50 BASES AND CRITERIA. 



been recognized. This is well illustrated by shallow lakes in the sandhills of 

 Nebraska, where the depth is so uniform that Scirpus is the sole dominant in 

 spite of the fact that neighboring lakes show Typha, Zizania, and Phragmites. 

 Absence as a result of destruction is usually difficult to determine and yet is of the 

 greatest indicator importance. The grassy parks of the Uncompahgre Plateau 

 in Colorado are so extensive and appear so permanent that their real signifi- 

 cance, as well as that of the absence of the trees, was finally determined only 

 by the discovery of burned wood deep in the soil. Similarly, much evidence 

 has been found to show that the absence of Stipa or Agropyrum over wide 

 stretches of the Great Plains reveals overgrazing of a type that has never been 

 suspected. Thus, while absence is necessarily correlated with the presence of the 

 related dominants in order to be usable, it does furnish indications of much value. 



Subdominants. — Subdominants are species which exert a minor contro 

 within the area controlled by one or more of the dominants of an association or 

 associes. They are the successful competitors among the species which accept 

 the conditions imposed by the dominants. As a rule they differ from the latter 

 in life-form, and their competition is largely mutual rather than with the domi- 

 nants. This is obviously the case in forests where the subdominants form 

 layers. In grassland, where fight controls in a minor degree alone, the layer- 

 ing is in the soil, but with a somewhat similar result that the dominants use the 

 water before it reaches the deep-rooted herbs. In prairie and meadow, there 

 is often enough water for both, a condition favored by the fact that subdomi- 

 nants reach their maximum at different times during the season, and hence 

 cause the characteristic seasonal aspects. During dry phases of the climatic 

 cycle, however, there is direct competition between dominants and subdomi- 

 nants, but usually at the expense of the latter. 



Within the limitations set by the dominants, subdominants follow the same 

 general principles as to indicator values. This applies to their association in a 

 community, either climax or serai,. their equivalence, their dominance as com- 

 pared with mere presence, and to their absence. They diverge, however, in 

 exhibiting a seasonal sequence in many associations, by which they appear to 

 escape too intense competition with each other. Prairies purple with Astra- 

 galus crassicarpus in April and May are covered with Amorpha, Psoralea, 

 Petalostemon and Erigeron in June and July, and these in turn yield to golden 

 rods, asters, and blazing stars in August and September. To a large extent 

 these successive societies occupy the sa ne ground and would seriously compete 

 with each other were it not for the fact that the maximum demands of Astra- 

 galus, for example, are over before those of Psoralea and Erigeron begin. 

 Societies thus have a time as well as a space value as indicators. While the 

 subdominants of the same aspect are equivalent to a large degree, those of the 

 three aspects, spring, summer, and autumn, differ in being progressively more 

 xerophytic, owing to the seasonal relations of rainfall and evaporation. 

 Societies are not only most numerous and best-developed during the early 

 summer because of optimum conditions, but they likewise reach a maximum in 

 those communities with optimum conditions, such as prairie and forest. In 

 the short-grass plains they are greatly reduced, and in desert they are relatively 

 few, except in the spring. This exception covers those deserts with two rainy 

 seasons in which the socies of winter and summer annuals are possible only 

 because of a relative excess of moisture near the surface at these times (plate 9). 



