324 PRINCIPLES UNDERLYING THE STUDY OF GROWTH 



In using the past growth of a stand on which to base the prediction 

 of its future growth, these records of past growth of the living trees 

 in diameter, height and volume are the only data available. This 

 prediction is based on one of two assumptions, either that the growth 

 for a future period will continue at the same rate as shown for a past 

 period, or that this future growth will be at a different rate, either increas- 

 ing or decreasing, and that the amount of this change may be deter- 

 mined by a study of past growth. 



In the use of either of these methods to predict the growth of trees, 

 the method may be applied either to the volume of the tree or to its 

 diameter and height instead. If a volume analysis is made for two 

 or more past decades, it may be assumed either that this rate of volume 

 growth will continue unchanged, an assumption which is practically 

 never correct, or that the curve of volume growth which may be plotted 

 from past volumes can be prolonged to indicate the growth of the next 

 decade. 



But the method more commonly employed is to substitute a study 

 of diameter and height growth for volume analysis. If future diameter 

 growth is assumed to be at the rate shown in the past decade, future 

 volume growth will increase (§ 270). If the past growth in diameter 

 is plotted, and a curve projected, the future diameter so obtained is 

 the basis of the predicted growth in volume. 



252. The Effect of Losses versus Thinnings upon Yields. The 

 first conception in the study of growth is apt to be that it consists chiefly 

 of measuring the growth in diameter, height and volume of individual 

 trees. Although it is true that growth per acre is based primarily upon 

 the rate of growth of the individual trees which make up the stand 

 and that according as this rate of tree growth is rapid or slow, the yield 

 per acre will be large or small, yet the total growth per aero, which is 

 the result desired in all growth studies, is the product of the growth 

 of individual trees and the number of trees surviving to the end of a 

 future period plus such growth as may take place on trees which die 

 and are removed during the period. The death of a certain number of 

 trees in the stand during the period will have this effect, that if these 

 trees can be removed as thinnings, their volume at the beginning of the 

 period, augmented slightly by growth which takes place in them before 

 they die, is part of the yield for the period, but does not appear in the 

 volume of the standing timber alive at its end. If these trees cannot 

 be harvested, their total volume as originally measured will disappear 

 from the live stand, and constitute a negative growth or loss which 

 must be deducted from the growth on the surviving trees before the actual 

 volume of the stand at the end of the period can be correctly ascertained 

 from its volume at the beginning. 



