376 



GROWTH OF TREES IN VOLUME 



as many trees as are required for the study of average diameter growth 

 on account of the greater consistency of height growth based on age. 

 From a curve of growth in diameter, based on age (§ 267 and § 268), 

 the diameters of the average trees at different ages are determined. 

 From a second curve of height based on age (§284), the heights of the 

 same average trees for different ages are found. Since diameter and 

 height determine the volume as classified in these standard volume 

 tables, the requisite volume is interpolated from the values in the table 

 for the nearest ^o-inch in diameter and foot in height. The successive 

 volumes found in this way indicate the growth laid on by the average 

 tree. This may be expressed in whatever unit of volume is represented 

 by the volume table employed. This method is almost universally 

 substituted for volume growth analysis wherever figures on average 

 volume growth of trees are desired. This method is illustrated by 

 Table LVIII.i 



1 The method of interpolation is illustrated as follows. The 60-year-old tree is 

 6.6 inches in D.B.H. and 46 feet high. The values in the standard table from 

 which to interpolate are, in cubic feet. 



The difference for 1 inch is 1.5 cubic feet for 40-foot trees, and for .6 inch, is 

 .9 cubic foot, giving for 6.6 inches, 5.1 cubic feet. The average difference between 

 40- and 50-foot trees is .85 cubic foot. For 46-foot trees it is .6 times .85 = .51 cubic 

 foot. Then 5.1 -|-. 51 = 5.61 rounded off to 5.6 cubic feet as the interpolated volume 

 sought. These interpolations are more expeditiously made from graphic plotting 

 of the values in the volume table. 



One drawback to the use of volume tables as a substitute for actual growth analy- 

 sis is illustrated in the attempt to measure growth at successive decades on sample 

 plots for scientific purposes. Even here, if a single volume table is carefully pre- 

 pared, combining aU age classes, the transition in form from young to old trees is 

 blended with the volumes shown in the table for small and large trees, but where, as 

 for instance with Western yellow pine, separate volume tables were made for black 

 jack or young trees and for yellow pine or old trees which differed by about 10 per 

 cent in the average volume due to difference in form, the application of a different 

 volume table to trees passing from one age class to the other caused a jump of 10 

 per cent in the volume due apparently to growth, but in reality due to the irregular 

 distribution of this growth by separation of form classes in these tables. 



