288 JOHN ERNST WEAVER 



We shall next consider the plant associations and evaporation 

 on the buttes and mountains. The factors causing evaporation — 

 humidity, temperature, and wind — are all so much greater in 

 degree upon the exposed south and southwest sides of the buttes 

 that they have exerted a marked effect upon plant distribution. 

 The conditions on Kamiak Mountain, a bold butte lying nine 

 miles north of Pullman, and one of the series extending in a 

 westerly direction from Thatuna Hills, illustrates this condition 

 very well. This butte, running from east to west for a distance 

 of over two miles and reaching an elevation of 300 meters above 

 the surrounding hilltops, is covered on the south side with the 

 prairie formation, while the steeper north slope bears a dense 

 forest of Douglas fir {Pseudotsuga mucronata) and Western larch 

 {Larix occidentalis) . On the south side trees occur only near 

 the summit, where outcropping rocks furnish in their crevices 

 a sufficient shelter and water supply for the establishment of the 

 seedlings. Here is a very open growth of yellow pine {Pinus 

 ponderosa) which flanks the fir and tamarack on both ends of 

 the butte and occupies the less sheltered north base below the 

 fir-tamarack zone. 



Eight miles east, near Viola, Idaho, is another butte slightly 

 lower than Kamiak, but with plant associations very similarly 

 distributed. Here again the south side is prairie, which the 

 pines, occupying the crest and northwestern slope, are gradually 

 invading. The sheltered northeast sides are clothed with a 

 forest of Douglas fir and tamarack. These conditions are rep- 

 resentative of numerous other situations. Only as the moun- 

 tains become higher and rainfall increases, does the prairie give 

 way to a forest growth, but in all places areas of prairie occur 

 near the summits on the exposed slopes. 



The butte prairies at Kamiak and Viola are not very unlike- 

 those upon the basalt as far as kinds of species and their dis- 

 tribution are. concerned; but, owing in part to the more open 

 soil and consequent smaller water-holding capacity, combined 

 with greater exposure to the wind, the vegetation bears a more 

 xerophytic stamp and there are fewer plants per given area and, 

 consequently, more soil surface is exposed. 



