THE SELECTION SYSTEM 49 



oldest trees. If a given stand is cut through each year, 

 there is removed an amount equal to the annual growth 

 of the whole stand. If the stand is cut every 10 years, each 

 cutting removes an equivalent of 10 years' growth. As all 

 age-classes are at hand, there are continuously trees grow- 

 ing into the merchantable class as a basis for the next cut. 



The theory may be illustrated by a concrete exam- 

 ple. Suppose that the stand comprises 100 acres, the 

 annual growth is found to be 250 board feet per acre, 

 and the cutting cvcle is 10 years; then 2,500 board feet 

 per acre, or 250,000 feet on the whole tract, may be 

 removed at each cutting, provided there are no special 

 conditions to modify this amount. In a stand in which 

 the age-classes are well represented, the proportion of the 

 total area uncovered by a given cutting, that is, the 

 aggregate of all openings taken together, ic approximately 

 equivalent to the rotation divided by the cutting cycle. 

 Thus, if the rotation is 150 years, and the cutting cycle 

 is 25 years, each cutting uncovers in aggregate of all 

 small openings one-sixth of the area. 



In the continued operation of a selection forest it is 

 exceedingly desirable to have the different age-classes 

 represented in their proper proportion. Obviously, if a 

 given age-class is not represented or is deficient it is not 

 possible to cut at regular intervals an amount of timber 

 equivalent to the full growth of the stand. Suppose, for 

 example, that in a selection forest handled on a rotation 

 of 100 years the oldest age-class, namely from 80 to 100 

 years, is represented in full proportion, while there are 



