THE SHELTER-WOOD SYSTEM. 217 



takes only a few, and in others more years, it is impossible 

 to define its duration accurately, and least of all can m be 

 placed equal to n, the number of years in the period. Under 

 these circumstances the arrangement of age classes can 

 be indicated only approximately, somewhat in the following 

 manner : 



Cuttings in the oldest age class commence when the crop 



is r ~ years old, and the last cuttings occur when the crop 



is r + years old. Assuming that s years pass, after the first 



2 



regeneration cutting, until the new crop is fully started, then 



A 



the annual cutting area, as before, = - - , and the area of the 



T -\- 8 

 



regeneration class = - X m. The latter contains the areas 

 r + s 



as yet blank, young trees from 1 to m s years old, and the 

 remaining old trees ranging in age from r ^ + lto 





years. Now, it may happen that m s = n ; in that case the 

 youngest age class does not exist by itself, but forms part 

 of C v . Again, it may occur, that m s > n, in which case C v 

 contains not only the youngest age class, but also a portion, 

 if not the whole, of the second age class. Hence the size of 

 the several age classes may be expressed as follows, assuming 

 five age classes : 



(1) m s<^n : 



C v = -- 

 r+s 



