National Geographic : 1899 May
168 THREATENED ARIDITY ON THE PACIFIC SLOPE which therefore furnishes an excellent indication of the limits of the quasi semi-arid and lower subhumid 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 largely 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 Rocky Mountain region, we meet a juniper much resembling the 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 murrayana, 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.