432 RYDBERG: PHYTOGEOGRAPHICAL NOTES 
Populus tremuloides, Abies lasiocarpa, Pinus aristata, Pinus 
exilis, and Pseudots uga mucronata, of which the last two really 
belong to the Montane Zone and extend into the lower part only 
of the Subalpine Zone. Their relative importance, size, and dis- 
tribution have been treated before.* 
The forest formations of the Subalpine Zone in the Rockies may 
be divided into more or less defined classes, according to the most 
characteristic trees found there. 
I. SPRUCE-BALSAM FOREST 
This formation in the Southern Rockies is mostly found on the 
northern slopes and along water courses. The predominant tree 
is the Engelmann spruce, Picea Engelmannii. Sometimes it is 
found in pure stand, sometimes mixed with the subalpine fir or 
balsam, Abies lasiocarpa, occasionally also with the aspen. In the 
lower part of the zone the red fir, Pseudotsuga mucronata, is often 
added. The following species constitute mainly. the flora of the 
spruce-woods, though several more might be added, especially such 
as are characteristic of the aspen groves, and occasionally accom- 
pany the aspen into the coniferous woods. | 
The corresponding formation in the Northern Rockies resembles 
that of the Southern very closely, except that Abies lasiocarpa’ 
becomes more common on the western side, and on the western 
slope of the Bitter Root Mountains in places becomes the dominant 
tree. Lyall’s larch, Larix Lyallii, and the alpine hemlock, Hes- 
peropeuce Mertensiana, are added in the Bitter Root Mountains 
and the Selkirks, and the former in the main Rockies north of 
latitude 48° 30’. The red fir, Pseudotsuga mucronata, scarcely 
reaches the subalpine region in the Northern Rockies, but the 
lodge-pole pine, Pinus Murrayana, often enters the lower portion 
of the zone. As stated in a previous paper, Picea Engelmannii and 
Abies lasiocarpa are rare in the Big Horn Mountains, and there 
often the upper limit of the lodge-pole pine constitutes the timber 
line. oe 
The undergrowth of the spruce-balsam forest in the Northern 
Rockies is similar to that of the Southern Rockies. Many of the 
plants are common to both regions, as seen from the following 
* Bull. Torrey Club 42: 14-20. 
