Populations of Two Sonoran Desert Plants, and Deductions as to Factors Limiting Their Northward Extension

Abstract

Recently discovered populations of ocotillo Fouquieria splendens, and smoke tree, Dalea spinosa in extreme southern Nevada were studied. Sparse distribution, reduced vigor and size all indicate that these populations have reached their northern geographic and ecologic limits. It is suggested that nothern dispersal and establishment of smoke tree is checked in extreme southern Nevada by lowered and more prolonged winter temperatures. Populations of ocotillo are disjunct and may be relict. Populations of ocotillo, Fouquieria splendens, Engelm and smoke tree, Dalea spinosa Gray have recently been located along the Colo- rado River in extreme southern Nevada. Neither species has been defi- nitely reported prior to this study as occurring in Nevada although Tidestrom (1925) and Muller (1940) suggest the possibility of oco- tillos being present in the most southern part of the state. Both species are found in the southern part of the Mohave Desert and are wide- spread in the more southern Sonoran Desert. The most northern ex- tension of the Sonoran Desert in California is in the vicinity of Needles, California where both species are found in suitable habitats. The population in Nevada represent a northern extension of the known range of both species into the northern Mohave Desert. No- menclature used in this paper follows Munz (1959). DESCRIPTION OF THE STUDY AREA. The area studied was the flood plain of the Colorado River on the Nevada side extending from south of Davis Dam to the California-Nevada boundary. The flood plain along this portion of the river is several miles wide, usually about three miles in width on the Nevada side, and varies from about 700 to 500 feet in elevation. The soil is primarily alluvial sand but small areas elevated above the main flood plain may be sandy loam. Several vegetation types are represented, the most common and widespread being creosote bush (Larrea divaricata) and burro-weed (Franseria dumosa). Thick stands of quail bush (Atriplex lenti- formis) are located near the river. Riparian populations of such species