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 serai 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 



