404 NORMAL YIELD TABLES FOR EVEN-AGED STANDS 



When constructed on this basis, yields for different site classes 

 increase at a greater ratio than do the indicating heights. 



In drawing the curve of yield based on age for a single site class, 

 it is best to first obtain the average yield for a given decade by arith- 

 metical means and connect these averages by straight lines. Even if 

 each plot were normal, the averages at different points might fall above 

 or below the mean for the site as the plots happened to be on the better 

 or poorer portions of this site class — and to this factor, the natural vari- 

 ation in density or yield is added. 



7. For this reason, the average curves so constructed, for each 

 site class, should now be assembled on a single sheet, as shown in Fig. 

 85. The curves of yield based on age can then be harmonized for all 

 site classes by the same principle as used for volume tables (§ 140).^ 



311. Rejection of Abnormal Plots. As shown in § 304, the intent 

 of this table is to estal)lish a standard of yield, termed normal or index, 

 with which the yields of any existing stand may be compared. After 

 the separation based on height growth is effected, the yields of plots 

 in the same site class will show great variation, due to the 



Natural range of site quality within the arbitrary boundaries 



established ; 

 Number of trees per acre in the natural stocking; 

 Completeness of the crown canopy. 



The eccentric behavior of the averages plotted in Fig. 85 indicates the 

 effect of these variations in yield. The question arises as to whether 

 all of the plots should be included in these averages or certain plots 

 rejected as abnormally stocked. A method of correcting the yields 

 by a factor of density of crown has been generally rejected as unsatis- 

 factory (§ 309). The area of plots is accepted as measured. There 

 are, then, two possibilities of rejection; first, by ocular selection in the 

 field, which eliminates those plots which are incompletely stocked; 

 second, by further inspection of the plotted volumes based on age. 



Baur's rule for rejection of plots is quoted by Graves as follows: 

 "Stands which have the same age and average height are compared, 

 and all are considered normal whose basal area lies within a range of 

 15 per cent; that is, the basal area of the best and poorest stocked stands 

 must not differ more than 15 per cent." ^ The application of this rule 

 rests upon the interpretation of the term " average height." Where 

 from three to five site classes are made as in Fig. 85, and a curve of 

 average height is found for each site class, which would fall midway of 



iThe yields shown in Fig. 85 are from an unpublished manuscript by S. B. 

 Show, U. S. Forest Service, California, for second growth Western yellow pine. 

 " Graves' Forest Mensuration, p. 319. 



