Sea-level rise can threaten the long-term sustainability of coastal communities and valuable ecosystems such as coral reefs, salt marshes and mangroves. Mangrove forests have the capacity to keep pace with sea-level rise and to avoid inundation through vertical accretion of sediments, which allows them to maintain wetland soil elevations suitable for plant growth. The Indo-Pacific region holds most of the world's mangrove forests, but sediment delivery in this region is declining, owing to anthropogenic activities such as damming of rivers. This decline is of particular concern because the Indo-Pacific region is expected to have variable, but high, rates of future sea-level rise. Here we analyse recent trends in mangrove surface elevation changes across the Indo-Pacific region using data from a network of surface elevation table instruments. We find that sediment availability can enable mangrove forests to maintain rates of soil-surface elevation gain that match or exceed that of sea-level rise, but for 69 per cent of our study sites the current rate of sea-level rise exceeded the soil surface elevation gain. We also present a model based on our field data, which suggests that mangrove forests at sites with low tidal range and low sediment supply could be submerged as early as 2070.
The majority of the world's mangrove forests occur on mostly mineral sediments of fluvial origin. Two perspectives exist on the biogeomorphic development of these forests, i.e. that mangroves are opportunistic, with forest development primarily driven by physical processes, or alternatively that biophysical feedbacks strongly influence sedimentation and resulting geomorphology. On the Firth of Thames coast, New Zealand, we evaluate these two possible scenarios for sediment accumulation and forest development using high-resolution sedimentary records and a detailed chronology of mangrove-forest (Avicennia marina) development since the 1950s. Cores were collected along a shore-normal transect of known elevation relative to mean sea level (MSL). Activities for lead-210 ( 210 Pb), caesium-137 ( 137 Cs) and beryllium-7 ( 7 Be), and sediment properties were analysed, with 210 Pb sediment accumulation rates (SARs), compensated for deep subsidence (~8 mm yr À1 ) used as a proxy for elevation gain. At least four phases of forest development since the 1950s are recognized. An old-growth forest developed by the late-1970s with more recent seaward forest expansion thereafter. Excess 210 Pb profiles from the old-growth forest exhibit relatively low SARs near the top (7-12 mm yr À1 ) and bottom (10-22 mm yr À1 ) of cores, separated by an interval of higher SARs (33-100 mm yr À1 ). A general trend of increasing SAR over time characterizes the recent forest. Biogeomorphic evolution of the system is more complex than simple mudflat accretion/progradation and mangrove-forest expansion. Surface-elevation gain in the old-growth forest displays an asymptotic trajectory, with a secondary depocentre developing on the seaward mudflat from the mid-1970s. Two-to ten-fold increases in 210 Pb SARs are unambiguously large and occurred years to decades before seedling recruitment, demonstrating that mangroves do not measurably enhance sedimentation over annual to decadal timescales. This suggests that mangrove-forest development is largely dependent on physical processes, with forests occupying mudflats once they reach a suitable elevation in the intertidal.
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