Accelerated sea-level rise is suppressing CO 2 stimulation of tidal marsh productivity: A 33-year study.

Accelerating relative sea-level rise (RSLR) is threatening coastal wetlands. However, rising CO 2 concentrations may also stimulate carbon sequestration and vertical accretion, counterbalancing RSLR. A coastal wetland dominated by a C 3 plant species was exposed to ambient and elevated levels of CO 2 in situ from 1987 to 2019 during which time ambient CO 2 concentration increased 18% and sea level rose 23 cm. Plant production did not increase in response to gradually rising ambient CO 2 concentration during this period. Elevated CO 2 increased shoot production relative to ambient CO 2 for the first two decades, but from 2005 to 2019, elevated CO 2 stimulation of production was diminished. The decline coincided with increases in relative sea level above a threshold that hindered root productivity. While elevated CO 2 stimulation of elevation gain has the potential to moderate the negative impacts of RSLR on tidal wetland productivity, benefits for coastal wetland resilience will diminish in the long term as rates of RSLR accelerate.