85 



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 giren period of time. The average number of 

 trees that will be available annually for felling during this 

 period is thus known. 



As an example, snppose a foiegt at present containing the following stock of 

 exploitable trees, and in whioli the average annual diametral increase ot the stems 

 when approaching the exploitable size of 2 feet in diameter is 0-2 iuehes— 



In the course of 80 years the growth in diameter of the larger trees will be 0'2 x 

 80=60 inches. Consequently in that time the trees of Class II, now 18 inches to 

 24 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 oertaia 

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

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

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

 the possibility, all the trees now in Classes I and 11, that is to say, 24,741 + 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 mature or are approaching maturity — and this is 

 the condition of crops which have not been worked or have 

 been much under- worked — 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 

 to 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 sufficient- 

 ly well represented, the possibility has been arrived at by 

 dividing the exploitable trees together with a proportion of 



