94 THE THEORY AND PRACTICE OF WORKING PLANS 



(4) The volume of old and medium trees may not be in the 

 proportion of 5 : 3, and their sum less than normal. This will 

 probably mean that the volume of old trees is deficient, and 

 must be increased by cutting less than the otherwise allowable 

 volume of old trees. 



(5) The volume of old and medium trees may not be in the 

 proportion of 5 13, and their sum more than normal. 



This could occur only with an excess in the old group. To 

 correct this, find the volume of old trees necessary to make the 

 ratio 5 : 3 with the volume of the medium trees, and which, 

 added to the volume of medium trees, will give a normal growing 

 stock. The difference between this volume and the actual 

 volume of old trees is surplus. This surplus must generally 

 be removed during the first third of the rotation, for the entire 

 area will be cut over once during that time. Even though it 

 were desirable to distribute this surplus over a longer period, 

 such a course would generally be impossible, because in virgin 

 forests, most of them of difficult accessibility, the first cut must 

 be heavy per acre to justify logging. Later cuttings may, 

 without hardship to purchasers, be made lighter. 



When several species occur in mixture all are regulated 

 together without affecting the method. If one species has a 

 more rapid growth and is shorter lived, requiring a shorter 

 rotation, its exploitable diameter should be made lower than 

 that of the other species. 



The whole calculation is checked by figuring what per cent 

 of the total volume is represented by the allowable cut. This 

 per cent, after subtracting the surplus, should be approximately 

 the growth per cent of the group of old trees. 



The area check on this method is appHed as follows : 



The whole working unit is to be gone over in one-third 

 of the rotation. Since the rotation may be long, this third 

 is further divided into periods during which the plan is to run 

 without revision. If these periods are too short an unneces- 

 sary expense will be incurred by frequent forest surveys, whereas 

 if they are too long there is danger that the effects of original 



