Southeastern Washington and Adjacent Idaho. 15 



out the region, the vegetational distribution is markedly influ- 

 enced by the edaphic and topographic conditions. The influence 

 of rainfall upon the distributional and seasonal activities of plants 

 is obviously exerted chiefly through its power to replenish soil 

 moisture. While rainfall is only indirect in its relation to plants, 

 soil moisture is direct. The retention of moisture by soil de- 

 pends not only upon the kind of soil, but also upon the slope and 

 exposure. These regional differences in precipitation, together 

 with topography and soil depth, delimit in a more or less definite 

 manner the plant communities. 



It may make the situation somewhat clearer if, before taking up 

 a detailed discussion of the structure and development of each 

 vegetational unit, a brief survey of the chief plant groups in the 

 succession is given. 



GENERAL VEGETATIONAL DISTRIBUTION 

 The Festuca consociation of the Agropyron-Festuca associa- 

 tion occupies the extreme eastern rim of the basaltic plateau, in- 

 cluding the lower buttes and the exposed southwest slopes of 

 some of the higher buttes and mountains (Figs. 21 and 22) . This 

 community extends from the yellow pine consocies of the Spo- 

 kane gravels southward beyond Snake River. It occupies the 

 part of the plateau having the deepest soils and the highest rain- 

 fall. Eastward it reaches to Tekoa Mountains, Thatuna Hills, 

 and other outlying spurs of the Bitterroot system. The breadth 

 of this belt is variable, depending upon the precipitation and soil 

 moisture. Perhaps at no point does it exceed 40 miles. West- 

 ward and southward it merges into the Agropyron consociation 

 (Figs. 19 and 20). 



In the Festuca consociation, besides the blue bunch-grass (Fes- 

 tuca ovina ingrata) and Balsamorhiza sagittata, Agropyron spica- 

 tum (including variety inerme) is the most important component 

 of the plant cover. In marked contrast to its bunch habit in the 

 consociation bearing its name, in the more mesophytic Festuca 

 community it is often a sod former. Other important grasses are 

 Koeleria cristata and Poa sandbergii, while numerous composites, 

 legumes, and other herbaceous species are abundant. Many of 



15 



