92 DETERMINATION OF THE INCREMENT. 



measurer, h is estimated, S is obtained by measuring the 

 X - - - diameter with a calliper, and s with the 

 ! assistance of an increment borer. F and 

 /must be taken out of form factor tables, 

 or estimated. If F is taken as = /, the 

 formula becomes : 



H 



Fig. 35. 



The method can only give approxi- 

 mately correct results, because h has to 

 be estimated. It must also not be over- 

 looked, that form factor tables give only 

 averages ; hence the method is not 

 adapted to the measurement of a single 

 tree, but only to that of a large number 

 of trees. 



I. Volume Increment of the Future. 



The increment which a tree may be expected to lay on in 

 the future can be estimated either from its own past incre- 

 ment, especially that of the immediate past, or by comparing 

 it with other older trees. 



i. ESTIMATE ACCORDING TO PAST INCREMENT. 

 The increment is represented by the formula : 

 I n = S X Hx F-s X h xf t 



where s x h x/is the value of the present volume, and S X H 

 x F that to be expected after n years. The formula shows 

 that, in order to obtain fairly accurate results, it is necessary 

 to estimate S, H and F from s, h, and /. How this 

 should be done as regards basal area and height, has been 

 explained above. The form factor F may be obtained from 

 tables, if such are available ; otherwise it must be estimated, 

 or it may be taken as equal to /. 



