io8 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 Mictium. 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 cor data 



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 cusickii Ribes aureum 



Betula microphylla Salix bella 



Cornus stolonifera Salix lasiandra caudata 



Crataegus columbiana piperi Salix pyri folia 



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. 



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