1 6 A Study of the Vegetation of 



these do not occur in the Agropyron community, while indeed 

 several species are confined to the moist north hillsides occupied 

 by this consociation. 



At lower altitudes westward and southward the well developed 

 prairie gradually merges into the Agropyron consociation. Like- 

 wise, on the rim-rock along the canyons of streams as well as on 

 dry mountain slopes this community is well represented. It is 

 characterized by the definite bunch habit of its chief species Agro- 

 pyron spicatum. These bunches are often more than 8 inches 

 in diameter, and may reach a height of over 3 feet. In addition 

 to the absence of numerous characteristically high-prairie species, 

 and the more open nature of the plant cover, the Agropyron con- 

 sociation is further characterized by the coming in of certain 

 plants of a more xerophytic stamp. Most conspicuous among 

 these are the rabbit brush, Chrysothamnus nauseosus (including 

 var. graveolens) , C. viscidiflorus, and several species of Erio- 

 gonum. The bunch-grass not only occupies the deeper soils of 

 the drier region, but is represented in the scab-lands and on the 

 rim-rock where the soils, only a few inche? deep, overlie deeply 

 fissured basalt. 



On the unbroken rock, covered with only an inch or two of soil, 

 Agropyron gives way to the Poa-Polygonum associes, a still earlier 

 stage in development (Fig. 18). This latter associes is well rep- 

 resented on the rim-rock throughout the region, as well as over 

 large areas of the scab-lands westward. The June grass, Poa 

 sandbergii, Polygonum majus, and Plantago purshii, are the most 

 important species. The knotgrass and plantain play the role of 

 grasses ecologically. This associes is preceded in the succession 

 by communities of mosses and lichens (Fig. 17). 



In the driest part of the region, in the scab-land, where the 

 basalt is almost free from soil, occurs the extreme eastward ex- 

 tension of the desert scrub formation. It is represented by the 

 Artemisia-Atriplex association of which Artemisia, rigida, the 

 scab-land sage of the Artemisia consociation, is the characteristic 

 species (Fig. 16). In the deeper soils impregnated with alkali 

 the greasewood consocies, dominated by Sarcobatus vermicu- 

 latus, occurs. 



16 



