I. Bij de Vaate1,2, M. Z. M. Brückner1, M. G. Kleinhans1, C. Schwarz1
Salt marshes form a natural barrier between land and sea. They can protect the coast from effects of climate change through attenuating storm surges, and accreting with rising sea level (Constanza et al., 1997). The effectiveness of salt marshes in doing this, is closely related to their channel networks (Leonardi et al., 2018), which is in turn influenced by their vegetation cover. Previous research suggests a dual effect of vegetation on marsh topography, where vegetation favours stabilization of sediment, yet also promotes erosion and channel incision (Schwarz et al., 2014). Past models used simplified vegetation properties to predict salt marsh channel development, disregarding the effect of various species-dependent growth forms (varying in space and time) on abiotic processes. The aim of our research is to investigate the effects of a set of common salt marsh species and their interactions on sediment stabilization and channel initiation.
To assess the long-term effect of vegetation on topography, we made use of a coupled biogeomorphologic model (based on van Oorschot et al. 2016, Brückner in prep.). This model couples vegetation development to Delft3D, which was set up using M2 forcing on a linear sloping bed. Species colonization was implemented by random establishment and growth modelled through species-specific growth and mortality functions. Here, the model allowed to consider both physical plant properties and spatio-temporal variation in growth. In this study we focused on three species that dominate NW European salt marshes: Spartina anglica, Puccinellia maritima and Salicornia procumbens. Their effect on topography was investigated for (i) each species respectively, (ii) species-assemblages and (iii) species shifts potentially occurring due to climate change or species invasions.
Our results demonstrate the importance of species-dependent vegetation properties in shaping the resulting marsh topography. Both Spartina and Puccinellia induce significant channel incision, while Salicornia does not lead to topographic change. Species assemblages resulted in comparable topographies, but with reduced channel development compared to the most spatially dominant species in the assemblage. Vegetation cover also enhances tidal asymmetry and hence influences the direction of net sediment transport. In both species shift-scenarios, the pre-shift channel network eroded because of an initial drop in vegetation cover under the new species, implying reduced protective capacity of the marsh.
Figure 1: Vegetation distribution (A) and related bed level change (B) after 20 years of simulation. Colours in (A) depict different species: Spartina (dark green), Puccinellia (light green) and Salicornia (black).
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Schwarz, C., Ye, Q. H., Wal, D., Zhang, L. Q., Bouma, T., Ysebaert, T., & Herman, P. M. J. (2014). Impacts of salt marsh plants on tidal channel initiation and inheritance. Journal of Geophysical Research: Earth Surface, 119(2), 385-400.