1914] VESTAL— PRAIRIE VEGETATION 385 



In clay or loam, sometimes subjected to flooding (margins of reservoirs, 



alkali flats). 



Salt-Grass Association 



Associations not typical of particular habitats, but scatteringly distributed 

 chiefly because they are primitive growths; early stages in development of 

 vegetation; growths typical of recently denuded or broken surfaces, or of 

 physiographically new environments. Vegetation temporary in character; 

 usually tending to change more rapidly than physical environments (Primitive 

 Associations). 



Growth dominated by annuals; short-lived growths typical in recently dis- 



turbed areas. 



Plains Ruder al Association 



Growths dominated by perennials; frequently preceded by a ruderal stage; 

 usually less temporary in character. 

 Dominant plants xerophytic bunch-grasses. 



Primitive Bunch-Grass Association 

 Dominant plants bushy xerophytes, deep-rooted, not grasses. 



Gutierrezia-A rtemisia A ssociation* 



Descriptions of the associations 



The descriptions of associations are necessarily brief. Only the 

 very important variations of each association are mentioned, and 

 only dominant or very characteristic plant species receive notice. 

 References to the same growths, or different representations of 

 these same growths, as described by various authors, are given. 

 It is the opinion of the writer that the time has come for correlation 

 of associations in different parts of the same region, in cases in 

 which the region is fairly well known. Effort has been made to 

 select the more important references to prairie associations, but 

 not to make a complete synonymy of the various growths. 



Buffalo-grass formation 



THE SHORT-GRASS ASSOCIATION 



Clements 



formation 



— Shaxtz 17, p. 26. 



4 



The Gutierrezia-Arletnisia association, while most frequent in gravelly mesa- 



tops, is a primitive growth rather than an association of this particular habitat. In 

 such situations the growth normally develops into short-grass. On the other hand, 

 certain appearances of the Stipa-Aristida association (the wire-grass association of 

 SHANTZ,see p. 394), may be, following conditions of disturbance, a primitive association. 

 Local associations, or growths relatively permanent in extreme habitats, may become 

 primitive or temporary associations in the less extreme habitats in which climatic 

 associations are dominant except for a short time after disturbance. Though primitive 

 and local associations cannot always be rigidly separated, it seems helpful to distin- 

 guish the two groups. 



