CHAPTER XXIX 



THE USE OF YIELD TABLES IN THE PREDICTION OF GROWTH 

 IN EVEN-AGED STANDS, WITH APPLICATION TO LARGE 

 AGE GROUPS 



315. Factors Affecting the Probable Accuracy of Yield Predictions. 



If the average yield on Quality I site for a species is taken as 100 per 

 cent, and but three qualities are distinguished, the relative yields shown 

 for Qualities II and III may be as low as 72 and 45 per cent of that on 

 Quality I, respectively. 1 This means gaps of 28 and 27 per cent in the 

 series between the points arbitrarily marked by the average curves 

 expressed in the yield table. The use of five qualities of site reduce 

 these intervals to about 15 per cent. For young stands, or areas just 

 growing up to timber, this is as close a prediction as can be expected. 

 If the site is properly classified, its future yield if normally stocked will 

 differ by an extreme of one-half of the above interval, either above 

 or below the standard. Once the site is identified by the use of average 

 height based on age, the future yields can be predicted by use of the 

 yield table, either for bare land or for partly grown young stands, 

 provided the degree of stocking agrees with that incorporated in the 

 table. 



The larger part of the area of any natural forest is not comparable 

 with these conditions. The variables of density of stocking, form of 

 age classes, and composition of species must all be dealt with before 

 yields on any considerable area can be predicted within the desired mar- 

 gin of accuracy. The degree of accuracy attainable in prediction of 

 yields in our wild forests is not yet known even approximately since 

 for many-aged forests and mixed stands, yield tables based on age 

 have not been attempted until recently (§314). This much can be 

 said — the degree of accuracy attainable, and hence required, is greatest 

 for short periods, i.e., for the current growth of a decade or two, and 

 diminishes as the length of the period increases. But the relative 

 importance of accuracy also diminishes with the length of the period, 

 thus permitting the use of yield tables based on averages. 



1 Norway Pine in the Lake States, U. S. Dept. Agr., 1914, Bui. 139, p. 15. 



412 



