1 
16 ] The University of New Mexico 
and cacti are found far north into the Dakotas and Wyom¬ 
ing:; and it is a matter of present observation that Acacias 
and mesquites (Prosopis) are steadily taking* over formerly 
grass-covered areas in Texas. Tropical vegetation has en¬ 
tered northwestern Mexico along the Pacific coastal plain, 
where it attains to northern Sinaloa. The mangrove (Rhiz - 
ophora mangle) extends along the strand into southern 
Sonora. From the above it can be seen that northwestern 
Mexico constitutes a primary Xerophytic vegetation unit 
which has been modified and augmented by tropical, north¬ 
ern mountain, and prairie invasions. The identity of this 
north Mexican vegetation region has been recognized by 
such phytogeographers as Seeman, Hemsley, and Harsh- 
berger. 
Vegetation Areas 
On the basis of intrusive and native floras, 2 modified by 
local climatic and edaphic factors, northwestern Mexico falls 
into four major vegetation areas, each susceptible to sub¬ 
division. These major divisions are expressions of latitude, 
continental position, and elevation in so far as these items 
condition temperature and precipitation. The Chihuahuan 
Desert area embraces the most continental interior portions 
of the elevated tableland that makes up Chihuahua and 
Durango. The Sonoran Desert area comprises the extra 
tropical high pressure lowland parts of Sonora and northern 
Sinaloa, which are somewhat in the rain shadow of Lower 
California's Peninsular Range. Between these arid areas 
is the Sierra Madre Occidental area, with a vegetation typi¬ 
cal of humid mesothermal highlands. The Sinaloa Tropical 
area coincides with the northern extension of the Pacific 
coast tropical forest and is characterized by a deciduous 
thorn shrub and forest. 
Chihuahuan Desert: The Chihuahua Desert area com¬ 
prises eastern and northern Chihuahua and eastern 
2. The botanical nomenclature here used conforms as closely as possible with the 
usaee in Paul Standley: “Trees and Shrubs of Mexico.” U. S. Nat. Herb.. Cont. vol. 23, 
1920-26. 
