CONIFERS OF THE ROCKY MOUNTAIN REGION. 19 



occurs very commonly in small pure stands and also in mixture with 

 white-bark pine, alpine fir, alpine larch, and Engelmann spruce, while 

 lowland white fir, lodgepole pine, and western white pine are 

 occasional associates at lower elevations. Large pure stands are 

 uncommon, but rather extensive mixed forests are occasionally 

 found containing as much as 85 per cent of this hemlock. 



Mountain hemlock is very tolerant of shade, surpassing all of its 

 associates, except western hemlock, in shade endurance. Its dense 

 shade, which in some instances almost excludes light from the ground, 

 usually prevents growth of all seedlings, even of its own, which appear 

 to require more fight than is available under the mother trees, but 

 which thrive under lodgepole pine, fir, etc. Seedlings grow more 

 thriftily in moderate shade than in full light. Seedlings and saplings 

 bear long suppression, rarely dying under it, and resume normal 

 growth when top light is admitted. Because of the mountain 

 hemlock's great shade-enduring qualities, the trunks are not readily 

 cleared of branches, even in dense stands. The branches become 

 so large before they die that they usually persist for a long time or 

 leave short stubs, thus forming loose knots in timber. 



Tsuga mertensiana is a prolific seeder, some trees beginning to pro- 

 duce cones when about 20 years old. More or less seed is borne 

 annually, but good seed years occur only at intervals of several years. 

 The seed has only a moderate rate of germination, and its vitality is 

 rather transient. With sufficient moisture, seed germinates on both 

 humus and mineral soils, but much more readily on the latter. 



LONGEVITY. 



The age limit of mountain hemlock is not known at present, but it is 

 believed to be a very long-lived tree, probably attaining an age of 

 from 400 to 500 years. Trees from 18 to 20 inches in diameter are 

 from 180 to 260 years old, while trees only from 5 to 7 inches in 

 diameter growing on high, wind-swept ridges are from 60 to 80 years 

 old. 



GENERIC CHARACTERISTICS OF FALSE HEMLOCKS. 



The generic name Pseudotsuga, false hemlocks, is applied to a 

 group of trees related to the hemlocks (Tsuga), which they resemble 

 in the contracted base of their leaves and in the habit and 

 character of their cones. The resin vesicles peculiar to hemlock 

 seeds, are, however, absent from the seeds of Pseudotsugas. Like 

 the balsam fir trees, the false hemlocks have small resin pockets, or 

 "blisters," in the bark of young trunks and branches. Woodsmen 

 and lumbermen know the trees of this group as "varieties" of "fir" 

 or "spruce," and even as "pines," this being due probably to the 

 superficial resemblance of the wood to that of spruces and pines, 



