106 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



The second is that one or more of the dominant species must range throughout 

 the formation as a dominant to a larger or smaller degree. The importance 

 of this lies in the fact that while no two dominants are exactly alike, those of 

 the same formation are so nearly equivalent that the presence of one indicates 

 the possibility of others. This is well illustrated by the behavior of Bouteloua 

 gracilis, which ranges as a dominant from the Missouri River to California 

 and from Saskatchewan to Mexico. While the climate of this vast stretch 

 varies greatly in physical or in human terms, the conclusion is unavoidable 

 that the extensive areas covered with Bouteloua have the same or a similar 

 grassland climate. This obviously permits the application to vegetation of 

 the principle that things equal to the same thing are equal to each other. 

 This approximate equivalence of dominants receives its best proof in the 

 grassland formation, in which the mixed prairie shows Bouteloua gracilis in 

 intimate mixtures with Stipa, Agropyrum, Bulbilis, Car ex, or Koeleria. The 

 third criterion is that the majority of the dominant genera extend through- 

 out the formation, though represented by different species. This is well 

 exemplified by the chaparral climax, in which Quercus, Prunus, Rhus, Cer- 

 cocarpus, and Ceanothus are found in the several associations. A corollary 

 of this is that most of the subdominants likewise belong to the same genera, 

 as, for example, Astragalus, Erigeron, Psoralea, Petalostemon, Solidago, Erio- 

 gonum, and Artemisia in the grassland associations. The fourth criterion is 

 developmental or successional and has several aspects. It is seen in the 

 behavior of such subclimax dominants as Aristida purpurea, which charac- 

 terizes certain types of disturbed areas in all the grassland associations, and 

 later yields to the climax dominants of each. It is equally well shown by 

 Andropogon scoparius and Bouteloua racemosa, which are subclimax in rough 

 areas as well as in meadows to the final dominants of the four most extensive 

 grassland associations. Finally, the degree of equivalence of dominants is 

 indicated by their mingling but is checked by their successional alternation. 

 The position of Andropogon in meadows, Agropyrum and Stipa on slopes, 

 Bulbilis and Bouteloua on the crests of the rolling prairies, is not only signifi- 

 cant of their physiological and successional relations, but also of their as- 

 sociational positions. Andropogon furcatus and seoparius are typical of the 

 subclimax prairie of the Mississippi Valley, Stipa and Agropyrum ot the climax 

 prairies, and Bulbilis and Bouteloua of the still drier plains. 



Structure and development. — By far the greater portion of a climatic region 

 is occupied by the climax characteristic of it. But all through it occur areas 

 of varying size in which new or denuded soils are available for colonization and 

 the development of the climax as a consequence of succession (Plant Succes- 

 sion, 3). As a result, every formation shows subdivisions or communities of 

 two sorts, namely, climax, and successional or serai. Initial serai communi- 

 ties, such as the colonies of water-plants in ponds and streams and of lichens 

 and mosses on cliffs and boulders, are readily distinguished from climax ones. 

 As succession proceeds, however, the communities more and more nearly 

 approach the climax in appearance. In the last analysis, they can be dis- 

 tinguished only by the fact that each stage slowly yields to the next until the 

 climax is reached, when the succession stops. In many cases where the dis- 

 turbance due to fire, grazing, or cultivation is continuous or periodic, the sub- 



