QUALITATIVE EXPLOITABILITT. 45 



to any fixed law. Hence no rule can be laid down with regard to 

 the age at which single trees ought to be cut so as to obtain the 

 maximum produce from the soil in the least possible time. It is for 

 this reason that, in foresters' parlance, we say that there is no Quanti- 

 tative Exploitability for single trees. 



§ 2. Qualitative Exploitability. 



It is well known that the degree of usefulness of timber increases 

 as a rule with its size. It hence follows that the usefulness of a 

 tree, for every cubic foot of wood it contains, is highest when the 

 tree has acquired the largest dimensions it is capable of attaining 

 ■without beginning to decay, that is to say, only when it is thoroughly 

 mature. Thus the age at which the conditions of Qualitative Ex- 

 ploitability are realized is the same as that of complete maturity. 



The trees forming any crop reach this stage one by one, never 

 simultaneously. Individual trees of the large forest species possess 

 remarkably different degrees of longevity. While one silver fir is 

 mature at 150 years, its immediate neighbour may not become so 

 until it is 250 years old. Of two oaks, growing side by side, one 

 decays perhaps at the age of 180 years, while the other is quite 

 sound at 300 years. Hence we could not exploit all the trees of a 

 crop according to their individual maturity, unless we were prepared 

 to fell them one by one at wide intervals of time. But to do so 

 would be to expose oneself to some capital disadvantages. It is 

 opposed to the requirements of a great number of species, and the 

 question of the condition or the quantity of the produce becomes 

 thereby subordinated to that of constantly maintaining the 

 leaf-canopy. 



It is thus apparent that Qualitative Exploitability is not one that 

 we could realize throughout the whole extent of a crop of trees of 

 the same age. 



§ 3. National Exploitability^ 



To obtain the most useful yield of material from a perfectly 

 uniform crop, it would be sufficient to fell it only at complete maturity. 



Before the trees begin to decay or the death of some among them 

 produces blanks, it is clear that the crop, as it stands, contains in 

 its large timber the highest sum of utility it can ever possess. 



