416 THE USE OF YIELD TABLES 



the stand in question with the standard yields, the second, the rate of 

 growth for future decades, which can be reduced to fit the existing 

 stand. 



318. Separation of the Factors of Volume, Age and Area. The 

 difficulties surrounding the prediction of yields lie in the fact that this 

 requires for any stand the determination of three factors: volume, which 

 can always be measured; age, which can be determined for a given tree 

 but is difficult to find for an entire stand of mixed ages; and area, which 

 can be measured, provided the boundaries of the age class are known 

 or defined. The trouble arises entirely from the mixture of trees of dif- 

 ferent age classes on the same area, the overlapping of crowns and root 

 spread, and the shifting of total areas occupied by each separate age 

 class in successive periods (§ 298 and § 299). Thus two of the essential 

 factors, age and area lose their clear definition. These two factors 

 are interdependent in such forests. Age classes cannot be confined 

 to stands of a single age but must include an age group. The area 

 occupied by such a group will be influenced by the number of separate 

 ages included in the group. 



It has been shown previously in this chapter that the area occupied 

 by a given age class, when determined by mapping, determines the 

 relative density of stands whose age is known. The yield table expresses 

 an arbitrary standard yield on 1 acre at a given age, representing 

 100 per cent density at each age. (This means that the table is accepted 

 as standard, but does not necessarily represent the maximum yields 

 possible on any acre, which may exceed this standard, by from 15 to 

 20 per cent.) When both area and age are determinable for a stand, 

 the exact relation as to density or yield when compared with the standard 

 can be found for each stand separately. When neither can be found 

 with accuracy, they must be found by such means as is possible, and the 

 results, while not as accurate, will be serviceable and worth attaining. 

 The general method of solving this problem is to work from the known 

 to the unknown, accepting averages and approximations when exact 

 determination is impossible. 



319. Determination of Areas from Density Factor. One of the 

 simplest and most useful applications of this principle is in the deter- 

 mination of the area occupied by each of several age classes, whose 

 age and volume are known but which have not been or cannot be mapped 

 separately. 



The total area of the tract can always be determined. If for any 

 reason it is impossible to map the area of each age class, these areas may 

 still be found by proportion if we are willing to assume that the average 

 density of the entire stand can be applied separately to each age class. 

 While admittedly less accurate than the separate determination of 



