14 MISC. PUBLICATION 11, U. S. KKPT. OF AGRICULTURE 



CALCULATION OF ALLOWABLE CUT 



Tlie literature of forestry is not lacking in the treatment of regu- 

 lation, particularly the calculation of the yield. It is not intended 

 here to go into the merits or demerits of the many and various 

 metiiods tiiat are in use or have been advocated by foresters here 

 and abroad. Wide vaiiations in forest conditions, utilization possi- 

 bilities, and availability and character of growth data justify similar 

 variations in the methods used in specific ca;^es. In the management- 

 plan work of the national forests in recent years there has been a 

 nuirkcd tendency to get away from the use of formula methods of 

 calculating the yield as a sole dependence. This is probably due to 

 the fact that it has not often been possible to get togetiier all of 

 the infoiination needed in suflicient detail and with sufficient accuracy 

 to form a safe basis for such methods of calculation. Then, too, 

 the silvicultural condition of the generally overmature virgin .stands 

 that characterize the national forests does not lend itself to methods 

 of calculation developed mainly for stands that have been under 

 management for many years. 



If the forester is interested, as he usually is, in a course of man- 

 agement that will promise sustained operations without a hiatus in 

 production and that will replace his poorly conditioned virgin stands 

 with fast-growing, fully stocked second growth representing some- 

 thing approaching a normal forest, he will first find out over how long 

 a period of time he must spread the cutting of his first crop of timber 

 in order to allow the second crop to reach merchantable size and 

 volume and take up the burden of supplying the needs of industry. 

 Thus, except in forest types like Douglas fir on the Pacific coast that 

 are cut clean and harvested only once in a rotation, the determination 

 of the cutting cycle is an important step in the calculation of the 

 allowable cut. 



One rough-and-ready method of calculating the yield in selection 

 forests, often used as a check on the results of more finished methods 

 or used alone where the basic data for other methods are lacking, is 

 as follows: 



Assume an average diameter of merchantable trees and a volume 

 per acre to represent a stand merchantable at the time of cutting. 



Ascertain by increment borings or otherwise the number of yeare 

 that it will take to bring the reserve stand left after removal of the 

 first harvest up to the assumed size and volume. This is the first 

 cutting cycle. 



The total volume of merchantable timber in a tj'pe, minus the 

 volume that is to be left for the second cut, divided by the number 

 of years in the first cutting cycle, gives the annual yield for that 

 cutting cycle and type. The sum of the allowable cuttings for the 

 tyjies is the total allowable cut for the working circle. 



"UHiilo the first cutting cyc\o is the only one used directly in the 

 calculation of the yield for the first budget period, it is very de- 

 si lablc to carrv the calculations through the second and remaining 

 cutting cycles m the rotation. It is important because it applies a 

 working test to the marking policy tentatively adopted and discloses 

 any possibility of a hiatus or a change in yield in the future. 

 Threatened lapses in yield may often be avoided or at least shortened 



