Apr. 14,1923 
Physiological Requirements of Rocky Mountain Trees 1 59 
The large moisture demands of yellow pine, with the normal amount of 
precipitation, can only be supplied in open stands, which first permit 
the safe establishment of the roots at a depth and later their extension 
into a large area of soil. This is of fundamental importance in the manage¬ 
ment of the species and explains the ultimate failure of one crop of seed¬ 
lings after another in stands which are already moderately crowded or 
apparently fairly open. 
Next in order on such sites we might expect to find Douglas fir, because 
it, also, produces a deep-rooted seedling. However, we should bear in 
mind that this species transpires less freely than yellow pine and hence 
may not be able to tolerate so much insolation. Extended observation 
shows that it will grow almost an3rwhere that yellow pine will grow, 
provided only the seedlings may have shade until they have passed the 
stage when most susceptible to heat injury. The fact that seedlings 
start in the shade of and in the densest root area of yellow pine trees 
shows that this species requires less moisture than yellow pine or, at 
least, that the moisture is not a controlling factor, and it is apparently 
for this reason that Douglas fir forms the climax forest except on the 
warmest yellow pine sites. n 
Limber pine and bnstlecone pine are also, by germination and rooting 
habit, adapted to well-insolated sites. The sap density of bristlecone 
pine, however, is apparently considerably higher than that of limber 
pine, and therefore it succeeds better on cooler sites and on heavier 
soils. The physiological development of both species and their growth 
rates are so low that neither can hold a place in fihe forest in competition 
with spruce or Douglas fir. 
On the other hand we have the poorly insolated sites, commonly 
described as “cool and moist," which are subject to comparatively slow 
and wide seasonal changes in their moisture conditions. 
Spruce seedlings on account of their growth habit are able early in 
the season to penetrate the layer of loose organic matter which is in 
many seasons thoroughly wet only after snow melting. The small seeds 
germinate at a lower temperature than those of other species. There¬ 
after the roots show little stimulation to further gro\^. Even the 
dryness of the fall period does not appreciably stimulate root growth in 
the new seedlings, and it is believed that this is clear evidence of the 
ability of the species to extract water from the soil at a low degree of 
availability. Possibly because of their generally higher organization, 
spruce seedlings even prefer a low moisture content which results in 
greater concentration of the soil nutrients. In such a situation Douglas 
fir has no theoretical advantage over spruce except in case of a drought 
so prolonged as completely to dry out the soil layer in which the spruce 
roots are found. Then the deeper rooting of the fir should, apparently, 
count in its favor. But in the established forest this can hardly be a 
material advantage, considering the evenness and depth of the drying 
where the soil is well occupied by older roots. 
The situation with regard to lodgepole is very different from that of 
our other forest trees, and the writer takes from the evidence the liberty 
of suggesting either that it is just approaching the physiological status 
of a full-fledged tree or that it is such a recent mi^ant to the Rocky 
Mountain region as to have failed by far to adapt its mode of growti 
to the poor moisture conditions usually found where its heat require¬ 
ments are best satisfied. Some of the evidence on the latter point has 
been presented by the writer (j), Clements (9) has classed lodgepole 
