Vegetation Zones of the Southwest - Design with the Desert: Conservation and Sustainable Development - page 136

and the Sonoran Desert (centered in Arizona and Sonora, Mexico). Because of its location
at higher latitude, the Great Basin Desert is often referred to as a cold desert , while the
remaining deserts in our region are regarded as hot deserts .
7.3.1 The Sonoran Desert
Because of its biotic diversity, the Sonoran Desert is typically further subdivided into
at least five subdivisions: the Lower Colorado River, Arizona Upland, Plains of Sonora,
Gulf Coast, and Vizcaino subdivisions. 11-13 The richness of the flora in the Sonoran Desert
arises in part because of climatic conditions (see Chapter 3) but also because its location
allows a mixing of species from both temperate zone and tropical zone floras. The
Sonoran Desert is quite geographically complex with numerous mountain ranges, often
called sky islands , occurring throughout the area. Although each of these subdivisions is
important, our primary focus will be on the Lower Colorado River and Arizona Upland
subdivisions. The Lower Colorado River surrounds the lower Colorado and lower Gila
Rivers and is the hottest and driest subdivision. The Arizona Upland, on the other hand,
is located on the eastern and northern portions of the Sonoran desert and is the highest
and coldest subdivision.
Across all of the Sonoran Desert, the biological diversity is a function of its unique climate.
The climate ranges from semiarid in the higher elevations to hyperarid along the lower
Colorado River and has two distinct seasons of rainfall. Winds originate from the west
during the winter, bringing moisture from the Pacific Ocean across the desert, resulting
in storms that are often widespread and of moderate intensity (see Chapter 3). During the
summer months, the direction of the wind shifts to a prevailing southern and eastern flow,
bringing moisture in from the Gulf of California, the tropical eastern Pacific, and the Gulf
of Mexico. When this increased humidity is combined with intense summer temperatures,
the results are convective storms that are often localized and intense. Another distinctive
feature of the Sonoran Desert is its mild winters with frost seldom occurring except in the
Arizona Upland.
The result of the bimodal pattern of precipitation is a desert that is both rich in species
diversity and complex structurally compared to many, if not most, other deserts. This
diversity is expressed through two principal themes. First, because of the abundance of
large shrubs or small trees, in contrast to the low shrubs characterizing the other deserts, the
Sonoran Desert is often considered an arboreal desert (Figure 7.1), or even a thornscrub at its
upper-elevation limits. These arboreal species serve as the signature species for the desert
and, for instance, in the Arizona Upland, include such species as saguaro cactus ( Carnegiea
gigantea ; Figure 7.2) commonly growing to a height of 40-50 ft, and the palo verde ( Parkinsonia
spp.), which grow to 12-16 ft. Second, two distinct ephemeral floras exist corresponding
to the bi-seasonality of precipitation. These ephemeral species comprise approximately
half the species in the region and include a large number of ruderal, invasive species of
Mediterranean origin that have recently entered the region representing a vegetation
change 14 equal to geologic time-frame changes. 15 These invasive species depress native
wildflowers and promote wildfires that may lead to the loss of some fire-sensitive species
such as cacti. The topic of invasive species and development is discussed in a later chapter.
Across the entire Sonoran Desert, creosote bush ( Larrea tridentata ) is the dominant shrub
(Figure 7.3). The creosote bush range also extends into the Mojave Desert to the west and
the Chihuahuan Desert to the east. Creosote bush is a small (3-10 ft in height) evergreen
shrub that provides an excellent example of the stress-tolerance strategy. It combines attri-
butes of resisting water loss (e.g., thick resinous leaves with high stomatal resistance and