THE THEORY AND PRACTICE OF WORKING PLANS 111 



If i equals mean annual increment it will usually be lo to 20 

 per cent less than the current annual increment. This makes 

 its use the more conservative of the two. 



(b) Example. — In a white pine forest of 2160 acres, site 

 quality II, with a rotation of sixty years, the stands thirty 

 years and older have a volume of 33,041,000 feet board measure 

 on 1 1 20 acres. The mean annual increment for the rth (sixtieth) 

 year equals 782 board feet per acre.* Then by formula: 



v+aXiX- , , 



Y= 4 = 33>04i,ooo+(ii2oX782)i5 



r 30 



2 



_ 33,041,000+13, 137,600 _ 



1,539,287 feet board measure 



30 



= annual cut. 



(c) Value and Application. — Hufnagl's method shows much 

 originality and is applicable to even-aged stands of only mod- 

 erate regularity, the very conditions encountered in many 

 American forests. Its age-class differentiation is very simple, 

 as is also the volume and increment determination. The latter 

 had best be the mean annual increment, and can readily be 

 calculated from sample plots if yield tables are lacking. 



A disadvantage of the method is the rigid fixation of the 

 rotation age, which should really be a flexible quantity; but if 

 this is offset by frequent revisions at regular intervals the method 

 will pass muster, especially in the irregular stands common to 

 most parts of America. If this method is adopted, it must 

 always be supplemented by a careful cutting plan (distribu- 

 tion of cut). 



* Table 7, Bui. 13, U. S. Dept. of Agric, n. s., " White Pine under Forest 

 Management." 



