A? BULLETIN 24, U. S. DEPARTMENT OF AGRICULTURE. 
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thinning could be partially or totally met by a market for the mate- 
rial removed, the possibility of increased returns would, of course, be 
still further improved. Until markets for small stock improve, how- 
ever, or until more definite conclusions can be made respecting the 
actual increase in growth due to thinnings, it is doubtful if thinning 
operations will generally be considered feasible in natural stands of 
second-growth cottonwood in the South. 
ROTATION. 
LUMBER. 
A rotation of approximately 35 years is sufficient to yield maxt- 
mum returns from natural stands of pure cottonwood. This is based 
on present market requirements, and corresponds almost exactly 
to the age at which a stand has attained its maximum mean annual 
yield of 840 feet per acre board measure, the trees ranging from 10 
to 30 inches in diameter, with an average of 19.7 inches (Table 3). 
An average stumpage value of $5 per thousand feet at 35 years 
is assumed. Below this age, with present merchantable sizes, the 
stumpage value of cottonwood decreases very rapidly and few-stands 
are now being cut much younger. 
Since all forest investments are reckoned at compound interest, the 
latter necessarily has an important influence upon the length of rota- 
tion. This is shown from the following figures, which indicate the 
necessary increase in stumpage value to warrant extending the rota- 
tion beyond 35 years, at which age a stumpage of $5 per thousand 
is assumed. The investment is reckoned at 7 per cent for a 35-year 
period. An initial cost of $5 per acre is allowed for establishing the 
stand by natural reproduction, taxes are assessed at 20 mills on the 
dollar for a land valuation of $3 per acre, which is assumed to in- 
erease $1 each succeeding decade. The sale value or cost of the land 
is placed at $5 per acre. 
Thus, in order to warrant lengthening the rotation from 35 to 40 
years, the value of the timber per thousand feet must increase from 
$5 to $6.73 during this period. This merely means that a stumpage 
value of $5 per thousand at 35 years will return a net profit equivalent 
to a stumpage value of $6.73 per thousand at 40 years. At rotations 
Ge 
of 45 and 50 years the stumpage value would have to attain $9.60 and 
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