A Comparative Study of Salt Marsh Vegetation Dynamics in an Eco-Morphodynamic Modeling Framework

Abstract

Salt marshes are key components of coastal and estuarine landscapes, providing important ecosystem services including carbon storage, nutrient filtering, and wave attenuation. Currently, marsh sustainability is increasingly threatened by sea-level rise and human pressures such as land reclamation, subsidence, and reduced sediment supply, leading to widespread and accelerating marsh loss worldwide.

Understanding salt-marsh evolution at relevant timescales typically relies on biogeomorphodynamic models that resolve hydrodynamics, sediment transport, and morphological change, coupled with biotic feedbacks from vegetation, which directly affect sediment transport and organic deposition. Vegetation dynamics in these models are commonly based on the assumption that each species has an optimal elevation range where its productivity peaks. This assumption derives from field observations of realized (i.e., observed) ecological niches, which display hump-shaped distributions along marsh elevation gradients.

However, both theoretical and empirical insights now challenge this hypothesis, suggesting that species productivity actually tends to increase monotonically with elevation—following a logistic growth pattern—and that the hump-shaped niches observed in the field are merely the result of interspecific competition. This discrepancy challenges the way vegetation feedbacks are represented in most salt-marsh models and calls for a re-evaluation of how species interactions are incorporated into predictions of marsh evolution.

Here we use a biogeomorphodynamic model that solves hydrodynamics, sediment transport, sediment mass balance, and topographic change, coupled to two alternative vegetation formulations: (i) a classical model based on hump-shaped elevation niches, and (ii) a spatially explicit dispersal–competition model based on monotonic (logistic) fundamental niches. Simulations are performed for a real case study in the Venice Lagoon under different rates of relative sea-level rise and suspended sediment supply.

The two vegetation formulations produce markedly different patterns of vegetation zonation and, more importantly, contrasting predictions of marsh resilience to rising sea levels. While the classical model predicts marsh persistence under a relative sea-level rise of 2.5 mm yr⁻¹, the dispersal-based vegetation model predicts marsh degradation, yielding qualitatively different outcomes under the same environmental forcings.

By explicitly accounting for dispersal and interspecific competition, our results show that vegetation dynamics exert a stronger control on long-term marsh survival than is captured by traditional niche-based models. This calls for a revision of current biogeomorphodynamic frameworks to improve projections of salt-marsh resilience under future sea-level rise.

Keywords: Echo-morphodynamic model, Dispersal colonization, Habitat quality, Sediment transport, Salt marsh