158 
Journal of Agricultural Research 
Vol. XXIV, No. a 
obscured by the relative sizes of the seedlings and their root developments. 
The seedlings of spruce and lodgepole pine are small and frail and in the 
first two or three months develop scarcely more than half the root 
produced by Douglas fir and yellow pine. As a result, even when care¬ 
fully protected from excessively rapid water loss, lodgepole pine seedlings 
show far less drought-resistance than the others. Spruce seedlings, on 
the other hand, show quite as great resistance as those of pine or fir 
when not excessively insolated, and possibly a little more if the drought 
condition is approached every slowly. Limber and bristlecone pine 
seedlings, as meagerly observed, resist drought with the best of the others, 
no doubt because of a low rate of transpiration. In this connection the 
soil conditions leading up to wilting of seedlings should be borne in mind. 
Rarely is it possible for the roots to reach and extract all of the moisture 
which it would be physically possible for them to absorb. The complete¬ 
ness of this absorption depends very much on capillary movement in 
the soil. If the amount required by the seedling is small, this movement 
may supply the needs. Therefore, the rate of transpiration by the seed¬ 
ling is very important in determining, to a fine point, the degree of 
drought which it will resist. 
In nature all possible rates of soil-drying are represented, dependent 
very much on the amount of insolation on the site and to some extent 
on the nature of the soil cover. The open south exposure will usually 
dry at tiie immediate surface very rapidly. Because of the lack of humus, 
however, the layer just below the surface may remain moderately moist 
so long as the quantity of water below is sufficient to maintain capillary 
movement. When that end is reached, the soil undoubtedly dries out 
very rapidly to a considerable depth. Even with the respite furnished 
by capillary movement, the whole process of^ drying, in continuously 
bright and dry weather, seems likely to be accomplished here sooner 
than in the contrasting site. This may be on bottoms or north exposures 
where the total moisture supply is sufficient to produce a closed stand, 
heavy shade, and the accumulation of humus. In this soil the surface 
litter and humus are rarely thoroughly wetted except dtrring and imme¬ 
diately after the melting of snow. The more decomposed humus below, 
however, due to a lack of insolation and being protected by the surface 
litter, is rarely dry except after prolonged drought. It dries out slowly 
and steadily, however, both through the demands of the roots below and 
by direct evaporation. It follows that, since these demands in the aggre¬ 
gate are very large, such a soil may at unusual times, or possibly in the 
usual autumn drought, become extremely dry, especially so in the 
physiological sense, because of its high wilting coefficient. 
On the one hand, then, we have the rapidly fluctuating moisture 
conditions of the well-insolated site, which, for the establishment of 
seedlings would appear to demand prompt germination and prompt 
deep rooting. Yellow pine is preeminently adapted to these conditions 
by reason of its large seeds, which produce large sturdy seedlings with 
a habit of immediate deep rooting. There is nothing in the evidence 
on this species to suggest conservatism in the use of water. Probably 
the extravagant use of water assists in protecting from excessive heat. 
Success is dependent on the roots reaching a layer of the soil which does 
not dry out dangerously through insolation. It follows logically that 
yellow pine can not attain success in the face of competition, either 
with the roots of established trees or with grasses and herbs whose use 
of the water would materially augment the drying of the surface layer. 
