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THE
CALIFORNIA DESERTS
The area encompassed by this study
includes the California portions of the Mojave and Colorado Deserts
as well as a fringe of the Great Basin Sagebrush Desert. It includes
the California distribution of the Creosote Bush Scrub, the so-called
"hot desert". The Great Basin Sagebrush Desert is "cool
desert" in the sense of Wheeler & Wheeler (1973).
Our attention has focused on the "hot
desert" portions and we consider the "cool desert"
to be marginal, for few of the species present there also occur
in the Creosote Bush Scrub. Pinon-juniper Woodlands in the higher
desert mountain ranges are generally excluded from this study for
they, too, are "cool desert" habitats with few species
shared with the "hot desert".
All botanical names used below have
followed the terminology established in the recently published The
Jepson Manual Higher Plants of California (1993, James C. Hickman,
Editor).
The
Colorado Desert
We
have persisted in using this term within this report although, as
far as ants are concerned, there is little justification for doing
so. "Colorado Desert" is a term of convenience only and
must be understood as such. The Colorado Desert is the northwestern
extension of the Sonoran Desert; it includes the drainage of the
lower Colorado River and the Gila River in Arizona, the northwestern
lowlands of Sonora and
northeastern Baja California. For our purposes, it is bounded on
the south by the International Border and on the west by the mountains
of the Coast Ranges: the Laguna, Santa Rosa and San Jacinto Mountains.
The eastern boundary is the Colorado River, north to Needles. In
the northwest the Little San Bernardino, Cottonwood, Eagle and Coxcomb
Mountains form a partial boundary; east of these ranges the desert
floor gradually rises to an elevation of more than 1100 feet and
merges into the Mojave Desert through a gradual ecotone.
Although the Colorado Desert is among
the most arid regions in North America, there is a bimodal pattern
of precipitation, in contrast to the unimodal pattern of the Mojave
Desert.
According to Shreve (1925) summer
rainfall increases from about 5% of the annual total along the western
margin to about 34% at the Colorado River. Annual totals for the
desert are, however, essentially meaningless: the total may be the
result of a single storm, mostly lost due to high runoff rate; further,
rains are often highly localized and any one site may go for years
with no rain at all.
The preeminent vegetation community
in the Colorado Desert is Creosote Bush Scrub, with Larrea tridentata
and Ambrosia dumosa as codominants. Other important components
within this biome include Tiquilia (=Coldenia)
palmeri, Croton californicus, Psorothamnus schottii, P. emoryi,
Pleuraphis (=Hilaria) rigida, and Ephedra trifurca.
This community is characteristic of the bajadas and is especially
well developed in soils of low alkalinity (0.02% or less) and of
coarse, well drained texture. Creosote Bush Scrub is interposed
between high, rocky hillsides and Saltbush Scrub in the lowlands.
Where the bajadas are crossed by washes
and arroyos a relatively dense growth of trees is formed. This is
Wash Woodland (Figs. 289, 290), characterized by Cercidium floridum,
Olneya tesota, Psorothamnus spinosus, Lycium andersonii, Prosopis
glandulosa var. torreyana, Chilopsis linearis, Ziziphus obtusifolia
var. canescens, and Baccharis sarothroides. Very flat
black pebble beds ("desert pavement") may occur between
arroyos; these are often entirely without perennial vegetation.
Cactus Scrub (Figs. 279, 280), located
mostly on south-facing slopes is our only community dominated by
stem-succulents. Although the soil type can usually be called rocky,
the soil is fine-textured and rocks are interspersed. Intermediate
sized, or gravelly, soil particles are usually absent. Dominant
plants here include: Opuntia spp. (including O. bigelovii),
Ferocactus cylandraceus, and Echinocereus engelmannii.
On sites, usually in rocky canyons, with a permanent water supply
we find Palm Oases, visually dominated by Washingtonia filifera
(Fig. 278). Soil salinity is low in the root zone but rises toward
the surface where salt crust is often present. Other important floral
elements may include Sporobolus airoides, Juncus acutus
subsp. leopoldii, Pluchea sericea and Isocoma acradenia.
Tamarix (an introduced tree native to the Arabian Peninsula), and
Prosopis glandulosa var. torreyana may also be present.
Once a prominent feature of the Salton
Sink, Saltbush Scrub is now largely under cultivation. This is a
lowland community of sandy loamy soils with a salinity range of
0.2-0.7 %. Dominant perennials in this biome are Atriplex canescens,
A. polycarpa, Prosopis glandulosa var. torreyana,
and Isocoma acradenia var. eremophila.
Along the Colorado River and the
margins of the Salton Sea are belts of wet, heavy soils with a salinity
ranging from 0.5-2.0 %. Codominant plants defining the Alkali Sink
biome are Allenrolfea occidentalis and Suaeda moquinii.
Other important elements are Tamarix spp., Pluchea sericea, Baccharis
salicifolia, Atriplex lentiformis, Prosopis pubescens, Salix goodingii,
and Populus fremontii.
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