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A land of extremes, this region of southwestern
North America is topographically and biologically diverse. Biological
communities grade from boreal zone forest to nearly frost-free
dry tropical areas, and from extremely hot and arid desert, maritime
strand and mangroves to sphagnum bogs and moist montane meadows.
Elevations range from sea level to 3,000 meters. The area is a
transition zone for faunas and floras, especially those of tropical
affinity. In general, total annual precipitation decreases from
higher to lower elevations, from south to north, and from east
to west. Towards the drier regions the rainfall becomes increasingly
sporadic and unpredictable. Precipitation is generally bimodal
with peaks during both the warm season and the cool season. Some
areas, however, receive the larger proportion of moisture in the
fall and winter, while to the east rainfall peaks during the summer
monsoon. The eastern portion of the region is comprised of numerous
north-south trending mountain ranges. Other ranges, generally
decreasing in elevation westward, are spread across the rest of
the region, interspersed with broad valleys and expansive plains.
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© Drylands Institute, 2007-2008
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