Climates and Vegetation Pattern across the Mojave/Great Basin Desert Transition of Southern Nevada

Abstract

Plant communities of the transition between the Mojave and Great Basin deserts of southern Nevada are under the primary control of climatic variables. Rainfall increases and temperature decreases according to large increments of increase in elevation of the drainage basins from S to N. Within the basins, the climates and vegetation pattern are primarily under the control of patterns of air circulation and nocturnal cold air accumulations and secondarily, of edaphic factors. Minimum temperature and maximum mean rainfall tolerances of Mojave Desert Larrea (creosote bush) communities are exceeded across this transition as, apparently, are the mean maximum temperature and minimum rainfall tolerances of the Great Basin Artemisia (sagebrush) communities. In those communities which characterize the transition (Coleogyne, Grayia-Lycium andersonii, Lycium pallidumGrayia, Lycium shockleyi), the Mojave and Great Basin temperature and rainfall regimes occur in various definable combinations. Only Atriplex confertifolia (shadscale) communities cannot be so defined; these occur along topographic gradients in both Mojave and Great Basin Desert climates. Contrasts in temperature regimes and their effects on vegetation in the lowlands of closed basins are illustrated by air temperature and plant data from two adjacent sites, with only 1.5 m elevation difference, near the playa of Frenchman Flat. All relationships are inferred from data collected during 10 years on a network of study sites on the Nevada Test Site of central-southern Nevada, whose drainage basins encompass the transition from the Mojave to the Great Basin Desert. INTRODUCTION In the basin and range topography of western United States the pattern of desert shrub vegetation of the intermontane valleys is often conspicuously defined and repeated from basin to basin. The area comprises the Basin and Range province of Fenneman (1931), which in part includes the Great Basin of Billings (1951) and most of the Great Basin Division of Cronquist et al. (1972). In the southern part-the Mojave, Sonoran and Chihuahuan deserts-the bajadas are mostly dominated by creosote bush (Larrea tridentata) communities. Along an irregular boundary extending from southwestern Utah across southern Nevada to southern California, these communities are replaced to the N by mostly sagebrush (Artemisia tridentata) and shadscale (Atriplex confertifolia) vegetation-the Great Basin Desert. These broad vegetation zones were recognized, mapped and described by Billings (1949, 1951) in relation to the macroclimate, topography 1 Work performed under U. S. Atomic Energy Commission Contracts AT (04-1) Gen-12 and AT(ll-1) 2307, and at the Nevada Test Site as a part of CETO Proj. 61.5.4.