Plant facilitation in extreme environments: The non-random distribution of saguaro cacti (Carnegiea gigantea) under their nurse associates and the relationship to nurse architecture

This study samples 247 saguaros (Carnegiea gigantea) and their nurses in 30 populations in the northern Sonoran Desert to determine whether saguaros are randomly distributed beneath their nurses’ canopies. Saguaro–nurse association patterns are compared across three nurse groups distinguished by canopy architecture characteristics (leguminous trees, small dense Ambrosia spp. shrubs, and the larger, open-canopied shrub Larrea tridentata). For all three nurse types, saguaros are significantly clustered near the center of their nurseʹs base. Clustering varies with nurse architecture, as saguaros are most tightly clumped under the most open-canopied species (L. tridentata), as more than 80% of saguaros were under the innermost 10% of L. tridentataʹs subcanopy area. Saguaros showed the least basal clustering about tree nurses, though not significantly less than Ambrosia spp. nurses. More saguaros establish under Ambrosia relative to its cover in sampled populations, while saguaros were underrepresented under L. tridentata. All analyses suggest that L. tridentata is an inferior nurse for saguaros. Different canopy architecture influences the extent of ground available to a nursed species to benefit from, for establishment and survival. While herbivory, rodents or other factors may play a role in these patterns, ultimately microclimate differences are the most likely factor shaping cacti–nurse associations in the harsh desert environment.