PLANT SUCCESSIONS OF MT. ROBSON 233 



Kinney flat next to the mountain slope is an older terrace a 

 couple of feet higher than the main body of the flat, and upon 

 this the subclimax forest of Pinus Murrayana, Betula papyrifera, 

 Populus balsamifera L., Picea Engelmanni, and Pseudotsuga is 

 well developed. Salix spp., Juniperus sibirica, Shepherdia 

 canadensis, Viburnum pauciflorum, and Lonicera involucraia 

 (Richards) Banks accompany the foregoing, and the ground cover 

 is composed mainly of Dryas Drummondii (most abundant, and 

 rehct from the earher stage), D. octopetala, Ardostaphylos Uva- 

 ursi, A. rubra {Mairania alpina), Pyrola idiginosa, Orchis ro- 

 tundifolia Pursh, Cornus canadensis L., Linnaea borealis. The 

 Une between the subclimax forest and the area dominated by 

 Dryas Drummondii, marked by the low escarpment of the 

 older terrace, is quite distinct. 



In the westernmost corner of the flat there is a remnant of 

 forest that was formerly true climax except that a few large 

 specimens of Populus balsamifera and perhaps other subclimax 

 trees were present. 



We thus see that the change from the wet to the dry flat 

 results in a changed succession, Dryas Drummondii replacing 

 the wet-flat pioneers, and a xerophytic forest stage taking the 

 place of the willow thicket. It may well be that this course 

 of succession is commoner than the other, since all flats are 

 likely to become dry at some point in their history, and fre- 

 quently this happens at a very early stage. In the Subalpine 

 Zone the stages upon dry shingle are somewhat different, as will 

 appear in the next paragraph. 



The shingle flat which covers ' the summit of Robson Pass 

 possesses some features of special interest. The general char- 

 acter of the drainage from the Robson Glacier has already been 

 described, and the fact was mentioned that a large area of the 

 flat in front of the terminal moraines has been protected by 

 them from erosion ever since the oldest deposit was made, the 

 evidence for this being found in the shallow stream depressions 

 in the flat which end abruptly against the outer face of the mo- 

 raine and which manifestly are continued beneath it. Upon this 

 protected area various stages of the Shingle Flat Succession may 



