432 THE DETERMINATION OF GROWTH PER CENT 



An attempt has been made to adapt the formula to rapid-growing 

 trees by substituting other values for 400, but the resulting formulse 

 have little practical value. 



331. Use of Growth Per Cent to Predict Growth of Stands. Growth 

 per cent is sometimes used to determine the growth of trees or stands, 

 by both the standard methods, that of prediction, and of comparison. 

 It is not well adapted to secure accurate results by either method. 

 Owing principally to the variability of the per cent relation, and its 

 direct dependence on and derivation from the two factors, volume and 

 increment, the problem of reversing this process and deriving increment 

 from growth per cent is apt to lead to error through a mistake either in 

 choosing the basis of volume for deriving the per cent figure, or in 

 applying this figure in turn to the wrong volume basis. 



The method of prediction of growth by means of growth per cent 

 consists of determining this per cent for a stand, either from sample 

 trees (§ 241) or by direct use of yield tables or other methods of measur- 

 ing the past growth for a decade. 



Schiffel states, " If in any period of life the current annual incre- 

 ment per cent of a tree is to be calculated, it would be contrary to nature 

 and incorrect to relate the increment to any former dimensions or 

 volume, but it must be related to the dimensions or volume of the previ- 

 ous year." 



The formula, growth per cent =(-77-1 — ) — when n=10 years, 



XV -\-v/ n 



bases growth per cent on volume five years ago, and is correct as an 



average per cent of the past ten-year period. If applied to the next 



decade, and based on V, or present volume, it assumes an increase in 



growth for this period. When this per cent is applied only to the current 



year, and is based on V the per cent is more conservative. 



While individual trees are growing rapidly in diameter, as dominant 



trees, their growth per cent for a time falls less rapidly than that of 



slower-growing trees. In even-aged stands, growth on individual trees 



is proportional to their diameters. Growth per cent in area is about 



twice the per cent of diameter growth. If determined for the trees 



which will be retained under management, this relation of growth to 



volume may be fairly consistent in such even-aged, thinned stands. 



Hence sample or average trees may give a close indication of the growth 



per cent or present status of the stand. But the assumption that this 



growth -per cent will continue to be laid on annually breaks down at 



once; hence the real assumption and the only one possible, if growth 



per cent is to be applied for predictions, is that the volume indicated 



by this per cent will continue to be laid on annually. And this in turn 



is inaccurate. 



