Warming and mangrove encroachment interact to drive belowground processes and wetland resilience to sea level rise

Climatic warming alters coastal wetland ecosystems directly and indirectly by altering plant community structure. Along the Florida coast, average temperatures have increased, likely having an influence on ecosystem processes, while decreasing freeze frequency has allowed mangroves (Avicennia germinans) to expand their range into higher latitudes. Though we are beginning to understand some consequences of mangrove encroachment, the implications of climatic warming for carbon storage soil elevation maintenance remain unknown. Across a chronosequence of mangrove invasion on the east coast of Florida we established passive warming experiments, we measured aboveground plant growth, root dynamics, decomposition rates, carbon stocks, and microbial communities with the goal of examining future wetland soil elevation maintenance and carbon storage. Warming chambers, placed at the ecotone between mangroves and salt marshes in eastern Florida, warmed the air temperature by an average of 2°C. We have found that warmed plots produced two times more salt marsh biomass and taller mangroves with greater leaf area. Warming can decrease salt marsh root biomass, particularly in deeper sediments but increased root biomass in areas where shrubby mangroves had invaded marshes. These warming-induced increases in root biomass resulted in higher relative surface elevation in less than two years. Despite these increases in organic matter due to the interaction of warming and mangrove encroachment, mangrove roots can decompose differently than marsh roots and create a different decomposition environment. Further, mangrove encroachment also changes the soil microbial community structure by decreasing the amount of obligate anaerobic microbes, indicating that mangroves enhance soil oxygen availability, which may also alter organic matter decomposition rates in the long term. As mangroves encroach in coastal marshes, the balance of greater organic matter production and more rapid decomposition will partly determine wetland resilience to rising seas.