Apr. 14,1923 
Physiological Requirements of Rocky Mountain Trees 155 
fir, and limber pine grow in a mixed stand, Douglas fir at first showed a 
more pronounced discoloration, but later the injury to yellow pine was 
seen to be much more severe, as only this species was defoliated. This 
injury was always much more pronounced on the west (windward) side 
of a tree, but it varied with different specimens, partly because the 
ground is strewn with large boulders which deflect the wind and also 
reflect sunlight. While in no case fatal (and even the general injury in 
the Black Hills in 1909 caused a very small percentage of deaths), this 
defoliation obviously must have a retarding effect on the growth of the 
whole tree. That the same kind of injury occurs at intervals of a few 
y^ars, and that it hits “twice in the same spots,” seems to be indicated 
by the one-sided development of most of the trees which were injured 
in 1916 (see PI. 7, A). Buds and branches were rarely injured in this 
case, and new foliage appeared almost as early as on unaffected trees 
or parts. 
In the nursery, where there was no snow to furnish protection during 
most of the winter, a better comparison of the species was possible be¬ 
cause of the uniform conditions of soil and exposure. Yellow pine stock 
was damaged more than Douglas fir; Douglas fir far more than spruce. 
In fact, in only a few cases was spruce even discolored. With lodgepole 
the injury was usually confined to an exposed branch or leader, suggesting 
incomplete ripening of the previous season's growth. 
This indicates, as do all other data, that spruce can bear drying to a 
greater degree than the other species, or at least that it resists the dry¬ 
ing better, which comes to the same result. It is perhaps significant of 
the moisture-conserving adaptation of limber pine, which has been 
indicated by the transpiration tests, that there was no apparent injury to 
this secies on the south slope where yellow pine was most plainly injured. 
It resisted wind-drying of this kind as well as any species. On the other 
hand, during the summer drought of 1917, limber pine was the only 
species showing injury to trees of large size. 
SUMMARY 
The relative qualities of the important forest trees of the Central 
Rocky Mountains, primarily from the standpoint of moisture relations, 
have been approached from five different angles. No one of these efforts 
has been free from errors, and no one would alone carry conviction, but 
the several results are corroborative with only insignificant exceptions. 
These comparisons of the species have been made on the basis of— 
1. Measurements of the water used in relation to growth and leaf ex¬ 
posure of 3- to 9-year-old trees, under uniform conditions for all species. 
2. Comparisons of sap density under uniform and varying growth con¬ 
ditions. 
3. Measurements of the moisture of soils not available to young seed¬ 
lings by direct comparisons of the species and also under varying con¬ 
ditions as to soil quality and atmospheric stresses. 
4. Observations on fatality among seedlings under high temperature 
conditions. 
5. Observations on the resistance to winter drought of specimens 
growing side by side, and as measured by the extent of injury to foliage. 
It will have become apparent that there are several aspects of the 
moisture relations, that the several species studied do not always stand in 
the same relation one to the other, and that it is not even possible to state 
