Apr. 14.1933 Physiological Requirements of Rocky Mountain Trees 105 
It will be fairly evident from the reading of the few treatises which 
have been mentioned that the region under discussion has not been neg¬ 
lected by ecologists. It will be equally evident that there is still room 
for much systematic effort in the study of the environmental factors in 
order that the theories advanced regarding the distribution of mountain 
forests may be more thoroughly tested by well-established facts. The 
most apparent fact, after considering all of these regional studies, is that 
so far ecology has given the physics of the soil-moisture problem entirely 
inadequate attention. 
II. PHYSIOLOGICAL STUDIES LEADING TO AN INTERPRETATION OF 
THE ENVIRONMENTAL DATA 
It is quite generally recognized that the result of studying any condi¬ 
tion in nature, even when the method of study is strictly quantitative, 
should not consist wholly in presenting the accumulated facts but quite 
as much in placing a logical interpetation on those facts. In the present 
study we are dealing not only with a great variety of natural conditions 
which require quantitative expression but with a variety of growing 
entities whose behavior and reaction to known conditions can not be 
determined by casual observation. For example, the mere fact of find¬ 
ing a spruce tree growing at the water's edge does not prove that the 
tree uses or requires an unusual amount of water, much less that it is 
growing in that particular spot primarily because of the moisture, or 
even indirectly because of the moisture. It would be as logical to say 
that because the aligator spends a good deal of his time in the water, he 
must drink and must require for physiological processes an extraordinary 
amount of water. This may not be true at all; he may be a most abste¬ 
mious animal. 
The point is that in ecology we dare not take the conditions of growth 
as prima facie evidence of the requirements of growth, even though it 
be true that none of the conditions can be altered without affecting the 
character of the growth. This is especially true when we are compelled, 
as in the present instance and in most ecological studies so far made, to 
speak of requirements in a relative rather than an absolute sense; that 
is, when we are simply trying to compare the requirements of several 
species rather than determine them absolutely for any species. This 
may be illustrated by a point which has appeared very forcefully in the 
present study. Taking the superficial appearance of soil conditions as a 
measure of relative requirements, foresters have repeatedly stated that 
the moisture requirements of spruce were greater than those of yellow 
pine. Now, there could be no objection to saying, and probably no 
error in saying, that spruce requires or at least devdops best in a fresh, 
moist soil of high water-holding capacity. This would be an absolute 
expression which would simply gain in accuracy as the soil conditions 
were further analyzed. We might infer, and would be likely to do so, 
because of the character of the soil occupied, that spruce must use a 
great deal of water in its development. Such an inference would be 
unwarranted, but would be especially dangerous if we should say, com¬ 
paratively, that spruce uses more water than pine. Here we are Read¬ 
ing on absolutely unsafe ground. On the face of it there is no 
scientific basis for such a statement, if we use simply the evidence of the 
field conditions observed. And even if this were true of the spruce 
forest in the aggregate, that bespeaks nothing as to the individual. 
