43 



difference between the ages of the oldest and youngest trees in one 

 and the same girth-class is, the more approximate to the true 

 mean age of the crop will be the arithmetical mean of the ages of 

 the sample trees. And the result will be all the nearer, when the 

 number of sample trees taken is, is large and the more closely the 

 system of forming the stem-classes approaches that of Hartig's 

 method. 



Chapter VI. 



Determination of the rate of increase of individual trees and 

 of canopied masses of trees. 



Single trees increase in height, girth, basal area and volume. 

 Similarly, crops of trees increase in height, aggregate basal area 

 and volume. 



The current annual increment of a tree or crop is the amount by 

 which it has increased during the past year. 



Periodic increment is the amount of increase gained during a 

 period of several years. 



The total increment of a tree or crop is its actual volume at any 

 given time (the sum of all the annual increments up to the given 



time). 



The expression mean increment, when used without any other 

 qualifying word, may refer to merely a period, or to the entire 

 age of the tree or crop, or to the age when the tree or crop becomes 

 exploitable. 



In determining the increment for a single year, it is customary 

 to assume for it the figure of. the .mean increment for a short 

 period of years, because the increment for a single year is not 

 only composed of factors too small to be accurately measured, but 

 is subject to disproportionably large fluctuations from year to 



year. 



For many purposes the increment is conveniently expressed as a 



percentage. 



2. Increment of indiv iduai.tr ees. 



A. Bate of-increate in height. 



The rate at which the height has increased can be determined 

 at once in the case of most conifers, since the length of each annual 

 shoot is apparent owing to their peculiar mode of branching. 



In the case of broad-leaved trees, the stem must be cut acroat at 



