Carbon Dynamics of a Coastal Wetland Transitioning to Mangrove Forest

Coastal wetlands play a vital role in the global carbon cycle and are under pressure from multiple anthropogenic influences. Altered hydrology and land use change increase susceptibility of wetlands to sea-level rise, saltwater intrusion, tidal flood events, and storm surges. Flooding from perigean spring tides and storm surges rapidly inundates coastal wetlands with saline waters, quickly surpassing vegetation tolerances, leading to shifts in soil microbial respiration, peat collapse, and plant mortality, followed by establishment of salt-tolerant vegetation. The Southeast Saline Everglades (SESE) is facing many of these pressures, making it a model system to examine the impacts of ecosystem state transitions and their carbon dynamics. Saltwater flooding from Hurricane Irma (2017) initiated a transitional state, where less salt-tolerant vegetation (e.g., Cladium jamaicense) is declining, allowing halophytic species such as Rhizophora mangle to colonize, altering the ecosystem's biogeochemistry. We utilized eddy covariance techniques in the SESE to measure ecosystem fluxes of CO2 and CH4 in an area that is transitioning to an alternative state. The landward expansion of mangroves is increasing leaf area, leading to greater physiological activity and higher biomass. Our site was presented initially as a small C source (47.0 g C m-2) in 2020, and by 2022 was a sink (-84.24 g C m-2), with annual greenhouse carbon balance ranging from -0.04 to 0.18. Net radiative forcing ranged from 2.04 to 2.27 W m-2 d-1. As the mangrove landward margin expands, this may lead the area to become a greater carbon sink and a potential offset to increasing atmospheric CO2 concentrations. Hurricane-caused storm surge and king tides have led to increasing amounts of saltwater being deposited inland. Inland flooding causes increased salinity in freshwater marshes, leading to mortality in marsh plants that are less tolerant to salt. At the same time, inland flooding pushes mangrove propagules inland and allows them to rapidly establish. Mangroves have high saline tolerance and accumulate salt in their leaves. Mangroves can further increase salt concentrations when they drop their leaves, which may increase the mortality of existing marsh vegetation and enhance the expansion of mangroves. As these mangrove trees increase in number and size, they can capture more carbon dioxide from the atmosphere compared to the previous vegetation. If this trend continues these new mangrove-dominated systems may aid in slowing or mitigating atmospheric increases in CO2. Storm surge and high tides are leading to marsh loss and mangrove establishment Mangrove expansion is increasing CO2 sequestration Mangrove colonization in marshes is driving a net cooling forcing