IV. CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



Nature. — The vegetation of a continent falls into a number of major 

 divisions or units. These are known as plant formations, and are regarded as 

 complex but definite organic entities with a characteristic development and 

 structure (Plant Succession, 124). They are the product of climate and are 

 controlled by it. Each formation is the highest expression of vegetation 

 possible under its particular climate, and hence it is also termed a climax. 

 As here understood, the formation and climax are identical, and the terms 

 are essentially synonymous. For the sake of emphasis as well as of conven- 

 ience, however, the two are used together. Hence the same unit may be 

 referred to as a climax, a formation, or a climax formation. Since it exhibits 

 a development as well as structure, it is further necessary to recognize that the 

 successional areas in the great grassland formation, for example, are an inte- 

 gral part of the climax, however much they may differ from it. Whatever 

 seems inconsistent in this is apparent and not real, since it is a matter of 

 common knowledge that the same organism may appear in two or more 

 unlike forms, such as the seedling and adult plant, or the larva, chrysalis, 

 and butterfly. 



Climaxes owe their character to the controlling species or dominants which 

 make them up. These climax dominants belong to the same vegetation- 

 form, which represents the highest type possible under the prevailing climate. 

 In grassland the climax dominants are all grasses or sedges, in forest they 

 are trees, in chaparral, shrubs, and so forth. The exceptions to this rule all 

 seem to be merely apparent. They are well illustrated by the so-called 

 savannah in which the trees or shrubs are more noticeable than the grasses, 

 but the latter are in actual control of the habitat. Moreover, in the prairies 

 the dominant grasses may be concealed for much or all of the growing season 

 by tall herbs, such as Psoralea, Amorpha, and Solidago. These are called 

 subdominants and characterize minor communities subject to the control of 

 the grasses. In addition to the climax dominants are the other species which 

 mark particular stages in the development of the climax. These are develop- 

 mental dominants, and are usually termed successional or serai because of 

 their role in the succession or sere which reestablishes the climax on a bare 

 area. 



Tests of a climax. — Each climax is regarded as the direct and complete 

 expression of its climate. The climate is the cause, the climax the effect. So 

 close is this relation that the climax must be regarded as the final test of a 

 climate. From the standpoint of vegetation at least, climates are to be recog- 

 nized and delimited only by means of climaxes, and not the reverse. No 

 matter how complete his equipment of meteorological instruments, the ecologist 

 must learn to subordinate his determination of climate to that of the plant if 

 his results are to be reliable and usable. The paramount importance of forma- 

 tions in indicating climates makes their objective recognition of the first 

 importance. In the search for criteria which would permit an objective and 

 consistent basis for formations, several guiding principles have become evi- 

 dent. The first of these is that the climax dominants must all belong to the 

 same great vegetation-form, since this indicates a similar response to climate. 



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