Density-dependent shading patterns by Sonoran saguaros

Woody plants produce shading patterns that can alter soil evaporation rates and provide nurse-plant effects for seedling establishment. These effects depend on the density of woody plants and associated characteristics of canopy architecture such as height, and can be particularly important in deserts. The tallest stature woody plants in some desert ecosystems are columnar cacti, which have distinctly different architecture than other woody plants for which shading effects have been systematically assessed. Focusing on columnar cacti of the northern Sonoran Desert, we used hemispherical photography to evaluate the effects of saguaro cacti (Carnegiea gigantea) on microclimate along a gradient of increasing cactus density. Notably, incoming annual near-ground solar radiation was reduced by up to ∼10% for the highest density stand (156 cacti ha^-1), with spatial variation in shading patterns peaking at nearly the highest stand density (133 cacti ha^-1). The annual near-ground solar radiation reductions were more directly related to cacti density than to density of surrounding shrubs. Our results document that at high density, saguaro columnar cacti can have substantial effects on microclimate and, more generally, our results contribute to the growing library of relationships quantifying how shading patterns vary with woody plant architecture and density.