THE GRASSLAND CLIMAX. 



119 



Floristic relations. — The floristic relationship of the associations is evident. 

 Of the five great dominant genera, Stipa, Agropyrum, Bouteloua, Aristida, 

 and Koeleria, all occur throughout the formation, though Bouteloua is rare in 

 the Coast region and Stipa in the southeast. Each of these is represented by 

 a species of peculiarly wide range, namely, Stipa comata, Agropyrum glaucum, 

 Bouteloua gracilis, Aristida purpurea, and Koeleria cristata, all of which occur 

 from Saskatchewan to Texas, California, and British Columbia or Alberta. 

 With the exception of Koeleria, which is monotypic as well as the least impor- 

 tant of the five, each genus has one or more corresponding species in different 

 portions of the area. Thus Stipa comata as a dominant is largely replaced in 



Fig. 4. — Map showing the percentage of annual precipitation between April 1 and 

 September 30. After the U. S. Weather Bureau. 



the Missouri Valley by S. spartea and in California by S. setigera and S. 

 eminens. Agropyrum glaucum yields almost wholly to A. spicatum in Idaho, 

 Oregon, and Washington. In southern Texas, New Mexico and Arizona, and 

 in Mexico, Bouteloua gracilis gives way largely to B. eriopoda, B. hirsuta, 

 B. rothrockii, and B. bromoides, while Aristida purpurea is represented for the 

 most part by A. divaricata, A. calif ornica, and A. arizonica. Of the ten most 

 important subclimax dominants, such as Andropogon scoparius, Bouteloua 

 racemosa, Sporobolus airoides, Stipa viridula, etc., all but one or two are found 

 from Canada to Texas and California. 



A similar relationship is shown by the climax subdominants which con- 

 stitute the societies of the formation, though this is naturally somewhat less 

 close so far as species are concerned. The most important societies are con- 

 stituted by about 35 genera, of which practically all range throughout the 

 formation, though they are naturally little in evidence in the California grass- 

 land to-day, owing to the intensive cultivation and the almost universal 

 invasion of ruderal grasses. The number of such societies is 45, represented 

 by as many species. Most of these are found in each of the associations, 



