Early warning signs of salt marsh drowning indicated by widespread vulnerability from declining belowground plant biomass

Intensifying stressors are weakening resilience in ecosystems across the globe. Here, we show that belowground biomass (BGB), a proxy for salt marsh health, has decreased over a large area of coastal marshes since 2014, indicating growing vulnerability to sea-level rise (SLR)-induced drowning. Using observational modeling, we show a shift in plant biomass allocation, away from belowground resources, which are critical for resilience, toward higher aboveground stocks. Marsh drowning is likely underway, and the window for action may be short lived.