448 COORDINATION OF FOREST SURVEY 



When these factors cannot be directly ascertained, the requisite basis 

 must be obtained for calculating them. The most fundamental and 

 useful basis is, 



5. Diameter of trees in stand b}- species, or a stand table. 

 Finally, because of its inadequate handling, special emphasis must 



be placed on obtaining 



6. The area stocked by immature age classes. 



345. Site Qualities — Separation in Field. Site qualities in the 

 forest should be separated by area. Where several types exist, such 

 as cove, lower slope, upper slope and ridge, which correspond closely 

 with difference in site, the division by types goes a long way toward 

 separating the site qualities (§ 228). 



Where site qualities must be determined directly, there are but two 

 methods possible of which the first is direct judgment based on obser- 

 vation of site factors, such as soil, altitude, slope, rock, moisture (as 

 swamps) and general character of the timber growth. This method 

 is subject to serious errors (§ 226). The second method ^ is based on 

 the height growth of dominant trees (§ 227). But to determine directly 

 the site class indicated by trees of different heights, their age must be 

 known. When the forest is composed of a few large age classes of even 

 age, direct determination of a few ages may give this basis. 



But where the age classes are mixed, the age of individual dominant 

 trees, rather than age of stand, must be relied on to indicate site quality. 

 If we could assume that diameter growth did not decrease for the average 

 tree, on poor sites, and that average trees of a given diameter were 

 as old on Quality I site as on Quality III, diameter could be substituted 

 for age; but average diameter growth varies with the site quality itself, 

 which prevents this substitution. 



To obtain the basis of field classification of site, the heights of dif- 

 ferent trees based on age are plotted and divided into site qualities 

 based on the standard chosen, as illustrated in Fig. 84 (§ 310) except 

 that in this case the data are obtained by plotting individual trees, 

 and by analysis of the height growth of trees, rather than from plots. 



To apply this table or set of curves, in determining the quality of 

 a given site, a selected tree or two is measured for height. If fully 

 matured, total height may indicate directly the site quality. If the 

 stand is young, age must always be ascertained. The average height 

 for the given age is then looked up on the chart. The trees chosen 

 should preferably be dominant and must never be suppressed. The 

 position of the height with reference to the curves or table indicates 

 the site quality. 



The unit of area on which sites are separated should be that used 

 1 Journal of Forestry, Vol. XV, 1917, p. 552. 



