82 DETERMINATION OF THE INCREMENT. 



3. Area Increment. 



The increment in basal area is calculated from that of the 

 diameter. Let D be the mean diameter of the whole section, 

 d the diameter of the same section // 3'ears ago, then 



Basal increment ] _ />- X tt _ f/- X tt 

 during n years > "^ 4 



= (^^-;^-^'=(/>=--/^, X-785. 

 4 



The basal increment can be ascertained for a limited number 

 of years only, or for the several periods of the life of a tree. 

 An estimate of the future increment is based upon that of the 

 immediate past, taking into consideration the proposed treat- 

 ment, as in the case of the diameter increment. 



4, Volume Increment. 

 a. Of /he Fast. 

 The past volume increment of a tree during a certain period 

 of years, n, is equal to the difference of volumes at the com- 

 mencement and end of the period. These volumes can be 

 ascertained by examining a series of sections at various heights 

 of the tree, or by basing the calculation upon measurements 

 made at the middle section, or by using form factors. 



i. DETEnMINATION OF THR IXCKEMENT I!Y SECTIONS. 



If the increment of only a limited number of years, n, is 

 desired, it can be ascertained by means of the increment borer. 

 The breadth of n rings is ascertained at regular intervals along 

 the stem and the differe)ice between the present volume and 

 that n. years ago calculated. 



The investigation of the progress of increment throughout 

 the life of a tree is called a stem analysis. It consists of a 

 combination of a height and a diameter analysis. 



