14 MISC. prBLiCATiox 1], r. s. dept. of ageiculture 



CALCULATION OF ALLOWABLE CUT 



The 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 

 methods that are in use or have been advocated by foresters here 

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

 bilities, and availability and character of growth data justif}^ similar 

 variations in the methods used in s^^ecific cases. In the management- 

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

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

 calculating the j^ield as a sole dependence. This is probably due to 

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

 the information needed in sufficient 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 

 onanagement for many years. 



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

 agement that will promise sustained ojDerations 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 years 

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

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

 cutting cycle. 



The"^ total volume of merchantable timber in a type, 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 

 types is the total allowable cut for the working circle. 



While the first ctitting cycle is the only one used directly in the 

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

 sirable to carry the calculations through the second and remaining 

 cutting cycles in 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 



