154 
Journal of Agricultural Research 
Vol. XXIV. No. a 
5. With this understanding of the situation we may say that in this 
test Engelmann spruce and lodgepole pine were most susceptible, while 
Douglas fir and yellow pine were about equally resistant. The factor 
which seems to control susceptibility is mainly structural rather than 
physiological—that is, it is the small mass of the spruce and lodgepole, 
and possibly their weak rooting, which causes them to stand out in 
contrast to yellow pine and Douglas fir under extreme drying conditions 
of relatively short duration. 
This pairing of spruce with lodgepole suggests as strongly as do the 
high wilting coefficients for lodgepole the very poor ability of the latter 
to supply itself with water; but, in the light of the other facts secured, 
the same cause will not fully explain the behavior of spruce. It is 
believed it would be fairer in the case of spruce to say that high tem¬ 
peratures in direct sunlight create high internal temperatxures and some 
direct heat injury. This hair-splitting distinction is necessary for the 
proper physiological interpretation which will agree with the other facts 
at hand. It may be added that the susceptibility of spruce to injury in 
sunlight has been very evident in many of the wilting coefficient tests. 
HVIDKNCie OF WINTFRKII.WNG 
Winterkilling of trees is generally recognized as the direct result of 
evaporation from the leaves or twigs at times when moisture can not be 
supplied to replace the loss, owing to a frozen condition of the soil. It is, 
of course, not confined to evergreen trees but may affect fruit trees, or 
even such hardy forest trees as honey locust, when devoid of foliage. 
The conditions for winterkilling are usually provided by a very rapid 
rise in air temperatures and by wind which facilitates evaporation. 
The soil, of course, warming more slowly than the air, may not free its 
moisture for many hours ^ter the beginning of the unseasonable air 
conditions. Likewise, if the tree stems have been thoroughly frozen, 
they may not be able to transport water until a great loss from the 
leaves and small twigs has occurred. 
The conditions conducive to winterkilling are especially likely to be 
produced near the base of the Rockies from northern Colorado northward. 
iTie coniferous forests which are subject to this form of injury are there¬ 
fore the low-l)dng yellow pine forests of the Black Hills and eastern 
Montana. Here the Chinook, a warm wind occurring at a season when 
the normal temperatures are below freezing, attains its most typical 
development. 
A typical Chinook has not been noted within the locality of the present 
study. It has been shown, however, that in the Pikes Peak region the 
winds from January to March possess the powers of a modified Chinook. 
While they do not often bring extremely large rises in air temperatures, 
they are of high velocity, the air is dry, and the soils at all elevations, 
unless strongly isolated, are likely to be deeply frozen and remain so 
throughout the duration of the wind, which is often two or three days. 
The Pikes Peak region therefore presents a good opportunity for fihc 
study of the relative resistance of the several species to this form of 
drought, for the desiccating influence is not coiffined to the low zone 
where only yellow pine occurs. 
The present writer {2) has described in some detail the cumulative 
effects of winds occurring at the Fremont Station in January and March, 
1916. It was shown that on a south exposure where yellow pine, Douglas 
