VOLUME AND AREA FOR TWO AGE GROUPS 419 



ured. In timber estimating, a stand table can be made giving the range 

 and distribution of diameters in the stand. The substitution of diam- 

 eters for ages thus furnishes a means of separating age classes in forests 

 of mixed ages. 



Choice of Methods. There are tnree gradations in the possible 

 applications of this method. 



1. Diameter is used merely to determmc the age of an average tree, 

 but the forest is separated into actual age classes as nearly as possible, 

 rather than diameter classes (§321). 



2. Diameter is used as the basis of separation into classes, whose 

 average age is then determined on the basis of these diameters (§ 323). 

 These, as shown (§275), are not true age classes since they do not 

 include all the trees of a given age. 



3. Diameter is substituted altogether for age, and the total age of 

 trees is not determined for these classes, but current growth is predicted 

 merely for trees of given diameters for short periods. This method is 

 discussed in Chapter XXXII. 



The use of diameter to indicate total age is most reliable when applied 

 to large areas and numbers and to forests of many age classes, for species 

 and stands whose actual and economic age agree, i.e., which usually 

 do not show a period of suppression. 



321. Determination of Volume and Area for Two Age Groups on 

 Basis of Average Age. While the method to be described is limited 

 in its application to two age groups, yet even this subdivision will be 

 found of great value in Mensuration and Regulation. In the French 

 many-aged forests, but two groups are made in timber above exploit- 

 able size. In our forests, when under management, the subdivision 

 into two groups will be equally effective. 



In natural stands containing decadent timber, three groups are 

 needed instead of two, for timber above the minimum diameter. These 

 may be termed " young merchantable," " mature " and " veteran." 



In the Western yellow pine stands for which this method was 

 developed, it was possible to separate the young merchantable timber 

 by the appearance of bark into a class termed " Blackjack," leaving 

 the remaining yellow pine timber for separation into mature and 

 veterans. In forests where this cannot be done, it is possible to first 

 separate the young merchantable timber on a diameter class basis, 

 leaving the larger mature and veteran timber for division by this method. 

 ^Vhere the forest is cut over, and but two age classes are required, 

 the method will separate the j'oung merchantable from the mature 

 timber. The three steps in this method are as follows: 



1. A standard yield table based on age for even-aged stands can 

 be made the basis of separation of the forest into two age groups. This 



