CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 295 



inants are derived from both these communities, it is generally 

 agreed that there is sufficient unity and distinctness to justify as- 

 sociational rank. Important dominants throughout the area are 

 Bouteloua gracilis, B. hirsuta, Andropogon scoparius, and, except 

 in the north, Bulbilis dacty hides. In the north, Koeleria cristata, 

 Stipa spartea, and 5. comata are added dominants, which suggest 

 the recognition of a northern faciation. Other important species 

 included among the dominants are Andropogon furcatus, Sporo- 

 bolus cryptandrus, and several species of Stipa. 





FlG. 158. Short grass plains pastured to sheep in Wyoming.— Photo by W. 

 D. Billings. 



The western limit of the association may be taken as the line 

 where tall grasses disappear and beyond which only short grasses 

 are dominant. Since the tall grasses require available soil moisture 

 to a depth of twenty-four or more inches during their active 

 growing season, the limit of mixed grass prairie is a line beyond 

 which precipitation is insufficient to provide moisture to this 

 depth. The eastern limit is not as sharply defined but is also de- 

 termined by soil moisture, since mixed prairie is marked by prairie 

 grasses in bunch-grass habit sharing dominance with permanently 

 established short grasses. 3 Thus the area forms a strip from Sas- 

 katchewan through the central Dakotas, Nebraska, Kansas, and 



