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 long been recognized, little information is available on the dynamics of these patches. We studied the changes in soil and vegetation associated with the presence of a long-lived tree, Acacia papyrocarpa, in arid lands of south Australia. The soil under young individuals was not different from the soil in the surrounding open spaces, confirming the assumption that establishment 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 canopy of the tree became more fragmented. The content of organic mater and total and available P remained higher than that in the matrix soil for at least fifty years after the death of the: tree. There were several species almost completely restricted to the canopy environment. Some, but not all of them, have bird dispersed seeds. One of these species (Enchylaena tomentosa) established and grew better in soil collected under tree canopies in a glasshouse experiment, independently of light environment. After the death of the trees the under-canopy species declined rapidly, and the patches were colonized by invasive annual species, and short lived perennials. Our results suggest that patch dynamics driven by the population dynamics of woody perennial species have paramount importance for the ecosystem, and community dynamics of arid lands.