Exposure mediates transitions between bare and vegetated states in temperate mangrove ecosystems

The resilience of mangroves is dependent on their regeneration capacity. Patchy mid-19th century clearing dramatically affected this capacity, creating stable vegetated and unvegetated states in a fragmented temperate mangrove ecosystem. Mechanisms of mediation between states were tested by monitoring the survival and growth of planted mangrove seedlings and propagules on formerly forested bare mudflats and inside patches of existing forest. Survival (1 to 76%) and growth (−0.83 to 10.45 mm mo−1 increase in plant height) of seedlings was affected by (1) differing levels of exposure found at varying proximities to remnant forest and (2) differing inundation regimes both within and between sites that were randomly selected from locations that varied in aspect relative to prevailing winds. Increases in hydrodynamic energy within and between sites corresponded to a decrease in survival that was much more pronounced at locations that were exposed to prevailing winds. Growth rates were also generally lower at sites in exposed locations, but inundation regime was a more important determinant within sites, where growth was reduced at lower heights on the shore. Results suggest that stability of the bare mudflat state (caused by historical clearance of the mangrove forest) is dependent on level of exposure to hydrodynamic energy, and a return to a forested state is more likely where this exposure is lower. These results have implications for planning and implementing mangrove restoration projects and illustrate the role that physical factors can play in determining the resilience of disturbed temperate mangrove ecosystems.