38 
A. G. Tansley . 
altitudes. (4) The Pine forest association, dominated by PinuS 
scopulorum and in places by P. flexilis. The pine forest has an 
undergrowth, more or less discontinuous, especially of Arctostaphylos 
Uva-ursi , with Pentstemon spp., Drymocallis ( Potentilla) fissn, etc., 
and the frequent tall grey-white spikes of Frasera speciosa. (5) The 
Douglas fir-wood association dominated by Pseudotsuga mucronata 
freely mixed with Engelmann spruce ( Picen Engelmnnni). The 
latter tree is dominant and forms forest at higher levels up to 
11,000 feet. Both shelter a fairly rich ground-vegetation essentially 
similar throughout. When growing in very close stand rejuve¬ 
nation in both forests ceases and the ground becomes almost bare 
of vegetation. The shrub-layer normally consists of Acer glabrum, 
Betula fontinalis, Opulaster opulifolia, Holodiscus dumosus ; and 
local dominants of the ground-vegetation are Thalictruin sparsiflorum, 
Erigeron macranthus , Solidago Parryi , Mertensia pratensis, Fragaria 
vesca. 
After a burn of the mature forest aspen ( Populus treinuloides) 
wood normally appears and is eventually replaced by Douglas fir. 
There is, however, another aspen association—the aspen-meadow, 
developed in the broader valleys at higher altitudes. The meadow- 
vegetation is much the same in the open as under the light shade 
of the aspens. Towards the wetter ground on the edges of the 
streams this is replaced by a birch-willow thicket. Potentilla ( Dasy - 
phova) fruticosa is a very conspicuous plant of such habitats. 
The stages of the main (gravel slide—Douglas forest) succession 
alternate very strikingly on the slopes of the valleys about 
Minnehaha. Generally speaking the south and west-facing slopes 
are in earlier stages of the succession than those facing north and 
east. 
In extreme cases a bare gravel-slide may face a Douglas wood 
on the opposite side of the valley. This, according to Professor 
Clements, is because of the prevalent west winds which help fires 
and keep the soil disturbed. On the south-facing slopes also the 
conditions are evidently drier and the succession less rapid. On 
the south-facing slope of Ruxton glen indeed there is a complex 
mixture of different stages of the sucession, and a great deal of 
telescoping often occurs, while many species come in out of their 
places, e.g., Pine and Douglas fir sometimes directly follow on 
gravel-slide. A certain amount of scepticism was indeed expressed 
by some members of the party as to the generality of the succession 
outlined above, the view being taken that the different associations 
