92 DETERMINATION OF THE INCREMENT 



where s x // X/ represents the present voUime, 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', // 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/. 



Instead of estimating the separate factors, the volume incre- 

 ment of the next n years may be estimated direct from that 

 laid on during the last n years, taking into consideration how 

 far the latter should be modified with regard to the age of 

 the tree, locality, future treatment of the wood, especially the 

 proposed degree of thinning, etc. 



According to Pressler's method, the probable increment can 

 be ascertained by estimating the probable diameter increment 

 and then proceeding by the formula : 



4 = (,S' - s) h, 



where s represents the present section in the middle, ;S' the 

 expected section in the same spot after n j^ears and h the 

 present height. The method applies only to felled trees ; 

 hence it necessitates the felling of one or more sample trees. 



Section II. — Determination of the Increment of Whole 



Woods. 

 It has been shown, that, in the case of single trees, the 

 accumulation of the volume, as well as the increase of the 

 factors, which lead up to it, height, diameter, or basal area 

 increment, can be followed backwards with a considerable 

 degree of accuracy. This is not the case as regards whole 

 woods, because trees die or are taken away in thinnings. 

 Investigations, made on sample trees selected in a wood, 

 show only the gradual development of the individuals 

 existing at the time of examination, but they throw no light 

 on that of those trees, which have disappeared in course of 



