38 BULLETIN 234, U. S. DEPARTMENT OP 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 multiplying 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 9X0.67X 1,570 = 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: — -j— = 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 correspondingly 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. 
