Temporal and Spatial Accretion Patterns and the Impact of Livestock Grazing in a Restored Coastal Salt Marsh

Abstract

A summer polder had developed a deficit in surface elevation of about 20 cm in respect to rising sea level during its almost one-hundred-year period of embankment. We addressed the questions whether the distance of the restored site to the intertidal flats and continuation of livestock grazing in the restored site could hamper surface-elevation change during the first 10 years after de-embankment of the summer polder. The surface-elevation change showed similar positive linear relationships with annual tidal flooding in both the reference salt marsh and the restored site, indicating that the surface-elevation change in the restored site was not moderated by the distance from the sea. The surface-elevation change had a clear seasonal pattern with positive values in winter and negative values during summer. The surface-elevation change was 11 mm/year in the grazed reference salt marsh and 7 mm/year in the grazed restored site, but amounted to 17 mm/year in ungrazed exclosures in the restored site, showing that grazing retarded the catching up of the elevation deficit in the restored site. The surface-elevation change within the restored site was higher close to the constructed creeks indicating the inception of levee formation. The surface-elevation change was also positively affected by the proximity of breaches in the embankment, but this effect was less clear than the effect of creeks. We conclude that the surface-elevation deficit may be compensated in the Wadden Sea summer polders by their de-embankment when sediment supply is high, whereas livestock grazing retards this process. Dug creeks increase spatial variation in the restored site.

Can Unleveed Agricultural Fields in Deltas Keep Pace With Sea‐Level Rise?

“…The measured elevations fit with the range of published estimates for the slope of the delta (Figure 3). Considering that the field is 40–50‐years old, these elevation data alone suggests that the field is significantly more resilient than equivalent leveed environments of other deltas, many of which have experienced meter‐scale subsidence over the same time period (Auerbach et al., 2015; Koppenaal et al., 2021; Rogers & Overeem, 2017). However, given the range of measured elevations at each site (∼50 cm std.…”

The importance of marshes providing soil stabilization to resist fast‐flow erosion in case of a dike breach

“…This only occurred for marshes with thin cohesive top layers over sandy subsoils, when the artificial crack penetrated to the sandy subsoil. The cohesive layer depth in marshes can increase with age, productivity, sediment availability and flooding frequency (e.g., Olff et al, 1997; Elschot et al, 2013; Koppenaal et al, 2021). Furthermore, grazing can also affect soil accretion, making this process very context dependent (Elschot et al, 2013; Koppenaal et al, 2021).…”

“…The cohesive layer depth in marshes can increase with age, productivity, sediment availability and flooding frequency (e.g., Olff et al, 1997; Elschot et al, 2013; Koppenaal et al, 2021). Furthermore, grazing can also affect soil accretion, making this process very context dependent (Elschot et al, 2013; Koppenaal et al, 2021). Cracks in the field may be bigger and deeper, therefore even with thicker cohesive layers the marsh could collapse if the sandy subsoil is eroded.…”

Biocompacting livestock accelerate drowning of tidal salt marshes with sea level rise

“…A scenario 'low' assumes a linear extrapolation of the current Dutch sea level rise of 2.4 mm/y, resulting in 25.2 cm rise in 2100, relative to the base year 1995. High-end projections for sea level rise that include rapid Antarctic ice sheet mass loss were specified for the Dutch coast by the Royal Dutch Meteorological Society based on Le Bars et al (2017) and applied for the Netherlands' Delta Commission (Haasnoot et al, 2018). In the scenario 'high' an accelerating sea level rise for RCP4.5 reaches a height of 108.1 cm in 2100, and in the scenario 'extreme' an accelerating rise for RCP8.5 reaches a height of 194.4 cm in 2100 relative to the base year 1995.…”

“…Globally, a well-used management practice on tidal marshes is grazing by livestock to maintain plant diversity (Bakker et al, 2002;Davidson et al, 2017;Esselink et al, 2017). Several studies tried to determine the impact of livestock on vertical marsh growth, but results have shown positive (Suchrow et al, 2012), neutral (Davidson et al, 2017) as well as a negative effects (Neuhaus et al, 1999;Schulze et al, 2021;Koppenaal et al, 2022). The impact of livestock grazing also differs between grazer type and intensity (Nolte et al, 2015;Davidson et al, 2017).…”