22 



In the case of crops the increment is neoessaiily expressed, 

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

 cannot be done for isolated trees, because the area covered 

 is uncertain and changes from year to year. In calculating 

 the average production of crops, the total production during 

 tlie period, including the material yielded at the passage of 

 thinnings, etc., must be reckoned with. 



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

 in thinnings 5,000 ouoio feet, are fonnd to contain 25,00) oubio feet of timber. The 

 average annual increment at 100 years is therefore '^•°3°°AM'"'° ='~lor = ^^ '^^^^'^ 

 feej par acre. 



2. Increments of single trees and crops. — The mean annual 

 increment of a single tree varies greatly with the age and 

 may be held to continually increase to maturity, while that 

 of a crop, occupying a given area, varies much less and^ in- 

 creases up to a certain time when it diminishes. 



Tlie following examples are taken from measurements- 

 made in India : — 



The reason for the slight variation of the production in the case of crops is easy 

 to nnderstiiiid. The quiintily of material prjduced each year on an acre of forest, 

 whether covered with young trees or old, does not differ so much as might be 

 expected, as this prodnction is due to the fertility of the soil, to the leaf-canopy, 

 the ajtion of liglit, ttc. When the crop is young it contains, say, 40,000 or 50,000 

 yoDDg seedlings, each producing a very small quantity of material. When old 

 although e9ch tree produces a hundred or a thousand times as much as e.ich seedling 

 each acre contains comparatively few large stems. la the case of isolated trees- 



