100 



When the possibility has been dttermined, the length of 

 the felling rotation, during which the whole forest will be 

 gone over by the selection fellings, should be fixed. In the 

 present case it should, however, be one-third or a ssub-multiple 

 of one-third of the exploitable age. 



To tliis method of determining thepossibilily of selection- 

 worked high forests the following advantages are ascribed : — 



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

 the stafe of the crops on the occasion of each revision of 

 the working-plat). 



(6) It tends to introduce the normal proportion betweea 

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



(c) It helps to secure a sustained yield, so far as the- 

 composition of the forest renders this possible. 



(iv) Von Mantel's melhod. 



The possibilityunder Von Mantel's method is calculated' 

 by the use of the formula : Annual Yield is equal to the Eeal 

 Growing Stock of the forest divided by half the number of 

 years in the exploitable age. This formula is based upon 

 the idea that the real yield must have the same propoition 

 to tlie real growing stock as that existing between the normal 

 yield and the normal growing stock.* 



Let us suppose it has been ascertained by measurement 

 that the growing stock on the ground amounts to 600,000 

 cubic feet, and assuming that the exploitable age has been 



estimated to be 160 : there would be felled each vear M«oo 



or 7,500 cubic feet of wood. ~^ 



7. JVatureof the fellings.— Whatever system be adopted in 

 order to calculate the possibility, whether this is expressed 

 by cultural rules, by number of trees, or by volume, the 

 fellings should be prescribed in a simple manner and aocord- 

 ing to the principles of the selection method. The manner 

 in which the marking of trees for felling is to be carried out 

 should be clearly laid down : the conduct of the fellings 



♦ That is, Eeal Yield : Real Growing Stock : : Normal Yield : Normal Growin? Stock 

 „ -,, ■■■.,.,, Normal Yield r. i ^ . ">^».«,. 



Or, Eeal Yield = formal Growing Stock " "^'^^ ^"^""^ ^took. ' ut, as crpUncd on 

 page 24, ^vhen discussing the calculation of the normal growing stock, the Normal Growing 



Stock is equal to the Normal Yield x i?. And, consequently, the Keal Yield 



, , Eeal Growing Stock 

 becomes equal to j oiploitable ago * 



