2 THE YOSEMITE FLORA 



Washington, Canada, and Alaska, where, however, they grow at 

 a lower altitude. Some of these species, as the White-bark Pine, 

 the Cassiope, and the Bitter-root, are found also in the Rocky 

 Mountains, while not a few of them, such as the Alpine Sorrel, 

 the Sibbaldia, and the Shrubby Cinquefoil, extend quite around 

 the world in circumpolar regions. 



As implied in the foregoing, the most influential factor in the 

 geographic distribution of plants is temperature. Now, in a 

 mountainous district, such as ours, the temperature depends 

 largely upon altitude, and we therefore find that species adapted 

 only to warm temperatures are restricted to the foothill belt, that 

 those adapted to moderate temperatures occupy the middle alti- 

 tudes, while species so constituted as to be able to carry on their 

 life functions with a relatively small amount of heat flourish 

 on the plateaus and peaks of the highest mountains. Since each 

 species thus comes to inhabit only those places where conditions 

 of temperature are suitable, the result is a grouping of plants 

 into more or less definite belts of vegetation. These have been 

 worked out for most of North America by Dr. C. Hart Merriam, 

 of the United States Biological Survey, who has designated them 

 as Life Zones and developed a system of nomenclature that is 

 generally adopted by biologists. One must not expect to find 

 these zones always clearly defined. Often the line between two 

 belts is as sharp as though cut by a knife; again the belts over- 

 lap and intermingle in so confusing a manner that even the 

 expert is baffled in an attempt to distinguish them. In the Yo- 

 semite National Park four such belts are recognized, as follows : 



1. Foothill Belt {Upper Sonoran Life Zone). This belt is 

 composed chiefly of endemic Californian species. The vegetation 

 is largely chaparral, that is, thickets of shrubs, mostly with stiff 

 branches, small, often thick or leathery leaves, and not rarely with 

 spines. Annuals grow in abundance between the shrubs but only 

 during the spring and summer months. The root systems of 

 plants in this belt are well developed and the herbage is often 

 woolly, or densely hairy, or coated with resin. Such qualities are 

 characteristic of plants obliged to conserve their moisture, the 

 Foothill Belt being mostly a dry as well as a warm zone. The 

 upper line of this belt lies normally at about 3000 feet altitude, 

 but it reaches 5000 feet on warm southerly or westerly exposures, 

 while on slopes facing the north it may descend to as low as 2000 

 feet. Characteristic plants are the Digger Pine, Wedge-leaf 

 Ceanothus, Bladder-nut, California Buckeye, and Poison Oak. 



2. Yellow Pine Belt {Transition Life Zone). In this belt we 

 find endemic and southern species intermingling with those of 

 northern origin. All of the more frequented portions of the Park, 



