PLANT SUCCESSIONS OF MT. ROBSON 231 



of the Fraser, extending from Lake Kinney to its source, the 

 Robson Glacier. The term "Shingle Flat Succession" is used 

 to include the processes upon all the alluvial deposits, whether 

 composed of fine silt or of coarser materials properly termed 

 "shingle." 



The normal beginning of the Shingle Flat Succession is upon 

 materials wet by surface or superficial underground flow. The 

 \'elocity of the surface flow may of course be so great as entirely 

 to prevent the establishment of vegetation. In such a case no 

 invasion can take place until the current slackens. 



The pioneers differ according to the size of materials. In 

 true shingle the two most important are annuals: Epilobium 

 latifolium and Saxifraga aizoides. A species of Salix appears 

 almost if not quite as soon as these two. In fine silt the first 

 arrivals are more numerous in species, and are mainly perennials 

 of sedge or rush type. Most important are Carex pauciflora 

 Lightf., Eriophorum sp., Equisetum variegatum Schleich., Tri- 

 glochin palustris L., and Juncus spp. The loss of my plant 

 collection is felt here with particular keenness, and in the willow 

 thicket stage as well. 



Regardless of size of materials, the next stage upon wet 

 alluvial deposits is a willow thicket, made up of several species. 

 Betula glandiilosa is added, and the climax forest follows. 



Such, in skeleton form, is the Shingle Flat Succession upon wet 

 deposits. Important variations in it are brought about by the 

 continued activity of the streams flowing over the flat. Sooner 

 or later, at some flood time, or merely because of increase of 

 velocitj^ due to partial loss of load, the streams will cut into the 

 alluvial deposits, leaving large stretches of them as low terraces. 

 The moisture conditions immediately change in an important 

 way. The sinking of the streams lowers the water table, and 

 the surface of the terrace becomes relatively xerophytic. The 

 coarser the materials, the more rapidly and thoroughly does 

 this change take place, since capillary rise of water varies in- 

 versely with size of materials, and drainage varies directly with 

 it. In such a case the wet flat pioneers are replaced by another 

 set of plants, among which Dryas Druvimondii is of supreme 



