242 THE NORMAL YIELD. 



Example : 



Let bew = 20; then rotation = 110 years. Annual cutting 



1000 

 area = -^ = 9-09 acres. Volume standing on an acre at the 



age of 110 years = 5820 + 243 = 6063 ; hence annual yield 

 = 6063x9-09 = 55,113 solid cubic feet. 



The intermediate yields would amount to = 35, 010 cubic feet, 

 or: 



Total annual yield = 90,123. 



The raising of the rotation has led to a reduction of the 

 yield. 



c. Selection Forest. 

 If all trees which are cut in one year were brought together 



A 



on a portion of the area, the latter would be = : hence the 



T 



yield is practically the same as in the case of clear cutting. 

 Another way of looking at the matter is, to determine the 



area on which cuttings are made in each year ; this has been 



^ 

 placed above (p. 226), = . Everything which has to be cut 



(j 



on this area forms the normal annual yield. 

 Example : 



Area of a selection forest = 1000 acres 

 Kotation . . . = 100 years 

 I . = 20 years. 



Then : 



f =-00 = 50 acre, 



On these 50 acres the following material is cut : 



(1) All trees which have reached the age of 100 years. 



(2) A certain proportion of trees in the younger age classes, 

 so as to reduce their number gradually to that number which 

 should reach maturity at the age of 100 years. 



Taking the data in the table at p. 190, the material men- 

 tioned under (1) should give = 58,660 cubic feet, that is to say, 



