Volume 12 



Number 8 



The Plant World 



A Magazine of General Botany 

 AUGUST, 1909 



THE CAUSES OF TIMBER LIXE OX MOUNTAINS; THE 

 ROLE OF SXOW.* 

 By C. H. Shaw. 



In the plant geography of the world there are few features 

 more inviting for study than the alpine Yegetation which oc- 

 curs on many mountains. Sui generis in itself, its character- 

 istics are heightened by contrast with the forest which usually 

 is found immediately below. The limit where the forest ceases, 

 and \egetation of a different and unusual sort begins, consti- 

 tutes a frontier of unusual interest. Contrary to popular im- 

 pressions, the cause of timber line is by no means well under- 

 stood. During the study of certain other problems of mountain 

 vegetation, in the last few years, I have had excellent opportun- 

 ities for observation of primeval conditions, and the results 

 are embodied in the present paper. Such observational work 

 leaves many questions unanswered; nevertheless it may lead 

 to certain conclusions with sureness. The illustrations are from 

 my photographs obtained at various times. 



Most of the work on the subject of timber line has been done 

 in Europe, and there the conditions are peculiarly unfavorable. 

 For in the Alps and Pyrenees, the timber line has been greatlv 

 modified by human agencies. When an observer has first to 

 decide whether that which is before him is due to natural causes 

 or not, his conclusions stand a double chance of error. Certain 

 European workers have published elaborate tables showing the 

 precise altitude of timber line and other facts of vegetation 



♦Read before the Botanical Society of America at the Chicago Meeting. December 31. 

 1907. By a coincidence, Dr. H. C Cowles read elsewhere at the same time .t paper blowing 

 that in many parts of western Yorth America timber line is dependent upon snow. 



