Abstract
Growth conditions and eco‐engineering effects of vegetation on local conditions in coastal environments have been extensively studied. However, interactions between salt marsh settling, growth, and mortality as a function of hydromorphology and eco‐engineering lack sufficient understanding to forecast morphological development of dynamic systems. We predict salt marsh establishment with an ecomorphodynamic model that accounts for literature‐based seasonal settling and life‐stage‐dependent growth and mortality of a generic salt marsh species. The model was coupled to a calibrated hydromorphodynamic model of an intertidal bar and, on a coarser grid, to the entire Western Scheldt estuary. To quantify the importance of eco‐engineering effects we compared the dynamic model results to a static model approach. The ecomorphodynamic model reproduces spatial pattern, cover, and growth trends over 15 years. The modeled vegetation cover emerges from the combination of a positive and a new negative eco‐engineering effect: vegetation reduces tidal flow strength facilitating plant survival while the developing salt marsh increases the hydroperiod, which limits large‐scale marsh expansion. The reproduced spatial gradient in vegetation density by our model is strongly correlated to their life‐stages, which underlines the importance of age‐dependence when modeling vegetation and for predictions of the stability of the marsh. Upscaling of the model to the entire estuary on a coarser grid gives implications for grid size‐dependent modeling of hydrodynamics and vegetation. In comparison with static model results, the eco‐engineering effects reduce vegetation cover, showing the importance of vegetation dynamics for predictions of salt marsh growth.
Highlights
We developed a new ecomorphodynamic model consisting of a dynamic vegetation model, which is coupled with a hydromorphodynamic model (HM-model)
Two main differences between model and data emerge: all model simulations predict salt marsh to settle in the western part of the bar, which is not found in the ecotope maps (Figures 4a–4e and 5)
Patch formation of salt marsh vegetation is not represented in the current model as we focus on large-scale morphology and salt marsh growth requiring larger grid sizes
Summary
Vegetation-landform interactions play a key role in shaping terrestrial-aquatic boundaries in fluvial and coastal environments (e.g., Jones et al, 1994; Kirwan et al, 2016; Kleinhans et al, 2018; Mariotti & Fagherazzi, 2010; Schwarz et al, 2018; Temmerman, Bouma, Govers, Wang et al, 2005; Temmerman et al, 2007). Salt marshes situated along the worlds temperate coasts are prominent examples resulting from vegetation-landform interactions Their aboveground biomass, such as stems and leaves, reduces flow velocities, dissipates wave energy, and promotes sedimentation (Kirwan et al, 2016; Leonard & Luther, 1995; Morris, 2006; Möller et al, 2014; Reed, 1990; Stevenson et al, 1986; Silvestri & Marani, 2004). Increasing rates of sea level rise and increasing vulnerability of populated coastal areas urgently require better understanding of how salt marshes shape coastlines and estuaries (Kirwan et al, 2010; Miller, 1987) This necessitates a better understanding of factors determining salt marsh establishment, growth, and mortality.
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