28 



technical purpose. Rotations fixed with a view to meet such 

 special requirements are called, respectively, the rotation [a) 

 •of the grf'atest volume production, (6) of the highest inconae, 

 (c) the financial rotation, {d) the physical, {e) the techniciil 

 -rotation. 



Thus, a forest exploited for quantity of fuel, or timber to furnish sleepers, 

 -would be exploitable when, the capital being properly constituted, the' oldest trees 

 ■were large enough to furnish in greatest quantity the fuel or sleepers of ihe size 

 desired. The age of the trees when this condition is reached would be the exploitable 

 -age. Similarly, a forest explrited with a view to the realisation of the highest rate 

 -of interest on the capital invested would be exploitable at the age when this obj--ct 

 was attained. 



The age at which trees are felled in properly managed 

 forests is not, however, always the exploitable age. Owing to 

 irregularities in the composition of the growing stock it 

 "frequently happens that, during a longer or shorter period 

 which may, and strictly speaking ought to be, the same 

 length as the exploitable age, trees or crops must be felled 

 before or after they become technically exploitable in order 

 that the correct proportion of age-classes may be secured. 



2. Calculation of the exploitable age.— The price and the 



utility of wood depends as a rule on its dimensions, that is 



4o say, on the age of the tree when. felled. In order to 



-determine the exploitable age in a forest exploited for its 



wood, it is suflBcient for present purposes to ascertain tii 



size of the trees that will furnish the greatest quantity of 



the most useful produce. The average age at which the 



trees attain this size can then be determined by observation. 



Where the trees are simply preserved with a view to indirect 



Tjenefits, such as the shelter they afford to the soil,, the 



exploitable age will generally approach their natural 



longevity. 



It may be objected that the age at which a crop yields the greatest quantity of 

 material, or the highest interest on the capital invested, is not th» age at which the 

 individual trees are the largest. This is perfectly true ; but there are cases in whic>', 



jf the trees were felled when the crop would yield the greatest amount of material, 

 the material might not be of the n.ost useful size or bring in most money- It wo' Id 



. not be of much use ascertaining that at the age of 120 years, it would give the 

 greatest quantity of material when the requirements were beams obtainable only 

 from large trees 150 years old. Even if this yrere not the case, there are hardly 

 any instances in which Sata are available for making such calonlations. The age at 

 which an exploitation famishes the greatest quantity of produce is that at which the 

 sum of the pro'luction, that is to say, the total quantity of mitteiial produced (includ- 

 ing whatever has been removed or has disappeared since the young growth firtt 

 sprang up) divided by the aga of the crop is greatest, that is, the age at which the 

 Mean annual increment culminates. The mat>'ri«l removed is of course hardly 

 ever known, and the experiments required to asoeitain the balance on the ground 

 ■would, in default of suitable orops, generally be impossible in India. 



Calcnlatirg the age at which the interest on the capital invested in the forest! s 

 Jiighest, involves ascertaining by experiment the qnantity of material the forest 



