386 FACTORS AFFECTING THE GROWTH OF STANDS 



The influence of these variable factors is tremendous, and it has 

 usually been considered necessary to eliminate them by constructing 

 yield tables for given fixed conditions only, such as for even-aged 

 stands, artificially grown and thinned, of normal or full stocking, and 

 of pure species. Where these conditions do not apply, as for instance 

 in mixed stands of broken density in forests of all ages, it has often 

 been considered impossible to determine the rate of growth per acre. 



296. Height Growth a Basis for Site Qualities. A'though it may 

 be possible, by rigid selection, to eliminate these four variables and thus 

 base the site qualities upon the rate of growth or the total yield per acre 

 based on age, yet when it comes to reversing the process and applying 

 this standard of site classes to the classification of lands on a larger 

 area, the remaining variables are present and must be dealt with. 

 This problem may be summed up as follows: 



1. The factors of site, such as climate, and soil, are too complicated 

 to be directly measured in the field as a means of site classification. 

 Results expressed in forest growth, rather than causes, must be used 

 as the indicator of site. 



2. Volume as a site indicator is incomplete without the determina- 

 tion of age. For most conditions the relative volume based on age 

 is too variable and difficult of determination to serve as a field basis 

 of classification of large areas. 



3. Dimensions of typical dominant trees in a stand may serve as 

 the required indicator, since the tree unit is independent of the variables 

 of age, form, composition and density which affect the stand. 



4. The dimensions which may serve for this purpose are diameter 

 and height. Of these, height alone is a reliable index of site quality 

 since it is affected but little by varying density or degree of stocking, 

 or by the treatment of the stand. Height based on age is a more 

 reliable basis than volume on age for stands of varying degrees of stock- 

 ing, and for both wild or unmanaged forests and thinned or managed 

 stands. This reduces or eliminates two of the five variables, namely, 

 treatment, and density of stand. Height growth is retarded by shade 

 to a marked degree; hence in forests of all ages, and in mixed stands 

 of several species, height based on total age ceases to be a reliable 

 index, since the factor of economic age is introduced. 



Total height or height at maturity remains, even in mixed stands, 

 a distinguishing characteristic of different site qualities. The growth 

 of dominant, unsuppressed trees, a few of which may be found in almost 

 every stand, may be ascertained in a very few tests and will hold good 

 for the stand or site. Thus the remaining two variables, form and 

 composition, may be eliminated by selection of dominant trees or fully 

 mature trees. 



