Southeastern Washington and Adjacent Idaho. 63 
of favorable modifications of the edaphic conditions such as an 
increase in amount or an increase of the water holding capacity 
of soils. 
Likewise, on its high eastern border the Festuca consociation 
contains a potential shrub community. Almost everywhere Sym- 
phoricarpos, Rosa, and Spiraea are present but are held in check 
by competing herbs and grasses. A local disturbance of the 
habitat such as the building of a fence or the burrowing of squir- 
rels modifies it favorably by increasing the water holding capacity 
of the soil and swings the balance in favor of the shrubs. Once 
established the shrubs may furnish a suitable nursery for seed- 
lings of the yellow pine and thus the prairie may give way to 
forest. These modifications of edaphic conditions foreshadow 
those which would be caused by a favorable swing of climate, 
while the reverse of these conditions would follow an unfavor- 
able change. Indeed, the balance between prairie, shrub, and 
forest is so nicely adjusted that even a few years of exceptionally 
dry or unusually wet weather would be recorded in the movements 
of the vegetation. 
The seral stages of the Agropyron-Festuca association will next 
be considered, and in this place we shall discuss those of the 
xerosere only. This will be followed by a brief treatment of the 
desert scrub formation, after which the xerosere will be traced to 
its culmination in the climax cedar forest. 
(See outline of vegetational units, page 19.) 
LICHEN-MOSS COMMUNITIES 
Where the basaltic rock is exposed along the canyon walls and 
in the scab-lands, as well as upon outcropping ledges of crystalline 
rocks on buttes and mountains, a flora of lichens and mosses 
abounds. During the dry summer months the rocks appear quite 
bare and devoid of vegetation, but upon the advent of the autumn 
rains, not only lichens and mosses, but also the June grass, which 
closely follows the latter in point of succession, takes on renewed 
growth. Then these situations, with their new verdure, become 
quite inviting. Three associes are easily distinguishable, of which 
that dominated by various crustose lichens is the pioneer. 
63 
