The distribution and abundance of dominant species and their relationships with soil and climate were investigated using a variety of multivariate statistics across 30 plots that spanned 50 000 km2 of the Sonoran Desert. Relationships between the distribution of Carnegiea gigantea and several of its nurse plant species (Ambrosia deltoidea, Ambrosia dumosa, Cercidium microphyllum, Larrea tridentata, Prosopis spp., Olneya tesota) were documented. A general east-west gradient was observed where temperature increases and precipitation decreases westwards and calcium levels, total organic carbon, particle size, and soil pH increase westwards. Western areas with high temperatures and low precipitation may have elevated levels of calcite and thus a high pH; the low rainfall limits calcium dilution within the soil. The gradient in soil pH likely governs the range of several species (e.g., Ambrosia deltoidea, A. dumosa, and Larrea tridentata) that were delineated by calcic soils in western areas from eastern regions with lower soil pH and higher precipitation. Thus, the distribution of the dominant species reflects a temperature—precipitation—calcium—pH gradient. We found that the pH gradient follows the precipitation gradient as much as the dominant species follow the pH or rainfall gradients. Although climate is thought to dominate the distributions of these species, we found that soil pH and texture are intimately intertwined and that their removal from analyses resulted in poorer explanatory power of species distributions.
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Vol. 19 • No. 2