88 



Fellings limited by the productive capacity of the soil.— The 

 method last described meets the diflSculty, so frequent in 

 India, involved in calculating the possibility of a mixed crop 

 containing only one or two saleable species, such as the teak 

 forests of Burma, the deodar forests of the Himalaya, and 

 many others. The more accurate methods employed in 

 European forestry are not intended for such forests, and it is 

 a question whether they can be made use of except where 

 the whole crop is saleable. 



The most common of these European methods consists in 

 determining the number of trees to be felled by estimating 

 the production of the soil. The number is, if possible, deter- 

 mined by ascertaining the mean increment of a complete 

 crop of the exploitable age on the same kind of soil, and by 

 dividing this increment by the volume of an average tree of 

 the exploitable size. 



Thus, if the average annual production of the soil were 120 cubic_ feet per acre, 

 and the volume of an average exploitable tree 60 cubic feet, the possibility would bo 

 fixed at two trees per acre per annum. 



As, however, each portion of the forest might not be able 

 to furnish per acre the number of exploitable trees so deter- 

 mined, no minimum girth limit is fixed with regard to the 

 trees to be felled. In this way, a quantity of material less 

 than the possibility is removed from those portions of the 

 forest which are deficient in large trees ; while, where the 

 exploitable stems are in excess, the possibility is, for the 

 time being, exceeded. The result is the nltimate regulari- 

 sation of the crop as a whole. 



The method nay best be explained by an example. Let ns suppose that the 

 forest which it is wished to organize by the selection method, and the possibility of 

 which has to be ascertained, consists of 1,810 acres of mixed conifers and oaks, all 

 beiug saleable. We will also suppose that an enumeration of the trees has been made, 

 and that the forest has been found to contain 263,520 Bt«mB of all sizes and a^es, the 

 proportion between the two species being 90 firs to 10 oaks ; also that the average 

 number of stems of all ages per acre is 146 as follows : — 



Small trees under 12" in diameter ..... 92 

 Sledium-sized trees 12" to 24' in diameter ... 46 



Exploitable trees over 24" in diameter .... 8 



Total stems per acre . 146 



The production of the soil can be roughly estimated as follows. It may be as- 

 Bumed that the same kind of soil, when fully cropped with trees of a certain species, 

 produces a constant quantity of material per unit of area each year. Suppose that 

 a fir tree of two feet in diameter is exploitable, and that by experiments in the forest 

 it has been shown that a tree of this size requires a circular space of 25 feet across. 

 We will also assume that it has been determined that the mean volume of wood in a 

 typical fir of the above dimensions is 136 cubic feet, and that the tree requires 1 50 

 years to attain the diameter stated. The productive capacity or capability of the soil 

 will be as follows :— 



