GRASSLAND COMMUNITIES 249 



changing aspect in addition to that which is due to the prairie 

 grasses themselves. 



From the western edge of the true prairie to the fcx^tiiills of the 

 Rocky ]\Iountains is the great expanse of grasshmd that has com- 

 monly been called the short-grass plains. Studies during the past 

 quarter of a century or so have shown rather conclusively that the 

 climax over this vast area is a mixed prairie in which short grasses 

 and medium grasses are about equally important. The familiar 

 short-grass plains, in which the medium grasses are nearly or entirely 

 absent, are due to overgrazing, formerly, in some places, by buffalo 

 and in more recent times by domestic cattle. The short-grass plains 

 are often a continuous and unbroken sod of grama grass (Boideloua) 

 and buffalo grass (Buchloe), sometimes with the addition of the low 

 sedge iCarex fiUfolia), but in the climax mixed prairie there are in 

 addition to these several other short grasses, such medium grasses 

 as Stipa comata, Syoroholus cryptandrus, Agropyron smithii,Koeperia 

 cristata, and several others. The societies in the mixed prairie are 

 similar to those in the true prairie but, because of the more xeric 

 climate, the number of species concerned is much smaller. 



In the southern part of the great central grassland, near the Gulf 

 of Mexico, is the gulf coast prairie which resembles the mixed 

 prairie in having both short grasses and medium grasses as dominants 

 but differs from it in having a larger number of dominants, some of 

 which, notably Stipa leucotricha, are not found outside of this 

 community. Here, again, the societies are similar to those of the 

 true prairie but many of the species concerned are of southern origin 

 and are not found farther north. 



The desert grassland extends from southwestern Texas through 

 southern New ]Mexico and Arizona, mostly between the 3000 and 

 4500 foot contours. Its dominants are almost entirely short 

 grasses, species of grama grass (Bouteloua), wire grass (Aristida), 

 and mesquite grass (Hilaria). For this reason it very closely 

 resembles the short-grass plains except that it is usually dotted 

 with such shrubs as creosote bush (Larrea), mesquite (Prosopis), 

 catclaw (Acacia), Yucca, cholla (Opimtia), bisnaga (Ferocactm), 

 desert hackberry (Celtis), Morman tea {Ephedra), etc. These 

 shrubs are often increased by overgrazing, giving the community 

 the appearance of a savana. Societies of flowering herbs are in evi- 

 dence both after the summer rains and after the winter rains. They 

 are especially attractive after the January and February rains, mak- 



