The Causes of Timber-line on Mountains. 



177 



at timber line. Timber line is lowest in the relatively sheltered 

 valley heads — fine glacial basins as most of them are. To take 

 a specific instance— in ascending the north fork of the Spilli- 

 macheen, the first alpine meadows are found in the valley at 

 about 1,800 meters, 400 meters higher, on more exposed shoulders 

 of Mt. Copperstain, clumps of spire-shaped trees still occur. 



Where the forest is broken up, the characteristic forms of 

 wind cripples are scarcely seen. Most of the trees are spire- 

 shaped with healthy and flourishing tops. (These statements do 

 not apply to the groups of dwarf trees of higher altitudes.) All 

 considerations point to some other factor which in this case plays 

 a decisive part. What this may be is suggested by the fact that 

 on all sides the trees bear evidence of the mechanical action of 

 snow. They are often branchless and abraded at their bases 

 on the hillward side. Little trees are seen bent over and likewise 

 denuded of branches on one side. Significantly also, innumer- 

 able little trees are healthy only in their topmost branches, 

 the rest being dead, matted and infested with mycelia. Fur 

 ther still, the presence of the trees in groups and on local eleva- 

 tions appeared to be a snow relation. 



With these ideas in view, I went to the Selkirks in early 

 summer while the snow still lay in the sub-alpine forest and 

 covered deeply the fields above. The conditions seen came little 

 short of a demonstration (Fig. 3). In the forest near timber 

 line, the snow was interrupted only at the spots occupied by the 

 groups of trees into which the forest was becoming resolved. 

 No doubt less snow had accumulated there, and the trees them- 

 selves, as dark objects, must have hastened the melting, but the 

 relation was none the less significant. Passing higher up where 

 the snow beds were heavier, the tree groups were more widely 

 separated and more sharply confined to spots where from 

 local contour the snow had not accumulated so deeply. Thus 

 the forest is broken into scattered patches of trees, standing 

 mostly on mounds and hillocks. The vicinity of each group is 

 occupied by numerous small trees. All except the most sheltered 

 show the marks of the struggle with the snow. Only their tops 

 are in healthy condition. The lower branches are dying, covered 

 with fungus mycelia, or have entirely disappeared (Fig. 4.). 

 Such trees present an entirely different picture from that of wind 



