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

Salt marshes provide valuable ecosystem services but are vulnerable to drowning with accelerated sea-level rise (SLR). Marsh belowground biomass (BGB) production helps avoid drowning by building marsh surface elevation. Reductions in BGB can serve as an early warning sign of marsh deterioration, as they often precede decreases in aboveground biomass (AGB).