I. THE PRAIRIE-PLAINS GRASSLAND FORMATION. 



Extending from the Missouri River to the foothills of the Rocky 

 Mountains is a vast grassland formation. Pronounced climatic varia- 

 tions, especially in rainfall and evaporation, have clearly differentiated 

 the vegetation of the eastern portion, the prairies, from that of the 

 western, the plains. The former are dominated by a luxuriant growth 

 of many species of tall-grasses and numerous societies of tall-growing 

 herbs. The latter bear a sparser growth of shorter grasses and fewer 

 herbaceous societies. Thus the two extremes are cleaily defined. 

 But where short-grasses meet tall-grasses on more or less equal terms 

 over a considerable area, a third or mixed type of vegetation is clearly 

 evident (plates 1 to 3). Having repeatedly traversed this grassland 

 and having made careful vegetational studies at many stations through- 

 out the past five years, the writer is in hearty agreement with the 

 classification of Clements (1920), in recognizing in this area three plant 

 associations. For the area concerned these are: (1) the true prairie; 

 (2) the short-grass plains ; and (3) the mixed prairie. 



THE TRUE PRAIRIE. 



The true prairie (Stipa-Kceleria association), as determined by 

 Clements, is as follows: 



" It occupies a distinct belt between the subclimax prairie on the east and the 

 mixed prairie on the west, reaching from Manitoba to Oklahoma. The eastern 

 edge runs southward from Manitoba along the western boundary of Minnesota 

 and then swings southeastward with the Minnesota Valley, reaching its limits 

 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 Okla- 

 homa line. The western boundary begins in Manitoba between the one 

 hundredth and one hundred and first meridian and continues more or less due 

 south to near the Nebraska line, where it turns southeastward 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. It reaches its 

 greatest breadth of 7° of longitude along the forty-third parallel, and tapers 

 more or less irregularly in both directions to the width of 1 to 2 degrees in 

 Manitoba and Kansas." 



While the true prairie is a climatic association and, undisturbed, 

 would probably not be invaded by shrubs or trees, or at most only to a 

 slight degree, the subclimax prairies, with their greater rainfall, denser 

 sod, and more mesophytic and taller dominants, will be replaced by 

 scrub or woodland wherever fire, cultivation, or grazing does not pre- 

 vent (Weaver, 1919; Clements, 1920). 



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