90 



to replae€ the matare trees when felled, so as to enable the forest always to remain ia 

 move 01 less the same condition as regards its capability of furnishing a given number 

 of trees. 



Fellings limited by relative proportion. — This method is 

 based on considerations similar to those whichi influence the 

 determination of the possibility of regular high forests, com- 

 posed of properly graduated series of crops of all ages up 

 to the number of years comprised in the normal rotation. 

 These crops may be classed under one of three groups, each 

 of which would obviously occupy one-third of the total 

 area, vie. :— 



I. — Full-aged crops, of wbicb the ages will range 

 downwards to two-thirds of the total number 

 of years comprised in the exploitable age. 

 II. — Medium-aged crops, of which the ages will range 

 from two-thirds to one-third the number of 

 years in that age. 

 III. — Young crops, aged less than one-third the num- 

 ber of years in the exploitable age. 



It is evident that, in a regular high forest so stocked and 

 in which the elements of production are everywhere identi- 

 cal, a sustained yield will be assured if each group is in its 

 turn regenerated during one-third the number of years in 

 the exploitable age. The annual yield can therefore be 

 determined by dividing the volume of material contained in 

 the full-aged crops, with the addition, if possible, of its 

 increment up to the time of felling, by the number of years 

 in one-third of the exploitable age. When a forest does not 

 contain crops of all ages covering equal areas, it may be 

 necessary to make transfers from one group to another, so 

 that the areas to be exploited in equal intervals may be 

 equalised. 



If for a regular be substituted a selection- worked high 

 forest, it is still possible, notwithstanding the apparent 

 irregularity of the latter, to distinguish in it these three 

 groups, namely, of full-aged, medium-aged and young 

 crops. But the areas respectively stocked with these three 

 groups, instead of forming compact blocks, are scattered and 

 intermixed in the most irregular manner throughout the, 

 forest. It is usually impossible to ascertain what area each 

 group occupies ; so that it is necessary to determine whether 

 the volumes of material in each group are in norm.al pro» 

 portion to each other. It has been ascertained that, in a 



