THE BUNCH-GRASS PRAIRIE. 151 



the association, but nowhere in it have they been found in pure communities. 

 At the present, they are more characteristic of the Agropyrum-Festuca com- 

 munity. 



The role of Agropyrum glaucum in the bunch-grass association is still in 

 question. As a rule, it is subclimax in lowlands and especially in moist saline 

 areas. In northern California and in Oregon, it often meets Stipa setigera 

 or Agropyrum spicatum on what appear to be more or less equal terms. In 

 the Hampton Valley in Central Oregon, the removal of the sagebrush results 

 in the establishment of an A. glaucum sod instead of the usual bunch-grass 

 community. In fact, repeated observations in Oregon and Idaho during the 

 past summer indicate that Agropyrum spicatum frequently loses its bunch 

 habit under certain conditions, and comes to be almost indistinguishable from 

 forms of A. glaucum. Through the same region, Elymus condensatus is a 

 frequent associate of the bunch-grass. It reaches its best development in 

 saline lowlands, however, and must be regarded normally as a subclimax 

 dominant. Eriocoma cuspidata and Stipa speciosa likewise occur now and 

 then with the dominants when the soil is looser or sandy, but they are clearly 

 subclimax consocies of the xe rose re. 



Factor relations and sequence.— The presence of a prairie of tall-grasses in a 

 region with 10 to 12 inches of precipitation annually is due to several facts. 

 Perhaps the most important is the bunch habit, which enables each plant to 

 draw upon a relatively large area of soil for its water supply. The second is 

 that 60 to 90 per cent of precipitation comes during winter, with the result 

 that penetration and conservation of the water are at a maximum. As a 

 consequence, the root systems are mostly deep-seated, and their efficiency is 

 high. Along the coast of southern California, moreover, the low precipitation 

 is offset by the high humidity and reduced evaporation to the extent that 

 Stipa setigera and S. eminens reach a high development here. The best 

 expression of bunch-grass prairies to-day occurs in that part of the Palouse 

 with 15 to 25 inches rainfall (plate 31). 



Weaver (1917) has made a careful study of the physical conditions of the 

 Agropyrum and Festuca consociations in this region, as well as of the root- 

 systems of the dominants and subdominants. From June to September, at 

 Colfax, the evaporation in the former averaged 8 to 10 c. c. higher than in the 

 latter, while the water-content at 10 inches was 5 to 10 per cent lower. Since 

 the differences between northeast and southwest slopes of the Festuca con- 

 sociation were 9 to 10 c. c. and 5 to 12 per cent, respectively, it is evident why 

 the two consociations are frequently mixed. As would be expected from the 

 behavior in other associations, Koeleria stands close to Festuca in its water 

 requirements, while Poa is somewhat more xerophytic than Agropyrum. 

 These relations are confirmed by the successional sequence (Weaver, 1917: 68). 

 Of the Stipas, Stipa comata is the most mesophytic, followed closely by S. 

 setigera and this by S. eminens. Elymus sitanion is more xerophytic than 

 S. setigera and probably slightly more so than S. eminens. 



SOCIETIES. 



The bunch-grass prairies contain three groups of subdominants: (1) those 

 derived from the mixed prairie; (2) those characteristic of the Washington- 

 Idaho center; and (3) those found in central and southern California. The 



