The Causes of Timber-line on Mountains. 



171 



ont disparu par le fait de l'homme, apres avoir ete la cause 

 dominant de la production de ces gazons." 



His view is not shared by others, however. Schroeter,* 

 in his excellent resume of the subject, while recognizing that 

 the Alpine pastures have been much extended, seeks, in climatic 

 causes, the reason for their existence. So far as first-hand 

 knowledge goes, I may say that in a walking trip of several 

 weeks through the Alps, I had much difficulty in finding an 

 illustration of timber line that was beyond suspicion of being 

 artificially caused. 1 did, however, see the shepherds pulling 

 up little trees near timber line in a manner which left no room 

 for speculation as to its efficiency. 



Certain other "timber lines" are still less worthy of the 

 name. One on Mt. Katahdin, in Maine, said to descend as low 

 as 800 meters, was found by Cowles to be merely a case where 

 the steep and rocky character of the mountain side made the 

 development of a forest impossible. 



Real timber lines do exist, however, and the question of 

 their cause is one the ecologist is bound to answer. We may 

 accordingly consider some of the various factors which have 

 been assigned as its explanation. 



1. Cold. The view of the older naturalists was sufficiently 

 simple, namely, that the low temperatures of mountain tops 

 prevent the growth of trees. The rough relation between 

 vegetation zones of altitude and latitude gave support to the 

 idea and it has been widely held. When examined, however, 

 very little can be found in its support. It is without foundation 

 in physiological knowledge for little is known concerning the 

 effects of low temperatures upon hardy plants. That the idea 

 is incorrect is moreover shown by the fact that forests do exist 

 in the coldest districts known on the globe. f Furthermore, 

 so far as cartographic work in the Alps is to be relied on, isotherms 

 and timber lines do not run parallel. 



2. Shortness of Vegetation Season. If the portion of the 

 year which is sufficiently warm for physiological activity is but 

 short, and relatively cool, the success of the tree might thereby 

 be hindered. This might be true in two respects: (a) The 



*Schroeter. Das Pflanzenlebcn dor alpen. Zurich 1908. 

 tSchimper. Pflanzengeographie. 



