38 BULLETIN 234, IT. S. DEPARTMENT OF AGRICULTURE. 



90 cubic feet, the average normality of the 10-year age class is 0.67,. 



and the actual area occupied is 1,570 acres; consequently, the real 



growing stock is 90X0.67X1,570 = 94,671 cubic feet. The real 



periodic annual increment is determined by multipljung the normal 



periodic increment per acre, as given in Table 9, United States 



Department of Agriculture Bulletin 154, by the normality and by the 



area actually occupied. Thus, for the 10-year age class the real 



periodic increment is 9x0.67x1,570 = 9,467 cubic feet. 



The normal growing stock is based on the assumption that the 



forest will be managed on a 140-year rotation, and that in a normal 



forest each age class should have the same area. This normal area 



is found by dividing the total area by the number of age classes. 



62491 

 Thus: — ^— = 4,463.6 acres. The normal growing stock on this area 



is then found by multiplying the normal yield at any given age (as given 

 in Table 9, United States Department of Agriculture Bulletin 154) by 

 the normal area. For example, the normal yield at 10 years of age 

 is 90 cubic feet and the normal area of a 10-year age class is 4,463.6 

 acres ; consequently, the normal growing stock is 90 X 4,463.6 = 401 ,724 

 cubic feet. Similarly, the normal periodic annual increment is the 

 normal increment per acre (as given in Table 9, United States Depart- 

 ment of Agriculture Bulletin 154) multiplied by the normal area. 



The fact that all ages of merchantable timber were lumped together 

 in the estimates and that, as already stated, any stand running 2,000 

 or more board feet per acre was considered merchantable, necessarily 

 results in a comparatively large area and growing stock being assigned 

 to the 120 to 160 age classes and a corresponding^ small area and 

 low normality to the age classes just under 120 years. For this 

 reason the figures for volume increase tend to be conservative. Other 

 reasons why these figures are conservative are that no considera- 

 tion is given to the effect of future thinnings in young stands, to 

 reproduction in old stands, or to increased growth resulting from, 

 selection cutting. Moreover, certain areas less than 0.3 normal are 

 classed as grassland, although they bear an open stand of timber 

 which will actually figure in the final yield. Also, rather open stands 

 of low normality will become better stocked through the filling in of 

 blanks. On the other hand, there will undoubtedly be some losses 

 from fire and other causes. 



It will be noticed that the scheme of regulation is presented as 

 though the area would be managed under a clear-cutting system, 

 though actually the cutting will be done largely under a selection 

 system. The reason for this is that it is possible to figure much more 

 readily for a clear-cutting than for a selection system, while, in 

 any event, the main object is to obtain a fairly conservative estimate 

 of the probable volume production, which is likely to be as great 

 under the selection system as another. 



