14 BULLETIN 1291, U. S. DEPARTMENT OF AGRICULTURE 
further discussion of aspen and conifer sites will be found under 
" Yield." 
TOLERANCE 
Aspen is an exceedingly intolerant tree, being less able to stand 
shade than any of the coniferous species associated with it, with 
the possible exception of limber pine. Sprouting goes on with little 
success in most stands until they begin to open from old age. An 
analysis of light intensities (Table 7) shows that even where the light 
is as bright as half the normal full sunlight sprouting is unsuc- 
cessful compared to what it is in clearcut plots in full sunlight. 
Table 7. — Relation of intensity of light to number of sprouts per acre 
Light intensity * (full sunlight 
1.00) 
Number sprouts 
per acre 
Light intensity 1 (full sunlight 
1.00) 
Number sprouts 
per acre 
10 
40,000 
2,784 
2,175 
0.3 to 0.4. . 
2,436 
0.5 to 0.6 
0.2to0.3 
1,255 
0.4 to 0.5 
Less than 0.2 
Usually none. 
i Measured with the Clements photometer. 
Taking a specific example for illustration, it was found that the 
intensity of light under a stand where sprouting has for years 
shown itself to be only partly successful was approximately one-fourth 
full sunlight. Sprouts from this plot compared with those from 
the open near by very strikingly show the difference in sprout de- 
velopment beneath a parent stand and in one where clear cutting has 
taken place. Although the sites are the same except for the degree 
of light, 3-year-old sprouts average only 8.7 inches tall under the 
old stand, whereas in the open they are 42.3 inches tall and very 
much more vigorous in appearance. Under very dense aspen stands 
the light intensities may be as low as 0.07 normal; but, even in such 
places, an intolerant species like western yellow pine will remain 
alive, although growing very spindly, while Douglas fir, white fir, 
alpine fir, and Engelmann spruce will grow fairly readily. Aspen 
sprouts do not develop sufficiently to be generally apparent where 
light intensities are under 0.2 normal. Conifers can therefore readily 
suppress aspen, as intensities as low as 0.03 to 0.06 are found in 
average sp Tice stands, and even in lodgepole pine 0.07 to 0.08 is 
normal (5) Pearson's results in Arizona (10) indicate the following 
intensities of light as favorable to good growth of conifers: "Western 
yellow pine 0.41 to 0.57; white fir 0.027 to 0.068; Douglas fir 0.133 
to 0.192; Englemann spruce 0.033 to 0.060. 
Since the usual light intensity in aspen stands of moderate density 
is about 0.2, it is evident that they offer excellent opportunities for 
satisfactory regeneration of conifers. 
SUSCEPTIBILITY TO INJURIOUS FACTORS 
Aspen is a tree which is extremely susceptible to attacks of fungi 
and is also considerably injured by many other biotic and climatic 
agencies. Fungi (rots) and grazing damage, however, are practi- 
cally the only ones which have a great bearing on management. 
