THREATENED ARIDITY ON THE PACIFIC SLOPE 173 



AVith the exception of the limber pine, most of these species 

 are, in these regions, confined to the Cascades, and do not extend 

 very far from the high, upper slopes of the range. Only the 

 sugar pine and incense cedar come into contact with regions of 

 subhumidity. 



I have already noted that strong and broad projections from 

 the subhumid areas push far into the humid belts. This is 

 most marked along deeply eroded valleys, where high summer 

 temperatures prevail. Not only do the subhumid conditions 

 become conspicuous in the humid belts along such lines, but one 

 meets occasionally spots of true aridity in their midst. Such 

 localities present the anomalous spectacle of permanently de- 

 forested areas supporting species of grasses and other herbaceous 

 plants peculiar to the arid plains at subalpine altitudes, and in 

 regions where the situation seemingly should insure an abun- 

 dance of precipitation. Excellent examples of these phases are 

 seen in the Bitter Root forest reserve in Idaho. This region lies 

 within an area of sufficiently heavy precipitation to be generally 

 classed as humid above the 5,000-feet level ; but arid and sub- 

 humid conditions have extended up the Clearwater and Salmon 

 river valleys, in places reaching the main range, and overleap- 

 ing this barrier have joined the arid regions of the interior Rocky 

 mountain basins. The subhumid and semi-arid conditions have 

 spread upward from the valley bottoms along the mountain 

 slopes to elevations of 6.000 feet to 7,000 feet, carrying their pe- 

 culiar shrubs and trees with them. We cannot account for the 

 permanency of these arid and semi-arid extensions, except by 

 adopting the proposition that a progressive diminution of the 

 annual precipitation is now an established and general climatic 

 feature in this region. 



Coexistent with the advance of drier climatic conditions into 

 the humid areas, we find, as already noted, many of the trees 

 and shrubs of the semi-arid and subhumid tracts, while the 

 entire forest has been profoundly disturbed in its equilibrium. 

 Among the shrubs of the arid and semi-arid regions which have 

 thus penetrated into the humid areas may be mentioned Cerco- 

 carpus ledifolia-?, Kunzia tridentata, Artemisia tridentata, Artemisia 

 arbwcula, and one or two species of Forsellesia. These shrubs 

 abound on the arid regions of the plains, to which they properly 

 belong. They are found following the subhumid areas into the 

 humid ones in the Bitter Roots, in the Rocky mountains, and in 

 the Cascades. Their presence and distribution here proves an- 



