, but, while doing so, inevitably reducing either 
the intake of carbon dioxid or the effectiveness of sunlight so that 
photosynthesis and growth are reduced perhaps even more than is water 
loss. This seems to be the general line of protective development in the 
‘'weed’’ trees, limber pine and bristlecone pine, and to a lesser extent 
in lodgepole pine. 
Again, resulting from gradual adjustment to the moisture conditions 
which accompany certain heat conditions, the forest trees have different 
root habits, or (shall we say ?) are unequally stimulated to root develop¬ 
ment. It is believed that temporary stimulus has much to do with 
it, but inherited habit still more. Be that as it may, yellow pine and 
Douglas fir root much more vigorously at an early age than lodgepole 
pine or spruce. Almost as divergent are the germinating rates of the seed, 
lodgepole pine standing out as the most sluggish of the six species studied. 
In the strictly physiological sense, spruce is undoubtedly the most 
highly developed of the indigenous species we have considered. This 
is evidenced by the sap densities which the trees show after long seasons 
of photosynthesis and by the amount of growth made in relation to the 
total amount of the water consumed. 
In actual water consumed by a tree exposing a unit area to light (and 
wind) spruce is again the most economical, followed by Douglas fir, 
bristlecone, limber, yellow, and lodgepole pines. In this consideration, 
it is, almost without question, the special adaptations of the weed pines 
which put them down as only moderately extravagant, making them 
especially suited to exposed windy sites but wholly incapable of holding 
a permanent place in the forest. On the contrary, spruce maintains 
a moderate rate of transpiration under the driest conditions (so far as 
measured) for two reasons, namely, because it does not mechanically 
restrict losses but forges ahead with growth, and because when the water 
supply is low it is still more able than any of its competitors to supply 
its needs and is not so soon restricted either in transpiration or growth. 
These facts stand out very clearly. In this comparison we have placed 
Douglas fir next to spruce, believing that the actual position shown by 
Table XI is misleading, because the trees involved did not develop 
normally. 
In resistance to winter-drying, limber pine with its peculiar structural 
development and spruce with its high physiological resistance have 
shown themselves about equally effective. Douglas fir and yellow pine 
follow with increasing weakness. Lodgepole pine shows greater resistance 
than would be expected, a fact which we shall not attempt to explain at 
present. 
Considering the drought resistance of seedlings, through the wilting 
coefficients of a number of soils in which they have been compared, we 
find the same physiological properties evidently at work, though much 
