8 THE THEORY AND PRACTICE OF WORKING PLANS 



where n equals number of years in each age class (step of the 

 yield table) and a, b, c, . . . w = the volume per acre given 

 in the table for each age class. 



The method and correctness of finding 7iv (i) by formula, 

 (2) by summation from peld table, is illustrated on the accom- 

 panying diagram (Diagram B). For the values as given in this 

 diagram, which is based on a yield table for white pine con- 

 structed by W. J. McCarthy, M, F., in the vicinity of Ithaca, 

 N. Y., the normal growing stock would be as follows, assum- 

 ing a forest of 1000 acres: 



f . ri 



(i) 7lV = -~\ 



2 



r = rotation = 140 years ; 



i = mean annual increment per acre = 281.64 board feet; 



140X281.64 



nv = ~^--^ = 19,715; 



2 



19,715X1,000=19,715,000 board feet. 

 (2) nv = nia-\-h-\-c . . . +"); 



» = number of years in each step of yield table = ic 

 a, b, c, . . . are volumes per acre in each step 

 of yield table; 



m is volume per acre at the rotation age (140 years); 



«z;= 10/^3580-^8600+12580 . . . -\-^M^\ 



, ^^ 1000 2.996,950,000 , J r ^ 



= 2,g96,95oX = — ^^ — ^^^— ^ = 21,407,000 board feet. 



'^^ '^^ 140 years 140 ' 



A further comparison of the two methods of determining the 

 normal growing stock is given in the following table. These 

 calculations are based on Hanzlik's jield table for Douglas fir 

 as given in Forestry Quarterly, Vol. XII, No. 3, pp. 442-445. 



