12 BULLETIN 1291, U. S. DEPARTMENT OF AGRICULTURE 
SOIL AND MOISTURE REQUIREMENTS 
Aspen grows in practically every variety of soil found in the 
climatic belt to which it is suited, from loamy sands in parts of the 
western yellow-pine type to heavy clays characteristic of parts of 
central Utah; but the development of stands varies considerably on 
different soils. The chief direct influencing factor is rockiness of 
the soil, which by hindering the lateral spread of the shallow roots 
and interfering with the tendency to rise close to the surface, re- 
stricts the reproductive ability of the tree and its development. Soil 
has also an indirect effect on the development of the aspen type 
through its influence on conifers, for any factors which assist the 
growth and regeneration of conifers tend to reduce aspen through 
suppression. Rocky areas, therefore, have both a direct and indi- 
rect effect, as they are favorable to conifers. The best aspen develop- 
ment is on rich, deep-soiled flats supplied with plentiful moisture. 
Heavy-soiled flats are most difficult for conifer reproduction. There 
is evidence to show that on such sites recurrent surface fires have 
spread with exceptional ease in the uninterrupted stretches of dry 
grasses and herbaceous debris accumulated under aspen shade, de- 
stroying rival growth; and for this indirect reason large flats or 
rolling country tend to bear the largest pure aspen stands at present. 
But even here, without the aid of fire, conifers would probably have 
replaced aspen as a type. 
Aspen is apparently a little less drought resistant than either Doug- 
las fir or white fir (Abies concolor), which are very slowly extending 
into altitudes below the aspen. Aspen demands more moisture than 
western yellow pine, and the conifers of the pifion- juniper type, but 
appears to be more resistant than Engelmann spruce and alpine fir in 
the spruce-fir type, although probably not more so than limber pine, 
which is frequently found in this type. In the lodgepole pine region 
it seems hardier in this respect than the pine, for it usually runs out 
into the sagebrush more actively. The advantage which aspen has 
in occupying drier sites is not, however, so much a matter of actual 
water requirements of the full-grown trees as of the difference in 
the method of reproduction. 
The critical period with conifers is during germination and the 
seedling stage of life, a period through which aspen does not have 
to pass. Even though it was originally established on a moist site 
which has become drier on the surface, and in spite of retrogressive 
successions due to fire, heavy grazing, or erosive agencies, if not to 
actual climatic changes, aspen can hold its own as long as moisture 
in the deeper soil layers does not fail. However, conifers once estab- 
lished would probably form types on every aspen site, not excepting 
those on south and west slopes at high altitudes into which conifers 
are at the present time coming only sparingly or not at all. In fact, 
conifers develop into large trees, nearly as large as on the best sites, 
in places where aspen is scarcely more than a shrub. 
QUALITY SITES 
Owing to the extreme variability in the development and growth 
of aspen under different soil and climatic conditions, it is^ neces- 
sary to consider every phase of silviculture and management in con- 
