vn] CALCULATION OF THE POSSIBILITY 59 



block during the whole period, the other half remaining in a 

 succession of graduated crops on the ground after the passage 

 of the fellings. The increment may be expressed either as a 

 percentage of the growing stock, or as so many cubic feet per 

 acre per annum. It is usual to make a rather low estimate of it, 

 so as to keep on the safe side, and to have a reserve always in 

 hand in case of accidents, such as wind storms, etc. Any excess 

 of material is easily adjusted at the decennial revision. 



53. French method. 



In France, before making the calculation of the possibility 

 based on an estimate of the volume of the mature crop to be 

 removed during the coming period, it is customary to ascertain 

 whether the older age-classes in the crop which will come under 

 working in the near future, are approximately in correct grada- 

 tion; so that, if desirable, an adjustment can be made in the 

 regulation of the yield in case of any marked excess or deficiency 

 in the older parts of the crop. This method is employed both 

 with even-aged high-forest, and with Selection-worked forests. 



Suppose for example that 6 feet in girth were adopted as the 

 size of maturity; then an enumeration would be made, and the 

 whole stock would be divided into three groups. The youngest, 

 up to 2 feet in girth, the middle-aged third from 2 to 4 feet in 

 girth, and the oldest third would include all stems of over 4 feet 

 in girth. The estimated aggregate volumes of the oldest and of 

 the middle-aged thirds would then be compared, and if they 

 were found to bear respectively the ratio of five to three, it 

 would be assumed that the age-classes were in sufficiently correct 

 proportion. 



This does not mean to say that the crop is necessarily by any 

 means normal in volume, or 

 fully stocked. This calcula- 

 tion is based on the method 

 which we have already men- 

 tioned on page 46, and which 

 may be graphically demon- 

 strated by the figure, in 

 which the volumes of the three thirds of the growing stock 



Volume 

 propor- 

 tionate 

 to age. 



Area 



