THE GRASSLAND CLIMAX. 115 



as usage was coming to do more and more (Plant Succession, 124), and to use 

 association for the major subdivision, a relation likewise warranted by usage 

 as well as by the action of the Brussels Congress. Whatever the final solution 

 of this matter may be, there would seem to be no doubt that the grassland is a 

 major unit, coordinate with deciduous forest, sagebrush, chaparral, etc. 



When we turn to the internal proof of the unity of the grassland climax, 

 the evidence is more complete. In the first analysis of the grassland, Pound 

 and Clements (1898 : 243, 1900 : 347) recognized two prairie formations, 

 viz, the prairie-grass and buffalo-grass formation, a bunch-grass formation 

 of the sandhills, and a meadow formation. In the light of successional studies, 

 the last two are to be regarded as subclimaxes. In a few years (Clements, 

 1902) it had become clear that the buffalo-grass or Bulbilis-Bouteloua forma- 

 tion and the prairie-grass or Stipa-Agropyrum formation were the two great 

 communities of the prairie-plains region. This was essentially the view of 

 Shantz (1906, 1911) and of Pool (1914). This conception was still main- 

 tained in "Plant Succession" (180 : cf. note) after many additional years of 

 successional research. However, the developmental concept of the formation 

 had broadened its scope and afforded a clearer view of its structure. As 

 a consequence of a special study of these relations, it became necessary to 

 abandon the view of two separate grassland formations, and to recognize a 

 single formation composed of several associations. Meanwhile, it had become 

 increasingly evident that the Agropyrum spicatum consociation of the North- 

 west was closely related to the Stipa-Agropyrum prairie (Weaver, 1917 : 40). 

 This was first suggested by frequently finding the three dominants associated 

 in the field work of 1914 from Washington to Montana. It was confirmed 

 in 1917, but the true relationship was obscure until it became certain in 

 1918 that Stipa setigera and S. eminens were the original bunch-grasses of 

 California. As a consequence, it proved possible to recognize a fourth grass- 

 land association, composed of bunch-grasses and characteristic of the Pacific 

 region of winter precipitation. 



Unity of the grassland. — The conclusion that the grassland is a single great 

 climax formation is based in the first place on the fact that the three most 

 important dominants, Stipa, Agropyrum, and Bouteloua, extend over most 

 of the area, and one or the other is present in practically every association 

 of it. This would seem the most conclusive evidence possible, short of actual 

 vegetation experiments, that the grassland is a climatic vegetation unit. 

 Equally cogent is the fact that these dominants, together with Carex, Bul- 

 bilis, and Koeleria, mix and alternate in various groupings throughout the 

 Stipa-Bouteloua association. Indeed, this association appears so conclusive 

 as to the general formational equivalence of these seven dominants that it is 

 regarded as the typical or base association. In addition, the characteristic 

 societies either extend through several of the associations or are represented 

 by corresponding communities belonging to the same genus. The relation of 

 the associations to such subclimax species as Andropogon scoparius, Cala- 

 movilfa longifolia, Aristida purpurea, and Elymus sitanion further confirms 

 the relationship of the dominants. The most obvious difference between the 

 various associations are exhibited by the tall-grass prairies, Stipa-Koeleria 

 poium, and the short-grass plains, Bulbilis-Bouteloua poium. Yet these are 

 closely related, as shown not only by the criteria given above, but also by their 



