406 NORMAL YIELD TABLES FOR EVEN-AGED STANDS 



object sought both in field selection of plots and in their further sifting 

 in the office for the preparation of normal yield tables for natural 

 growth. 



312. Construction of Yield Table with Site Classes Based Directly 

 on Yields per Acre. The main objection to the direct classification 

 of site on the basis of yield or volume on age by Baur's method is the 

 impossibility of using this basis later as a means of classifying forest 

 lands into site qualities from field examination. Furthermore, yield 

 alone gives an unsatisfactory basis for correlating yield tables for given 

 species when made for different regions, or for correlating the yields 

 of different though similar species. It is this need of standardization 

 that has led to the adoption of height growth rather than volume as 

 the basic standard. 



A further objection to the direct use of yields lies in the method of 

 plotting, and the testing of plots for normal density. By this method, 

 the volumes of all plots, based on age, are entered on the same sheet as 

 shown in Fig. 86. The drawing of the maximum and minimum curves 

 is the next step. There is no way by which the abnormality of the plots 

 can be first tested as with heights. So the elimination consists wholly 

 of drawing these boundary lines to exclude certain plots whose yield 

 is so much greater or smaller than the remainder that their inclusion 

 would unduly influence the position of these limiting curves. 



The third step is to divide the space thus blocked off into equal 

 bands by the method used for height, i.e., by dividing the distance 

 on each ordinate into equal parts, and connecting the points so estab- 

 lished. 



Finally, a curve is drawn exactly midway of each space as described 

 for height (§ 310), and the values are read from this curve at each decade 

 to form the table of yield based on age. 



By this method yields increase with site quality by exact intervals. 

 No averages are attempted, and the result is entirely independent of 

 height and is influenced principally by the maximum and minimum- 

 yields rather than the general weight of the plots studied. 



Using as the basis the plots which have been classed as belonging 

 to each separate site by either of the above methods, curves showing 

 the average at different ages can also be prepared for the following 

 additional data: 



Number of trees per acre; 



Total, 



Above a minimum diameter. 

 Average diameter. 

 Average height of dominant trees. 

 Total basal area. 



