Warming and mangrove encroachment into marshes alter belowground processes with positive implications for resilience to sea level rise : findings from the WETFEET project

Background/Question/Methods

Though we are beginning to understand some consequences of mangrove encroachment into salt marshes, the implications of this species shift, and of sustained climatic warming, for the resilience of wetland habitats remains unknown. At three sites in northeast Florida, USA, we deployed warming chambers on both marsh-dominated and mangrove dominated plots to investigate how warming influences plant above- and below-ground growth, organic matter decomposition and surface elevation with respect to sea level rise. We also are modifying the Marsh Elevation Model (MEM) to assess how wetland surface elevation will respond to sea level rise in the presence of encroaching mangroves.

Results/Conclusions

Warming chambers have warmed the air temperature by an average of 2°C, which is similar to the climate change that is predicted for northeast Florida. Warming strongly increases mangrove aboveground growth after two years, especially at the most northern site.We also found that area-based relative growth rate was five times higher across all treatments than height-based relative growth rate, indicating that mangroves are growing wider rather than taller in these ecotonal environments. Root growth was stimulated by both mangrove presence and warming, with potential positive effects for surface elevation. Neither warming nor mangrove presence altered organic matter decomposition rates in soils. The new Coastal Wetland Equilibrium Model (a newly developed version of MEM) show that mangroves have a greater capacity for maintaining surface elevation with respect to sea level rise than salt marshes. The timing of mangrove encroachment into marshes determines how much this species shift can increase surface elevation resilience to sea level rise.