The Sonoran Desert Region
Authors: Violet Gray, David Stoms, and Frank Davis
Plant Community Types
The portion of the Sonoran Desert which lies west of the Colorado
River and north of the Mexican border (also known as the Colorado
Desert), is characterized by low annual rainfall and extreme heat.
The region covers 29,187 km² and is generally described as
the drainage areas of the Salton Sea Basin and the western bank
of the Colorado River Drainage (Figure SON-1). The division between
the Sonoran and Mojave deserts is relatively indistinct, but generally
the Sonoran region is lower, flatter and warmer (Hickman 1993).
Shaded relief image of the Sonoran Desert Region.
the lack of severe winter weather, the mountains have tended to
weather in place. Weathered material does not travel far, but falls
short distances and collects at the base of the mountains in deep,
broad bajadas. These are stable environments and though the soils
are poorly developed, tending to consist mostly of large gravel
and cobble, do provide one of the richest biotic environments in
the region. The plains between these ranges are varied in soil structure
ranging from clay to shifting sand dunes. Low elevation soils contain
the finest particles of the desert environments. While clays and
silts hold water well by bonding with the hydrogen atom in water
molecules, they tend to hold the water so tightly in very arid environments
that plants are not able to make use of the moisture. Also these
soils are often alkaline because salts are carried in running water
down into these basins where the water evaporates leaving minerals
in the soil.
Much of the
Sonoran Desert, which covers most of Arizona, and much of northern
Mexico, is characterized by a bimodal pattern of rainfall. The thirty
year normals for Arizona weather stations reflect a strong pattern
of high rainfall during the month of August, that month being the
monthly period of highest rainfall throughout the year. This pattern
is largely thought to account for the lushness of the Sonoran Desert
at large, and for the presence of extensive thorn and stem succulent
woodlands. The thirty year precipitation normals for the Colorado
Desert do show this bimodal pattern although it receives less precipitation
than in Arizona. There is a strong east-west gradient in seasonal
rainfall in this region of the desert with winter precipitation
diminishing rapidly from west to east and summer precipitation diminishing
from east to west. Summer rains in California are not nearly as
pronounced as those in Arizona where many stations report considerably
higher rainfall during the month of August than in January or December.
Desert is the most varied and floristically rich of the four American
deserts. The Colorado Desert portion in California, however, has
a relatively small number of perennial species, and of these, few
are rare. Those perennial species which have been able to invade
and persist in a particular desert habitat tend to occur as dominants
wherever that habitat type recurs. The small number of perennials
is made up for by a large number of herbaceous ephemerals which
make up about half of the entire complement of species in the desert.
The primary floristic character of this region is the overwhelming
dominance of microphyllous shrubs, such as Creosote Bush (Larrea
tridentata) and Burro weed (Ambrosia dumosa) across most
of the plains and mesas of the desert. The upper bajadas and rocky
slopes provide habitats for most of the other dominant life forms
of the desert. They provide one of the richest biotic environments
in the region. Many plants cannot tolerate the alkaline conditions
that characterize the lowlands, so that plant species diversity
increases with an increase in elevation, and then decreases again
because of the poor growing conditions on the unweathered rocky
slopes (Shreve and Wiggins 1964, Parker 1991). On these slopes can
be found most of the stem and leaf succulents, as well as extensive
stands of Ocotillo (Fouquieria splendens) . In addition they
host large stands of arborescent shrubs such as Smoke Tree (Dalea
spinosa), and Palo Verde (Cercidium floridum) which line
the drainage reticulations and incised ravines.
contribute strongly to the life-form variation of desert vegetation.
The highest diversity of these types is along the western border
of the desert in the vicinity of the Lower Borrego Valley. Wetland
types are also quite diverse, and include palm oases dominated by
the Fan palm (Washingtonia filifera), freshwater marshes,
saltwater marshes, riparian streams, desert washes, desert arroyos,
salt-impregnated playas, and saline lakes.
Much of the
study area has been subjected to intensive land use. Most of the
mountainous areas of the desert have been mined, some quite heavily.
Urban and agricultural uses are concentrated in the low valley areas
of the Salton Trough and the Colorado River Basin. In order for
agriculture to be undertaken as a major industry several large aqueducts
were built, effecting not only the irrigated lands, but also those
lands through which the aqueducts were built. There is a strong
recreational component to land use in the desert which ranges from
urban resort use such as at Palm Springs, to outdoor recreation
activities such as water-based sports on the Colorado River, and
off-road vehicle travel and competition. The development of bajadas
for ranchettes and vacation homes has caused considerable disturbance
in areas of high plant diversity.
In 1994, Congress
passed the California Desert Protection Act (CDPA) which established
dozens of new wilderness areas in the Sonoran and Mojave Desert
regions and upgraded national monuments to national parks. The changes
made by the CDPA were incorporated into the CA-GAP stewardship map
from a preliminary 1:1,000,000 scale digital map of the Bureau of
Land Management state office. Both pre- and post-bill versions of
the land management data were compiled to evaluate the changes caused
by the legislation. At the time these maps were compiled, the CDPA
had just been enacted and only the version of the boundaries from
an earlier version of the bill were available. Only the areas designated
in the bill were incorporated into the GAP land stewardship coverage,
but it should be noted that the boundaries may be different in the
final version of the legislation than those used for GAP.
Desert Protection Act nearly quadrupled the proportion of Status
1 managed areas from 5.8% to 23.1% in the Sonoran Desert region
(Figure SON-2; Table SON-1). Roughly one-quarter of the land area
is in private ownership. Only the Mojave Desert and the East of
the Sierra Nevada regions have smaller proportions of private land.
Public land is predominantly managed by the Bureau of Land Management
(50.9% of the total land area), Department of Defense (7.5%), National
Park Service (1.5%), USFWS (0.9%), Indian reservations (1.7%), USFS
(0.3%), state parks (7.5%), California Department of Fish &
Game (0.6%), State Lands Commission (1.6%), and TNC (0.2%). The
GAP land ownership database also includes 250 ha of county park.
The checkerboard configuration of public and privately owned lands
in much of the region poses a difficult challenge for biodiversity
management. The dominant managed area of the region is Anza-Borrego
Desert State Park, covering more than 2,000 km².
Management status of lands in the Sonoran Desert Region. See text
for definitions of management levels.
Table SON-1. Area
and percentage by management status level of the Sonoran Desert Region
(The table compares management levels before and after the passage
of the California Desert Protection Act of 1994, excluding the Salton
levels are distributed unevenly across elevation zones (Figure SON-3).
In particular, the lands below sea level in the Salton Trough are
predominately in private ownership (84.5%), while higher elevation
zones are at least 80% in public ownership. Lands below 500m elevation
are represented at below average levels while those above this elevation
are better represented than the regional average. Quantified as
a bias index, the region shows a statistical bias in representation
of elevation zones, but this bias is less than in most other regions
of the state.
Comparison of the proportion of managed areas with all lands in the
Sonoran Desert Region by elevation zones.
CDPA certainly increased the number and area of designated areas
(Figure SON-2), we were curious how much it changed the actual level
of protection. For instance, did the legislation reduce the representational
bias index or did it merely perpetuate any existing bias by expanding
protection of well-represented environments? Since some of the new
wilderness areas were already managed as Areas of Critical Environmental
Concern, did the Act really increase the level of protection significantly?
The CDPA increased
the representation of the region in status 1 lands from approximately
6% to 23% and increased the mean size of status 1 BMAs from 30 to
37 km². While there is still a significant bias in representation
expressed by elevation zones, the bias did decrease after the legislation.
The CDPA improved the representativeness of the network of BMAs
in the Sonoran Desert region, but some bias still remains. This
bias is of less importance, however, because most plant communities
have relatively high levels of protection in current biodiversity
The basic approach
in delineating areas was to divide the desert region into rocky
slopes, bajadas, and valley floors as viewed on 1990 TM images (see
Gray 1994 for details of the mapping process). An extensive field
survey of the dominant perennial vegetation of the entire region
was undertaken for determining the floristic information. Because
of the extent of the area to be inventoried, the greatest coverage
of the desert could be affected by using the major highways and
other roads and tracks of the desert as transects. Approximately
10,000 miles of road were traveled and 86% of the map units were
viewed. Subsequent to the draft GAP land-cover map, the Bureau of
Land Management and the California Department of Fish and Game conducted
a joint effort to enhance the map with added detail for critical
wildlife habitats in their Northern and Eastern Colorado Desert
planning area (Dorweiler 1997). These modifications were incorporated
into the final GAP land-cover map.
The study area
covers more than 2.9 million hectares, and is divided into 421 polygons.
The largest polygon is 232,125 hectares, and the smallest is 17
hectares. Mean polygon size is 6,933 hectares.
A total of
33 species were mapped as overstory dominants within the Colorado
Desert. Another six taxa were mapped at the genus or family level.
Twenty-five of these species are considered to be characteristic
of the Colorado Desert flora, another four are species with widespread
distribution in the coastal chaparral communities, and two are species
with Mojavean affinities. Two species of conifers, Pinyon pine (Pinus
monophylla), and California juniper (Juniperus californica)
generally occur at elevations higher than those typical of this
low desert, and thus are found only near the boundaries between
the desert and bordering regions. While the saltbush (Atriplex)
species are strongly characteristic of alkaline environments in
the low desert, the field methods and scale of this map were not
appropriate for mapping at the species level. While one species
may dominate a small area, dominance shifts among several species
at a grain below the landscape scale of the map in response to microenvironmental
effects. Therefore only one species with fairly coherent and wide-ranging
dominance, saltbush (Atriplex canescens), was mapped individually.
In the analysis of areal distribution, A. canescens is listed
both individually and in the class of Atriplex spp. The grasses
were divided into annual and perennial types, because once again
it was not possible to map individual species at this scale. Most
of the perennial grass areas mapped were dominated by Galeta (Hilaria
rigida) and mostly occur on sandy soils. The annual grasses
are generally introduced species, Schismus spp. being particularly
widespread. Species characteristic of wet environments such as members
of the Salicaceae and the Cyperaceae, while expected
to occur within the desert, can be found only rarely in the desert
under unusual conditions. The endemic Fan palm (Washingtonia
filifera) occurs throughout the canyonlands of the Salton Trough,
but is only included in the special species category of the database,
because the areal extent of palm oasis is quite small compared to
the landscapes mapped here.
A total of
16 CNDDB types were mapped for the region (Table SON-2) plus 4 land
use and non-vegetated cover types. Types not mapped are largely
wetland types which would be expected to occur at a finer scale
than the GAP land cover map. In addition there are several upland
types which occur in stands of very small extent when they do occur
or are types which are more characteristic of Arizona's Sonoran
Desert vegetation and may only occur in California as very small,
Table SON-2. Percent
area of each CNDDB community type at each management status level
in the Sonoran Desert region.
Community Name (Holland 1986)
Mapped Distribution (km²)
Creosote Bush Scrub
Desert Mixed Scrub
Creosote Bush Scrub
Mixed Woody Scrub
Mixed Woody and Succulent Scrub
Cottonwood-Willow Riparian Forest
Dry Wash Woodland
Pinyon and Juniper Woodlands
Pinyon and Juniper Woodlands
Total - Natural Communities
Total - All Lands
Most of the
sixteen community types are relatively well represented in Status
1 or 2 management areas since the passage of the California Desert
Protection Act in 1994. More than 20% of the mapped distributions
of ten of the 16 types are within biodiversity management areas.
The Peninsular Pinyon and Juniper Woodlands community has over 70%
of its distribution in the Sonoran Desert region mapped as Status
1 or 2. The two most widespread types of the region are very well-represented:
Sonoran Creosote Bush Scrub (25%) and Sonoran Desert Mixed Scrub
(48%). Desert Dry Wash Woodland has just under 20% of its distribution
in such managed areas. Desert Sink Scrub is currently mapped with
over 50% in Status 2 areas but was only mapped over 8 km².
Because this type has been largely lost to agricultural uses and
the flooding of the Salton Sea, we are concerned about its long-term
On the basis
of management status alone, only two relatively rare types and one
relatively widespread type would be of concern: Sonoran Cottonwood-Willow
Riparian Forest and Mesquite Bosque, both of which were mapped almost
entirely on private lands, and Alkali Playa which is predominantly
on unreserved public land. The former two types are rated by the
Natural Heritage Division as rare and highly threatened (California
Department of Fish and Game, unpublished list, dated 4/12/91). Mojave
Mixed Woody Scrub and Mixed Woody and Succulent Scrub also have
less than 10% in Status 1 and 2 and over 80% in Status 4. These
types however are more characteristic of the Mojave Desert region
to the north where they are very well-represented and are therefore
of less concern in the Sonoran region.