44 QUANTITATIVE EXPLOITABILITY . 



§ 1. Quantitative Exphitdbility. 



In a canopied collection of trees the quantity of wood produced 

 each year does not vary in the same manner as it does in the indivi- 

 dual trees themselves. In the former case, the annual increment 

 represents one year's production of the whole wooded surface. Since 

 the number of the trees diminishes as the crop grows older, the 

 average annual rate of growth at any given age is at once obtained 

 by dividing by the number of years in question the sum of the con- 

 tents of the standing timber and of the trees already removed. lu 

 this manner it has been ascertained that the mean rate of growth ^ 

 or the ratio of the cubical contents to the age of the forest at any 

 time varies with the age itself of the forest according to a certain 

 fixed law. This mean annual growth reaches its maximum at an 

 advanced age, which is nevertheless considerably below that of 

 maturity. The moment that age is reached, the forest has attained: 

 its Quantitative Exploitability. This age varies according to soil 

 and climate, differs considerably for the various species of trees, and 

 is not the same in a high forest as in a copse, except that under 

 either Regime it is approximately midway between the birth and 

 decay of the forest. If the age at which the maximum rate of 

 growth is reached is very different for the different species, the 

 pine and the silver fir for instance, the degree in which the rate of 

 growth itself varies, is also very different ; from being well marked 

 from year to year it may become scarcely appreciable. This fact can 

 often be noticed by attentively observing tbestate of the forest and of 

 the soil, but to establish it thoroughly requires actual experiments. 



In the case of an individual tree, the annual rate of growth goes 

 on increasing during a considerable portiou of its life; it then 

 remains constant up to an extremely advanced age, and if it decreases 

 at all, it does so to an inappreciable degree just before decay sets in. 

 The above fact is explained by the manner in which the organs of 

 growth are developed, for a long time increasing progressively, th^i 

 remaining more or less constant. The result is that the mean 

 annual rate of growth of the tree generally goes on increasing until 

 death supervenes, so that its maximum is reached only with the end 

 of lijfe. Nevertheless no conclusions can be drawn from these facts 

 as to the sum of production of the soil, for the area occupied by the 

 tree goes on increasing like its rate of growth, and this not according 



