13 



The average annual production or mean increment during 

 a given period, -which it is sometimes of importance to 

 ascertain, is calculated in the same way. 



Increments of isolated trees and crops.— The mean annual 

 production of an isolated tree varies greatly with the age 

 and may be held to continually increase, while that of a crop, 

 occupying a given area, varies much less and increases up 

 to a certain time when it diminishes. 



The 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 understand. The quantity of material produced each year on an acre of fore-st, 

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

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

 the action of light, etc. When the crop is young it contains, say, 40,000 or 

 50.000 young seedlings, each producing a very small quantity of material. When 

 old, although each tree produces a hundred or a thousand times as much as each 

 seedling, each acre contains comparatively few large stems. In the case of isolated 

 trees, however, the area is not taken into account. Each tree starts as a young 

 seedling producing very little, and ends as a large tree producing a great deal by 

 more or less constant rings of growth applied to an ever increasing circum- 

 ference. Hence there is a constant increase in the average production of single 

 trees. 



POSSIBILITY. 



Meaning of the term.— The most important calculations 

 and provisions in a working-plan are those which relate to 

 what is called the possibility. 



