16 
BULLETIN 
U. S. DEPARTMENT OF AGRICULTURE. 
Age 20 Years 
(800 Trees Per Acre) 
Age4-2 Years 
( 4 o OTrees Per Ac re) 
Age33Years 
(580 Trees Per Acre) 
Valley region the first general period ends somewhere between the 
ages of 40 and 50 years, depending upon the local situation; in the 
central Atlantic coast belt apparently between 55 and 70 years. 
Figure 6, showing progressive stages of natural thinning and crown, 
classed according to 
age, represents actual 
numbers of trees and 
outlines of crowns as 
they existed in four 
fully stocked stands 
in Arkansas measured 
for growth and yield. 
The 20-year-old stand 
contained 800 trees per 
acre; the 33-year-old 
stand, 580 trees; the 
42-year-old stand, 400 
trees ; and the 52-year- 
old stand, 320 trees 
per acre. 
Shortleaf pine shows 
progressive changes in 
the character of the 
forest canopy other 
than the mere reduc- 
tion in number of trees. 
These changes are well 
illustrated in figure 6 
for stands from 20 to 
50 years old. In early 
life the tree crowns are 
approximately circular 
hi outline and closely 
approach each other, 
leaving very little un- 
A£e52Years • i gj. 
(320Trees Per Acre) occupied space. At 
the age of 50 years, 
Fig. 6.— Progressive change in tree density by natural thinning , ' , 
in pure even-aged stands of shortleaf in Arkansas: D, Dominant LLOWever, tUe tree UaS 
classes; I, intermediate; S, suppressed classes. Areas, 33 by 66 become leSS tolerant 
feet. (Drawn from actual stands.) 
the crowns are quite 
irregular in outline, and crown isolation leaves relatively large light 
spaces in the canopy. The slow rate of natural thinning after about 
50 years undoubtedly is accompanied by relatively small changes in 
the tolerance of the tree. The climax of lateral growth or spread 
of the branches characteristic of the species seems to be closely ap- 
