THE THEORY AND PRACTICE OF WORKING PLANS 17 



sixty years old, and 60 acres of trees only forty years old. The 

 average age here would be 61 years; for: 



160X70+100X60+60X40 , 



'- — ■ ^=61 years. 



320 



Where, in uneven-aged forest, the age classes are so inter- 

 mingled that they cannot be distinguished by area but only 

 by volume (from the diameter-classes in the estimates), the 



average age = the . ; e.g., if the uneven-aged forest 



mcrement 



has three main age classes: 



100 year class with 2000 feet board measure 

 60 1 200 



50 800 



xi ^1 111 2000+1200+800 



then the average age would be — = 71.4 years. 



2000 1200 800 



100 60 50 



Prof. Chapman of Yale has suggested a method of dividing 

 the total volumes by the total number of trees, then finding 

 the d.b.h. and height which, in the volume table, corresponds 

 to this average volume and determining the average age from 

 stump analysis, increment boring, or growth table. This 

 method appHes only to the merchantable classes. 



The normal selection forest would show the following dis- 

 tribution of ages by area: 



E.g., 900 acres 'of selection forest with a rotation of 150 years 



and a cutting cycle of 30 years would normally contain -^ = 5 

 age classes as follows: 



Trees ■ ^^^^'^^ 



