Spatial relationships and competition in a Chilean desert cactus.

In the Atacama Desert of northern Chile, cacti of the genus Copiapoa occur in extensive, relatively dense, monospecific stands. The spatial distribution patterns within several stands of Copiapoa cinerea v. columna-alba were analyzed for evidence of competitive interactions among individuals. There was no indication that competition was affecting stand density. Mean annual precipitation is only 25 mm/yr, but estimates of maximum possible water use within a stand were much lower. It was shown that the time between rainstorms is long compared to the water storage capacity of the plant stems, especially in the case of small individuals. We conclude that population densities in Copiapoa are limited by the difficulty of establishment, not by competition.