A gradient analysis of Coleogyne communities in Southern Nevada
Species richness, density, and abundance in Coleogyne ramosissima (blackbrush) shrublands were compared on 15 elevational transects in the Spring and Sheep Mountain Ranges adjacent to the Las Vegas Valley. Coleogyne shrublands shared relatively broad upper and lower ecotones with Pinus-Juniperus and Larrea-Ambrosia vegetation, respectively. Lower Coleogyne ecotones generally had the highest species richness, and Pinus-Juniperus woodlands had the lowest species richness; Detrended correspondence analysis (DECORANA) suggested that elevation and soil depth were significantly associated with the distribution of stand and species groups identified from two-way indicator species analysis (TWINSPAN) in the Spring and Sheep Mountain Ranges. Precipitation and soil moisture were positively correlated with elevation; air and soil temperatures were negatively correlated with elevation; Coleogyne density was positively correlated with soil moisture, soil organic matter, Coleogyne water potential, Coleogyne leaf biomass, and Coleogyne stem and leaf phosphorus. Soil moisture and soil organic matter appeared to influence the distribution of Coleogyne at its lower elevational boundary in southern Nevada.