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     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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Date of this version 18, October 2003
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