422 THE USE OF YIELD TABLES 



In the above case, the per cents are: 

 Veterans 76.4 

 Mature 23 . 6 

 The total stand is 2,583,940,000 feet B.M. 

 The stand of veterans is then 1,974,130,000 feet B.M. 

 and of mature is 609,810,000 feet. B.M. 

 To secure this division, a little over 1 per cent of the total stand was tallied and 

 estimated for the basic data, while the total estimate was secured by ocular means 

 (§ 208) (Coconino National Forest). 



323. Determination of Volume and Area for Age Groups on Basis 

 of Diameter Groups. Where the second alternative is chosen (Method 2, 

 § 320) to obtain the separation of age classes, namely, diameter rather 

 than age, the following changes in procedure are necessary. 



1. The volume of the so-called age classes is directly obtained from 

 a stand table, in which the number of trees of each diameter class must 

 be shown. 



2. The diameter of the average tree is obtained by first finding the 

 average volume for the group, and second, the tree of this volume 

 from a local volume table based solely on diameter, which is obtained 

 from a curve of average heights and a standard volume table. 



3. . The age of a tree of this average diameter is then found, not 

 from the yield table as before, but from the curve of growth based on 

 diameter, which gives directly the ages of trees of given diameters. 

 The ages indicated will be those of the respective age groups into which 

 the forest has been separated. As indicated, this method works back 

 from diameters to age, while the first is based on age directly. 



By either of these methods, the area in each age class may now be 

 found by following the precedure described in § 319. The age, and 

 consequent normal yields for 1 acre at these ages, have been determined 

 for each age class. The total normally or 100 per cent stocked area 

 can be found, and from this the reduction per cent and the area in each 

 age class. From the reduction per cent an empirical yield table can 

 be computed, which will be used as the basis for predicting the yields 

 of the forest or site class as a whole (§ 250). 



Since the above-described methods of determining areas of age 

 groups are based primarily on the factor of relative density of the stands 

 as determined by volume, they apply only to the age groups which 

 have already grown to merchantable sizes. The problem of determin- 

 ing the area of immature age classes is treated in § 348, and must be 

 considered in working out a plan for growth predictions for any large 

 area, in connection with the above methods. 



324. The Construction of Yield Tables Based on Crown Space, for 

 Many-aged Stands. The above methods depend upon the construc- 

 tion of yield tables from plots whose average age is determined, so that 



