132 CUMAX FORMATIONS OF WESTERN NORTH AMERICA. 



fire, or grazing does not prevent. Here again much of the broad transition 

 between the two prairies would probably develop into forest where disturbing 

 processes are not too great, but the Stipa-Koeleria prairie is a climax associa- 

 tion through practically its entire area. In a few especially favorable locations 

 and during the wet phase of the climatic cycle, forest may encroach upon it, 

 but not to an important degree. -Finally, the societies of the subclimax 

 prairie differ from those of the climax in containing more eastern species and 

 fewer western. The majority of the societies, however, are the same for both, 

 and this is likewise true of their luxuriance and complexity (plate 23) . 



Range. ^The Andropogon associes has never been clearly recognized 

 before, and in consequence it has received little direct attention. The few 

 studies have been local ones dealing chiefly with succession in dunes or swamps, 

 and have consequently emphasized the serai stages more than the climax. 

 The region lies east of the Missouri River for the most part and has been 

 visited but little in the course of the special survey of the past six years. As 

 a consequence, its outlines can be traced only in the most general manner. 

 The area includes southeastern Nebraska, eastern Kansas, northern Missouri, 

 eastern Iowa, small areas in southeastern Minnesota and southern Wis- 

 consin, and more considerable areas in Illinois and Indiana. In addition, it 

 runs into Oklahoma and Texas, but little is known of the extent covered. As 

 successional fragments, it is found also in Arkansas and Mississippi, but these 

 are wholly extra-regional. Similar extensions occur throughout the valleys 

 of the prairies and well into the plains, but here they are subclimax to the less 

 mesophytic grass associations. A remarkable development of this sort occurs 

 in the great sandhill region of Nebraska, where Andropogon is again the domi- 

 nant genus. Here the important dominants are bunch-grass, as demanded 

 by the more rigorous water conditions, and the climax is the Stipa-Bouteloua 

 prairie. 



The western limit of the subclimax prairie as known at present is fairly well 

 indicated by the isohyete of 30 inches, as it runs through Minnesota, Iowa, 

 Nebraska, and Kansas, and northern Oklahoma. It is impossible to draw the 

 limits in the east, north, or south, not merely because of lack of knowledge, 

 but also because its occurrence is more and more local in character and suc- 

 cessional in nature the farther east one goes. 



CONSOCIATIONS. 



Andropogon furcattjs. Andropogon saccharoides. Panicum virgatum. 

 Andropogon nutans. Boutelotja racemosa. Spartina cynosuroides. 



Andropogon scoparius. Elymus canadensis. 



The Andropogons are by all odds the most important dominants of his 

 association. They give it the distinctive impress everywhere except in 

 transition areas. Because of its characteristic alternation as a subclimax 

 with forest on the one hand and true prairie on the other, it often contains 

 subdominants from the former and dominants from the latter. In the low 

 prairies and meadows of Nebraska, Iowa, and Minnesota, practically any of 

 the above may be found intimately mixed with Stipa spartea, Agropyrum 

 glavcum or Koeleria cristata (Pound and Clements, 1899, 1900 : 345; Thorn- 

 ber, 1901 : 66, 86; Weaver and Thiel, 1917 : 11). As a consequence, the 



