340 



DETERMINING THE AGE OF STANDS 



of the test trees or their age determination by borings or choppings 

 is the standard practice in determining the age of stands. When the 

 stand is treated as a single group, the average of the ages of the test 

 trees, all of which will be of the same average diameter, is taken as the 

 age of the stand. When two or more sub-groups have been separated, 

 the age of the entire stand must be calculated by weighting the pre- 

 determined ages, of the sub-groups, in the proper proportions. 



The following illustration will bring out the different methods possible in doing 

 this. An " even-aged " stand composed of 30 trees is divided into two groups as 

 follows: 



1. If each of these groups occupies an equal area and is given equal weight, the 

 average age may be found by adding the ages of the sample trees and dividing by 2. 

 This gives eighty-five years, and is known as the arithmetical mean sample tree 

 method. This method does not conform to the basic principle of weighted ages 

 sought. 



2. When the trees are weighted by number the result is : 



10X100 = 1000 



20 X 70 = 1400 



Total, 2400 -=-30 = 80 years 



This overemphasizes the number of trees rather than their volume, hence is unsat- 

 isfactory. 



3. Trees are weighted by volume on the principle by which weighted volume 

 averages are always obtained: 



100 years X 5000 = 500,000 



70 years X2500 = 175,000 



Total, 675,000 +7500 = 90 years. This method is acceptable. 



4. The sum of the mean annual growth for the groups is obtained. The total 

 volume divided by this sum gives the average age. This method is considered 

 by European investigators to be more accurate than the others. As applied: 



5000-7-100 = 50 



2500 -f- 70=35.7 



Total mean annual growth for stand, 85 . 7 



7500-e-85.7 = 87 years. 



By either method 3 or 4, it is seen that the average age is influenced by volume 

 rather than by area or number of trees. 



