94> 



:and their rates of growth being known, it is possible to esti- 

 mate with fair accuracy the number that will pass from one 

 size-class to the next higher class, or that will become ex- 

 ploitable in a given period of time. The average number of 

 irees that will be available annually for felling during this 

 period is thus known. 



Ab an example, suppose a forest at present containiag the following stock of 

 -exploitable trees, and ia which the average annual diametral increase of the stems 

 when approaching the exploitable size of 2 feet in diameter is 0"2 inches — 



Class 



In the course of 30 years the growth in diameter of the larger treas will be 02 X 

 30=6'0 inches. Consequently in that time the trees oE Clasj II, vov 13 inches to 

 ^4 inches in diameter, will be replaced by those in Class III ; they may therefore be 

 removed. Most of them will have attained the exploitable diameter, but a certain 

 number being suppressed or crowded out may, unless felled while still below the ex- 

 ploitable siz9, be ultimately unutilisable. Assuming, therefore, the stock to bj com- 

 plete and normal, we may theoretically fell in the course of 30 rears, without exceeding 

 -the possibility, all the trees naw in Class I and II, that is to say, 21,741 + 17,867 = 

 42,608 trees or at the rate of —^^ = 1,420 trees a year. 



In the preceding remarks it has been assumed that the 

 crop contains a sufficiency of trees of the lower age- classes. 

 But in a forest where, for example, most of the trees are 

 matured or are approaching maturity — and this is the condi- 

 tion of crops which have not been worked or have been much 

 underworked — fellings determined in this manner would 

 remove in a single period not the possibility but practically 

 the whole forest capital. 



In India it has sometimes been the practice to base the 

 calculation on the number of trees already exploitable, and 

 i;o limit the annual fellings to this number divided by the 

 number of years in which all the stems in the next lower 

 size- class will become exploitable. Thus, in the above 

 Example, the annual fellings might be limited to 24,741 

 divided by 30, or to 825 trees. 



In other cases, where the younger age-classes are suffi- 

 ciently well represented, the possibility has been arrived at 

 by dividing the exploitable trees together with a proportion 

 of those to become exploitable during the felling rotation by 

 the number of years in that rotation. 



The calculation can, however, be made in a less crude 

 fashion. Supposing that the ages are evenly graduated, 

 the average number of trees that can attain exploitable 



