66 



can be taken as representative of its class. It must also 

 be borne in mind that so far as the standing timber alone 

 is concernedj thinnings cause the rate of increase to fall. 

 Lastly, we have the fact that what is now an average 

 stem of the crop was at one time an overtopping or domi- 

 nant tree, and may in the future become a dominated or 

 overtopped or even a suppressed one. 



The simplest way of determining the increment of. a crop with 

 the nearest approximation is to determine first the per- 

 centage of increase of several stems of each and every 

 component class. If the crop is at the time under a 

 regular valuation survey, this percentage is calculated for 

 the section passing throiigh the middle of the felled 

 sample trees, and the data thus obtained should also be 

 supplemented by measurements made on standing trees. 

 If there is no valuation survey going on, then measure- 

 ments made on standing trees must supply all the required 

 data. 



If Fj, V2, have been ascertained to be respectively 



the volumes of the several stem-classes^ and jBj, p^, 



...their average increment per cent., then the increment 



V «i- 

 for one year (*) of the several classes will be, i^ = '^ ' 



V n 

 is = .^,»^ , •.. and the total increment of the 



crop 



= I3 = «i + i^ ; 



so that the increment per cent, of the crop will be 

 100 1 



= ^'=^ + F, + 



The formula just investigated should be used only when it is 

 required to know the increment for a single year, or for a 

 short period. If it is sought to determine the increment 

 for a long period of n years, another procedure must be 



adopted. Pi,P2> being found to be the percentages 



of increase of the respective stem-classes during this period, 



the volumes Vj, n^, of these classes n years ago 



m&y be determined from Pressler's formula thus — 



