1 Primary succession on coastal sand dunes has generally been presumed to be driven by autogenic environmental change assocaited with dune stabilization and gradual soil development. 2 An extensive chronosequence of dune-capped beach ridges is found adjacent to northern Lake Michigan and the youngest 13 ridges (aged 30-440 years old) show a clear pattern of primary succession and development of a forest ecosystem. 3 Seed-addition and seedling-transplant experiments indicated that colonization of young sand dunes by late-successional Pinus and Quercus species is constrained by limited seed dispersal, seed and seedling desiccation, and seed predation rather than seedling success being constrained by low soil nitrogen availability. Their establishment may therefore depend on coincidence of chance seed dispersal, favourable weather conditions and low rodent densities. 4 In addition, episodic burial by sand prevents most species from colonizing young dune ridges, while burial of seedlings by litter limits recruitment on older dune ridges with developing forest. The intensity of competition increases during succession. 5 Dune succession is better described as the transient dynamics of colonization and competitive displacement rather than the result of gradual soil development and competitive displacement.