Cross-shore gradients of physical disturbance in mangroves: implications for seedling establishment

Mangroves may grow in an active sedimentary environment and are therefore closely linked to physical coastal processes. Seedlings colonize dynamic tidal flats, after which mangroves have the potential to change their physical environment by attenuating hydrodynamic energy and trapping sediments. Disturbance from hydrodynamic energy of waves or currents and the resulting sediment dynamics appear to be a critical bottleneck for seedling establishment on tidal flats and at the forest fringe. However, knowledge about the mechanisms at the single plant level and the spatial pattern of disturbance is limited. By means of a flume study, we demonstrate that a surface erosion threshold of as little as 1-3 cm depth can lead to failure of young seedlings. By monitoring accretion/erosion for 8 months along cross-shore transects in southwest Thailand, we show that, especially on the bare mudflat, the physical sediment disturbance regularly exceeds the critical erosion thresholds derived from the flume study. Physical sediment parameters along the same transects were analysed to deduct patterns of hydrodynamic energy attenuation. Grain size analysis and erosion/accretion data showed only limited energy dissipation within the fringing Avicennia/Sonneratia zone; sediment dynamics only dropped below lethal values for seedlings within the denser Rhizophora zone. Overall, present results emphasize that (i) seedling survival is extremely sensitive to physically driven sediment dynamics and (ii) that such physical disturbances are not only present on the tidal flats but can penetrate a significant distance into the forest. Spatio-temporal patterns in sediment dynamics should hence be considered when conducting restoration of mangrove ecosystems.