llO EOTATION FOE COMMERCIAL EXPLOITABILITY. 



and the continued rise in the market value of builders' and artifi- 

 cers' wood which is likely to take place, is a sufficient reason for 

 entirely leaving out of account the effects of cover. We may even 

 limit ourselves, in the majority of cases, to comparing the value at 

 different ages of the trunk only, of formed trees, paying no atten- 

 tion whatever to the contents of the small crowns. The calculation 

 thus simplified would still give sufficiently correct results. 



§ 3. Age of Commercial Exploitahility of the Timber of 

 a Forest worked hy Selection. 



The growth of the trees in a forest worked by Selection is 

 irregular and unequal and often differs to an extreme degree from one 

 tree to another. While one individual has a diameter 8" at the base 

 at the age of 50 years, another, although measuring no more, may be 

 loo years old ; in other words, there is no definite proportion of 

 any kind between size and age. But as the very first condition of 

 all is to keep the leaf-canopy continuous, the exploitations oug^ht not, 

 speaking in a general sense, to touch any trees but what are already 

 fertile, nor, in a particular sense, any trees but those surmounting 

 a young growth capable of taking their place. This gives us the 

 lowest limit of age at which the forest becomes commercially 

 exploitable. To remain below it would be simply the ruin of the 

 forest and if proprietors, goaded on by a speculative fury, are often 

 led to destroy their forests, it is not with the idea of treating the 

 latter as forest property but as an entirely different class of 

 investment. 



Now the trees of a high forest worked by Selection, when tKey 

 have reached the age of fertility, stand, as a rule, well apart, thanks 

 to the removal of their original immediate neighbours. From this 

 time, therefore, their annual increments can be measured. These 

 increments all tend towards a common mean, if not throughout the 

 whole Working Circle, at least for entire compartments taken 

 in groups. Suppose, for example, that silver fir trees gain a radial 

 increment of 1/10 inch per annnm. Granted so much, we can tell 

 how many years it would take for a certain tree to attain a given 

 size or acquire a given market value. 



