PLANT SUCCESSION OF MT. ROBSON 237 



SUMMARY 



In the mountains of the Mt. Robson region there are two 

 forest zones. In the lower, or Montane, the dominant and 

 chmax formation is made up of Thuja plicata, Picea Engelmanni, 

 Tsuga heterophylla, Pseudotsuga mucronata, and Abies lasiocarpa. 

 This forest is very similar in character to that of the lower zone 

 of the Selkirks. Its upper Hmit is approximately 3500 feet 

 (1077 meters). The chmax forest of the upper or Subalpine 

 Zone, whose upper limit is 6500 feet (2000 meters), is much 

 poorer in number of species and in size of trees, which are Picea 

 Engelmanni (dominant), Abies lasiocarpa, and Pinus albicaulis. 

 The successions in the two zones are similar in general, but 

 differ in certain details. They are studied to the best advantage 

 in the Subalpine Zone and in the upper parts of the Montane, 

 since the earUest stages, following the retreat of the glaciers, 

 are still present in these regions. 



Two primitive surfaces are left bare by the retreat of the ice: 

 rock surfaces and moraines. Each of these soon generates 

 another, so that four habitats are offered to invasion by plants: 

 rock surfaces and talus; moraines and shingle flats. Talus and 

 shingle flat are continually increasing at the expense of rock 

 surface and moraine. In each of these a succession originates, 

 and all of them lead to the estabUshment of the climax forest. 



The successions are all short and much telescoped. 



The Rock Surface Succession was not studied. 



The course of the Talus Succession upon the lower slopes of 

 Mt. Robson, where the blocks are of medium size, includes no 

 pre-forest stage of consequence. Trees and tall shrubs are the 

 first arrivals: Betula papyrifera, Salix spp., and the climax 

 conifers. At first Betula is greatly predominant because it is 

 not permanently harmed by falling and rolling rock fragments; 

 while conifers are killed thereby. Later, because of lessening 

 bombardment or partial protection by birches or both, the climax 

 conifers attain dominance. Up to this stage the surface of the 

 talus slope may be practically bare of lower growth. The 

 undergrowth follows the trees, and the last plants of the final 

 stage to become estabhshed are the chmax herbs and mosses. 



