264 Forestry Quarterly 



The same thing should usually be done for each species and, of 

 course, separately for each type. 



The next step is how to use these data to determine the allow- 

 able cut. The increment may be used directly, as described by 

 Hufnagl, that is the simple summation of the current annual in- 

 crement per acre which, in the case of hemlock, adds up to 5.478 

 board feet per acre, but a better method, which we may call the 

 "Swiss Method" is described in an anonymous article on selection 

 forest management in the Swiss Forestry Periodical for 1913, 

 briefed in Forestry Quarterly, vol. XIII, pp. 260-2, as follows: 

 "Divide the volume of the oldest size classes by the annual in- 

 crement of the entire stand. This will give the number of years 

 during which the volume of the oldest size class must last. If this 

 be 45 years, then the cut for the next decade would be from one 

 fifth to one quarter of the volume of the largest size classes." 

 Expressed mathematically : 



Let X = the volume of the size classes below the diameter 

 limit 

 " Xi ^= the current annual increment thereon 

 " Y =: the volume of the size classes above the diameter 

 limit but within a current annual increment of, 

 say, 1 per cent 

 " Yi = the current annual increment thereon 

 " Z =: the volume of the size classes beyond a current 

 annual increment of, say, 1 per cent — that is over- 

 mature timber — surplus growing stock 

 " Zi = the current annual increment thereon 

 " CC = the cutting cycle 



y-t-z 



Then CC = 



xi-\-yi-{-zi 



Y+Z 



And annual cut — ^^ 



This is the strict interpretation of the method as described, but 

 since Z is surplus growing stock, it should play no part in deter- 

 mining CC; nor should the increment thereon (Zi). 



Y 

 Then CC^ —^, — : 

 xt-\-yt 



y-\-z 

 And annual cut— -^ 



