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 formulae 

 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 = f v ) — when n=10 years, 



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. 



