Vegetation recovery on neighboring tidal flats forms an Achilles' heel of saltmarsh resilience to sea level rise

Abstract

Coastal wetlands such as saltmarshes are valued as prominent buffering ecosystems to global climate change and sea level rise (SLR), yet their long-term persistence may also be threatened by these global change stressors. While saltmarshes are increasingly thought to be resilient to SLR owing to high vertical marsh adaptability, their long-term stability remains uncertain due to our poor understanding of marsh resilience at the marsh-tidal flat interface, where wave disturbance can progressively shift vegetated marsh toward a bare tidal flat state. Here, we explore how SLR affects vegetation recoverability on tidal flats using cordgrass, a globally common saltmarsh foundation species, as a model plant. Combined field and model results demonstrate that small increases in wave forcing due to raised water depth over tidal flats can dramatically weaken or even block vegetation recovery from eroding marsh edges, through hampering seed persistence. Vegetation recovery on tidal flats next to the marsh edge thus represents an unrecognized Achilles' heel of marsh resilience to SLR, which if ignored may cause underestimation of marsh vulnerability. These findings are highly relevant for a more comprehensive assessment of marsh susceptibility to SLR in systems where seeds play an essential role in revegetation of tidal flats, and highlight the importance of maintaining either a wave-protected or well-elevated tidal flat near the marsh edge that allows for quick vegetation recovery for supporting resilient marshes.