Sonoran Desert Granite Boulder Scrub

This ecological system occurs in foothills and mountains of Sonora, Mexico, and extends north across the border into southern Arizona. It is found on low- to mid-elevation granitic outcrops. Tropical genera of Jatropha and Bursera become codominants in dense to sparse vegetation transitioning upslope from Sonoran Paloverde-Mixed Cacti Desert Scrub (CES302.761). Diagnostic species are Bursera microphylla, Jatropha cuneata, Nolina bigelovii, Parkinsonia microphylla, or Rhus kearneyi.

[from M088] This warm-temperate to subtropical, semi-desert type occurs in the southwestern U.S. and adjacent Sonora and Baja California, Mexico. It forms the vegetation matrix in broad valleys, lower bajadas, plains and low hills in the Mojave, western Sonoran and Lower Colorado deserts. Elevation ranges from -75 to 1200 m. Sites are gentle to moderately sloping. Substrates are typically well-drained, sandy soils derived from colluvium or alluvium, and are often calcareous with a caliche hardpan and/or a pavement surface. Precipitation is markedly unimodal with most falling in the winter months associated with winter storm tracks reaching the desert from the Pacific Ocean. Stands extend north into the broad transition with the Great Basin and at higher elevations on desert mountains above Larrea tridentata - Ambrosia dumosa desertscrub and below the lower montane woodlands (700-1800 m elevation) that occurs in the eastern and central Mojave Desert. Stands in the Arizona Sonoran Desert occur on lower slopes of mountains, foothills, hillsides, mesas, upper bajadas, and less commonly in valleys and plains in southern Arizona and extreme southeastern California. Elevations range from 150-1070 m (Shreve and Wiggins 1964). Climate is semi-arid. Summers are hot and winters rarely have freezing temperatures. Freezing winter temperatures limit the elevational and northern extent of these stands. Annual precipitation has bimodal distribution with about half of the rain falling during July to September and a third falling from December to March. Farther west, the proportion of summer precipitation decreases until there is not enough summer moisture to sustain Carnegiea gigantea (Barbour and Major 1977). Stands in the subtropical central Gulf of California coast and adjacent portions of the lower Colorado River valley region of the Sonoran Desert occur on gentle to steep, rocky sites. It extends north into the extreme southwestern U.S. and northern Sonora.

At Organ Pipe National Monument, stands typically occur on southerly aspects between 550 and 765 m (1800-2500 feet) elevation. In general, sites have gentle to steep slopes. Sites in northern Baja and southern California occur on isolated maritime coastal bluffs and terraces. Sites in the Vizcaino Region of central Baja California reach several kilometers inland. These areas are frost-free and receive the least annual precipitation of the California and Baja California coastal shrublands, most of which falls in winter. Climate is extremely arid with mean annual precipitation of less than 100 mm, which occurs mostly in the summer-early fall season (monsoon). Precipitation is augmented by summer fog drip. Sonoran stands are extremely arid with mean annual precipitation of less than 100 mm, which occurs mostly in the summer-early fall season (monsoon). Extended drought is common which favors plants with water storage (Turner and Brown 1982). Semi-desert vegetated and sparsely vegetated sandsheets and dunes that are stabilized or partially stabilized are included in this macrogroup. They occur as small to large patches or as a complex of active and stabilized dunes. These sand deposits often form on the leesides of desert playas and basins that serve as a source for the sand. Substrates are variable, but typically shallow, well-drained, rocky or gravelly, coarse-textured soils derived from colluvium or alluvium, except for the sand deposit vegetation included in the macrogroup, which is eolian. Parent material is usually gravelly alluvium and colluvium, derived from basalt and other igneous or metamorphic rocks.

[from M088] This type occurs in warm to subtropical semi-arid regions. Most characteristic species are frost-sensitive as only vegetation in the Mojave Desert or at high elevation or in the northern extent of the Sonoran Desert experience frost or extended freezing temperatures.

Occurs in foothills and mountains of Sonora, Mexico, and extends north across the border into southern Arizona.