12 A Study of the Vegetation of 
drained by the Palouse River and in the extreme northeast by 
tributaries of Spokane River (Fig. 1). 
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 
