108 A Study of the Vegetation of 
of the region. These usually occur in situations where the stream 
has a double channel or where, having abandoned one channel, it 
has built up a rich alluvial flat. It is quite clear how after a 
spring flood the wind-blown seeds of the poplars find excellent 
conditions for germination in such situations and even if only a 
few become established the extensive underground parts of Popu- 
lus tremuloides would soon increase the area dominated by trees. 
In this manner the trembling aspen frequently extends its terri- 
tory beyond the flood-plain onto the moist, sheltered banks of 
canyons. I have observed one or two cases in the deeper canyons 
where yellow pines had established themselves among the aspens, 
and seemed to be replacing them. 
The Salix-Crataegus Mictiwum—Much larger areas of the flood- 
plains are dominated by shrubs or trees of the following species ; 
these may occur singly, but more often form a mictium. 
Alnus tenuifolia Salix bebbiana 
Amelanchier florida Salix cordata 
Crataegus brevispina Salix exigua 
Prunus demissa Salix scouleriana 
Salix argophylla 
In addition to the above dominants numerous secondary tree 
species and several important undershrubs should be mentioned 
here. The following are the most important: 
Alnus rhombifolia Rosa pisocarpa 
Amelanchier cusicku Ribes aureum 
Betula microphylla Salix bella 
Cornus stolonifera Salix lasiandra caudata 
Crataegus columbiana piperi Salix pyrifolia 
Rosa nutkana 
The alder sometimes becomes so abundant locally as to consti- 
tute perhaps half of the flood-plain woody vegetation. The 
chokecherry is never so abundant. 
Of the willows, only one, Salix lasiandra caudata, assumes the 
proportions of a tree. As might be anticipated, the willows are 
usually among the first species to appear. 
108 
