California deserts

    
      The Mojave Desert
       Bounded on the west by the Tehachapi and Transverse Ranges, the Mojave Desert extends north through the Owens Valley to about Big Pine and eastward to Eureka Valley; in these areas it is complexely interdigitated with Great Basin Sagebrush Desert. In the east the boundaries are the Nevada State Line and the Colorado River. In the south, the San Gabriel, San Bernardino, Little San Bernardino, Cottonwood, Eagle, and Coxcomb Mountains form a convenient margin. Beginning at about Needles on the
Colorado River, and east of the above ranges, the Mojave merges gradually into the Colorado Desert. Within the Mojave Desert, basin elevations range from about 1000 to 3300 feet, but include Death Valley, below sea level. The Chuckwalla Mountains in Riverside County seem also to be Mojavean and may be considered an isolated fragment.
     Precipitation in the Mojave Desert is unimodal, winter rains and/or snow. Scattered summer rain may occur in the eastern portions, usually drifting north from the Colorado Desert. Unlike the Colorado Desert, the Mojave does not have as pronounced a rain shadow along the western margins and, in general, total annual precipitation decreases from west to east.
      As is true in the Colorado Desert, Creosote Bush Scrub (Figs. 276, 277) is the main vegetational feature, but the diversity of perennials, especially of trees, is much less. Within the Creosote Bush Scrub, which covers some 70% of the Mojave (Shreve, 1942), the codominants are Larrea tridentata and Ambrosia dumosa. Other perennials, in varying combinations, are Yucca schidigera, Achnatherium speciosum, Ephedra nevadensis, Atriplex spp., Opuntia spp., Lycium andersonii, and Grayia spinosa.
      The Shadscale Scrub occurs as a community between Creosote Bush Scrub and Sagebrush Scrub, usually on mountain slopes with dense, rocky soils. Dominants in this association are Atriplex confertifolia, Artemisia spinescens, and Sarcobatus vermiculatus.
      One of the most conspicuous biomes in the Mojave Desert is that of the Joshua Tree Woodland (figs. 286, 287), for Joshua Trees are large and often bizarre in appearance. This community is found between Creosote Bush Scrub and Pinon?juniper Woodlands on gentle slopes with fine gravelly?sandy loams. In addition to the Joshua Tree (Yucca brevifolia) there is a well developed understory featuring combinations of Juniperus californica, Ephedra spp., Krascheninnikovia lanata, Isocoma spp., Ericameria spp., Pleuraphis rigida, Salazaria mexicana, Hymenoclea salsola, Opuntia ramosissima, Muhlenbergia porteri, and Yucca baccata.
      Saltbush Scrub of the Mojave Desert is roughly equivalent to Alkali Sink in the Colorado Desert but trees are usually absent. Dry soils with low salt content are dominated by Atriplex confertifolia, A. hymenelytra, and A. polycarpa; these characterize the xerophytic phase of the Saltbush Scrub. This xerophytic phase (Figs. 282, 284) is widespread in basins and valleys and often exists between the halophitic phase and the surrounding biome, usually Creosote Bush Scrub. Soil of high mineral content on playas, sinks and seeps supports the halophytic phase Saltbush Scrub. Principal components here include Allenrolfea occidentalis, Nitrophila occidentalis, Salicornia subterminalis, Sarcobatus vermiculatus, and Suaeda spp. A Riparian variant of the halophitic phase occurs along semipermanent water courses and includes cottonwood and mesquite (Fig. 285).
      Scattered throughout the western Mojave Desert, above the Creosote Bush Scrub, there exists a vegetation of low, dark shrubs, often in combination with Joshua Tree Woodland, designated the Blackbush Scrub. The characteristic plant is usually blackbush (Coleogyne ramosissima). However, Blackbush Scrub has been extended to include communities with this characteristic appearance even when Coleogyne is absent. Eriogonum fasciculatum is an important member of this community which includes a wide and varied assemblage of such species as Salazaria mexicana, Ephedra spinescens, Krascheninnikovia lanata, Ericameria cooperi, Atriplex spp., Opuntia spp., and Juniperus californica. It often has the appearance of Joshua Tree Woodland without the Joshua trees.
      Most of the above information is synthesized from other sources. Data for the Colorado Desert are largely from Burk (1977). For the Mojave Desert we drew principally from Vasek and Barbour (1977). Other important sources included Munz (1970) and Johnson (1968).



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Date of this version 18, October 2003
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