PLANT SUCCESSIONS OF MT. ROBSON 227 



to define a true plant connnunity. There are potentially three 

 communities, represented by the herbs, the shrubs, and the one 

 or more trees. 



Upon the third moraine we find the first definite plant com- 

 munity. Here the vegetation is sparse enough, but is much 

 more abundant than upon the fourth, and certain species emerge 

 as true dominants. These are creeping shrubs and herbs, and I 

 would name the community, from its two most important species, 

 the Dryas ociopetala-Arctostaphylos rubra Stage of the Moraine 

 Succession. Other members, less abundant, are Dryas Drum- 

 mondii, Epilohium latifoliujn, Hedysarum Mackenzii Richards, 

 Castilleja pallida, Silene acaulis L. It is hardly more than a 

 rudimentary community upon the third moraine, but in de- 

 velopment is decidedl}^ in advance of the shrubs and trees, 

 which are represented by occasional willows and very occasional 

 spruce seedlings. 



Coming to the second moraine we find a great advance. The 

 difference is so great that the third and second moraines are 

 plainly distinguishable at a distance of two miles by the different 

 amounts of vegetation which they carry. The Dryas-Arcto- 

 staphylos community is far better developed than upon the third 

 moraine, large areas being dominated by it. But it is now in 

 competition with a second community made up of shrubs, among 

 which several species of Salix are the most important. Betida 

 glandulosa Michx. is occasional, and increases as the community 

 grows older. These form a true thicket which is the dominant 

 growth upon the second moraine, although comparison with the 

 first moraine shows that it has not yet attained its full develop- 

 ment. It may be termed the Salix-Betula glandulosa Stage of 

 the Moraine' Succession. The trees are also represented here. 

 They are larger and more numerous than upon the third moraine, 

 but are not yet seriously competing with the shrubs. Frequent 

 young spruces were seen, the largest being six feet in height. 

 A few seedlings of Pinus albicaulis were also found. Some of the 

 spruces were prostrate and sprawling. Humus is naturally 

 not abundant upon the second moraine, and there are still con- 

 siderable areas nearly bare of plants. 



THE PLANT WORLD, VOL. 19, XO. 8 



