Do published tolerance ratings and dispersal factors predict species distributions in bottomland hardwoods?

Species flood and shade tolerances are commonly used to explain regeneration and distribution of canopy trees in bottomland hardwood forests. Restoration planning and management decisions for these highly threatened forests often match canopy species to site conditions based on published tolerance ratings. We identified ‘regeneration strategies’ by combining flood and shade-tolerance ratings with plant colonization traits, specifically seed size and dispersal mode, and asked if they predict field regeneration patterns of floodplain canopy species. From the literature, we identified four groups of species with theoretically similar regeneration strategies. We then used empirical evidence from three floodplain datasets to test whether the a priori groups were useful predictors of regeneration responses to gradients in canopy openness, flooding, and distance from forest edge. Species with different published tolerance levels overlapped substantially; minimum and maximum positions along each gradient overlapped, and the medians alone were useful in the empirical tests of three of the four defined strategy groups. Predicted responses for the strategy groups and fidelity of species to the groups were not always met over flooding or canopy openness and distance-to-seed-source gradients in the field. However, responses of species within the groups, and the groups themselves, suggest that flood and shade tolerances are coarse filters, or that they filter species establishment following a dispersal filter on species colonization. Thus, regeneration strategies based on a suite of characters related to colonization and species sorting over elevation and canopy openness heterogeneity have implications for BLH management and restoration. More empirical work is needed to evaluate the full range of species responses to multiple environmental gradients.