CLASSIFYING FOREST SITES 377 



SOME FEATURES AND POSSIBLE DEVELOPMENTS OF THE PLAN 



There are some difficulties which must be settled if the case for 

 height-growth is to receive serious consideration. What, for instance, 

 is the justification for selecting one particular age as a reference point? 

 How can the quality of site be judged on the basis of height except by 

 the use of the entire growth curve? How can the site requirements 

 of trees with different height-growth curves be compared on the basis 

 of the height at any given age? And if they can be so compared, of 

 what use is the comparison if trees of this particular age are not avail- 

 able for measurement? These questions strike to the heart of the 

 plan ; answers to them should go a long way toward explaining its 

 purpose and its limitations and advantages for practical use. 



Growth curves versus the height at a uniform age. 



The reason for selecting a single common age is simplicity — an 

 essential attribute considering the diversity of the material to be 

 classified. By way of analogy, if volume per acre were a practicable 

 site index the same consideration would have to be regarded ; instead 

 of whole yield tables or yield curves we might use, for the classification 

 of sites, the yield at a stated age. For the field identification of sites, 

 however, the entire curve or table would be essential, for if only the 

 yield at a single age were provided the site could not be determined 

 when occupied by a stand of any other age. Similarly, in the height 

 classification plan, a considerable part of the height curve or table is 

 necessary for the identification of sites. If it were possible to arrange 

 a classification of height-growth curves or tables for all the important 

 species on their different sites which would be simple enough for use 

 in ordinary field work, such a classification might be superior to the 

 single age-height reference point. As it is, the height growth curves 

 are a necessary adjunct to the height-age reference point, which is 

 used simply because curves or tables do not lend themselves to prac- 

 tical classification. 



The single age-height figure may then be regarded as simply an 

 index designating the intersection point of the curve which it repre- 

 sents with the ordinate at 100 years. In other words, while the site is 

 catalogued in terms of height of a species at one stated age, it is fully 

 expressed only by the entire curve ; and it is only by reference to the 

 entire curve that it can be determined for trees of any other age. The 

 points of intersection of the growth curves with the ordinate repre- 



