Vegetation Zones of the Southwest - Design with the Desert: Conservation and Sustainable Development - page 138

FIGURE 7.2
Saguaro ( C. gigantea ) is one of the signature species of the Sonoran Desert.
While the Arizona Upland is an arboreal desert, the drier and warmer Lower Colorado
subdivision is dominated by sparse stands of creosote bush, white bursage ( Ambrosia
dumosa ), and brittle bush ( Encelia farinosa ). Frost-sensitive species that are able to survive in
this subdivision, particularly around the numerous ephemeral washes, include ironwood
( Olneya tesota ), brittle bush, and smoke tree ( Psorothamnus spinosa ).
The Sonora Plains, located entirely within the state of Sonora, Mexico, has a greater
percentage of summer precipitation with the attendant denser vegetation. The arboreal
species that have increased abundance include legumes, especially mesquite, while species
common to much of the Sonoran Desert, such as creosote bush, have limited distribution.
The Central Gulf Coast subdivision, located around the Gulf of California, is hyperarid
with undependable summer and winter rains. The flora is dominated by large stem
succulents including the massive cardón ( Pachycereus pringlei ) and trees that often remain
leafless throughout the year including palo verde, ocotillo ( Fouquieria macdougallii ), and
elephant tree ( Bursera spp.). The Vizcaino subdivision, on the Pacific Ocean, or western
side of the Baja California peninsula, has undependable rains that fall primarily in the
winter. The flora has many unique species; the most notorious being the eccentric-looking