Seasonal variation in stem morphology critically influences long-term saltmarsh development

Saltmarshes provide numerous ecosystem services, contributing to climate change mitigation (carbon sequestration) and adaptation (coastal protection). While capable of accreting sediments in a dynamic equilibrium with changing sea levels, uncertainty remains regarding their continued resilience under accelerated rates of sea level rise (SLR). Ultimately, an improved understanding of how saltmarsh systems develop and evolve under changing conditions is needed to inform management and restoration strategies. Numerical frameworks that couple hydro-morphodynamics and vegetation dynamics (“eco-geomorphic” models) are emerging to support such advancements. Challenged by conflicts of scale and high computational costs, however, saltmarsh modelling studies often implement simplifications that ignore short-term vegetation dynamics such as seasonal growth cycles. Consequently, it remains poorly understood how seasonal variation impacts saltmarsh eco-geomorphic processes on sub-annual to multi-decadal timescales, and if there are implications for ecosystem vulnerability to SLR.To address this, a numerical study was developed based on seasonal stem measurements of Sporobolus alterniflorus from the St. Lawrence Estuary (Québec, Canada). Coupling hydro-morphodynamics (TELEMAC-2D, GAIA) with a cellular automaton for vegetation dynamics, a novel eco-geomorphic framework was applied to simulate saltmarsh development under scenarios with explicit seasonal variation, versus vegetation properties averaged over the growing season. For each scenario, the model was used to simulate 180 years of eco-geomorphic development for an initially bare, idealized tidal domain.This study demonstrates, for the first time, how sub-annual seasonal processes contribute to ecosystem development over the long-term (decades to centuries). Simulations that incorporated explicit seasonal variation in stem characteristics yielded more rapidly accreting saltmarsh platforms, with denser tidal channel networks; both supporting improved resilience under SLR. Sediment delivery to saltmarsh interiors was promoted during seasons of low biomass, while seasons of peak biomass strengthened flow routing around vegetation patches, enhancing channel network development. Identifying new mechanisms underlying long-term saltmarsh evolution and resilience, this work highlights the critical importance of integrating seasonality into eco-geomorphic models.