258 CLARENCE F. KORSTIAN 



of the association, society or community as an integral should be 

 resolved to the individual species, to which indicator significance 

 may be attached. In any ecological group of plants in which 

 there are many diverse growth forms within a biological unit it 

 is impossible to define accurately the boundaries of the associa- 

 tion without encountering many overlapping species, especially 

 in arid or semi-arid regions. An herb or a shrub can complete 

 its full life cycle in a much shorter time than a forest tree; there- 

 fore, succession proceeds at a faster rate and a greater number 

 of successional series may be observed than if the forest trees 

 alone were under consideration. 



The use of shrubby and herbaceous vegetation as forest site 

 indicators may be applied with greater ease in the comparatively 

 open, park-like forests of western yellow pine than in the denser 

 Douglas fir, lodgepole pine, and Engelmann spruce forests. 

 Changes in the density of the crown cover may change the com- 

 position of the vegetative undergrowth although the potential 

 forest productivity of the site remains unchanged. Herbaceous 

 plants are rather scarce in the denser forests except in partial 

 openings or where the stand is below normal density. The 

 character of the shrubby and herbaceous layers of forest vegeta- 

 tion is conditioned largely by the shade of the forest canopy. 

 The forest floor in stands of fir, spruce and lodgepole pine is 

 heavily and more continuously shaded than that of the densest 

 stands of yellow pine. The proposed criterion is believed to be 

 especially applicable to yellow pine forests or those of compara- 

 ble density. 



It is not the purpose of this paper to detail specific corre- 

 lations between forest growth and the character of the associated 

 vegetation. However, a few noteworthy cases in which the 

 native vegetation presents rather conspicuous and convincing 

 evidence of site character will be cited. 



The studies conducted by the writer 10 in central New Mexico 

 showed that, where two western yellow pine sites were recognized, 

 the species limited to site I were mesophytic while those found 



10 Loc. cit., pp. 277-279. 



