GROWTH PER CENT TO DETERMINE GROWTH OF STANDS 433 



The sources of inaccuracy in this method are: 



1. Predicting the volume growth of a stand from that of one or two 

 selected or average trees. The growth per cent of a stand is practically 

 always less than that of the average trees which survive, due to loss 

 of numbers and falling growth rate of the suppressed class. 



2. Applying a growth per cent obtained from a past period on a 

 smaller volume, to the present volume of tree or stand, under the assump- 

 tion that not only will the rate of growth in volume continue the same 

 but the per cent will remain unchanged, when, as shown, growth per 

 cents always fall as wood capital increases. 



3. Assuming that the growth per cent as derived from average 

 trees, or even from sample plots, will apply to larger areas and to dif- 

 ferent proportions of age classes in mixture, when in fact, so doubly 

 sensitive is this per cent relation, that any difference in average age 

 and volume between the forest and the sample areas will result in a 

 large error in determining the true weighted per cent by this means. 



The possible errors may be illustrated as follows : 



From a yield table for White Pine ' the actual known yields are, 



At 30 years 3750 cubic feet 



40 years 6590 cubic feet 



50 years 8035 cubic feet 



60 years 9075 cubic feet 



By Pressler's formula, the current annual growth per cent for these decades is, 



30 to 40 years 5.5 per cent 



40 to 50 years 2.0 per cent 



50 to 60 years 1.2 per cent 



If the growth for the decade from thirty to forty years be taken to indicate the 

 current growth in the fortieth year, of 284 board feet, this gives a current growth per 

 cent for that year on 6590 board feet, of 4 . 3 per cent. Assuming that this growth 

 per cent will continue for the next decade, we have a total increase of 43 per cent or 

 2834 board feet. The actual growth is 1445 board feet. The error is 96 per cent 

 excess. 



Such errors are the result of use of the growth per cent, even when the basic 

 data are correct. The errors may be greatly increased when growth per cent is 

 obtained from single trees and the losses in the stand are ignored, since too high a 

 current growth per cent will be obtained. 



332. Use of Growth Per Cent to Determine Growth of Stands by 

 Comparison with Measured Plots. The only merit which growth per 

 cent has as a method of determining growth lies in the possibility of 

 using it as a means of comparison. Since per cent does not express 



1 Forest Mensuration of the White Pine in Mass., H. O. Cook, Office of State 

 Forester, 1908, p. 21. 



