Facilitation in multiple life-history stages: evidence for nucleated succession in coastal dunes

Understanding plant interactions during succession is a central goal of plant ecology. The nucleation model of succession proposes that facilitative interactions lead to outward radiations of colonization from established plants. I tested the nucleation hypothesis for early successional plants on the primary dunes of barrier islands in the southeastern United States. On the coast of Florida and Georgia, I sampled the above-ground vegetation and the seed bank, and conducted a seedling emergence experiment in the field. Vegetation sampling at both field sites showed more positive correlations in percent cover of adult species than expected by chance. At both sites, there were significantly more seeds under plants than in open microsites. Seeds of the same species as the adult plants in the vegetated microsites were removed from the analysis, so the accumulation of seeds under adults is not due simply to a seed shadow effect. Seeds of six species were added to plots under plants and in open microsites, and seedling emergence was significantly higher under plants than in the open. The findings support the predictions of the nucleation hypothesis for the early life-history stages of seed dispersal and germination, and help to explain the clumped distribution of dune plants seen in the field. The results add further support for the contention that facilitation is an important mode of plant interaction in stressful and disturbed habitats.