SHORTLEAF PINE: IMPORTANCE AND MANAGEMENT.. 35 
RELATION BETWEEN TREE DENSITY AND YIELD. 
A thinning is very desirable between the ages of 10 and 15 years. 
It should be somewhat earlier for the better than for the poorer sites 
and regions of growth. Subsequent thinnings should be made at 
regular intervals of 5 years up to the age of from 40 to 60 years, and 
thereafter about every 10 years to the close of the rotation. If, for 
any reason, it is impracticable to repeat the operation so often, the ~ 
interval may be increased to from 7 to 10 years. Ten years is satis- 
factory for older stands of timber managed under a longer rotation. 
It is thoroughly practical to start thinnings even at considerably 
later ages than those mentioned. The limit has not been definitely 
determined, but vigorous recovery after suppression has been ob- 
served up to 80 and 100 years of age. Perhaps ages of 50 to 60 years 
on the better situations and best regions of growth are approxi- 
mately near to the average limit of the period of good recovery. This 
allows a period of 20 to 30 years prior to the culmination of height 
and diameter growth. On the drier and thinner soils the correspond- 
ing upper limit seems to be reached from 10 to 20 years earlier. 
Too heavy thinning stimulates leaf development and wood produc- 
tion over the lower branches, correspondingly reduces the rate of 
height growth, and is injurious also through the exposure of the 
soil and humus to the unfavorable action of sun and wind. The ideal 
thinning removes a sufficient number of the trees to relieve over- 
crowding without creating large openings in the canopy. Obviously, 
long intervals between thinnings make necessary the removal of a 
greater amount of material than shorter intervals, and increase the 
danger of soil exposures and the development of long dense crowns. 
It should be borne in mind, however, that young stands which are so 
open as to be considered understocked may often close up as they 
grow older and be fully stocked at maturity. 
The close relation between the number of trees per acre and the 
resultant yield of saw timber is indicated by Table 18 derived from 
seven different portions of a 30-year old shortleaf stand of irregular 
density. The best yield resulted from a density of 350 trees per acre, 
with decreasing yields at about the same rate from both understock- 
ing and overstocking. Thinnings made in Hanover County, Va., 
for the accurate determination of the resulting growth in a typical 
shortleaf stand of similar character and age removed 42 per cent of 
the trees but only 6 per cent of the total cubic volume of wood. 
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