TIMBE KLINE. 



29 



result is that this .slope, unless too steep or otherwise unsuited to tree 

 growth, or withiu the track of avalauches, is commonly covered with 

 white-bark i)ines. As a rule the timbered area on these westerly slopes 

 takes the form of a broadening- tongue, beginning at the highest alti- 

 tude attained by trees on the crest of the ridge and increasing in width 

 at lower altitudes until the bottom of the valley is reached and skirted, 

 and the limit of avalanche movement passed, when the trees again 

 strike out boldly. The pines in the basins are much larger and more 

 erect than those on the summits of the ridges; they decrease in size 

 with increase in elevation. The long oblique line which on the west 

 side of each ridge marks the lower limit of tree growth commonly 



.^^" 





'ito •>"'•- /^-.- 'f^^WM't^mJ-''^' m ." 



:4.« 







V 



;1i 



Fig. 14.— niiili tiiiiliriiuic ml. 



I.s of sl(, 



(west) side ot the ri(lj;e are dwarf wliite-bark jjines. 



■xiiosiuf. I'lic (lark |jatelies on the left 

 (Photographed by John H. Sage.) 



marks also the upper limit of the area in shadow during the late 

 afternotni. In local s})ots other factors may account for the absence of 

 trees. Thus, they are always absent from the avalanche-swept bot- 

 toms of the valleys, and from ground kept wet by springs or melting- 

 snow. Studied with these facts in view, comparatively few treeless 

 areas will be fouiul which can not be explained, and the position of 

 true timberline may be fixed with some degree of certainty. This is 

 really a very important matter inasmuch as it affects the zone position 

 of a great many species. 



It is necessary to remember that the reason trees are absent from 

 the cold east and northeast slopes of the ridges whose summits are 



