THE SELECTION SYSTEM 77 



which is desired, but the growth of the stand after 

 lumbering. The forester therefore makes a study of 

 growth, which enables him to predict the rate of growth 

 of the middle-sized trees; that is, those which will be the 

 merchantable trees at the next cutting. In calculating 

 the number of trees which will be standing after the lum- 

 bering, it is necessary to assume that the lumbering will 

 take all trees above a diameter limit. Of course, the 

 rigid diameter limit will not be used in the cutting, for 

 the reasons already explained. The trees cut below the 

 limit about offset in volume those left standing above 

 the limit. 



But it is not known in advance what general average 

 limit is desirable. The forester therefore makes trial 

 calculations of growth on the basis of several limits. 

 Thus, in the case of spruce a superficial study shows that 

 the limit may be below 14 inches, breast-high. The 

 trees above this size are old and mature, and should 

 be cut and utilized as soon as possible. Moreover, a 

 higher limit would ordinarily not yield a profitable cut. 

 One can readily see that a cut below 10 inches would 

 deplete the pole-class so severely that the production of 

 the forest would be reduced by an excessive amount. 

 Hence the forester makes trial computations of the 

 rate of growth, assuming a cut first to 10, then to 12, 

 and then to 14 inches. 



A comparison of the various possible limits, and the 

 possible present and future returns, together with a con- 

 sideration of the problem of reproduction, enables the 



