is that North America can be divided into Hfe zones which, accord- 

 ing to Merriam, represent a vegetational response to temperature. 



The following discussion of Hfe zones in California is adapted 

 from the introduction to W. L. J epson's Manual of the Flowering 

 Plants of California published in 1925. Considering the time at 

 which Jepson's observations were published, they exhibit a re- 

 markably modern insight into fundamental ecological principles. 

 They also provide a valuable general, overall description of the 

 vegetation of CaUfornia. 



As a result of the varying combinations of climatic factors in 

 the state, intensified by distance from the ocean and by altitudes, 

 the vegetation of California is markedly stratified into horizontal 

 bands called life zones. Six life zones are recognized: 1 . Lower 

 Sonoran; 2. Upper Sonoran; 3. Transition; 4. Canadian; 5. Hud- 

 sonian; 6. Boreal. The isothermal lines of a temperature chart of 

 California correspond in a general way, though not exactly, with 

 these Hfe zones, while a contour rain chart shows similar corres- 

 pondence. Annual rainfall, which is slight in the deserts of the 

 Lower Sonoran, increases one-half inch (1 .3 cm) for every one 

 hundred feet (30.5 m) in proceeding up the west slope of the 

 Sierra Nevada. Insolation increases to the southward and also in- 

 creases markedly with altitude. Humidity is greatest along the 

 coast and diminishes toward the interior and in the south. 



The Lower Sonoran Zone comprises three distinctive areas: 

 (1) Colorado Desert or Colorado Sonoran; (b) the Mojave Desert 

 or Mojave Sonoran; and (c) the Great Valley or Valley Sonoran. 

 The two deserts are characterized by a typical desert climate. 

 They have a low humidity and a low rainfall, the annual precipi- 

 tation varying from to about 5 inches (0-13 cm). They have 

 high summer temperatures, averaging from 90° to 130°F (32° 

 to 54°C); they have low winter temperatures, varying from about 

 15° to 50° F (- 9° to 10°C); and they have a great annual tem- 

 perature range and a great diurnal range. Drying winds of gale 

 force are prevalent. The vegetation of the Lower Sonoran Zone 

 has the characteristic aspect of plants of desert regions, that is, 

 there is everywhere exhibited a marked development of struc- 

 tures to inhibit transpiration or of physiological devices for the 

 conservation of water. 



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