Establishment patterns of saguaro cactus (Carnegiea gigantea) at the microsite scale help explain saguaro regeneration and distributions in heterogenous, regional habitats

Premise: Establishment of long-lived perennial plants is a pivotal event that often leads to reproductive maturity. The population dynamics of the giant saguaro cactus (Carnegiea gigantea) have been investigated over large spatial areas, but establishment patterns have not been studied at the microsite (1 m) scale. Recent encroachment of non-native buffelgrass (Cenchrus ciliaris) has introduced an additional layer of complexity to our site at the Desert Lab on Tumamoc Hill in Tucson, Arizona, United States, with uncertain impact on saguaro establishment. We hypothesized that both biotic and abiotic microsite characteristics are correlated with saguaro establishment and that these correlations help explain saguaro distributions over larger spatial areas. Methods: We investigated microsite characteristics correlated with saguaro establishment, the degree and direction of those correlations, and microsite effects on growth rate and saguaro abundance using 40 years of repeat survey data from saguaro plots at the Desert Lab. Results: Saguaros established in microsites with higher native vegetation cover, intermediate rock cover, at more level sites, or sites closer to the north–south axis. Establishment was nearly zero in areas of high buffelgrass cover. The relative growth rate of young saguaros was determined in part by complex interactions of native vegetation cover with eastness and elevation. Abundance was positively affected by native vegetation cover and negatively by buffelgrass cover. Conclusions: Microsite characteristics help explain patterns in saguaro regeneration. Our results suggest that microsite characteristics be considered in future studies of the saguaro. Our findings will be useful for conservation, restoration, and management of saguaro populations.