Competition and Spacing in Plant Communities: The Arizona Upland Association

Spacing and competition were studied within and between species of the 'Arizona upland association' in Organ Pipe Cactus National Monument. Larrea tridentata, Franseria deltoidea, Opuntia fulgida, Carnegiea gigantea, and Fouquieria splendens comprise 95% of the individuals and 94% of the plant cover in the area studied. All intraspecific nearest-neighbour comparisons show that competition is occurring. Larrea tridentata competes with all species studied except Carnegiea gigantea, Franseria deltoidea competes only with Larrea tridentata, while there is no evidence of Carnegiea gigantea competing with other species (its interaction with Opuntiafulgida could not be determined). The root system of Larrea tridentata occupies a position intermediate between and overlapping those of Franseria deltoidea and Opuntia fulgida and as a result competes with both. Opuntia and Franseria do not compete as their root systems are segregated vertically from each other in the soil. It is suggested that vertical separation of root systems is the mechanism through which interspecific competition is reduced and coexistence maintained between these associated species of plants.