Do Cholla Cacti (Opuntia spp., Subgenus Cylindropuntia) use or need nurse plants in the Mojave Desert?

(1) Three species of cylindropuntia cacti, or chollas, occur in the Mojave Desert of southern California: Opuntia echinocarpa, O. ramosissima, and O. acanthocarpa . The spatial distributions of chollas of different sizes, and presumably ages, were examined at three sites of similar elevation, soils and uniform topography, but at which the densities of the different chollas and of certain other shrubby plants are different. (2) The youngest chollas are strongly associated with particular nurse plant species (especially the perennial grass Hilaria rigida), but differentially among cholla species and between sites with different composition of potential nurses. (3) Nurse plants are generally replaced by the growing chollas, but some nurse species recur with, and are nursed by, adult chollas. Other shrub species have no importance as cholla nurses, but occur as cholla protegés. (4) Data are reported on the potential for fruit trapping and herbivore protection by the nurse plants, and for light and water competition between chollas of different ages and other species in the vegetation. The roles of such processes in the dynamics of the cholla populations, and alternative explanations for the nurse plant phenomenon, are discussed.