Soil nutrients are not responsible for arrested succession in disturbed coastal dune forest

Abstract

Coastal dune forest succession frequently proceeds via the Acacia karroo pathway, but may become arrested. We examine whether soil fertility arrests forest succession in A. karroo stands in coastal dune forest in KwaZulu-Natal province, South Africa. We examined soil fertility of A. karroo stands, the adjacent forest, and forested dune slacks at Cape Vidal, and four rehabilitating A. karroo stands (13- to 28-yr-old) at Richards Bay. The effect of nitrogen supplementation on growth of three tree species (a forest pioneer, a late successional forest species, and A. karroo) was compared between A. karroo stands and adjacent dune forest at Cape Vidal. Soil fertility in A. karroo stands and the adjacent forest at Cape Vidal was similar and neither total nor readily mineralisable nitrogen were limiting in either habitat. At Richards Bay, where the dunes were previously strip-mined, total nitrogen accumulated rapidly (2.1–8.0 g N m−2 yr−1) and the oldest rehabilitating A. karroo stands (26–28 yr) had similar total nitrogen and other soil nutrient levels as stands twice their age at Cape Vidal. Seedling growth was unaffected by nitrogen supplementation. All species grew fastest in A. karroo stands demonstrating that soil nutrient levels in disturbed forest colonised by A. karroo are suitable for the growth of forest tree species. Soil fertility, including available nitrogen, is not limiting secondary succession at Cape Vidal, yet forest species are not replacing A. karroo stands at this site. Post-emergence factors, such as herbivory, are likely responsible for the arrested succession of forest in A. karroo stands.