1921] Smiley: Flora of the Sierra Nenada of California 57 



above timber line. In the Sierra north of Lake Tahoe no summits 

 rise to timber line except Mt. Rose, which is said by Heller'^ to have 

 a true alpine summit above the 10,000-foot contour. To the west of 

 Lake Tahoe, Pyramid Peak and Dick's Peak have treeless summits 

 with an ill-deJ&ned tree-line at about 9,900 feet. The summits of Castle 

 Peak and Mt. Tallac are devoid of trees because of lack of soil and 

 force of the wind. In the high mountains to the southward of the 

 Tahoe district as far as Mt. Olancha in Tulare County, many of the 

 sunnnits are truly alpine; in the Yosemite district, the line marking 

 the forest limit runs at approximately 10,200 feet on Ragged Peak; 

 10,300 feet on Mt. Dana; 10,700 feet on Mt. Lyell; 10,400 feet in 

 Farewell Gap; 10,700 feet on Mt. Kaweah; 10,600 feet on Sawtooth 

 Peak ; and 11,000 feet on Mt. Olancha ; south of the last no Calif ornian 

 summits rise into the alpine zone except Mt. San Bernardino (11,485 

 feet) and possibly Mt. San Jacinto (10,805 feet).'^ The Southern 

 Rockies of Colorado and New Mexico have approximately the same 

 latitude as the Sierra Nevada; tree-line in Colorado rises from about 

 11,000 feet on Longs Peak to 11,500 feet on Pikes Peak«° and 12,000 

 feet on the mountains in the southwestern part of the state. ^^ In New 

 Mexico, Truchas Peak has a tree-line at about 13,000 feet,^- the highest 

 elevation for timber line known from the United States. Mt. San 

 Francisco, in northern Arizona, has a timber line to which Mearns 

 assigns an altitude of 11,468 feet.^^* 



The data presented above at considerable length are of interest in 

 attempting to arrive at some conclusion with regard to the migration 

 and colonization of mountain ranges by arctic or circumboreal plants. 

 The gradual rise of timber line and therefore the rise of the minimum 

 altitude at which the typical arctic-alpine flora finds life conditions 

 most favorable for occupation, is seen to be essentially similar in both 

 the Rocky Mountains and the Cascade-Sierra system, though the actual 

 altitudes differ by approximately 1,000 feet at the same latitude. This 

 difference is owing to two reasons at least : greater aridity of the 

 Rockies coupled with their continental type of climate causes the 

 extremes of temperature to be greater than in the coast mountains; 

 the Rockies are colder in winter and warmer in summer, but nowhere 

 does the degree of winter cold exclude the forest; its limit is rather 

 determined by the sum of the effective temperatures during the growth 



* South of the mountains of northern Arizona and New Mexico, no peaks in 

 North America rise to above timber line till the great volcanoes in central Mexico 

 are reached, about sixteen de^ees of latitude further south, yet their tree-line is 

 scarcely a thousand feet higher.s* 



