168 THREATENED ARIDITY ON THE PACIFIC SLOPE 



which therefore furnishes an excellent indication of the limits 

 of the quasi semi-arid and lower suhhumid conditions which 

 mark the front of the semi-arid advance and the rear of the sub- 

 humid retreat. The mountain mahogany occurs, therefore, in 

 numerous localities all along the edge of this debatable ground 

 and mingles not alone with the yellow pine, but in many in- 

 stances also with the lodgepole pine, ascending to elevations of 

 7,000 feet. 



Crossing from the eastern Oregon plains to those of the Snake, 

 in southern Idaho, we find a growth of the one-seeded juniper, 

 Juniperus monosperma. Comparatively little is known of the 

 growth and distribution of the junipers on the Snake River 

 plains, but this species is one which prevails large]} 7 on the arid 

 regions in Utah, and should, perhaps, be regarded as being 

 pushed toward the north through the stress of increasing aridity 

 farther south. Coming into the interior Rock}' Mountain region, 

 we meet a juniper much resembling tht Virginian, juniper or red 

 cedar of the east. It is the species named mountain juniper, 

 Juniperus scopulorum, a small tree or shrub. It occupies more 

 or less closely the semi-arid regions on the west slope of the 

 range, doubtless extending across to the eastern declivities along 

 the lines of semi-aridity. This juniper can endure a greater 

 degree of humidity than the other two species mentioned. So 

 far as it has come under my observation, it reproduces itself 

 freely. It has not yet encountered a stress of arid conditions 

 excessive enough to lower its seed-producing capacity beyond 

 the balance point. It extends along various of the mountain 

 streams into the plains of eastern Washington, usually keeping 

 close to the streams. It does not spread into the open plateau 

 region of this state to any noticeable extent, indicating that the 

 semi-aridity of the interior Rocky Mountain basins, where the 

 tree grows on hillsides and in valleys alike, is not so intensive 

 as on the open plains of eastern Washington. 



THE SUBHUMID REGIONS 



Adjoining the region of semi-aridity lie the subhumid belts. 

 Four species of conifers are of common occurrence here. They 

 are: Western yellow pine, Pinus ponderosa; red fir, Pseudotsuga 

 mucronata; lodgepole pine, Pinus murrayaria, and great silver 

 fir, Abies grandis. Their endurance of dry soil and climatic con 

 ditions is in the order named, the yellow pine ranking highest 

 and the great silver fir lowest in the scale. 



