RYDBERG: PHYTOGEOGRAPHICAL NOTES 435 
decumbens, Poa crocata, Festuca saximontana, Agropyron Scribnert, 
and Bromus Richardsonii. 
The general feature of the corresponding formation in the 
Northern Rockies is the same as that just described, but the 
principal tree of the Southern Rockies, Pinus aristata, is not found 
there. Its place is taken by Pinus albicaulis. This tree grows 
even more scattered and the undergrowth is composed of plants 
which are still more characteristic of the grass-covered hog- 
backs. Some of the plants found in the Southern Rockies, as for 
instance Pseudocymopterus montanus, are lacking. Calamagrostis 
purpurascens and Agropyron latiglume are rather common. So 
are also Xerophyllum tenax and X. Douglasii in places, especially 
west of the mountains. 
3. ASPEN GROVES 
The aspen in the Southern Rockies grows mostly on more 
gentle slopes than the pines, both on the north and south sides of 
the mountains, especially where the ground is richer and moist. 
It often takes possession of the ground after the coniferous forest 
has been destroyed by fire or otherwise. Usually it grows in 
groves, but sometimes mixed ‘with the spruce, balsam and pines, 
especially along the edges of woods. 
The aspen is not so common in the Northern Rockies, and this 
is especially true of the Subalpine Zone. In the Montane Zone 
it grows usually mixed with other species of Populus, birches and 
alders, while in the Subalpine Zone it is found along the edges of 
the spruce woods, in company with alders and the mountain ash. 
The following plants are characteristic of the aspen groves, or 
else have followed the aspens where they grow mixed with the other 
trees: 
Common to the Southern and Northern Rockies 
Trees 
Populus tremuloides 
| Shrubs 
Salix brachycarpa _ Distegia involucrata 
Sorbus scopulina Vaccinium scoparium 
Herbs 
Poa crocata Bromus Richardsonti 
