Southeastern Washington and Adjacent Idaho. 1 1 1 



form in itself an intensely interesting and very practical field of 

 investigation. 



CONCLUSION 



In the foregoing pages it has been shown that in this region 

 lying in southeastern Washington and adjacent Idaho, three plant 

 formations occur. In the western part of the region, that of least 

 elevation and consequently of lowest precipitation, the desert 

 scrub formation occurs. As the altitude becomes greater and the 

 climatic conditions more favorable eastward, the desert scrub 

 gives way to the grass land of the prairie-plains formation. In 

 its early development this grass land is of a very open bunch-grass 

 type, with much of the ground being either destitute of vegeta- 

 tion or carpeted with interstitial plants only in early spring. As 

 conditions become less xerophytic at higher altitudes the ground 

 becomes rather completely occupied by grasses, some of which 

 are sod-formers, and by non-grassy herbaceous vegetation which 

 shows a great increase both in the number of species and in the 

 number of individuals per unit area. The best developed prairies 

 reveal a potential shrub community which, while held in check 

 usually by competition with prairie plants, develops in the eastern 

 edge of the grass lands and .especially upon the non-basaltic soils 

 into thickets of chaparral. Under cover of the chaparral, the 

 yellow pine, a pioneer community in the development of the 

 Pacific Coast forests, often becomes established. The pine may 

 be replaced by Douglas fir and tamarack, and these in turn by 

 mixed communities dominated by tamarack and white fir, or by 

 white fir and cedar. Finally, in the most mesophytic areas the 

 cedar community reveals the climax stage in the development of 

 the Pacific Coast forest formation. 



It has been shown that the desert scrub formation is charac- 

 terized by soils with lower water content and higher temperature, 

 and with higher evaporating power of the air than any of the 

 other plant communities. Likewise, there has also been found to 

 exist in the several plant communities of the xerosere, from the 

 bunch-grass community of the prairies to the climax cedar com- 

 munity of the forest, a direct relation between the water content 



1 1 1 



