93 



To this method of determining the possibiiity of selection 

 worked high forests the following advantages ai-e ascribed : — 



(a) It allows the possibility to be fixed according to 

 the statt) of the crops on the occasion of each revision of 

 the working-plan. 



(6) It tends to mtroduce the normal proportion be- 

 tween the different age-classes or size-classes of trees. 



(<?) It helps to secure a sustained yield, so far as the 

 composition of the forest renders this possible. 



Fellings limited by proportionate volume.— This method of 

 determining selection fellings by volume of material, known 

 in France as the systeme Masson, is based on the assumption 

 that as, in a high forest worked by the regular method, a 

 definite proportion of the crop on the ground is always 

 felled, RO, in a selection-worked forest, by invariably limit- 

 ing the fellings to a certain percentage of the volume of the 

 crop the fellings may be kept within the possibility. 



Applying this theorem to a 'fairly homogeneous fir forest, containing a wooded 

 area ot 1,600 acres in which the average annual production or capability of the Soil 

 has been asoert-ained, in the manner explained at page 89, to be 63 cubic feet of wood 

 per acre a year, and assuming that the forest is worl^ed by the regular method on 

 a rotation of 150 years, each acre would produce during the rotation 150 x63=S,4oO 

 cubic feet, and the volume of wood at any time duiin^ this rotation would, ou the 

 whole area, be one-half of 1,6U0X 9,450 or 7,560,000 cubic feet. Now, the area of 

 the forest being 1,600 acres and the number of years in the rotation 150 years, the 

 size of the annual coupes (the forest being worked by the regular method) would 

 be 1,600 -j- 150 = 10"66 or 10| acres, and the volume of material felled each year 

 would be 10| X 9,450 cubic feet = 100,800 cubic feet, or 1-33 per cent, of the total 

 fflaterial Sn the whole forest. 



Similarly, were the rotation reduced to 100 years, the volume of wood on the 

 gronnd would be (63 X 100 X 1,600) -=- 2 = 5,040,000 cubic feet ; while the material 

 removed at each felling, made on an area of 1,600 -r- 100 = 16 acres, would still 

 be 16 X 63 X 100 = 100,800 cubic feet. In this case, 2 per cent, of the material on 

 the ground would be felled. 



Therefore, in a high forest worked by the regular method, there is a fixed pro- 

 portion, depending on the length of the rotation, between the volume of material 

 felled each year and the total volume of material or the wood capital of the 

 forest. This proportion, following a general law, varies inversely as the length of the 

 rotation ; that is to say, it is higher- as the rotation is shorter and vice versd. 



Let us suppose that it is wished to ascertain by the volumetric method the 

 possibility of the fir forest of 1,600 acres, excluding blanks, for which it has been 

 ascertained that the productive power ot the soil is 63 cubic feet ot wood a year. 

 Let us also assume that the exploitable size of the trees is 2 feet in diameter, and that 

 it requires on an average 150 years for the trees to attain that dimension. 



Under ihese conditions the material oti the ground being, as stated, 

 (150 X 63 X 1,600) -7- 2 = 7,560,000 cubic feet, and the size of the annual coupes 

 being 1,600-4- 150 = lOf acres, there would be felled each yearlOf X 63 x 150 = 

 100,800 cubic feet of wood, or 1- 3 per cent, of the total material on the gronnd. On 

 the assumption that, were the forest worked by ihe selection method, an equivalent 

 yield could be obtained, it wonld be possible, without exceeding the capability, to fell 

 1-33 of the material on the entire area, or 13-3 per cent, of the material on one- 



