16 THE THEORY AND PRACTICE OF WORKING PLANS 



prevailing storm direction. However, this theoretical ideal is 

 never achieved; it suffices that each age class has an approxi- 

 mately equal representation on the area which is to have a 

 sustained yield; in fact, without a fairly even distribution of 

 the age classes sustained yield on a given area is impossible. 



Twenty years is commonly taken as one age class, though 

 sometimes ten and sometimes thirty-six years is used. In any 

 case the rotation must be a simple multiple of the age class. 

 It is customary to number the age classes from I up, beginning 

 with the youngest.* Thus for an eighty-year rotation there are 

 four age classes of twenty years each; a fifth age class would 

 contain all stands older than r (the rotation). 



It is of the utmost importance to get some conception of 

 how the age classes are distributed. 



In even-aged stands or stands even-aged in groups, the age 

 may be determined by finding the average tree (any of the 

 standard methods) and then getting the age from stump 

 analysis or boring to the center at breast high with an incre- 

 ment borer, or from diameter-age tables (if available and 

 applicable!). 



Where stands are fairly even-aged, but conditions are too 

 extensive to permit the exact assignment to definite age classes, 

 the general classification into 



overmature (more than rotation age) ; 



M mature (of rotation age down to \ thereof) ; 



F young (from lowest age to \ rotation) 



will serve the purpose. 



The selection forest, of course, has all age classes inextricably 

 intermingled. But where the age differences are not to exceed 

 ^ or I of the rotation, the stand can be classified according to 

 its average age, or, more exactly, according to the proportion 

 of space each age occupies. For example: 320 acres of spruce 

 might contain 160 acres of trees seventy years old, 100 of trees 



* In Prussia this is reversed, I. is the oldest age class. 



