12 



INCREMENT. 



Meaning 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, lias 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 years, the average is obtained by 

 dividing the total increase during 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 diflScult to measure 

 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 production from the 99th to tlie 100th year was 1-5 cubic feet. Its 

 annual production at 100 years is 1"5 cubic feet, while its a-yerage annual production 

 or mean increment during its life-time at 100 years is 115 = 1-15 cubic feet. Ten 



years later the tree is found to contain 128 cubic feet. Its average produc- 

 tion during that period is -y^ = 1' 1 cubic feet. 



In the case of crops the production is necessarily ex- 

 pressed, with reference to the area, at so much per acre. But 

 this cannot be done for isolated trees; because the area 

 covered is uncertain and changes from year to year. In cal- 

 culating the average production of crops, the total production 

 during the period, including the material yielded at the 

 passage of thinnings, etc., must be reckoned with. 



Three acres of high seedling forest, 100 years old, which have already furnished 

 in thinnings 5,000 cubic feet, are found to contain 25,000 cubio feet of timber. The 



average annual production or mean increment at 100 years is therefore ' oytob' — "^ 



■^^ = 100 cubic feet per acre. 



