continued for 27 years, ultimately creating the typical zonation of the salt marsh plant community (Pétillon et al. , 2010). In the Yellow River Delta in China, positive recovery and growth of Suaeda glauca was noted near tidal creeks in the subtidal zone, whereas, for common reed Phragmites communis, being further away from creeks in the intertidal zone was conducive for recovery (Wu et al. , 2020).
3. Factors that can affect outcomes
Elevation/slope: The existing elevation and slope of a site will determine the feasibility of the action and the vegetation that develops (Han Winterwerp, pers. comm.). Shallower slopes will allow for creation of a wider intertidal zone. Experience suggests a slope could be around <0.04 or 1:1000 (WAVE, 2001). Topography : Variation in topography influences water depth and vegetation coverage (Ma et al. , 2010). Experimental evidence from a freshwater environment found that a varied micro- topography (changes in elevation of up 3 cm above- or below-ground) can support greater plant species richness and diversity, with many species showing preferences for hollows or hummocks (Vivian-Smith, 1997). Deeper depressions have been shown to trap more seeds and form larger vegetation patches than smaller ones (Wang et al. , 2018). Water depth: Water depth will determine how accessible pools are for foraging birds, depending on their beak and leg length (Ma et al. , 2010). An average water depth of 0.5 – 15 cm across a relatively large area is generally recommended to maximise shorebird diversity (Rogers et al. , 2015). The amount of flooding can affect the rate at which vegetation establishes, with one study finding daily flooding to be the most successful (Pitre & Anthamatten, 1981).
4. Implementation
Create depressions/basins: Basins can be excavated by digging which, depending on the size, could involve the use of heavy machinery. One study in China found that larger, deeper basins (depth: 15 cm; width: 100 – 150 cm) trapped more seeds and formed larger patches of vegetation than smaller, shallower basins (depth: 5 cm; width: 20 cm) (Wang et al. , 2018). Change site elevation/slope: Assuming there have been no other modifications (e.g. installation of dams or culverts), restoring the natural elevation can restore the natural hydrological regime. Raising or lowering the elevation of the whole site will reduce or increase, respectively, the frequency and duration of inundation. Altering the slope will affect the relative inundation across the site. A site in the IJzer estuary in Belgium that successfully created a salt marsh plant community with zonation created a slope with water inundation frequencies ranging from 0.01 – 70% per year (Pétillon et al. , 2010). Use of heavy machinery: It can be challenging to use heavy machinery in wet, soft, intertidal sediments. Vehicles can displace or compress any vegetation present. Access can be facilitated and impacts reduced by using modified vehicles (e.g. with extra wheels, tracks and/or reduced tyre pressures), reducing the weight carried, using a few designated routes for access rather than driving across the whole marsh or tidal flat, accessing the site when the
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