176 THREATENED ARIDITY ON THE PACIFIC SLOPE 



Passing to the white-bark pine, we find it extending over all 

 the ridges and spurs of the Bitter Roots having elevations above 

 6,000 feet. On the north the range of the tree is intercepted by 

 the valleys of the Clark fork and the Bitter Root river. On the 

 south it follows the crest of the ridges into the Rocky mountains. 

 The species is lacking in vigor and is not maintaining its former 

 stands. It is a conspicuously shy cone producer throughout all 

 this region. The staminate blossoms or aments are borne in the 

 greatest profusion, but the pistillate are very rare. In conse- 

 quence, but few cones are seen, and the seedlings, while not 

 wholly absent, are very sparse and scattered. 



The mountain hemlock occurs on the ridges above 5,500 feet 

 elevation throughout the central areas of the Bitter Root system. 

 It is cut off on all sides from connection with the, species else- 

 where by wide stretches where it is wholly lacking. In the north- 

 ern portion of its range it is an abundant cone and seed producer, 

 and is maintaining the average densities of-its stands. Its south- 

 ern boundary in this region lies along the crest which separates 

 the north and middle forks of Clearwater. All along this south- 

 ern edge it abuts upon the subhumid tracts which spread up- 

 ward along the slopes of the low-lying valley of the middle or 

 Lochsa fork of the Clearwater. A low ratio of cone and seed- 

 bearing capacit}' marks the southern edge of its range, and its 

 seedlings are far from sufficient in number to keep the stands at 

 their maximum density. Throughout the entire Bitter Root 

 region the declining vitality of the species is indicated by its 

 small cones, which do not average one-half of the normal size 

 for the more vigorous type of the species. The habit of the 

 three summit species is inimical to survival under very great 

 stress of subhumidity. Their place of growth is invariably on 

 drained slopes. If through adaptation they should acquire the 

 power to grow in wet or saturated soil, they would stand a far 

 better chance of survival, but no evidence exists of any such 

 modifications. 



Below the summit group of trees are the species of the second 

 group. Among these the western larch possesses the greatest 

 power of adaptation, next the mountain white pine, then the 

 Pacific arbor-vitas, and last the western hemlock. The western 

 larch is able to endure subhumid conditions which, in places, 

 almost border on semi-aridity. Of the trees distinctly belonging 

 to the humid areas, it is the last to retreat before the advancing 

 line of climatic siccation. All these species are at home in wet 



