508 



GENERAL CONSIDERATIONS 



I I 



1 Arctic ; Tundra 



\Ms^Qrisdand 



W^ Desert 



^^Declduous Forest 



I Western CcnifcTvus forest 

 ^Norlk West Comfervas Forest 

 \ Southern Coniferous Forest 

 \ Northern Coni^rous /brest 



Fig. 311. — A diagrammatic map of North America showing the vegetation regions 

 north of the tropical region. {Areas in the United States based upon Shantz and Zon, 

 "Atlas of American Agriculture," Part I, Sec. E; those in British America upon Weaver and 

 Clements, "Plant Ecology"; and those in Mexico upo7i both.) One line separates the prairie 

 (Pr) from the plains (PI), and a second the sagebrush area (S) from the southern desert 

 region (D). The existence of elevated areas presenting conditions corresponding to the 

 northern coniferous forest, the tundra, or the arctic zone in the Alleghenies and the western 

 mountains is indicated by areas of black. 



