NATURAL AREAS AND REGIONS 



577 



especially in the tableland region where 

 it slopes down to the lower Rio Grande, 

 and in Lower California. Much of the 

 peninsula of Yucatan, however, is 

 comparatively arid as are also some 

 sections of the low Gulf strip in Vera 

 Cruz and the narrow coastal plains 

 bordering the Pacific. The dryest re- 

 gions of the Republic are the north- 

 western part of Sonora and the section 

 of Lower California extending from 

 about the thirtieth parallel of latitude 

 to La Paz. 



In these regions, and to a somewhat 

 lesser extent on the plains of north- 

 eastern Mexico, the annual precipitation 

 is slight. The aridity of the Pacific 

 coastal plains of southern Mexico is due 

 to the distribution of the rainfall during 

 the year, rather than to any lack of 

 total precipitation. The year is divided 

 into a wet and dry season, and it is to the 

 length of the dry season that the gen- 

 erally arid condition so well reflected 

 in the character of the vegetation is due. 

 The climate of the northern part of the 

 Sierra Madre Occidental in summer is 

 similar to that of the southern part of 

 the Rocky Mountain region in southern 

 Arizona and New Mexico. During this 

 season there are abundant rains, but 

 there is much less snow in winter. Low 

 temperatures due to the elevation of the 

 central plateau and the great height 

 of the mountains of southern Mexico 

 carry Boreal conditions far into tropical 

 latitudes. The rainfall of particular 

 areas is strongly influenced by local 

 conditions, especially variations in alti- 

 tude, slope exposure, and direction of 

 prevailing winds. As a result sections 

 of "rain forest" may occur on mountain 

 slopes in close proximity to areas of 

 extreme aridity. Comparative meas- 

 urement data concerning the rainfall 

 of particular areas are not readily 

 available, but field observations with- 

 out the use of instruments indicate 

 that the wettest regions of the Republic 

 are the Gulf slopes of high mountains 

 in Vera Cruz, Oaxaca, and Chiapas. 

 The rainfall is also excessive on island- 

 like areas on the Pacific slopes of some 



of the mountains in Michoacan, Guer- 

 rero, Oaxaca, and Chiapas. 



C. NATURAL LIFE AREAS 



Several major North American life 

 zones, or plant and animal associations 

 below the Arctic-alpine belt are easily 

 recognizable in Mexico within which, 

 owing to highly diversified conditions of 

 local environment, a great number of 

 minor associations may be discerned. 

 These minor associations, or plant and 

 animal communities commonly coincide 

 with and are obviously dependent upon 

 changes in landscape aspect such as 

 forests of varying type, grassland, rock 

 formations, sand dunes, lakes, marshes, 

 and streams. Minor ecological groups, 

 closely allied but varying in combina- 

 tion of component parts tend to appear 

 and disappear at irregular intervals 

 throughout the zones. Many animals 

 are not definitely assignable to a partic- 

 ular type of landscape aspect, as for 

 example, they may frequent the grass- 

 land for much of their food and retire 

 to the adjoining forest for shelter. 

 Combinations of ecological groups are 

 often conveniently recognizable as floral 

 or faunal regions or districts in which 

 many forms are allied to but often speci- 

 fically or subspecifically separable from 

 those of neighboring districts in the 

 same major life zone. The major life 

 zones have been incompletely delimited, 

 on the basis of field observation of plant 

 and animal assemblages. Only a mere 

 outline can be presented here. Alti- 

 tudes are omitted as a rule in this brief 

 treatment as these vary widely in 

 accordance with latitude, slope ex- 

 posure, base level, and other local 

 climate determining factors. 



The Lower and Upper Austral Zones 

 and the Transition Zone are easily 

 traced over the tableland region, in- 

 cluding its mountain ranges. Along 

 the southern borders of this area, how- 

 ever, especially where high mountains, 

 directly face tropical lowlands boreal 

 and tropical elements are less widely 

 separated due to peculiar climatic 

 conditions, as a result of which the 



