THREATENED ARIDITY ON THE PACIFIC SLOPE 177 



or swampy localities, and are therefore better fitted for a lengthy 

 resistance than would otherwise be the case. Their northward 

 range extends indefinitely to the limits where the mean temper- 

 ature becomes too low for their growth. Their south w'ard bound- 

 ary in the Bitter Root region lies a few miles north of a line 

 drawn east and west through the crest of the divide which sep- 

 arates the Salmon river from the Clearwater drainage. North- 

 eastward they cross the Bitter Roots into the Rocky Mountain 

 ranges, while in the northwest they extend through the moun- 

 tains between the Fraser river and the Columbia into the Cas- 

 cades. Owing to the circumstance already mentioned— that the 

 species can exist in swampy ground — they hold their own against 

 the subhumid encroachments everywhere but along their south- 

 ern edge. Their retreat is here marked, exactly as in the case 

 of other species, by deficient cone and seed production and by 

 the occurrence of detached bodies of the species along the line 

 of retreat. 



The third group of the humid series of trees contains species 

 whose adaptability to varying altitude and moisture conditions 

 is of the highest. This is owing to their capacity for enduring 

 very diverse habitats. They are equally at home on dry, well- 

 drained slopes, or in wet places, where their roots are continually 

 immersed in circulating water. Among the three, the lodgepole 

 pine has the greatest endurance, and all appearances indicate 

 that it is the species which eventually will supplant the other 

 species in the humid regions. 



The Alpine fir ranges throughout the entire extent of the 

 Bitter Root mountains, and extends indefinitely north and south, 

 east and west, along the crests of connecting ridges. It is a fair 

 producer of cones and seeds, and is maintaining its stands in most 

 localities. Its susceptibility to adverse subhumid conditions is 

 found in the occurrence of large deforested tracts occupied by 

 the tree within comparatively recent times, but which now show 

 no evidence of a return to forest cover. Such tracts are frequent 

 everywhere throughoutits range in these regions. Generally they 

 front on some broad valley, along whose slopes the subhumid or 

 semi-arid changes are advancing into the mountains. 



The Engelmann spruce and the lodgepole pine have a uni- 

 versal range throughout the mountains in this region. Both 

 have developed forms to meet drier conditions. Engelmann 

 spruce never reaches its greatest development except in swampy 

 local i tie.-', where it grows to be a large, well-formed tree. On dry 



