Untangling Dynamic Drivers of Salt Marsh Migration

Coastal marsh transgression into upland ecosystems is driven by complex hydrological processes, stressing vegetation through root‐zone salinization and hypoxia, but interacting roles of these stressors remains poorly understood. We use high‐resolution field observations at six sites along the Delmarva Peninsula, USA, to better understand how these processes alter subsurface conditions in changing coastal marsh‐to‐upland systems. We show vertical salinization occurs through episodic storm‐driven infiltration, and lateral salinization is driven by long‐term and intermediate‐term (seasonal) processes. Groundwater rise drives persistent root‐zone saturation, reducing soil redox potential and inducing hypoxic subsurface conditions. When flooding is transient, elevated freshwater levels can buffer SWI and flush salinity. Added site‐specific complexities, like low hydraulic conductivity ( K < 0.2 m/d), extend recovery times from <70 to >200 days. Across sites, interacting drivers operate across varying temporal and spatial scales, complicating attribution, and limiting our understanding of the rate of upland transition. Vegetation surveys revealed live foliage cover and sapling density decline with increasing porewater salinity and groundwater flooding, linking hydrologic stress directly to forest loss. Our results demonstrate marsh migration is driven by interacting processes and coupled salinization and saturation stress rather than a single driver, improving our understanding of shifting hydrological regimes.