The Causes of Timber-line ox Mountains. 



175 



fact, as fast as soil conditions permit, the forest is advancing to 

 the summit of the mountain. To the writer this seems scarcely 

 probable. While doubtless a condition of perfect stasis has not 

 been reached, it seems likely that above-ground factors are 

 largely responsible for present conditions. 



The mountains of the northwest, especially the Selkirks, 

 present altogether different conditions both as to topography 

 and vegetation. In height, ruggedness and the prevalence of 

 glaciers and perpetual snow fields, they strongly resemble the 

 Alps. They are more or less wind swept, but in the summer, 

 at least, they are far less so than are the mountains of the east 

 just considered. The whole character of the more elevated 

 forests is very different from that described above. In ascend- 

 ing beyond about 1,600 meters, the balsams (Abies lasiocarpa) 

 and spruces (Picea Engelmanni) which are the dominant species 

 of the sub-alpine forest, begin to appear in little groups separated 

 by shrubs (Azaleastrum albijlorum, Vaccinium membranaceum) . 

 Higher up this tendency becomes more pronounced; the intervals 

 are occupied by heather plants (Cassiope, Phyllodoce) till finally 

 at 2,000 meters, more or less, one emerges into open alpine 

 fields dotted with scattered clumps of spire shaped trees (Fig. 2). 

 On mounds and elevated spots, these clusters of trees wander 

 far upward. A thousand feet above the beginning of the alpine 

 fields, rows and groups of trees still look down from hillock 

 and ridge. 



European workers note the distinction between forest line 

 (timber line) and tree line, and some difficulty arises as to defi- 

 nitions, especially where krumholz or large isolated trees are 

 involved. In the Selkirks the outposts of the forest never occur 

 as single trees, but always in groups. In cases so numerous as 

 to be remarkable, the groups at timber line consist of a central 

 spruce or two, of patriarchal size, surrounded by numerous smaller 

 balsams. Possibly tins condition might indicate much 

 to him who is able to read it. 



The ceasing of the forest at about 2,000 meters to give place 

 to the prevailing open fields can scarcely be explained by any 

 of the accepted causes. Cold can not be the efficient factor, 

 from considerations already mentioned. Neither is it the wind, 

 as is strongly witnessed by the distribution and form of the trees 



