i6o 
Journal of Agricultural Research 
Vol. XXIV, No. a 
as even more intolerant of shade than yellow pine, and a fairly low 
photosynthetic efficiency is clearly indicated in this paper. Yet, while 
evidently demanding a great deal of light as well as moisture during 
the growing period, it shows no such habit as that possessed by yellow 
pine, of prompt germination or deep rooting. It is evident, therefore, 
that it is adapted only to sites with a steady supply of moisture and de¬ 
mands more than can usually be supplied by either of the contrasting 
situations which have been described. It is probably for this reason that 
it reproduces readily only where competition is decidedly lacking, ad¬ 
heres to the higher elevations where moisture is more abundant but 
where its growth rate is surprisingly slow, and has not penetrated to Ae 
south where there is a very sharp contrast between the summer rainy 
period and the clear, dry weather of autumn. In its range there is, to 
be sure, a generally steady decrease in the precipitation from May or 
June onward, but this is usually so gradual as to permit a great degree 
of adjustment. 
CONCLUSION 
In concluding this paper, it may be said that certain physiological 
relationships between the species which are of great importanc^, espe¬ 
cially for a proper understanding of forest growth, have been wrought 
out and tentatively established by approach from several angles, but 
tliat from the standpoint of natural reproduction and in relation to all 
questions of natural distribution of the species, these relative physio¬ 
logical qualities are not shown to be more controlling than some adapta¬ 
tions of form and characteristics of behavior which may be adequately 
described only by the word “habit.’’ Technical forestry or silviculture 
might be said to be based on the venerable concept that the several 
species of the forest vary in their demands for light or their tolerance 
of shade. This concept is not only not altered by the present results 
but is confirmed, and the relation of the photosynthetic capacity of a 
given species to its heat and moisture requirements is made much more 
clear and definite than it has appeared heretofore. Spruce is shoTO 
the most efficient of the species considered, not only because of its higU 
photosynthetic capacity, but also because when this capacity is exercised 
the species automatically becomes economical in its use (by transpira¬ 
tion) of water and at least in this sense has low moisture requirements. 
At the same time it may be rendered sensitive to excessive temperatures. 
Qn the contrary, yellow pine, commonly thought of as very drought 
resistant, is found to require much light and heat and, with these, to 
use comparatively large amounts of water per tree of given size. Ui 
course, the facts fit more closely with preconceived ideas when spruce 
stands are compared with pine stands, the much smaller number of in¬ 
dividuals in the pine stand not only compensating for ^e high individ¬ 
ual water use but being a most vital concomitant of this individual 
requirement. In other words, with the low moisture supply com- 
monlv available in the low elevations and warm situations (which alone 
insure proper development of the pine), wide root spread and an open 
stand are vitally necessary to insure the water supply of the individual 
tree. The practical importance of this fact in forest management should 
l^e v^erv clear 
It vnll be seen, then, that these physiological relations principally 
clarify our conceptions of growth. Spruce is a better grower, a more 
^cient mechanism for growth, than Douglas fir, fir more emaent 
