Organization of vegetation mosaics in the Acacia Schaffneri-Opuntia streptacantha association, Southern Chihuahuan Desert, Mexico

(1) A preliminary analysis of the species-rich Acacia schaffneri-Opuntia streptacantha association in the southern Chihuahuan Desert, Mexico, used data gathered from simpler vegetation mosaics within the association. Every combination of all common species in this association was found in these mosaics, giving the association an appearance of great complexity. (2) Within each mosaic there were representatives of species occupying three distinct microhabitats: a perennial legume at its centre; an erect (shrubby or arborescent) form of platyopuntia near or under the legume; and a decumbent form of platyopuntia at the periphery. (3) One of two species occupies each of these three microhabitats. The two species of each pair are separated from one another altitudinally over the 200-m gradient on the hillside studied. (4) At least one species, Opuntia streptacantha, is involved in a temporal replacement of Acacia schaffneri. (5) The differential utilization by different species of microhabitats within the vegetation mosaic over an altitudinal gradient and their successional replacement within the vegetation mosaic are the factors contributing to the complexity of this plant association.