414 THE USE OF YIELD TABLES 



table to the larger area involves the same steps for this area as are 

 required in the construction of the normal yield table itself, or for the 

 preparation of an average empirical yield table. These are as 

 follows: 



1. Determine the volume, the area occupied, and the age of each 

 separate age class. 



2. From these data in turn compute the volume per acre for the given 

 age. 



3. Determine the relative density by dividing this unit volume by 

 the yield of an acre of the same age from the yield table; this is expressed 

 as a per cent of the standard yield for that age. Per cent density can 

 thus be found separately for each age class, or for each separate stand 

 if desired. 



317. Application of Density Factor in Prediction of Growth from 

 Yield Tables. Future yield can now be predicted for all stands from 

 the same yield table, by applying the reduction per cent to this table 

 which is required by the stand or age class in question. 



Influence of Number of Trees per Acre. There is one valid objec- 

 tion to this assumption that relative density as expressed at a given 

 age in terms of volume will remain constant for future yields and that 

 is that under the laws of growth of stands partially stocked this stand 

 will, tend to become fully stocked (§ 301). A knowledge of the number 

 of trees per acre required for full stocking at the age of cutting is also 

 obtained from a normal yield table, and this knowledge may be directly 

 applied in determining the per cent of density in immature stands, 

 not on the basis of crown cover existent but of the ultimate yield to 

 be expected from the trees which will probably survive. In the same 

 way, for older stands, when volume per acre is less than that in a nor- 

 mal stand, but the number of trees per acre is sufficient, the reduction 

 can be lessened as applied to these partially stocked stands as long as 

 the trees are so distributed as to utilize the area; e.g., in one case, 

 a 50 per cent average stocking may represent 100 per cent stocking on 

 50 per cent of the area, with the rest blank. No correction should 

 be made. In another case the entire area is covered with a stand whose 

 volume is 50 per cent of normal, but trees are well placed. In this case 

 the yield will probably be normal at the age at which the normal num- 

 ber of trees per acre drops to about the average number now present in 

 the natural stand. 



The former or simpler method is of course extremely conservative 

 and allows a margin for the continuance of natural losses by fire, wind, 

 insects and diseases, while the latter may be applied to more intensively 

 managed and better protected forests. 



