16 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 inches 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 
sandbergu, Polygonum majus, and Plantago purshi, 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 
