21 



In act aal practice we coald not, of coarse, dlstinguisli le'weea trees differing i n 

 age by only one year. If the trees in such a forebt as the above had to be couuted 

 ^they n-Quld be grouped in age-classes oi', say,— 



T. — SteiLS aboTO [20 years old ; 



II.— „ from 19 to 12 „ 



III.— „ „ U to 5 „ 



IV. — Young seedlings .... below 5 „ 



and the crop would thus contain of — 



Class I 110 stems. 



,. H 1,530 „ 



„ III 5,650 „ 



„ IV . . . . . . . abundant. 



This would not indicate whether the age-classes were properly constituted or no! 

 We could only say that the crop was complete, and that all stages of growth were wel. 

 represented j for we cannot even approximately estimate the relative nnmber of trees 

 in each age-clan's which ought to be found in an irregular forest. Indeed, the relative 

 proportion is constantly changing, and this fact is one of the ohief drawbacks to the 

 -ayplication of the selection method of working, and is the reason why there ure so 

 many different ways of attempting to oalonlate the possibility of such forests. Where • 

 the crops of different agrs occupy distinct areas nothing is simpler than to mensare 

 these areas and to arrive at a correct estimate of the qnaLtity of - material the forest 

 can, and ought tn, produce ; but, in the case of irregular cropi", we can neither measure 

 the area occupied by each age-class, nor derive by other means, from the number of 

 trees found to be grcwii gin the forest, absdntely accurate information as regards the 

 sufficiency or otherwise of the growing stock. 



Section IV.— The increment. 



1. ffeaniug of the term.— The increase due to growth, -which 

 "takes place during any given time in. the volume of material 

 in a tree or crop, has received the special name of increment. 

 A distinction, is drawn between the annual increment and the 

 .average or mean increment. 



When the length of the period referred to is one year, 

 the increase is called the annual increment. When the 

 period includes a number of year', the average is obtained 

 hy dividing the total increase durinor the whole length of the 

 period by the number of years. The result is called the 

 mean annual increment or mean increment. The terms 

 annual production and mean annual production are also 

 used. 



The annual increment is extremely difficult to me.'isure or 

 to estimate except empirically ; and, as its calculation is in 

 India of little practical importance, the methods employed 

 to determine it elsewhere need not be here explained. 



A tree 100 years old is found to contain 115 cubic feet of wood, and it is calcu- 

 lated that its increment from the 99th to the 100th year was I'o cubic feet. I's 

 annual increment at 100 years is 1'5 cubic feet, while its average annual increment 



or mean increment during its life-time at 100 years is jgg = 1'15 cubic feet. Tea 

 years later the tree is found to contain 126 cubic feet. Its average increment during 

 tthat period is "°~j"° =l-l cubic feet. 



