338 DETERMINING THE AGE OF STANDS 



indicates or determines the rate of volume production per year at 

 which the stand has grown; therefore, the average age must be a 

 weighted age based on volume. The determination of average age 

 applies only to those stands which fall under the definition of even-aged 

 stands, yet have within the limits of the group a sufficient range of ages 

 so as to require a further investigation in order to fix the weighted or 

 average age of the group. For many-aged stands, the average age of 

 each age class must be determined separately. 



For a given age class or even-aged stand as thus defined, the average 

 age is the age which would be required to produce an even-aged stand 

 containing the same volume as that of the uneven-aged stand in ques- 

 tion. 



The methods possible for determining the weighted average age 

 of the trees comprising the age class usually involve the choice of 



1. Treating the entire age class as a single group, or subdividing 



it into from two to three, usually not over two, sub- 

 groups. 



2. Determining the average tree, for the entire class, or sepa- 



rately for each sub-group. 



3. Ascertaining the age of these average trees. 



4. Weighting the resultant ages of average trees of sub-groups, 



to determine the weighted average age of the age class. 



261. Determining the Volume and Diameter of Average Trees. 



Subdivision of a group into two or more sub-groups will be made, if at 

 all, on the basis of diameters, by the diameter group method (§ 251). 



In determining the average tree for the age class, or for a sub- 

 group, there are two reasons for basing this selection on average volume. 

 In the first place, if these selected trees are to be felled, and their ages 

 taken as indicating that of the stand, the larger trees must be avoided, 

 for in aU probability they are advance growth, several years older 

 than the rest or possibly belonging to an entirely different age class. 

 The smaller trees would also be rejected since they may be late seedlings 

 some years younger than the average, or in extreme cases, so badly 

 suppressed that a certain number of rings may be lacking and the 

 growth difficult to determine. Trees of about average size for the group 

 or stand must then be chosen. Where two or more groups are made, 

 an average tree for each group is separately selected. 



Volume is the determining factor upon which the weighted average 

 age is to be based, hence the tree whose age is taken to indicate that of 

 the stand must be a tree whose volume is an average of the stand. 

 This principle applies not merely to cubic volume, but to the merchant- 

 able volumes expressed in units of product, such as board feet. Since 



