THE WESTERN VALLEYS 135 



six to ten inches in four years, but accelerating for a time during 

 later years. In tliis respect the tree is at some disadvantage as 

 compared with other species like the fir and hemlock and is 

 saved only by its great tolerance of shade. Thus it may con- 

 tinue for some time under deep shade without suffering sup- 

 pression. The trees grow to great age and are very tenacious of 

 life; often with much of the trunk and crown dead a few living 

 branches may survive indefinitely. There are few parasites 

 which attack the arbor vitae though owing to its thin bark it 

 easily succumbs to fire and the ground which it occupied may 

 become covered with Douglas spruce or lodgepole pine, or as 

 appeared in one case, with a dense growth of the more rapidly 

 growing white pine and grand fir. 



Among the mesophytic companions of the white pine men- 

 tion should be made of the western yew ( Ta.nis h re vi folia) while 

 almost a rare tree in western Montana, it reaches a diameter of 

 a foot and a height of twenty to thirty feet, where it occurs 

 in some of the deep, moist canyons east of Flathead Lake, always 

 thriving best in the vicinity of permanent water, but forming 

 a shrubby undergrowth in the shade of old forests. It seldom 

 is abundant, even locally, which may partly be accounted for 

 by the paucity of its seeds and the slowness of its growth. Like 

 some of the other species, however, it may be taken as a good 

 indicator of the humid qualities of the soil and climate. 



The broad-leaved, deciduous element in the forests of this 

 zone is seldom conspicuous, but along open river bottoms, Pop- 

 ulus trichocarpa is everywhere the dominant tree, sixty feet or 

 ^ore in height, three feet or over in diameter, and sometimes 

 with a clear, cylindrical trunk of 40 feet in length. The sec- 

 ondary species on the river bottoms are Populus tremuloides, 

 Alnus tenuifolia, Betula fontinalis. B. papyrifera, Prunus demis- 

 sa, Crataegus Douglasii, Salix fluviat-ilis, 8. Bebbiana, S. cordata, 

 and 8. Scouleriana. Betula papyrifera is much more restrict- 

 ed in its range in Montana than any of the other species and is 

 confined to the northwestern slopes and valleys, where also the 

 aspen shows larger and more abundant development, with Cra- 

 taegus rhndaris, Ceanothus sanguineu-s, Ribes lacustre, Rhamnus 

 Pursliiana, R. aini folia, Nolodiscus ariaefolius, Spirea lucida 

 and numerous other shrubs. The trees which appear most con- 



