270 



THE NORTH AMERICAN GRASSLAND 



homa to Illinois approaching 40 inches. This drops to about 23 inches 

 in southern Manitoba, while along the western limit the precipitation 

 decreases from 30 inches in Oklahoma to 25 in Nebraska and about 20 

 in northeastern North Dakota. By virtue of lesser extent from south 

 to north, evaporation also exhibits a smaller range, and this is likewise 

 true to some extent of temperature. Drought periods are correspond- 







'M 



Fig. 62. — True prairie in eastern Iowa; iStipa spartca, Sporobolus hcterolcpis, 



and Bouteloua curtipendula chief dominants, with Koeleria, Andropogon sco- 



pariiis, and Sporobolus asper also present. (Photo by Edith Clements.) 



ingly less frequent and severe, and hence rarely affect the density and 

 survival of the dominant grasses and subdominant forbs (cf. Weaver, 

 Stoddart, and Noll, 1935; Weaver and Albertson, 1936; Savage and 

 Jacobson, 1935). 



Dominants. The following dominants constitute the true prairie: 



Stipa spartca 

 Sporobolus asper 

 heterolepis 

 Andropogon scoparius 

 Bouteloua curtipendula 



Koeleria cristata 

 Agropyrum sniithi 

 Muhlenbcrgia cuspidata 

 Panicum scribnerianum 

 Carex pennsylvanica 



