SHOETLEAF 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 

 bortleaf stand of similar character and age removed 42 per cent of 

 i ; ■ n ees but only 6 per cent of the total cubic volume of wood. 



