52 



BASES AND CRITERIA. 



indicator relations change more or less slowly but inevitably from one stage to 

 the next. While the developmental areas of a formation are usually less in 

 aggregate extent than those occupied by the climax stage, they are so numerous 

 and various as to demand constant attention. The relative permanence of an 

 indicator relation depends wholly upon whether it is determined by develop- 

 mental or climax conditions. Since the use of any area for cropping, foresta- 

 tion, or grazing either demands or effects constant changes in it, succession is 

 the basis of all utilization of communities or dominants as indicators. This is 

 especially true in the case of land classification, as Shantz has shown (1911 : 18), 

 and it applies also to all engineering and construction operations in which the 

 soil is disturbed or new habitats produced. 



Sequence of indicators. — Succession has been defined and analyzed as the 

 development of a complex organism, the climax community or formation 

 (Clements, 1905 : 199; 1916 : 3). It is a chain of causally related functions or 

 processes. Development begins at certain definite points, pursues a regular 

 course, and ends in the final or mature stage, the climax. As a result, each 

 serai dominant or community has indicator values beyond those arising from 

 the basic relation between plant and habitat. Each stage is the outcome of 

 those that precede and the precursor of those that follow until the climax is 

 reached. It indicates not merely the existing conditions, but it also points 

 backward through successively remote stages to the beginning of the sere, and 

 forward through those which lead up to the climax. Since the development of 

 the habitat proceeds step by step with that of the formation, each stage is an 

 indicator of earlier and later habitats as well as communities. Succession, 

 moreover, is always progressive, and makes it possible to forecast not only the 

 direction of development but something of the rate as well. It depends 

 primarily upon the production of new, denuded, or disturbed habitats, and thus 

 serves as an indicator of the many processes, physiographic, biotic, etc., which 

 initiate new habitats or denude existing ones. 



The several indicator values of a serai community depend primarily upon 

 the climax and the sere to which it belongs. The climax determines the domi- 

 nants and subdominants from which the stages are drawn, indicates the climate 

 in general control of the habitat changes, and constitutes the final stage toward 

 which all the successions are moving. It is in itself an indicator of succession, 

 since it permits the prediction of the general course of development that 

 results from any disturbance in it. The division of seres into primary and 

 secondary rests upon the double basis of habitat and development, and explains 

 why each sere has indicator significance in itself. The primary sere or prisere 

 indicates an extreme condition of origin, such as water or rock, slow reaction 

 on the part of the earlier communities especially, and hence a large number of 

 successive communities. The secondary sere or subsere begins on actual soil 

 in which the conditions are not extreme, requires less reaction, exhibits few 

 stages as a rule and runs its course to the climax with much rapidity. All 

 seres, but primary ones in particular, are distinguished upon the basis of the 

 climax and the water relations of the initial area. The great majority of seres 

 are mesotropic, that is, they progress to a mesophytic climax. In desert regions 

 they are xerotropic and in the tropics may be hydrotropic (Whitford, 1906). 

 Their indicator meaning varies accordingly, but it is even more subject to the 

 water-content of the initial area. Seres are termed hydrarch (Cooper, 1912: 

 198) when they originate in water or wet areas, and xerarch when the initial 



