122 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



to trace. This difficulty is increased by the breadth of the two ecotones 

 between the three parallel associations. However, the general limits of the 

 area may be drawn with some definiteness. The eastern edge runs southward 

 from Manitoba along the western boundary of Minnesota and then swings 

 southeastward with the Minnesota Valley, reaching its limit between 92 and 

 93" W. It stretches across northern and central Iowa in the vicinity of the 

 ninety-third meridian, and then trends southwestward across northwestern 

 Missouri and eastern Kansas, where it turns south to the Oklahoma line. 

 The western boundary begins in Manitoba between the one hundredth and the 

 one hundred and first meridians and continues more or less due south to near 

 the Nebraska line, where it turns southeast around the sandhill region, beyond 

 the ninety-ninth meridian. It then follows this in a general way into northern 

 and central Kansas, and finally approaches the Oklahoma line in the vicinity 

 of the ninety-eighth meridian. The association reaches its greatest breadth 

 of 7** of longitude along the forty-third parallel, and it tapers more or less 

 irregularly in both directions to a width of I*' to 2" in Manitoba and in Kansas. 



CONSOCIATIONS. 



SnPA SPARTEA. AgROPYRUM OLAUCIIM. 



KOELERIA CRI8TATA. AnDROPOGON SCOPARIUS. 



Stipa COMATA. 



Each of the 5 species may occur as a pure dominant, though this is excep- 

 tional for Koeleria. The latter has been found in pure communities covering 

 several square miles only in the Dakotas, where this condition was also found 

 by Griffiths (Williams, 1898 : 22). Koeleria is sometimes dominant in 

 meadows and swales, but it is usually associated with Stipa or Agropyrum. 

 While it possesses the most extensive range of any of the 5 dominants, it is 

 generally the least important locally, its abundance rarely being more than 

 30 per cent and often as low as 10 per cent. Stipa spartea and S. comata are 

 complementary species which overlap as dominants in northeastern Nebraska 

 and the central Dakotas. The ecotone between them runs in a general way 

 along the ninety-ninth meridian, though either occurs locally beyond this line. 

 In Kansas, Stipa spartea ranges over the eastern half of the State, while S. 

 comata is reported for only five counties in the extreme west. It is probable, 

 however, that the consociations are in contact with each other in the central 

 portion. Both occur as pure communities over large areas in their respective 

 regions, but they are generally associated with Andropogon scoparius or 

 Agropyrum glaucum. Stipa spartea is the most typical dominant of the true 

 prairies, while S. comata belongs primarily to the mixed prairies. 



Andropogon scoparius is the normal associate of Stipa spartea and Koeleria 

 eristata, giving the grass tone to the prairies during late summer and autumn, 

 as they do in spring and early summer. It is one of the most widespread of 

 dominants, and plays a climax or serai role in all the grassland associations 

 except that of the Pacific Coast. It shows two life-forms, appearing as a sod- 

 grass in the true and subclimax prairies, and as a bunch-grass in the sandhills 

 and "breaks" of the mixed prairies and the plains. Like Stipa comata, 

 Agropyrum glaucum is found throughout the West, but its dominance is 

 local or subcUmax in nature outside of the prairie association. It exerts a 

 greater control on the habitat than any of its associates, owing largely to its 

 many and vigorous rootstocks. It is purest on the gumbo plains and rolling 



