Global Determinants of the Spartina alterniflora Seaward Boundary: Quantifying Compound Constraints Through Inundation Probability

Abstract

The seaward distribution limit of Spartina alterniflora represents a critical ecogeomorphic boundary between salt marshes and tidal flats, with its dynamics being pivotal to coastal ecosystem stability. Traditional studies have predominantly relied on relative elevation as a proxy; however, this approach often suffers from spatial scale inconsistencies and fails to fully capture physiological constraints. Consequently, the dominant drivers and underlying mechanisms governing this limit remain elusive on a global scale. Here, we propose inundation probability as a robust, universal physical indicator. By integrating Sentinel‐2 imagery (2018–2022) with the FES2014 tidal model, we investigated 60 representative study areas worldwide. A pixel‐level data chain was constructed through time‐series classification of water and vegetation, enabling the precise identification of edge transition zones. The lower distribution limit was defined as the 90th percentile of the inundation probability within these zones. Quantitative analysis using Random Forest and SHAP methods reveals that the tidal range is the primary controller, contributing over 30% to the variance. The tidal form index and tidal flat slope emerge as secondary factors, while intercontinental differences reflect biogeographic context and population genetic histories beyond native status. Globally, the inundation probability at the lower limit exhibits a significant nonlinear decline as the tidal range increases, characterized by high sensitivity in micro‐tidal systems and a tolerance plateau in macro‐tidal environments. A sigmoid model incorporating tidal range and continental factors was established, providing a unified, physically constrained framework for assessing coastal vulnerability under future sea‐level rise.