Fern community assembly: The roles of chance and the environment at local and intermediate scales

We evaluated the roles of the abiotic environment and dispersal in the assembly of fern communities at contrasting spatial scales within an old-growth, temperate deciduous forest. Specifically, we examined correlations among the geographic location of sampling plots separated by either 135-3515 m (mesoscale) or 4-134 m (fine scale), the abiotic environmental characteristics of the plots, and their constituent fern species. Ferns had predictable distributions along a soil moisture gradient at both spatial scales: six of eight common fern species showed repeatable environmental optima along the soil moisture gradient. By sampling in such a way as to decouple the correlation between distance and environmental variation, we showed the dominant role of environmental variables such as soil moisture in determining fern distributions at the mesoscale. At the fine scale, however, strong spatial autocorrelation in the abiotic environment precluded assigning any definitive role for either dispersal or environmental determinism alone in affecting fern distributions. The expectations of neutral theory that are rooted in dispersal limitation and those of niche theory that are rooted. in environmental adaptation converge at fine spatial scales where natural environments have strong spatial structure. The structure of the environment at fine spatial scales may foster the persistence of dispersal-limited plants in the community; neighboring environments are likely to be similar, and thus suitable for propagules dispersing short distances. While patterns of fern distribution in this locality are not consistent with purely neutral or random models of species coexistence, alternative models that rely on strict niche requirements without accounting for dispersal effects and the inherent spatial structure of the environment are inadequate because they neglect the important interaction of these factors. This outcome supports the relevance of developing theory that considers the joint effects of environmental determinism and dispersal on the distribution and abundance of plant species.