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Sonoran Desert Biotic Province
(from Brown, 1994)
Arizona contains two of the five subdivisions recognized as constituting the Sonoran
Desert –the Lower Colorado River Valley subdivision and the Arizona Upland
subdivision. The other three lie to the south in
Mexico.
The flora (plant-life) of the Sonoran Desert
has its ancestors in the subtropical forests that
lie to the south in Mexico and beyond.
The modern Sonoran Desert formed roughly
8000 years ago as climate became drier.
The bimodal rainfall pattern within the
Sonoran Desert has resulted in a greater
structural diversity than in the other deserts of
North America. While the other North
American deserts are dominated by lowgrowing shrubs the Sonoran Desert has a large
arboreal (tree) component, large cacti, shrubs,
and numerous succulent plants.
Lower Colorado River Valley Subdivision (lower Sonoran Desert)
This is the largest and most arid subdivision of the Sonoran Desert. Rainfall averages
less than 8” of rain per year.
It is differentiated from the Arizona Upland Subdivision due to its warmer winters and
drier summers.
The most common plant community (Series) in the Lower Colorado River Valley
Subdivision of the Sonoran Desert is
the creosote-bush - white bursage
Series (Photo at left). These two
plants (below) either together or
alone occur over thousands of acres
in the broad valleys of the basin and
range. These two plants are
remarkable in their ability to survive
under extremely dry, hot conditions.
creosote
white bursage
Plant growth is both open and simple, reflecting the intense competition between plants
for water resources (right).
The desert is so dry that
vegetation is often
restricted to drainage-ways
(wash obligates), where
sand may hold moisture
from previous runoff events
(see below). Where plants
do manage to grow at a
distance from these
drainages they are often
smaller than the same plant
would be if growing along a
wash (wash facultatives).
Small trees are common along the larger drainage-ways, particularly trees of the
pea/bean family (Leguminaceae). Common small trees include: mesquite; palo verde;
ironwood; and smoketree. These trees have tiny leaves (microphyllous) and have
chlorophyll in or
beneath their bark
to help them
photosynthesize.
These small trees
also may grow at a
distance from
washes, except
smoketree, but will
be even shorter and
smaller.
Other plants grow only in these larger desert wash habitats due to their higher water
requirements (wash obligates). These species include Desert Willow (a tree); chuparosa;
desert honeysuckle; and canyon ragweed (all shrubs). Other shrubs are found growing in
the drier tiny shallow water-courses: catclaw acacia; burro-brush; Anderson thorn-bush;
and desert broom.
The most arid parts of this subdivision are extensive areas of desert pavement, gently
sloping surfaces covered by tightly fitting pebbles. Desert pavement supports only a
sparse seasonal cover of tiny ephemeral plants. Again however, tiny drainages crossing
through desert pavement areas will support small populations of perennial plants
demanding more water: saguaro; ocotillo; brittle-bush; palo verde, ironwood; bursage;
and creosote-bush.
Other plant communities take over for Creosote - White bursage where soil becomes
either sandier or finer.
Arizona Upland Subdivision (Upper Sonoran Desert)
Forms a narrow curving area to the north, northeast, and east of the Lower Colorado
River Valley Subdivision (see map).
This subdivision is slightly cooler and wetter than the LCRV Subdivision. Rainfall
ranges from 8 inches to 16 inches per year. *Phoenix gets 7 inches per year.
Most of this subdivision lies on slopes, broken ground, and on sloping plains dissected
by streams while very little lies on flat alluvial basin floors of the basin and range, as
does the LCRV Subdivision.
This plant community appears as a scrubland or low
woodland of leguminous (pea/bean family) trees with
open spaces filled with low shrubs and perennial
succulents. The trees are the same species as found in
the LCRV Subdivision farther west except here they are
not confined to the drainage-ways because there is
greater rainfall. The trees include Palo Verdes;
Mesquites; Ironwoods; Acacias.
The vegetation of this part of the Sonoran Desert is so
luxurious that some ecologists do not even consider it a
desert, but instead as a sub-tropical thorn-scrub.
Cacti are a large part of the vegetation in this
community. Cacti are classified in terms of their life
form as being stem succulents. The following cacti are
either restricted to this subdivision or are best represented in this community:
Buckhorn Cholla
Cane Cholla
Staghorn Cholla
Chain Fruit Cholla
Teddy Bear Cholla
Desert Christmas Cactus
Pencil Cholla
Klein Cholla
Devil's Club Ground Cholla
Saguaro
Organ Pipe
Senita
Night-blooming Cereus
Fishhook Pincushion
Thornber Pincushion
Fish-hook Barrel Cactus
Compass Barrel Cactus
The most common plant community (Series) within the Arizona Upland Subdivision of
the Sonoran Desert is the
Paloverde-Cacti-Mixed Scrub
Series (at left). This
community is dominated by
palo verde trees, with tall
columnar saguaro often
reaching above the trees. Often
the ironwood tree is found in a
secondary role to the palo
verde. Beneath the trees and in
the open spaces between the
trees can be found the
following plants:
trees
forbs
small shrubs
hackberry
desert zinnia triangle-leaf bursage
smaller acacia
little-leaved ratany
palo verde
brittle-bush
ironwood
feather duster
cacti
see above
large shrubs
creosotebush (but in much
greater numbers than in the
LCRVS
limber bush
ocotillo
jojoba
The creosote-bush-white bursage Series of the Lower Colorado River Valley
Subdivision and the palo verde-cacti-mixed scrub Series of the Arizona Upland
Subdivision come into contact with each other throughout the basin and range of Arizona.
The area in which they come into contact is called the ecotone. Within the ecotone a mix
of species of both communities is found as one community transitions to the next. This
ecotone is found along the margins of the desert valleys (basins). The creosote-bushwhite bursage Series dominates the warmer drier valleys while the Arizona Upland Series
dominates the pediments, bajadas and higher edges of the basins.
Animals
Mammals found in the Sonoran Desert as well as other N. American deserts include:
California Leaf-nosed bat; Coyote; Ring-tailed Cat; Black-tailed Jack-rabbit; Desert
Cottontail; Merriam's Kangaroo Rat; White-throated Woodrat; Desert Pocket Mouse; and
Desert Kangaroo Rat.
Mammals found only in the Sonoran Desert include: Bailey's Pocket Mouse; Roundtailed Ground Squirrel; and Arizona Cactus Mouse.
The young age of the Sonoran Desert is also reflected in its birdlife. The region is not
ancient enough for birds to have evolved that are restricted to the Sonoran Desert.
Common birds include: Road runner; Mourning Dove; Lesser Nighthawk; Verdin; Cactus
Wren; Black-tailed Gnatcatcher; Phainopepla; and Black-throated Sparrow
Birds that are found mainly in the Sonoran Desert include: Gambel's Quail; Costa's
Hummingbird; Gilded Flicker; and Gila Woodpecker.
Reptiles found in the Sonoran Desert as well as the Mojave Desert include; Chuckwalla;
Desert Tortoise; Desert Iguana; Spotted Leafnosed Snake; Rosy Boa; and Western
Shovelnose Snake.
The Sonoran Desert shares the Zebra-tailed Lizard with the Great Basin Desert, the
Mohave Rattlesnake with the Chihuahuan Desert, and the Banded Gecko; Desert Spiny
Lizard; Patch-nosed Snake; Glossy Snake; Western Ground Snake; and the Western
Diamondback Snake with other dry regions of the southwest.