Shifting allocations between biomass and soil carbon pools drive net loss of carbon in retreating coastal forests

Abstract:

Sea level rise is leading to the migration of coastal ecosystems and the replacement of terrestrial forests with tidal wetlands. Wetland soils are well known to accumulate carbon at faster rates than terrestrial soils, implying that sea level rise may lead to enhanced carbon accumulation. Here, we quantify biomass and soil carbon stocks across four rapidly migrating forest-to-marsh ecotones in the Chesapeake Bay (USA), a hot spot for sea level rise and coastal forest retreat. We find that the amount of carbon stored in marsh soils, herbaceous vegetation, and shrubs all increase across the forest-marsh ecotone. However, we find that the carbon stored in tree biomass greatly exceeds carbon stored in adjacent marsh soils, so that sea-level driven marsh migration reduces total carbon stocks by approximately 50%. Continued marsh soil carbon accumulation may eventually offset forest carbon loss, but we estimate that the time for replacement is similar to estimates of marsh survival (i.e. centuries), suggesting that forest carbon may never be replaced. These findings reveal a new carbon source in coastal carbon budgets, driven by migrating ecosystems and shifting allocations between carbon stored in soils and biomass.