THE BUNCH-GRASS PRAIRIE. 149 



Serotinal Societies: Serotinal societies— continued. Clans— continued 



Gutierrezia sarothrae. Artemisia gnaphalodes. Lesquerella fendleri. 



Gnndena squarrosa. Carduus undulatus. Lotus mollis 



Isocoma hartwegii. Evolvulus argenteus. 



Kuhma rosmanmfolia. Clans: Desmanthus jamesii. 



Vernonia baldwinu. Argemone platyceras. Hofmanseggia stricta. 



Liatns punctata. Aster ericoides. 



THE BUNCH-GRASS PRAIRIE. 



AGROPYRUM-STIPA ASSOCIATION. 

 Nature.— The grasslands of the Northwest and of the Pacific coast differ 

 from those already described in the characteristic bunch-grass habit of the 

 dominants and in their relation to winter precipitation. The first visit to 

 them in 1914 led to the suggestion that they were essentially prairies, resem- 

 bling in many respects the climax prairies of the Missouri Valley. The 

 difference in habit appears greater than it really is, since the prairies of the 

 great sandhill region of Nebraska are characterized by bunch-grasses also. 

 This association consists of tall-grasses, which are species of Agropyrum and 

 Stipa, as in the eastern prairies. Three of the dominants of the latter, Stipa 

 comata, Agropyrum glaucum, and Koeleria cristata, occur throughout the 

 bunch-grass prairies, though the latter is the only one of much importance. 

 Aristida purpurea is likewise important, especially in California, while Stipa 

 viridula, Elymus sitanion, and Eriocoma cuspidata all play a part as subclimax 

 dominants. Bouteloua gracilis and Bulbilis are the only ones of the great 

 dominants of the formation that are rare or lacking. The closer relationship 

 with the prairies shown by the dominants is explained by the fact of a fairly 

 continuous connection on the north, while the bunch-grass prairies are 

 separated from the plains by the wide stretch of the Colorado and Mohave 

 deserts. This fact is further reflected in the societies and clans. In the 

 Agropyrum consociation, the genera and many of the species of subdominants 

 are identical with those of the mixed prairie. These generally change south- 

 ward, and in southern California many of the genera and practically all of the 

 species which form societies are different. However, it is difficult to draw 

 exact comparisons here, since the relict areas of Stipa are too small to permit 

 the original structure to reach full expression. 



Range. — The bunch-grass prairies find their best expression to-day in the 

 Palouse region of southeastern Washington and adjacent Idaho. Typical 

 areas also occur in northern and eastern Oregon, but these are only fragments 

 of what were once extensive stretches. Cultivation, grazing, and fire have 

 combined to destroy bunch-grass or to handicap it in competition with the 

 invading sagebrush. In the form of outposts, this association is found east- 

 ward in Montana to Helena and Livingston, in western Wyoming from Yellow- 

 stone Park to the Green River region and southward through northwestern 

 Colorado and northeastern and northern Utah. Over most of this region, it 

 occurs on dry rocky hillsides surrounded by sagebrush, indicating that it 

 formerly covered much larger areas. This is confirmed by the fact that 

 burning or clearing the sagebrush from an area permits the development of 

 typical bunch-grass prairie (plate 30). 



The southern part of the association is much more fragmentary, so much so 

 in fact that it has had to be reconstructed from widely scattered relicts. The 



