BY FORM FACTORS. 



91 



iii. DlVlEliMINATION OK THE IXCHEMKNT liY FoRM FAnOKS. 



Let S be the basal area of a tree taken at chest height; s the 

 basal area of the tree // years ago taken at the same height ; 

 H and h the corresponding heights, and F and ./' the corre- 

 sponding form factors, then the increment — 



I = S X H X F - s X h X f. 



In the case of a standing tree, H is measured with a height 

 measurer, h is estimated or taken from tables of height growth, 

 the proportion between the outer 

 and inner diameters being utilised 

 to determine //. S is obtained l)y 

 measuring the diameter with a cal- 

 liper, 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 = f, 

 the formula becomes : 



I = {S X H - sx h) X F. 



The method can give only approxi- 

 mately correct results, because h has 

 to be estimated. It must also 

 not be overlooked, 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 ti 

 case, the results can be only aj^proximately correct. 



Z*. Vohoup Inrremeni of the Fuivre. 

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

 the future, can be estimated from its own past increment, 

 especially that of the immediate past. 



The increment is represented by the formula: 

 7,^ = ,S' X 7/ X 7' - s X A X ./', 



ees. Even in the latter 



