THE THEORY AND PRACTICE OF WORKING PLANS 75 



F = the volume of the size classes above the diameter 

 limit but within a satisfactory current annual 

 increment; 

 Yi = the current annual increment thereon; 

 Z = the volume of the size classes beyond a satisfactory 

 current annual increment — that is overmature tim- 

 ber — surplus growing stock; 

 Zi = the current annual increment thereon; 

 CC = the cutting cycle. 



Then CC 



and annual cut 



Xi-^Yi+Zi 



Y+Z 

 CC ' 



This is the strict interpretation of the method as described, 

 but since Z is surplus growing stock it should play no part in 

 determining CC, neither should the increment thereon {Zi). 



Then CC- 



and annual cut 



Xi+Yi 

 Y+Z 



CC ' 



The current annual increment can be easily determined by 

 Pressler's or by Schneider's formulae. Where reliable diameter 

 growth figures are available, these can be used as well as incre- 

 ment borings or stump analyses, since what is wanted is the 

 number of years required to grow from i inch class to the next. 



The chief data required are an exact determination of the 

 current annual increment and stand and stock tables. The 

 diameter-class distribution must approach the normal relation. 

 It is a further essential of this method that the increment and 

 growing stock be redetermined, by identical means, at frequent 

 regular intervals of not to exceed ten years. If the growing 

 stock then shows an unintentional diminution, the cut was 

 set at too high a figure; if, conversely, the growing stock, at 



