Upland Greening Compensates for Wetland Browning in Aboveground Coastal Carbon Change

Carbon cycling in coastal ecosystems is driven by interactions between sediment transport and vegetation growth. The response of coastal carbon stocks to interacting facets of climate change is poorly understood because sea level rise facilitates the accumulation of organic rich soils, while also threatening the persistence of coastal ecosystems. We quantified regional changes in vegetation type and aboveground biomass (AGB) using Landsat data between 1984 and 2020 for the mid-Atlantic coast of North America, a region experiencing declining coastal sediment supplies, accelerated sea-level rise (SLR) associated with recent warming, and rapid coastal erosion. For elevations less than 0.7 m above sea level, we detected consistent browning associated with erosion and submergence of marsh vegetation, and migration of marshes into upland forests. In contrast, adjacent upland forests exhibited strong greening in response to elevated precipitation and prolonged growing season during the study period. Greening trends increased continuously with elevation from 0.7 to 1.9 m, suggesting a shift from sea-level driven browning at low elevations to climate-driven greening at higher elevations. Total AGB increased by 1.1 Tg at high elevations (0.7-1.9m), more than offsetting AGB loss of 0.05 Tg at low elevations (0-0.7m). The close correspondence between AGB trends and elevation suggests that sea level rise has decoupled the response of coastal lowlands and uplands to climate change. Nevertheless, climate-driven upland greening compensated for SLR-driven lowland browning, resulting in an overall increase in the coastal biomass carbon stock.