36 BULLETIN 1291, U. S. DEPARTMENT OF AGRICULTURE 
aspen remains sound, which is itself largely controlled by site. Even 
on the best sites rotations of more than 80 years will be infrequent, 
particularly as the mean annual increment on these sites culminates 
at about this age, if the tree is used to a small diameter limit. Some- 
times, however, it may be advisable to let the trees grow to 110 years. 
On poorer sites decay is the limiting factor, and trees should seldom 
be grown longer than TO years. On sites 3, 4, and 5 there will usually 
be no merchantable yield, except for some special products employing 
exceptionally small diameters. 
RELATIVE VALUES OF ASPEN AND CONIFERS IN MANAGEMENT 
Aspen must be utilized to a low diameter to get the most out of it. 
Even on the best sites it will not run as high as conifers in saw- 
log material, although it requires rather less time to attain that size, 
and on second-quality sites the saw-log material is of very little im- 
portance. Utilization in the form of cordwood is, therefore, ideal 
for aspen, both for realizing the most out of the crop and as a means 
by which clean-cutting may be practiced. 
Aspen stumpage values are lower than the stumpage values of the 
associated conifers, so that to bring in equal annual income the aspen 
must outstrip conifers in yield, or have secondary income to compen- 
sate for the lower value of its wood. Aspen saw timber stumpage 
brought about $1.50 per thousand feet board measure in 1922, as 
against $2.50 for Engeimann spruce and Douglas fir, and $1.50 for 
white fir and alpine fir. The mixed-conifer type has a stumpage 
value of about $2 per thousand on the average. Aspen sold for 
cordwood brings about 75 cents per cord stumpage, and for mine 
props, 0.5 cent per linear foot. 
The general relations between the annual incomes from aspen and 
conifers can not, in the very nature of things, be worked out in detail 
on a money basis. Markets will always vary, and local conditions 
have a great bearing upon both yields and values. There are also 
secondary values of some degree or other possessed by every acre 
occupied by aspen or conifers, the chief of which is grazing. This 
runs about 14 cents per forage acre per annum for cattle and 11 
cents for sheep, or an average of I2V2 cents at present grazing rates 
(1923). There is more forage under aspen than under conifers on 
account of the lighter shade in stands. 
Figuring the grazing return in terms of the site area, a careful 
approximation shows that aspen stands average 0.62 of a forage acre 
to the surface acre, yielding about 7.7 cents a surface acre. The 
average for conifers is 0.47 forage acre to the surface acre, yielding 
5.9 cents. The return on conifer sites varies inversely with the den- 
sity of stand, from 7 cents from 0.56 of a forage acre on areas yield- 
ing 1,000 feet per acre to 1.9 cents from 0.15 of a forage acre on areas 
of 9.000 feet per acre. 
The aspen yield of 7.7 cents per acre per year must, however, be 
reduced slightly to allow for protection during the period of regen- 
eration, an average of 5 years in every 80, or 6 per cent, leaving 7.2 
cents per acre as the corrected value for grazing under aspen. 
A fully stocked acre of conifers should run at least 12.000 feet per 
acre at maturity on site 1 in the aspen belt. Under the selection sys- 
