REPORT ON FORESTS. 35 



inches; 125 years, 21 inches; 150 years, 20 to 27 inches; 190 

 years, 30 inches ; 250 years, 28 inches, and 300 years, 38 inches 

 in diameter. The curves show the growth in diameter to be at 

 a constant rate up to 65 years on the better soils and about 40 

 years on the thinner soils, and thereafter it decreases. The rate 

 of growth in height decreases throughout. 



It is instructive to note the increase in volume of a tree as 

 shown by the curves. The cubic feet of lumber at various ages 

 figures out as follows : 



Age. Best Soils. Thin Soils. 



20 years, 1.8 cubic feet, 0.7 cubic feet. 



40 " 11.4 " " 4-7 " " 



60 " 31.2 " " 9.2 " . " 



80 " 48.0 " " 14.7 " 



100 " 63.4 " " 20.4 " " 



This is the gain in volume of the individual tree. Now if 

 there is a constant yearly increase in the yield per acre from 40 

 to 60 years, such as wotild be called for by the ordinary rule 

 of one cord per acre annually, it can only be accounted for by 

 the crowding out or dying of many trees. Thus if on the best 

 soil there are 300 trees per acre at 40 years old, the total volume 

 of wood will be 3,420 cubic feet, and if this increases propor- 

 tionately with the age, at 60 years old it will amount to 5,130 

 cubic feet, which would require only 164 trees per acre. On the 

 thin soils the decrease would be from 300 trees at 40 years old 

 to .230 trees at 60 years old. Such a rule of yield is therefore 

 more likely to hold good on the. poorer soils than, on the better 

 soils. The figures show how rapid the gain in merchantable 

 volume is from 40 to 60 years of age, for at the age of 40 years 

 the trees have just reached a merchantable size, and even if we 

 concede that measured as cord-wood the yield per acre thereafter 

 does not rapidly increase, it is clear that the gain in the more 

 valuable merchantable volume is great. If so large a number 

 of the smaller trees are crowded out as is above suggested, this 

 is a strong argument in favor of thinning-out these trees from 

 time to time to be utilized for cordwood, thus leaving the best 

 trees room to grow at a still more rapid rate. 



The curves of growth for short-leaf pine are platted from a 

 table given by Mr. Pinchot on page 49 of his paper in the 



