Evaluating dispersal limitation in passive bottomland forest restoration

Dispersal limitation can retard natural establishment of desirable species on restoration sites, especially where landscapes are fragmented, but dispersal limitation is assumed to become less critical with time as early colonists become reproductively mature. Distribution patterns of recruiting trees in a 20-year-old passively restored bottomland in northeast Louisiana suggested persistent dispersal limitation in some bottomland hardwood species and influence of dense shrub patches on colonization. To test these hypotheses, we measured seed rain as a function of distance to seed source and association with shrub cover. Seed rain of the wind-dispersed Fraxinus pennsylvanica was highest near the forest edge, except where mature recruits occurred. Although shrub presence did not influence dispersal of E pennsylvanica, its negative influence on probability of occurrence in the sapling layer suggests that shrub cover may limit its regeneration. The bird-dispersed Crataegus viridis and Hex decidua were found in the seed rain and as reproductive individuals within the field; neither had a positive relationship with shrub presence. Dispersal of heavy-seeded Quercus spp. and Carya aquatica was limited to within 20 m of the forest edge. These results imply that dispersal limitation is diminishing in wind- and bird-dispersed species with maturation of in-field recruits and that shrub patches may influence these patterns. Heavy-seeded species, however, remain restricted to field edges that directly abut a seed source. If canopy closure by wind- and bird-dispersed species precedes dispersal of heavy-seeded species into the field, establishment of Quercus and Carya spp. may remain low for the foreseeable future.