CLIMATIC AND EDAPHIC INDICATORS. 349 



It is assumed that all dominants have different water requirements, and 

 that each in consequence indicates a different water-content. It is believed 

 that the results of further quantitative studies will show that the dominants 

 of a sere can be arranged in a linear sequence from the pioneer stage to the 

 climax. At the same time, it seems completely established that this sequence 

 falls naturally into stages or associes, characterized by dominants of the same 

 life-form and similar requirements. As a consequence, it becomes possible 

 to use the dominants or consocies of a sere to indicate the successive small 

 steps in the changing water-content from the initial bare or denuded area to 

 the climax, while the associes indicate the stages of longer duration which are 

 characterized by a certain set of water conditions. In the prisere, such con- 

 ditions and their indicators have some relative permanence, but in the subsere 

 the successional movement is much more rapid and the stages sometimes 

 obscured. In both cases, however, the basic principle holds that a complete 

 series of indicators marks the changes of water-content from an originally 

 hydrophytic or xerophytic bare area to the relatively mesophytic forest climax. 

 The exact value of each community or dominant as an indicator must await 

 more general quantitative study, but the approximate values that can be 

 assigned them at present are of genuine service in forest problems. 



Light indicators. — The general principles which underlie light indicators in 

 the forest have been discussed at some length in Chapter III, and the light 

 relations of the dominants of the various forest associations have been touched 

 upon in Chapter IV. The tolerance of western dominants has been indicated 

 by Zon and Graves (1911:21), Sudworth (1908), Larsen (1916:437), and 

 others. In a study of the tolerance of New England forest trees, Burns (1914, 

 1916) concludes that tolerance "really expresses not a light relationship, but 

 the total relationship of a tree to all the factors of its habitat." While the 

 results of Fricke (1904) and Burns have shown that competition for water 

 must be taken into account in studies of tolerance, light is still to be regarded 

 as playing the paramount role. Burns's further conclusion that light readings 

 in the forest are of little value is not in accord with extensive experience in 

 making and utilizing such readings in ecological studies. On the contraiy, 

 one of the chief difficulties in the correlation of edaphic communities with 

 their habitats is the absence of measurements of light intensity. Where 

 these have been made with care and in large number through several years, 

 as in the Pike's Peak region of the Rocky Mountains, they have proved 

 invaluable in the study of reproduction, development, and plant indicators, 

 as well as in that of leaf adaptation and photosynthetic efficiency. Measure- 

 ments of light intensity in the forests of the West have been made by Clements 

 (1905, 1910), E. S. Clements (1905), Pearson (Zon and Graves, 1911:46), 

 and Bates (1917:233). Studies of the quality of forest light have been 

 carried on for several years by means of a portable spectrophotometer (Cle- 

 ments, 1918: 291), but the detailed results have not yet been published. 



Site indicators. — The term site, like forest type, has a wide range of mean- 

 ing among foresters. While it is regularly employed to denote the habitat, 

 it is applied to all possible divisions of the latter. This is understandable, 

 since this is the present ecological practice in the case of habitat. But just 

 as it has proved necessary to distinguish habitats of different character and 



