Age Structure, Distribution and Survival under Grazing of the Arid-Zone Shrub Acacia burkittii

Age structure and spatial distribution of Acacia burkittii FvM. ex Benth., a tall shrub widespread in southern arid Australia, were studied at Koonamore, South Australia. This species is largely confined to a sandy soil habitat, with a contagious and self-perpetuating distribution pattern. Stands are uneven-aged. Age-size relationships were determined from long-term quadrat records, making it possible to estimate ages of individuals. Age structures were sampled in areas with different histories of grazing by introduced herbivores. Both sheep and rabbits were shown to inhibit seedling input to the populations, and together they completely suppressed reproduction. It is predicted that within 100 yr this shrub species will be extinct in some areas and greatly reduced in others if present land use practices are continued.