PLANT SUCCESSIONS OF MT. ROBSON 219 



once attain dominance being that the breaking off of their 

 trunks by falling fragments is always fatal to them. The 

 birch, having met such a catastrophe, merely replaces its lost 

 shoot with a cluster of stump sprouts, and so is helped rather 

 than harmed by the disaster. 



The tree seeds germinate in the crevices and cavities among the 

 boulders — obviously the only situation where there is moisture, 

 soil of any kind, and shelter. Possibly the reason why the climax 

 conifers are able to estabhsh themselves so early in so apparently 

 forbidding a habitat is that the shade of the rocks performs the 

 same service for them that the shade of the subclimax trees 

 performs in the normal succession. By the time the shoots 

 have emerged from the crevices the plants are well estabhshed. 

 Herbaceous plants, especially those without the power of upward 

 elongation, if they germinated, would be likely to die because of 

 insufficient Ught, and they might easily be covered by debris 

 from fresh falls of talus which might not harm well established 

 woody plants. Of course this hypothetical explanation is to 

 be apphed only to the particular talus studied, in which the 

 blocks are of medium size — from a few centimeters to several 

 decimeters in diameter. If the fragments are very small and 

 the mass is well consolidated, conditions are suitable for crevice- 

 and mat-plants as pioneers, and the trees should appear later. 

 With very coarse fragments the cavities would be so large and 

 deep and the ground water so far from the surface that all in- 

 vasion except by surface lichens would have to await further 

 disintegration of the rock masses. 



The trees and shrubs seen upon open talus were: Betula 

 papyrifera, Salix spp., Picea Engelmanni, Pseudotsuga mucronata. 

 Practically every tree upon open talus is scarred upon the upper 

 side; many have large blocks resting against them. Broken 

 trunks and branches are strewn over the slope. In some places 

 rock falls are so frequent as greatly to retard the development 

 of vegetation, or even to prevent it entirely. 



As the birches become larger and more numerous they lessen 

 the destructive force of the rolling and bounding talus blocks. 

 More and more conifers escape destruction, and the forest 



