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THE NORTH AMERICAN GRASSLAND 



though much interrupted by mountain ranges. It no longer touches 

 the California prairie, owing to the intervening desert, but probably 

 did so in the Pleistocene. 



Climate. The mixed prairie has the widest range in latitude, alti- 

 tude, and longitude of all the grassland associations, and a correspond- 

 ing variation in rainfall and temperature. It is semi-arid to arid in 

 character, its eastern boundary shifting from about the isohyet of 27 

 inches in the south to 13 in the north, and the western falling near the 

 line of 10 inches in general. As a rule, more than half the precipita- 

 tion comes between April first and September first. The seasonal evap- 

 oration is approximately 60 inches at the south and less than 30 at the 

 north. The mean annual temperature and the length of the frost-free 

 season decrease steadily with increasing latitude and altitude, but here 

 also the temperatures of the summer period depart less widely. 

 Drought periods are a recurrent feature, with marked effects upon 

 the growth of vegetation. 



Dominants. The dominants of the mixed prairie are as follows: 



Stipa comata 



pennata 



viridula 

 Sporobohis cryptandrus 

 Agropyrum smithi 

 Hilaria jamesi 



Kocleria cristata 

 Aristida purpurea 



Elymus sitanion 

 Poa scabrella 



nevadeufiis 



arid a 

 Muhlenbergia montana 

 Eragrostis spcctabilis 



Bouteloua gracilis 

 Buchloe dactyloides 

 Carex stcnophylla 

 filijolia 



Carex pennsylvanica 

 Bouteloua hirsuta 

 Muhlenbergia torreyi 



Oryzopsis hymenoides 

 Bouteloua racemosa 

 Festuca ovina 

 Agropyrum pauciflorum 

 Stipa richardsoni 

 Elymus m,acouni 



The first column contains the mid grasses, the third the short 

 grasses and sedges, while the middle one comprises species of mid- 

 grass habit but of lower stature. The break in each list separates the 

 species of greater from those of lesser importance. Of the entire num- 

 ber, three alone are sod-formers, namely, Agropyrum smithi and Carex 

 stenophylla, by virtue of root stocks, and Buchloe dactyloides by means 

 of stolons. Bouteloua gracilis and hirsuta and Muhlenbergia torreyi 

 often form mats that approach a sod in the north, but these are com- 

 posed of tufted culms as a rule. The remaining species are all bunch 

 grasses, both habit and spacing corresponding to the xeric climate. 



A few species are abundant throughout the association, or were 

 before the opening of the historical period; such are Stipa comata, 

 Bouteloua gracilis, Sporobolus cryptandrus, Agropyrum smithi, and 

 Aristida purpurea. Some, like Hilaria jamesi, Stipa pennata, and 



