3 58 THE STUDY OF PLANT COMMUNITIES * Chapter XII 



structure occupy similar sites, it is possible to judge a site and the 

 nature of the dominants from the ground cover alone. Thus recog- 

 nition of the herbs, mosses, and lichens on the forest floor with an 

 estimate of their relative proportions might suffice for evaluation 

 of the stand and the quality of the site on which it grows. 



Perhaps the most comprehensive attempt to apply the method 

 in North America was made in the Adirondack Mountain area. 121 

 Elsewhere smaller areas with fewer communities have been studied. 

 Although special phases of the method have proved useful in cer- 

 tain situations, the method as a whole has found limited applica- 

 tion. Although herbs undoubtedly affect the dominants by modi- 

 fying soil structure and water relations, and likewise through 

 competition with seedlings of dominant species, there are argu- 

 ments against the validity of information based on herbs alone, 

 particularly since they derive water and nutrients from different 

 soil horizons than do the dominant trees. It is, therefore, suggested 

 that all the lesser woody vegetation should also be included. There 

 is evidence that the same herbaceous species predominate on more 

 than one soil type, and, therefore, their significance is questioned. 

 Often the indicator types are of limited extent, and several may be 

 present within a single stand. Interpretation, then, becomes diffi- 

 cult in terms of management. Undoubtedly, the foresters' not 

 uncommon lack of familiarity with lesser vegetation and frequent 

 inclination to ignore it entirely have been factors in limiting the 

 testing and application of the method in American forests. 



Because the subordinate vegetation changes after lumbering or 

 fire and because height of trees, the commonest criterion of site, 

 cannot then be known, it is desirable that some relatively simple 

 means of evaluation of site be available that can be applied at any 

 time. T S. Coile 65 has approached this problem through physical 

 measurement of the soil, as others have attempted before, and, 

 after extensive investigation, has found that the site index can be 

 accurately determined if only the depth of the A horizon and the 

 soil type are known. Using the xylene equivalent (determined like 

 moisture equivalent) of the B horizon (known for the soil type) 

 and the depth of the A horizon, a positive statement of site quality 

 can be made whether the land is in forest, cultivated, or abandoned 

 and regardless of slope or exposure. This would seem to be the 



