CHAPTER XXX 

 THE DETERMINATION OF GROWTH PER CENT 



327. Definition of Growth per Cent. Growth per cent is an expres- 

 sion of the relation between growth and volume. 



Current growth per cent is the relation of growth during a given 

 year to the volume at the beginning of the year. 



Periodic growth per cent is the relation of the growth during a period, 

 to a basic volume, which may be taken as the mean or average volume 

 for the period (§ 328), but is usually that at the beginning of the period. 



Mean annual growth per cent is the per cent which the mean annual 

 growth (§ 245) for a given age bears to the total volume at that age, 

 and represents the average rate of growth per year, at which this volume 

 has been produced. Growth per cent requires for its determination 

 a knowledge of two factors, the growth for a period and the volume 

 upon which this growth was laid. The primary purpose for which 

 growth per cent is utilized is to test the maturity or ripeness of individual 

 trees and of stands of timber. Those trees or stands which show the 

 lowest per cent of increment on their present volume compared with 

 other trees or stands, should be selected for cutting. The object of 

 such selection is to withdraw from the forest the greatest possible volume 

 of wood capital, while at the same time reducing the volume of expected 

 growth by the smallest possible amount. If carried out, the effect is 

 to transform the forest capital from a condition in which the ratio of 

 growth to volume is low, to one in which this ratio is materially increased 

 for the forest as a whole. 



On individual trees the difference in volume or growth for the decade 

 may be found by analysis (§ 287 and § 288). For stands, the difference 

 is taken from yield tables for the decade. In each case one year's 

 growth is one-tenth of the growth for a decade. The growth per cent 

 of average test trees is frequently assumed to be that of the stand. 



328. Pressler's Formula for Volume Growth Per Cent. To deter- 

 mine growth per cent as a means of judging the ripeness or maturity 

 of stands or trees, the same methods apply whether the unit is the tree 

 or the stand. Since volume growth is measured for periods of a decade, 

 the growth for one year is found by division. Let n equal the period 

 representing a decade. This may be a longer or shorter period if neces- 



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