Synergistic effects of elevation loss and environmental extremes trigger salt marsh die-off in the Yangtze Estuary

Complex extreme events increasingly threaten coastal ecosystems, yet quantitative understanding of critical thresholds remains limited. Using a five-year dataset (2019–2023) that captured an unprecedented vegetation die-off in the Yangtze Estuary, we quantitatively derive ecological thresholds through GAM analysis. Our results establish critical thresholds of approximately 34.1 °C for temperature and 19.77 PSU for salinity; vegetation die-off arises when both thresholds are exceeded simultaneously. River discharge emerges as a critical regulatory variable that determines vegetation sensitivity to temperature-salinity extremes, with a strong negative correlation (r = -0.77, p = 0.014) between discharge and high salinity events. Low discharge conditions (<800 × 10⁸ m³/month) increase high salinity exposure seven-fold compared to high discharge periods. Our analysis reveals that the interaction between press (elevation loss) and pulse (environmental extremes) disturbances contributes 47.5 % of vegetation response variance, far exceeding their individual effects. These results illuminate how “press” disturbances (elevation loss from reduced sediment) increase susceptibility to “pulse” events (extreme temperature, salinity, and reduced discharge). The identified thresholds provide quantitative targets for early warning systems, suggesting dual management approaches targeting both press and pulse disturbances. These findings offer a transferable framework for predicting ecological transitions in river-dominated estuaries worldwide, enhancing coastal wetland management under intensifying climate extremes.