SRER consists of 53,159 acres about 35 miles south of Tucson in Pima County, Arizona (fig. 1). It lies at the foot of the northwestern edge of the Santa Rita Mountains. It is characterized by small areas of steep, stony foothills and a few isolated buttes but the greater part consists of long, gently sloping alluvial fans. Upper fans slope rather steeply and are cut by canyons and arroyos. At lower elevations, the slope diminishes to about 100 feet/mile and drainages become relatively shallow. Some parts of the lower range are characterized by terraces, breaks, or low escarpments and numerous gullies (Youngs and Poulson 1931). Elevations range from 2,900 feet in the northwestern corner to about 5,200 feet in the southeastern part. Average annual rainfall increases with elevation, from 10 inches at 2,900 feet to almost 20 inches at 4,300 feet.
The soils are representative of those developed under southwestern arid conditions. Most consist of, or developed from, recent alluvial deposits. Three soil orders (Aridisols, Entisols, and Mollisols) and 21 soil series have been described by Clemmons and Wheeler (1970). The soils present an interesting range of characteristics due directly or indirectly to difference in elevation and proximity to the Santa Rita Mountains. With greater elevation and proximity to the mountains, rainfall increases, temperatures decrease, soils are darker, soils have a higher content of organic matter, and soils are more deeply leached of soluble salts. Erosion is most pronounced in the lower elevations coincident with vegetation density. Additional soil characteristics are provided in appendix A.
Major vegetation changes have occurred since the early 1900's. Velvet mesquite (Prosopis juliflora var. velutina) is the dominant overstory species on 20,000-30,000 acres where shrub-free grassland dominated 80 years ago. Mesquite and prickly pear cactus are major species above 4,000 feet, but other species including acacia (Acacia greggii, A. angustissima), mimosa (Mimosa biuncifera, M. dysocarpa), and falsemesquite (Calliandra eriophylla) comprise 65% of the cover in this zone compared to 21% below 3,000 feet. Mesquite, burroweed (Haplopappus tenuisectus), and cholla cactus (Opuntia fulgida, O. spinosior, and O. versicolor) attain highest densities between 3,200 and 3,600 feet elevation (Martin and Reynolds 1973). Lower elevations (<3,200 feet) are dominated by creosote bush (Larrea tridentata).
Species composition of perennial grasses changes with elevation and rainfall (Martin and Reynolds 1973). Santa Rita threeawn (Aristida glabrata), Rothrock grama (Bouteloua rothrockii), and bush muhly (Muhlenbergia porteri) are important species of middle and lower elevations. Various species of gramas (Bouteloua eriopoda, B. curtipendula, B. filiformis, B. chondrosioides, B. hirsuta) are widely distributed at higher elevations. Arizona cottontop (Trichachne californica) and species of threeawns (Aristida hamulosa, A. ternipes) are common at all elevations. Lehmann lovegrass (Eragrostis lehmanniana) is presently the dominant grass over nearly 40% of the range.
Annual vegetation is most abundant in areas with light to moderate density of perennial grasses and in areas where native grasses persist over lehmann lovegrass (Medina 1988). The prevalence of many dry washes and small gullies afford microhabitats where cool- and warm-season annuals can propagate. See appendix D for a complete list of plant species found on SRER.
A diverse fauna characteristic of the semidesert habitats is found on the SRER. Important game species include mule deer (Odocoileus hemionus), white-tailed deer (O. virginianus), collared peccary (Tayassu tajacu), Gambel's quail (Lophortyx gambelii), scaled quail (Callipepla squamata), mourning dove (Zenaida macroura), white-winged dove (Zenaida asiatica), and desert cottontail (Sylvilagus auduboni). Many other species of mammals, birds, and reptiles are common (see appendix B).
Hunting, nature studies, and bird watching are the principal recreational activities. Hunting has been largely restricted to small game, with occasional special-season big game hunts. Thousands of visitors travel through SRER each year enroute to Madera Canyon, a popular bird-watching area nearby.
The geology varies from simple strata of recent deposits of alluvium to extensive thrust faulting. Alluvial caps cover about 95% of the area. These gravel strata vary in thickness from about 400 feet in higher elevations to well over 2,000 feet in the lower parts. Small outcrops of granodiorite, limestone, sandstone, and other conglomerates constitute the remaining 5%.