IS CLIMATIC ARIDITY IMPENDING ON THE PACIFIC 

 SLOPE? THE TESTIMONY OF THE FOREST 



By J. B. Leiberg 



The extension of explorations and observations in the region 

 of county west of the Rocky mountains tends in many ways 

 to develop and confirm the proposition that a steadily progress- 

 ive aridity is slowly replacing former more humid climatic con- 

 ditions. This change is manifest in various ways — most conspic- 

 uously in the decreasing volume of water in many of the lakes 

 and streams throughout the region, as shown by the existence 

 of former beach lines at higher levels, and in the profound dis- 

 turbances and modifications taking place in the native flora. 

 The phenomena which follow the advance of aridity are not 

 limited by altitude ; for, while the desert conditions at low ele- 

 vations exhibit them in their most intense aspect, the} 7 are also 

 clearly traceable to the highest summits, where gradually dwin- 

 dling glaciers and abnormally high extensions of certain lowland 

 types of forest show the general trend of the climatic change. 



In the general exhibition of increasing aridity there are to be 

 noted two important distinctions. One is dependent upon cli- 

 matic effects, the other upon the relief of a region as affecting 

 the drainage, and is termed soil aridity. Excellent examples 

 of the latter occur on the plains of the Columbia, where the 

 great coulees or sunken water channels, which traverse the plains 

 in all directions, are separated by comparatively narrow blocks 

 of plateau-like country. The drainage from these elevated tracts 

 is extremely rapid. As a consequence, their summits and slopes 

 are left without sufficient soil moisture during the growing season 

 to maintain a forest stand, although the annual precipitation is 

 high enough to make tree growth possible, were the drainage 

 conditions different. Similar examples occur in the forested 

 subhumid and humid regions, where any large area on which 

 temperature and precipitation are practically the same through- 

 out often shows a growth of species belonging to the drier areas 

 in the midst of the humid groups of trees, merely because the 

 angle of slope in some localities favors a more rapid drainage 

 than upon the contiguous areas. Similar effects are sometimes 



