Southeastern Washington and Adjacent Idaho. 73 



The Sarcobatus Consocies 



Communities of the greasewood, Sarcobatus vermiculaius, 

 occur in alkaline soils of the flood plains or depressions in the 

 scab-lands. This scrubby growth, with its pale branches and 

 fleshy leaves, stands out in distinct contrast to surrounding vege- 

 tation. The consocies is represented only locally and careful 

 study of it was not made. 



THE PACIFIC COAST FOREST FORMATION 

 The coniferous woodland occurs in the eastern part of the re- 

 gion and chiefly in Idaho. While the forests in the main grow 

 on the soils produced by the weathering of old crystalline rocks, 

 it is not uncommon in certain parts of the region to find under- 

 lying rocks of basalt. Likewise, the sheltered slopes of isolated 

 quartzite buttes may be clothed with trees, or woodland may ex- 

 tend as tongues along the deeper canyons far into the prairie. 



There exists a close relation between the development of the 

 physiography of a region and the life history of its forest com- 

 munities. This is nowhere better shown than in semi-arid cli- 

 mates where a slight change in elevation may so modify the pre- 

 cipitation and other life conditions as to turn the balance slightly 

 or wholly in favor of a tree form of vegetation. 



A study of the chemical composition of the soils of basaltic 

 origin as compared with those originating from quartzite, granite, 

 and other crystallic rocks, shows that they are not greatly dissimi- 

 lar (Table III). I am convinced that it is not the chemical com- 

 position of the soils of this region but rather their physical char- 

 acters that determine the type of vegetation which they support. 

 Of course the controlling factor of the physical composition 

 is, in this region of dry summers, their ability to hold water, pro- 

 vided always that sufficient humus is present to furnish the neces- 

 sary nitrates. Not only do these forest soils have a higher water 

 holding capacity than those of the basaltic prairies, as has already 

 been shown, but this property increases, due to increased humus 

 content, proportionally as the succession advances toward the 

 climax stage. That coniferous forests will grow upon the basaltic 

 soils is evidenced by their presence over considerable areas in the 



73 



