Mojave-Sonoran transition

Mojave-Sonoran transition
Name	Mojave–Sonoran transition
Country	United States; Mexico
States	California; Arizona; Nevada; Sonora; Baja California
Biome	Deserts and xeric shrublands

Contents

Geography and Boundaries

The transition occurs where physiographic provinces recognized by the United States Geological Survey and mapped by the Geological Society of America intergrade between the Mojave Desert high basins and the lower-elevation Colorado Desert portion of the Sonoran Desert. Boundaries are delineated using ecoregions from the World Wildlife Fund and floristic provinces referenced by the Jepson Herbarium, Arizona Game and Fish Department, Desert Botanical Garden, and the National Cooperative Soil Survey. Major landmarks within the zone include the Little San Bernardino Mountains, San Bernardino County, Yuma County, Imperial County, Colorado River, and drainages such as the Gila River and the Bill Williams River. Transportation corridors like the Interstate 10 and the Interstate 8 traverse the area, intersecting land managed by the Bureau of Land Management, US Fish and Wildlife Service, and tribal lands of the Quechan Indian Tribe and Cocopah Tribe.

Climatic gradients are defined by data from the National Oceanic and Atmospheric Administration, Western Regional Climate Center, and paleoclimatic reconstructions published by researchers at Scripps Institution of Oceanography and the University of California, Santa Barbara. The region exhibits a mix of winter-dominant precipitation patterns typical of higher-elevation Mojave Desert sites and bimodal precipitation associated with Sonoran Desert monsoon influences described in studies by the American Meteorological Society and the Desert Research Institute. Temperature regimes reflect records kept by the National Weather Service with seasonal extremes documented near Death Valley National Park and moderated lowlands near the Gulf of California coast adjacent to Puerto Peñasco. Microclimates created by orographic effects in ranges such as the Chuckwalla Mountains and riparian corridors along the Colorado River Indian Tribes influence species distributions noted in reports by the Nature Conservancy.

Biotic assemblages show admixture of taxa cataloged in collections at the Natural History Museum of Los Angeles County, Royal Botanic Gardens, Kew correspondence, Field Museum, and regional herbaria including the Jepson Herbarium and University of Arizona Herbarium. Characteristic plants include elements typical of Joshua Tree National Park associations (e.g., Yucca brevifolia documented in studies by the National Park Service) alongside Parkinsonia florida and Carnegiea gigantea occurrences verified by the Arizona-Sonora Desert Museum. Faunal overlap includes records for Mojave ground squirrel specimens curated by the California Department of Fish and Wildlife, desert bighorn sheep monitored by the Arizona Game and Fish Department, and avifauna such as Gila woodpecker and LeConte's thrasher observed in surveys by the Audubon Society and Cornell Lab of Ornithology. Herpetofauna studies by the American Society of Ichthyologists and Herpetologists report shared ranges for species like the Common chuckwalla and Mojave rattlesnake. Plant community mosaics—from creosote bush scrub noted in monographs by the Ecological Society of America to microphyll woodlands recorded by the International Union for Conservation of Nature—reflect complex zonation.

Geologic mapping by the United States Geological Survey and publications in the Geological Society of America Bulletin describe faults, alluvial fans, and basin-and-range structures that create edaphic heterogeneity influencing soil classification in the United States Department of Agriculture soil survey and the National Cooperative Soil Survey. Substrates include granitic outcrops, volcanic tuffs noted near the Chocolate Mountains, and playa deposits adjoining the Salton Sea region studied by researchers at the Scripps Institution of Oceanography. Soil salinity, alkalinity, and gypsum concentrations are documented in reports by the Soil Science Society of America and influence plant distributions referenced in floristic treatments by the Jepson Manual.

Indigenous presence is recorded for the Cocopa, Hia C-ed O'odham, Chemehuevi, and Quechan peoples in ethnohistoric accounts archived by the Smithsonian Institution and the Library of Congress. Historic land uses include routes of the Southern Emigrant Trail, waterworks associated with the All-American Canal, mining claims listed in records at the Bureau of Land Management, and grazing pressures chronicled by the California Department of Fish and Wildlife and Arizona State Land Department. 20th-century developments by entities such as the Southern Pacific Railroad, Union Pacific Railroad, and military installations like Marine Corps Air Station Yuma altered landscapes, while renewable energy projects sited by companies regulated under the California Energy Commission and the Bureau of Land Management introduce contemporary land-use conflicts addressed in environmental assessments prepared for the National Environmental Policy Act process.

Conservation initiatives by organizations including the Nature Conservancy, Desert Protective Council, National Park Service, Bureau of Land Management, and US Fish and Wildlife Service focus on protected areas such as Joshua Tree National Park, Saguaro National Park, and designated wildlife refuges. Threats identified in assessments by the Intergovernmental Panel on Climate Change, US Global Change Research Program, and state agencies include climate warming, altered precipitation regimes, invasive plants documented by the California Invasive Plant Council, habitat fragmentation from infrastructure like Interstate 10 and Interstate 8, groundwater depletion associated with irrigated agriculture in Imperial Valley tied to the All-American Canal, and policy challenges addressed in litigation at the United States District Court for the Central District of California and planning hearings before state agencies such as the California Coastal Commission and Arizona Corporation Commission. Conservation planning incorporates data from the US Geological Survey, modeling by the National Center for Atmospheric Research, and community engagement with tribal governments including the Colorado River Indian Tribes.