320 PRINCIPLES UNDERLYING THE STUDY OP GROWTH 



tain its position in the stand and subdue its competitors. The net 

 growth on an acre is the sum of the growth of the surviving trees. 



At any given period or year in the life of a stand, the number of 

 trees is considerably less than were present and living at any previous 

 period or decade, and is considerably greater than the number which 

 will be alive at any given period or decade in the future. This loss in 

 numbers, accompanied by rapidly lessening rates of growth of a portion 

 of the surviving trees, plus the normal growth of the remainder, produces 

 the net result or increase in the stand for the period, and any method 

 of study of growth which does not take this natural loss and change 

 into account will be ineffectual in predicting or measuring the growth 

 of forests or stands. 



248. Yields, Definition and Purpose of Study. The past growth 

 of the surviving portion of stands is represented by their present volume, 

 the measurement of which is dealt with in Part II. This present 

 volume represents the yield of the area, provided nothing has pre- 

 viously been removed as thinnings or otherwise. But without a knowl- 

 edge of the period required to produce this volume, the word yield is 

 meaningless as it cannot be expressed in terms of the rate of produc- 

 tion per year or mean annual growth. An estimate of standing timber 

 is merely a statement of the volume at present found on the area. A 

 yield, on the other hand, is a statement of the volumes produced on 

 the area within a definite period of time. If the total volume is to be 

 expressed as a yield, then the total age of the stand must also be known. 

 If the yield for a shorter period, such as a decade, is to be stated, then 

 only that portion of the volume of the standing timber must be shown 

 as was laid on during this period. Otherwise, the rate of growth per 

 year is not indicated. 



The growth of forests is studied primarily for the purpose of pre- 

 dicting future growth on forest land. On the basis of past records of 

 growth of trees and stands as shown by measurements of present 

 attained volumes and of age, predictions can be made as to the future 

 growth of these and of similar stands. 



This application or prediction may be made in one of two ways: 



1. By projecting the rate of growth of an existing stand into the 

 future. This is done either by assuming that the rate shown in the 

 immediate past will continue unchanged in the immediate future, or 

 else that this rate will change and that this tendency of future growth 

 may be predicted by the shape of the past growth curve. Of these 

 two assumptions the second is apparently the more accurate, but in 

 neither case is it possible to predict the growth for more than a short 

 period. 



2. Some better method of prediction is required to cover longer 



