LIFE ZONES 



NORTH AMERICA 



THE relationships existing between climate and plant and ani- 

 mal life, so apparent everywhere in nature, have long since 

 been expressed by biological laws. The result is a mapping of the 

 earth's surface into a system of so-called Life Zones, or natural 

 divisions as determined by temperature and moisture, and 

 characterized by the species of plants and animals found therein. 



Since temperature is controlled by altitude as well as by lati- 

 tude the same succession of Life Zones is met in climbing a high 

 mountain as in traveling toward polar regions. These Zones, 

 in North America, are: Arctic, or Arctic-Alpine ; Hudsonian; 

 Canadian; Transition; Upper Sonoran; Lower Sonoran; Tropical. 

 The three highest are known collectively as the Boreal Region, 

 while the Transition and Sonoran Zones constitute the Austral 

 Region. These last-named Zones, in the eastern or humid half of 

 the United States, are known as the Alleghanian, Carolinian, 

 and Austroriparian. (See map of North America.) 



The Arctic Zone, lying north of the limit of forest trees, is 

 equivalent to our mountain summits above timber-line, with 

 similar plants and birds. 



The Hudsonian Zone reaches from ocean to ocean, across 

 central Canada and Alaska. It is the Zone of spruces in which 

 forest growth is stunted and dwarfed in its northern portion and 

 finally conquered by cold. On our high mountains it is represented 

 by the Engelmann spruce and. fox-tail pine, and terminated by 

 the distorted, timber-line forest where sprawling, wind-swept 

 limber pines form a matted cover on exposed ridges. 



The Canadian Zone is distinguished chiefly by firs mixed with 

 spruce in its northern part and with aspens in the south. It 

 stretches across southern Canada, extending northwest along the 

 base of the Rockies and south on the mountain-tops into Arizona 

 and New Mexico. It caps the highest peaks in Tennessee. De- 

 tached areas clothe the upper slopes of the high Sierras in 

 California and the Cascade Range in Oregon. 



The Transition is the Zone of pine forests and of spring-wheat 



