Numerical modelling of biogeomorphological processes in salt marsh development: Do short-term vegetation dynamics influence long-term development?

Abstract

Salt marshes are critical coastal ecosystems that provide numerous services. They are governed by complex biogemorphological interactions on multiple spatiotemporal scales. To simulate long-term salt marsh development in numerical models, smaller- and shorter-term vegetation processes are often neglected. This study investigates the importance of spatiotemporally and seasonally varying vegetation dynamics on decadal salt marsh development in an integrated numerical biogeomorphological model simulating a representative, idealized case of the Dutch Western Scheldt Estuary. The focus lies on the influence of seasonal vegetation dynamics, interspecific interactions between two salt marsh species, and the stabilizing effect of below-ground biomass with increasing vegetation age on critical shear stress, which results in spatially non-uniform erosion resistance of sediment and vegetation. By incorporating seasonal growth periods and varying characteristics of vegetation, we demonstrate the impact of species-specific traits on vegetation stability and morphological changes. Introducing an additional pioneer species that can establish further seaward reveals the sheltering effect and the long-term implications of this zonation for marsh development. Additionally, the influence of spatially and temporally varying below-ground biomass on sediment stability and vegetation's resistance to uprooting is significant for the biogeomorphological interactions. Comparison of the different model simulations to topographical data from the Dutch Western Scheldt Estuary shows that the model results are representing the morphological variability occurring in the field. Our findings highlight the complex biogeomorphological feedback that govern salt marsh development and show that models that neglect shorter-term seasonal and grid-scale (25 m²) spatially varying processes may overlook critical interactions that influence the lateral extent and overall morphological development of salt marshes. Therefore, this study underscores the need to integrate seasonal dynamics, species diversity, and below-ground biomass variability into biogeomorphological models to enhance predictive accuracy. This approach provides a more comprehensive understanding of the large-scale, long-term consequences of seasonal and spatiotemporally varying vegetation processes, which is essential for informing conservation and management strategies under changing climatic conditions.