202 ECONOMICS OF FORESTRY. 



the arithmetical progression represented by the 



accumulated increments of the age classes, andthatit 



rl 

 assumes the general expression vS„ = — ; that is to 



say, half the accretion which takes place through- 

 out the rotation forms the normal stock, which 

 must be maintained for a sustained yield manage- 

 ment, the other half furnishes the harvest or 

 yield during the rotation. On p. 130 examples of 

 the actual volume and value of normal stock under 

 different conditions were given. 



While we have assumed, for the sake of simplic- 

 ity of conception, that the stands of different age, 

 the age classes, are separate in area one from the 

 other, it is readily conceivable that all, or some 

 of them, may be mixed together, on the same 

 area as in the selection forest, where all age 

 classes, from the seedling to the mature timber, 

 are mingled ; and if there are enough trees in 

 gradation from the older to the younger, allow- 

 ing for losses, so that the younger age class can 

 replace in amount the older as it is removed or is 

 growing out of its class, we would have arrived at 

 normal condition for the selection forest. 



In the actual forest some one condition or all 

 conditions will usually be found abnormal. The 

 normal accretion may be deficient, because the area 

 is not fully stocked or the timber is past its prime, 

 old timber growing at an inferior rate, or rot off- 

 setting increment. The age classes are usually not 



