256 THE NORTH AMERICAN GRASSLAND 



Savage and Jacobson, 1935; Weaver and Albertson, 1937; Weaver, 

 Stoddart and Noll, 1935). 



The xeric limit of the grassland confronts the desert in the south- 

 west along the line of 5-6 inches of rainfall, and the sagebrush to the 

 northwestward with an effective rainfall a few inches higher. The as- 

 sumption that grassland is restricted to regions with a spring-summer 

 precipitation of about three-fourths the total, though still more or less 

 prevalent, is no longer tenable in view of the fact that the Pacific and 

 Palouse prairies obtain most of their moisture during the winter. 

 Where freezing temperatures do not obtain, as in southern California, 

 the native bunch grasses may start growth in any month from Sep- 

 tember to February and are not infrequently in bloom in December. 



STRUCTURE AND UNITY (Fig. 54) 



The grassland under consideration is divisible into six types or 

 associations, namely (1) the mixed prairie, occupying a large central 

 area lying east of the Rocky mountains and in the main west of the 

 100th meridian; (2) the true prairie, which lies east of mixed prairie 

 and in contact with forest; (3) the gulf coastal prairie, lying near 

 the Gulf of Mexico; (4) the desert plains, mainly in southern Arizona, 

 New Mexico, and Mexico; (5) the California or Pacific prairie; and 

 (6) the Palouse prairie in the northwestern states. These associations 

 are distinguished by the prevailing dominant grasses and influent 

 animals, as well as by secondary differences in appearance. They are 

 bound together by certain major features of physiognomy and by 

 plants and animals in common, which may be called binding species. 



Binding Dominants of the Prairie.^ Binding dominants are species 

 of perennial grasses that occur as climax species in three or more as- 

 sociations of the grassland biome and are all found in the ancestral 

 mixed prairie. They are as follows, the sequence indicating a gen- 

 erally decreasing Wideness of range. ^ 



Sporobolus cryplandrus Boutcloua hirsuta 



Koeloia cristata Elymus sitanion 



Stipa comala Poa scabrella 



Siipa viridula Festuca ovina 



Agropynim smithi Andropogon scoparhis 



Boutcloua gracilis Buchloe dactyloidcs 

 Bouleloua curlipendula 



1 The names employed are those of long-accepted species of definite eco- 

 logical significance rather than the more recent minor species, or better, sub- 

 species (Clements and Clements, 1913; Hall and Clements, 1923). 



