27 



It •will be' seea that the proportion between the produce- 

 felled and the gro .ving stock would remain constant so long- 

 as the exploitable age remained unalterad but would vary 

 with every change in this age. 



Thus, suppose the area dealt with in the above example to be 20 acres, divided 

 into 20 coapss of one acre each, and that the average annual proiuotion ha? been 

 found to amount to 10 J cubic feet solid per acra per auaum. Felled at the agd of 20 



years, the capital on the ground would always be very nearly ^""•'"""^ cubic feet 



or 20,000 cubic feet, wbije the quantity of mateiial removed at each. ftsUing wonlf 

 amount to 1x100x20 cubic feet =2,000 cubic feet, or to one-tenth or 10 per cent- 

 of the capital. 



Agda suppose the age of felling to, be raised to 40 years, and that the area- 

 has been consequently divided into 40 coupes containing the proper gradation of age- 

 classes, as in fig. No. 3. The growing stock on the ground, even assuming the average- 

 production to be slightly higher than before, or 120 cubic feet per acre per annum, 

 would be approximately ?2iii^ili2= 48,000 cubic feet, or more than doable what it^ 



was before ; while the quantity of material felled (a crop 40 years old occupying half 

 an acre) would be |x 120x40= 2,400 cubic feet (very little move than before), and, 

 wonld amount to one-twentieth or 3 per cent, of the growing stock. 



Section VI. — The rotation or exploitable age. 



1. meaning of the term.— A forest like any other under- 

 taking must be managed and worked, or, as it is called,, 

 exploited, with a definite purpose. This purpise may of 

 course be one of a great many and its realisation is expressedl 

 by the term exploitability. 



The crop in a forest is said to be exploitable when it has 

 attained the condition that is required in order to fulfil the 

 purpose with which it is worked : and the age of the tree 

 when this condition is reached is called the exploitable age^ 

 The time required by new growth in a crop, which is being: 

 regenerated, to attain exploitable dimensions is also termed 

 the exploitable age or rotation. 



The use of the word rotation or revolution (French, revolution) to express the 

 exploitable aga is dne to the fact that under the methods of treatment very generally 

 iu use «n/brOTer rfay«, the interval of time between successive fellings in the samfr 

 area was the number of years in the exploitable age. At present this is generally only 

 true in the case of coppice. In most other cases the fellings pass over the area several 

 times during the course of the rotation : the period intervening between each such' 

 felling being termed a, felling rotation or felling cycle. 



It is usual to classify the various kinds' of exploitability 

 according to the objects of management. The chief purpose 

 for which a forest is exploited may be : to obtain from the- 

 land the largest possible average annual or periodic return 

 {a) of material, (6) of money, (c) of interest on the capital 

 iovested : or, it may be sought to adopt the rotation best- 

 suited {d) to natural regeneration, or (e) to some special 



