Patch dynamics in arid lands: localized effects of Acacia papyrocarpa on soils and vegetation of open woodlands of south Australia

Although the importance of plant-created heterogeneity in arid lands has lo ng been recognized, little information is available on the dynamics of thes e patches. We studied the changes in soil and vegetation associated with th e presence of a long-lived tree, Acacia papyrocarpa, in arid lands of south Australia. The soil under young individuals was not different from the soi l in the surrounding open spaces, confirming the assumption that establishm ent does not occur preferentially in high fertility patches. The amount of organic matter, total N, total S, total and available P, and soil salinity increased with the age of the tree until maturity, and declined as the cano py of the tree became more fragmented. The content of organic mater and tot al and available P remained higher than that in the matrix soil for at leas t fifty years after the death of the: tree. There were several species almo st completely restricted to the canopy environment. Some, but not all of th em, have bird dispersed seeds. One of these species (Enchylaena tomentosa) established and grew better in soil collected under tree canopies in a glas shouse experiment, independently of light environment. After the death of t he trees the under-canopy species declined rapidly, and the patches were co lonized by invasive annual species, and short lived perennials. Our results suggest that patch dynamics driven by the population dynamics of woody per ennial species have paramount importance for the ecosystem, and community d ynamics of arid lands.