74 A Study of the Vegetation of 



upper Palouse Valley, but the best example is to be found in the 

 woodland growth of that high upfold of basalt in southeastern 

 Washington known as the Blue Mountains. Under the present 

 climatic conditions, the prairies appear to be a subclimax forma- 

 tion. 



The higher elevation of these forest-clad foothills and moun- 

 tains is sufficient to considerably increase the precipitation. The 

 prevailing southwest winds sweeping across the many miles of 

 open prairie and desert of northern Oregon and south central 

 Washington, but gradually ascending, here lose much of their mois- 

 ture. It is not unusual to observe on a morning in autumn or 

 late spring the surrounding mountains covered with snow while 

 no moisture has fallen upon the lower hills. Sometimes even iso- 

 lated buttes only 700-1,000 feet higher than the lava plateau are 

 snow clad and stand out in bold relief in contrast to the surround- 

 ing country. 



A rain gauge of the government pattern placed with Mr. J. B. 

 Kissinger 4 miles northeast of Viola, Idaho, showed, during the 

 years 1912-1914, an annual increase over the rainfall at Pullman 

 during the same period of 3 and 4.5 inches respectively. A simi- 

 lar gauge placed with Mr. Stratton on Cedar Mountain and at 

 an elevation of about 4,000 feet gave 4 inches greater precipita- 

 tion during the months of July and August, 1914, than was re- 

 corded at the base station. Not infrequently, I have witnessed 

 rather heavy summer showers in the mountains while no rain fell 

 upon the plateau 1,500 feet below. 



Not only does the change in altitude affect the amount of pre- 

 cipitation, but it also shortens the growing season. Snow lies on 

 the ground until late in May or indeed on the more protected 

 slopes even until June. Thus, the season of drought is consider- 

 ably shortened. Likewise, the cool nights lessen the evaporating 

 power of the air. 



The forests of the region make up a part of the Pacific Coast 

 forest formation. This formation is represented by the Thuja- 

 Tsuga association. A single consociation, dominated by Thuja 

 plicata, forms the climax community. Tsuga heterophylla, an 



74 



