VOLUME GROWTH A BASIS FOR SITE QUALITIES 385 



which influence this growth, chief among which are quaHty and depth 

 of soil, average moisture contents, slope and exposure, altitude and 

 climate. Site factors cause a variation in total possible yields of from 

 200 to 300 per cent. Hence for a given stand or area the yield cannot 

 be predicted within a reasonable degree of accuracy unless the quality 

 of site is taken into account. This difference in yield on good and on poor 

 sites is caused by the more rapid growth in height, diameter, and volume, 

 of the trees in the stand, when growing on more favorable sites. Fewer 

 trees may mature on good sites than on poor, because of the larger 

 sizes and crown spread attained, but the sum of their volumes will 

 exceed those of the trees maturing on the poorer sites. When the 

 period of years required to produce these yields is considered, and the 

 mean annual growth is computed (§ 245) it will be seen that the more 

 rapid growth on good sites produces even more striking differences in 

 the annual rate of growth between poorer and better sites. These 

 differences are further increased when the value of the yield is compared 

 with the cost of production, so that it becomes of utmost importance 

 in forestry to determine, for any large area of forest land, the acreage 

 embraced in each of several grades or qualities of site. 



295. Volume Growth a Basis for Site Qualities. Forest types some- 

 times show abrupt transition from one to another, corresponding to 

 sharp differences in soil moisture; but more often the change is gradual 

 and the separation of areas in each type, as made in the field, is arbitrary. 

 The differences in site quality within a type form an unbroken series 

 of gradations, which must be separated, on a purely arbitrary' basis, 

 into a convenient number of site classes, whose average yields may 

 be expressed in tables. In European practice five qualities are recog- 

 nized when a few species occupy a wide range of conditions. In America 

 three qualities have so far sufficed to cover the range of a single species. 



The problem of classifying site qualities is two-fold. First, the 

 plots whose yields are measured to determine the average rates of 

 growth for different sites must be separated into the predetermined 

 site classes. Second, some convenient means must be found to apply 

 this site classification to forest lands during a forest survey in order 

 that the total area may be subdivided on this basis for the pi'ediction 

 of growth on the forest. 



The most direct method of classifying plots measured for yield is 

 by the rate of growth per year actually produced, i.e., the total yield 

 based on age of the stand. This has been the basis of most of the yield 

 tables constructed in America, and might suffice were it not for the 

 four other factors which modify the yields per acre independent of site; 

 namely, form of stand, treatment, degree of stocking, and composition 

 of stand. 



