12 A Study of the Vegetation of 



drained by the Palouse River and in the extreme northeast by 

 tributaries of Spokane River (Fig. i). 



GENERAL PLANT-LIFE CONDITIONS 



Hemmed in on all sides by mountains the region under consid- 

 eration has a climate much warmer and much drier than one 

 would expect at this altitude and latitude. The Okanogan High- 

 lands on the north shelter it from cold winter winds, while on the 

 east and south it is protected by the Bitterroot Mountain system 

 and the Blue Mountains, respectively. The Cascades on the 

 west intercept the rain-bearing westerly winds from the Pacific 

 Ocean. Even where the prevailing southwest wind cutting 

 through the mountain gap of the Columbia River and rising over 

 the high plateau drops much of its moisture near the high eastern 

 border, the annual precipitation is only 21.6 inches. Westward 

 it decreases steadily with decreased elevation, giving an annual 

 precipitation of only 10-13 inches near the western boundary of 

 the region. 



In this semi-arid region where evaporation rates are very high, 

 a knowledge of the distribution of rainfall and humidity is very 

 important, for it is well known that scanty rainfall throughout the 

 year, or relative dryness of the air and soil during the growing 

 season, favors a sparse vegetation and the development of xero- 

 phytic forms. Since vegetation is not only an expression of 

 present conditions, but to a greater extent a record of conditions 

 that have obtained during a period of years, and since the record 

 is not likely to be altered by a year or two in which conditions 

 may depart from the normal, a study of the precipitation in Fig. 

 2 is instructive. This gives the mean monthly precipitation at 

 Pullman, covering a period of twenty-four years, and is very 

 representative of conditions in the high prairies. It may be seen 

 at a glance that over two thirds of the precipitation occurs dur- 

 ing the non-growing season, and that the light showers of July 

 and August seldom have much influence upon the water content 

 of the soil. 



In order that the reader may realize the striking difference be- 



12 



