1921] Smiley: Flora of tlie Sierra Nenada of California 55 



of the ridges between the peaks of the main Sierran divide, rising 

 some five miles west of Lake Tahoe. Southwest of the lake, this zone 

 is found on Mt. Tallac, Angora Peak, and to the west on Lucile Crest, 

 Ralston Peak, and at various points on the intervening ridges. East 

 of Lake Tahoe in the Carson Range, only Mt. Rose and Freels Peak 

 rise above the Canadian zone. Southward of the Tahoe region, the 

 general altitude quickly rises and the highest of the boreal forest zones 

 spreads out over the high country but is still intersected by tongues of 

 the Canadian flora through the valleys. At Sonora Pass, near the com- 

 mon point of Alpine, Mono and Tuolumne counties, the northern limit 

 of the high southern Sierra is reached, and from there on to the south- 

 ward the Hudsonian life-zone is practically continuous except where 

 intersected by the Canadian flora at Tioga Pass and, more widely, at 

 Mammoth Pass. The zone is developed on all the high summits and 

 ridges to the west of the main divide in the Yosemite district, appearing 

 on Mt. Hoffman and perhaps on the very summit of Clouds Rest and 

 spreading on all the flanks of the crests that center about Mt. Lyell. 

 South of the gap at Mammoth Pass, this zone is continuous to beyond 

 Cirque Peak and reappears on Mt. Olancha in a narrow band at above 

 10,000 feet. Hudsonian plants are also found to the westward on Kaiser 

 Crest in Fresno County and on most of the higher ridges between that 

 divide and the high mountains west of Kern River as far south as 

 Sheep Mountain, the southern termination of the Great Western 

 Divide in Tulare County. Though the lower limit of the Hudsonian 

 is difficult of definition since subject to conditions of slope exposure, 

 the upper limit of this zone should be, at least in theory, easy of 

 determination since the accepted boundary is at that altitude where 

 the forest finally succumbs to the alpine climate. Practically, the 

 exclusion of tree growth depends upon so many factors, climatic, 

 edaphic, perhaps also biologic, that its delimitation is by no means an 

 easy matter. 



In the mountains of western North America timber line is still 

 a purely natural phenomenon ; artificial deforestation, which renders 

 its accurate determination so difficult in Europe has not affected it, 

 and its gradual rise from north to south attests the gradual increase 

 with lower latitudes in the sum of effective temperatures during the 

 season of growth. It appears that at the northern end of the Rockies, 

 in the Athabasca-Mackenzie region, timber line is found at about 

 2,000 feet f^ farther south in the mountains about the head of Stewart 

 River, the forest yields to the alpine meadow at between 3,700-4,700 



