OF THE FUTURE. 93 



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 : 



I* = (S-8) h, 



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

 expected section in the same spot after n years, and h the 

 present height. The method applies only to felled trees ; 

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



ii. ESTIMATE BY COMPARISON WITH OTHER OLDER TREES. 



The increment I which a tree A now a years old, and con- 

 taining a volume v a is likely to lay on during the next n years, 

 can be ascertained by examining another tree B grown under 

 the same conditions but now a + n years old, which with a 

 present volume = v a + n had, when a years old, a volume 

 equal v a . The increment which the second tree has laid on 

 during the last n years, may be assumed as equal to that which 

 the first tree is likely to lay on during the next n } r ears. The 

 assumption holds good only if the weather and treatment are 

 the same during both periods. 



It is, however, not always easy to find exactly the tree 

 wanted, and as a rule a tree differing somewhat in volume and 

 age must be taken, which involves the following corrections : 



Supposing the age of the second tree is a + t instead of 

 a + n, and its volume =v a + t> then, if the increment has 

 been steadily laid on, the following equation holds good : 



t : n = I t : I n = v a+t -v a : I n 

 and 



In=(v a+i -V a }X~. 

 t 



