102 THE THEORY AND PRACTICE OF WORKING PLANS 



this method with y obtained by current annual increment (method 

 No. 4 above) and, if necessary, the use of only the first one or 

 first two of this series of expressions so as to make the results 

 comparable, and also periodic revisions of the data on 

 which the method is based. The class represented by ai, z^i, 

 and «i, is usually the one just below the diameter limit of 

 cutting. 



Since the method is particularly intended for selection 

 forests it is Hufnagl's theory that the cutting cycle shall equal 

 approximately a^ — a^ years, i.e., the time required for the 

 highest merchantable diameter class to be replaced by the one 

 next below it. 



In its practical application it is customary to choose a 

 maximum as well as the usual minimum diameter limit; that 

 is, the maximum diameter of trees which it is practicable to 

 produce. This usually leaves a surplus of still larger trees 

 produced under virgin conditions. This surplus may well be 

 distributed over the cutting cycle or disposed of more rapidly 

 or more slowly as local exigencies dictate. 



(b) Examples. — Example of Variation I. — It is desired to 

 determine the cut of hemlock in the slope type of Catskill 

 forest, illustrated by the stand and stock tables quoted above 

 from Bulletin ii of the N. Y. State Conservation Commission.* 

 A breast-high diameter limit of 12 inches is to be observed 

 which, according to U. S. Dept. of Agriculture Bui. 152, n. s., 

 " The Eastern Hemlock," table 10, corresponds to a rotation 



of 160 years. The same table shows that at eighty years (-1 



a diameter breast high of 4 inches is attained. 



The volume of all hemlock trees over 4 inches is 403.08 

 board feet per acre (Table 5, Bui. 11). The current annual 

 increment thereon is 5.478 board feet per acre (see example of 

 method No. 4.) The annual cut then 



* Bulletin 11, "Forest Surv^ey of a Parcel of State Land," Albany, N. Y., 

 1915, Tables 4 and 5. See pages 47 and 48. 



