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

Abstract

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 <i>Avicennia/Sonneratia</i> zone; sediment dynamics only dropped below lethal values for seedlings within the denser <i>Rhizophora</i> 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

Modelling mangrove propagule dispersal trajectories using high‐resolution estimates of ocean surface winds and currents

“…), such as physical disturbance by hydrodynamics and accretion‐erosion processes (Balke et al . , ), predation (Cannicci et al . , Van Nedervelde et al .…”

Dynamics of a fringe mangrove forest detected by Landsat images in the Mekong River Delta, Vietnam

“…linearity and brightness) to progradation or erosion events. These large‐scale connections are then discussed and explained based on the small‐scale feedbacks between vegetation and erosion/deposition patterns presented in Balke et al ().…”

“…Here we use the results on the effects of erosion and deposition on mangrove seedlings presented in Balke et al () to interpret the large‐scale dynamics of the mangrove fringe (e.g. Figure d).…”

“…We note that, as indicated by Tamura et al (), there is a clear seasonal cycle of deposition and erosion along the shoreline of the Mekong River Delta, with large sediment inputs during summer favoring accretion and energetic waves during winter triggering erosion. The inherent sediment mobility is particularly detrimental to the establishment of Sonneratia spp., which are very sensitive to both erosion and deposition (Blake et al , ). The ragged marsh boundary in the central part of the fringe (Figures a and ) indicates that the forest is encroaching over an area with non‐uniform bottom elevation, or that localized erosion has disturbed the forest preventing its expansion.…”

Mangrove Sedimentation and Response to Relative Sea-Level Rise

“…Each of these factors will vary at individual sites as a function of distortion of the tidal www.annualreviews.org • Mangrove Sedimentation and Sea Level 16.9 Changes may still occur before final publication online and in print 13:40 curve as well as additional parameters, such as distance to a creek ( Janssen-Stelder et al 2002). On open coasts, the lower bound to mangrove ecological accommodation space is set by the lowest elevation at which seedlings can persist and grow to become trees (Balke et al 2013). The upper bound is the highest level to which tides or storms rise, although bounding water levels differ in mangrove forests that are located well inland and infrequently flooded (see Figure 2).…”