Apt. X4. *933 Physiological Requirements of Rocky Mountain Trees 161 
than pine. In forest planting, where use is made of trees which have 
been carefully nurtur^ beyond the stage of greatest susceptibility to 
heat and drought injury, this difference in the efficiency of our species, 
particularly the more effective use of water by spruce and fir, may be 
advantageously ^pldyM, the ranges of these species being artificialiy 
extended downward, without any injurious effects, while by natural 
reproduction such extension would be completely prohibited or would 
be very slow. With even greater care the economic loss resulting from 
the planting of any species in a situation too high or too cool, or in stands 
too dense for its proper development, must be avoided. 
In short, when it is consider^ that any cutting of a forest, by admitting 
more light and creating higher temperatures in the smface soil and more 
rapid fluctuations of the moisture on which young seedlings are depend¬ 
ent, tends to encourage a species more ‘‘hardy” but of a lower order 
of development than the one which dominates the stand (not necessarily 
of lower technical value), it is not difficult to see that all forest management 
hinges on these relative physiological properties for which we have been 
groping. Finally, it may be said that all of the physiological relations 
are embodied in the now rather general conclusion of foresters that the 
highest returns can be had from forestry only when cutting is followed 
by planting. 
This brings us to the original object of the present study, which has 
been to explain the existing natural forest types, to explain the distribu¬ 
tion of the species. As has been pointed out, natural distribution is 
plainly influaiced at present by habits and adaptations which have 
developed in each species and which to a considerable extent compensate 
or balance the more deeply embedded physiological qualities. Without 
again going into the details that have been brought out, we may illus¬ 
trate to show how these developments affect natural distribution. In 
figure 5 an attempt is made to show the influence of these developments 
on distribution, in a broad way. It has been indicated that spruce 
may be very sensitive to high temperatures, but especially so at a very 
early age when the seedling is small and tender. In consequence of this 
weakness, which is a result of its most fundamental organic character, 
probably for ages no spruce seedling has been able to develop at a low 
elevation or on a very well-insolated site at a middle elevation. Such 
sites as are suitable in respect to insolation and heat must show almost 
invariably quite even moisture conditions and usually soils characterized 
by a surface layer of litter and humus. Consequently spruce has de¬ 
veloped a rooting habit suited to these moisture and soil conditions—a 
relatively slow and feeble rooting habit which does not suffice for quick 
establishment under any other conditions. As a further consequence, 
spruce has evolved very small seeds, there being no need for large, sturdy 
seedlings or for deep rooting before the seedling may itself manufacture 
food. It is seen, then, without enumerating any otier similar develop¬ 
ments, that the species would have great difficulty in extending naturally 
to any sites other than the cool and moist ones on which it is com¬ 
monly found. The common conception has been in error only to the ex¬ 
tent of assuming that the essential feature of such sites is a large moisture 
supply. It is now fairly evident that the individual spruce tree does not 
require a large moisture supply even though this may insure the full¬ 
est development of the stand and, in view of this fact, that spruce 
may be used in planting where the moisture supply is relatively low. 
30616—23 -^6 
