58 CALCULATION OF THE POSSIBILITY [ch. 



pared with the normal: V is the volume of the actual growing 

 stock, and nV is found by multiplying the actual mean annual 

 increment by half the number of years in the rotation. 



In the statement of the formula given above, it is supposed 

 that the actual growing stock is in excess of the normal growing 

 stock, as may often be the case in virgin forests; but the differ- 

 ence between the volume of the actual growing stock and the 

 normal growing stock corresponding to the actual mean annual 

 increment may of course be either positive or negative. 



52. Regulation by area and volume. 



The best way, however, in everyday practice is to determine 

 the yield on a basis of area combined with volume, and this is 

 the method that will generally be employed in the Uniform 

 method (except in the rare case when a succession of annual 

 coupes can be employed and the working be arranged by area 

 only, as mentioned on page 42), or in its variations, the Group, 

 and the Strip methods. 



Here we have a periodic block which is to be regenerated 

 within the period of say twenty or thirty years. The yield is 

 calculated solely with regard to this block and this period. 

 Enumerations are made and an estimate is prepared of the 

 actual growing stock, neglecting young growth if any exists. 



Then the annual yield for this period will be Y= — —^ , 



where I is the future increment accruing during the period p, 

 and V is the present volume of the actual standing crop which 

 is to be extracted during the same period. 



The reason for adding only one half of the increment expected 

 to take place during the period is that the other half will remain 

 on the ground after the passage of the fellings. If we have made 

 our enumerations and estimates this year and make the first 

 felling next year, we shall only realise one year's increment on 

 the first coupe, and two years' increment on the second coupe, 

 and so on, so that it wiU not be till we make the last felling of 

 the period that we shall harvest twenty years' increment added 

 on to the original volume. Thus for the whole period we shall 

 harvest half the increment taking place over the whole periodic 



