The Importance of Scale in the Future of Mangrove Blue Carbon Under Sea-Level Rise

As efficient carbon sinks, mangrove forests are crucial for climate change mitigation. However, their vulnerability to sea-level rise (SLR) and human activities influencing sediment supply introduces significant uncertainty regarding their future carbon storage capacity. Given the complexity of mangrove landscapes, current projections may be limited in their ability to capture the broad range of potential responses. Here we investigate the distinct drivers of local- and landscape-scale changes in mangrove carbon accumulation under SLR and changing fluvial sediment supply, by developing a novel eco-carbon-morphodynamic model. The model incorporates interactions between hydro-morphodynamic processes, mangrove dynamics, and carbon dynamics while tracking changes in bed composition accounting for organic and inorganic fractions. Numerical experiments are conducted using a schematized tidal embayment to systematically explore the impacts of different SLR rates and sediment supplies. Our results indicate that carbon accumulation can increase locally under SLR, in agreement with empirical data. However, at landscape-scale, SLR reduces total carbon accumulation by limiting areas for mangrove colonization. Also, mangrove mortality and erosion of carbon-rich soils cause a transition from a carbon sink to source. At intertidal scale, our findings indicate strong spatial variations in SLR rate thresholds for mangrove survival and carbon accumulation trajectories, linked to channel network dynamics. Local changes in carbon sequestration therefore do not necessarily reflect longer-term landscape-scale carbon stocks. Overall, our study highlights the scale-dependency of carbon accumulation responses and drivers, with internal system dynamics shown to be equally important as external forcings in determining mangrove vulnerability and carbon accumulation under future disturbances.