110 THE THEORY AND PRACTICE OF WORKING PLANS 



15. BY AREA AND VOLUME.— BASED ON AGE CLASSES. HUF- 

 NAGL'S METHOD. 



(a) Description of Method. — Hufnagl's premise is that the 

 sustained yield can be determined directly if the volume and 



the increment of the stands now more than - years old is known. 



2 



This method presupposes a tabulation containing the volume 

 and area of all stands of over - years. To this volume is to be 



2 



added, also, the increment of these stands in the next - years; 



4 

 for since the area of these stands diminishes each year, and in 



the year - = o, the increment can only apply, on an average 



2 



to half the area. 



As to the increment, Hufnagl distinguishes two variations 

 of his method according as I the current, or II the mean annual 

 increment is used. 



I. The current annual increment of each stand over - years 



2 



old having been determined (by yield tables or by field meas- 

 urements, Chapter I, Section i), the sum of these increments 

 is used in the formula which follows: 



II. The mean annual increment equals -. It can be deter- 



r 



mined from yield tables or, empirically, by measuring average 

 stocked stands of average site quality whose age approximates 



r years. - then equals the mean annual increment. 



Hufnagl's formula then follows (letting v equal the volume 



of stands - years and over, a their area, i the increment in 



2 



board feet per acre per annum, current or mean) : 



v+aXiX- 



F = 4 



r 



2 



