The Causes of Timber-une on Mountains. 



179 



in their basal portions. Such mechanical action seems, on the 

 whole, however, much less important than the "drowning" 

 by wet snow just referred to. 



To digress a moment from the main question, it seems small 

 wonder that places where snow creep occurs to any extent are 

 inhabited only by the toughest and most supple species. In 

 the Cevennes, the higher summits as mentioned above are 

 normally occupied by forests of beech. In attempting to re- 

 forest the areas which have been denuded by man, pines have 

 been planted as nurse trees and now occur together with scat- 

 tered beeches in the pseudo-alpine meadow. If one chooses a 

 spot where the creep of the snow is pronounced, the pines are 

 seen to be broken and dying while the beech remains unaffected 

 and flourishing. Without going so far as to assign this as a 

 reason why these areas are naturally occupied by the beech, the 

 facts are mentioned as suggestive. 



In the Selkirks, as one ascends above the alpine grassland 

 of the valley heads to the elevated outposts of the forests the 

 effects of wind are once more visible. The trees which are per- 

 mitted by the snow to grow on ridges and hillocks exhibit at an 

 altitude of 2,200 to 2,500 meters the characteristic forms of 

 wind cripples (Fig. 4). These often show the most bizarre 

 forms, due to crippling from both snow and wind and, conject- 

 urally, from the browsing of wild goat and caribou as well. 



Finally the farthest outposts dwindle awav to prostrate 

 dwarfs or krumholz. Such decumbent trees occur as high as 

 at least 2,600 meters. It is noteworthy that the wind cripples, 

 krumholz, are found only in groups and where the snow does not 

 accumulate deeply. The open grassland due to snow is entirely 

 destitute of them. Possiblv herein lies a true distinction of 

 such grassland. 



SUMMARY. 



The view that timber lines on mountains are brought about 

 by the action of the wind, holds good for the isolated mountains 

 of eastern Xorth America, but can not be applied to those of the 

 northwest, in particular the Selkirks. Two kinds of true timber 

 lines may therefore be distinguished. : 



1. Timber lines caused by wind. Such are recognized 

 by a gradual and ultimately great reduction in height of trees, 



