58 University of California Puhlications in Botany [Vol. 9 



period and this sum is greater in the Rockies than in the western 

 mountains at the same altitude. In other words, the elevation, where 

 the total of effective heat received is insufficient for tree growth, is as 

 much higher in the Rockies than in the parallel Pacific system as the 

 difference between their tree-lines. The other reason believed to 

 explain in part the higher timber-line of the Rockies is the fact that 

 in the Cordillera the mass of land raised to equal elevations is far 

 greater than in the Cascade-Sierra system; the latter is a long but 

 relatively narrow mountain axis, while the Cordillera is rather an 

 enormous plateau from which the several mountain chains rise to yet 

 greater heights. It is well known that isotherms rise over plateaus. 

 These reasons then seem competent to explain the increased height of 

 tree-line along the same parallels in western North America. 



The data presented above take no account of the difference between 

 timber-line and tree-line, the latter referring to the elevation above 

 which no trees are found, the former denoting the limit of the forest. 

 Between these two limits is the area in which the forest and meadow 

 formations are in unstable equilibrium, other factors than those termed 

 climatic determining the issue as to which formation shall occupy a 

 given terrane. In the Sierra it is very difficult indeed to distinguish 

 the forest line from the tree-line, due to the fact that so much of the 

 high mountain country is at present incapable of supporting the 

 forest, not because trees are excluded by climatic factors, but because 

 of the absence of soil, which again is a consequence of the extreme 

 recency of Sierran glaciation. (See plate 4, Desolation Valley.) 



One result of this brevity of post-glacial time is that over much 

 of the boreal region within and above the Hudsonian zone, soil is 

 nearly non-existent and the forest is now absent where perhaps it at 

 one time existed and where it may reappear. R. S. Marshall, in his 

 survey of the Mt. Lyell region, found nearly one half of the area 

 of the quadrangle to be above timber line, devoid of timber, grass, or 

 soil.^^ 



A further consequence of the recency of glaciation has been to 

 exclude from the Sierra, till soil shall form again, the alpine meadows, 

 which are so characteristic of the mountains of Washington and 

 British Columbia and of the Rockies of Colorado. The "alps" with 

 their brilliant flowers and dense turf of grasses, sedges, and rushes, 

 which lend so much of charm to the other high mountain regions of 

 the west, are present in the Sierra in pocket-edition size, being repre- 

 sented by small patches among the roches nvoutennees or among the 



