Scirpus mariqueter (a dominant salt marsh species in the estuary) finding that the most economically efficient treatment was to transplant low-density corm seedlings without sediment, costing ¥10,100 Chinese Yuan (about US$1,400; Feb 2024 conversion) per hectare. Seeds can be collected from natural marshes or nurseries and are typically sown around 1.5 – 5.0 cm deep, but the optimum depth will vary between species (Groenendijk, 1986; Hu et al. , 2016). Although easier to handle than plants, seeds are more at risk of being washed away. The number of seeds planted will vary based on the species and the size of the site. Studies report a range of 80 – 4,000 seeds/m 2 (Groenendijk, 1986; Varty & Zedler, 2008; Guan et al. , 2011; Hu et al. , 2016). The survival of the new plants depends on the local conditions. For example, at a site in California, over 21,000 seeds were planted but only 17 seedlings grew (Zedler, 1993). This was attributed to the high salinity levels, sediment deposition that buried the plants, algal smothering and trampling by birds, such as American Coot Fulica americana . Sods of salt marsh vegetation have also been successfully transplanted to restoration/creation sites (Green et al. , 2009; Sparks et al., 2013). It has been suggested that this restores vegetation coverage more quickly, is more aesthetically pleasing and is more resistant to erosion than planting or sowing – but generally more expensive (Sparks et al., 2013). It may not be necessary to completely cover a restoration/creation site with sods: one study in Mississippi, USA reported that 50% coverage was more cost effective (and required less material from the donor site) than 100% coverage (Sparks et al., 2013). Plant material can be collected from nearby sites that contain target plant species and spread on a restoration/creation site (Emond et al. , 2016; Wolters et al. , 2017). For example, to restore a brackish marsh in the Netherlands, plant material was collected from the foot of a nearby sea wall, using agricultural machinery. Portions of 10 cm 3 were put in a manure spreader and 200 m 3 was spread over two thirds of the site in a 5 cm layer (Wolters et al. , 2017). If using vegetation from existing salt marshes, consider how to minimise impacts on donor sites, for example by leaving some vegetation in place and avoiding collecting vegetation during bird breeding periods. Any necessary permits should be secured before collection. Actions to help planted vegetation: A wide range of actions could be done before or after introducing vegetation to increase survival and/or growth rates. These include reprofiling (creating mounds and/or depressions), removing polluted/dry/crusty surface sediment, using nurse plants, using fences or barriers to exclude animals that may damage young plants, adding lime and adding fertiliser (Taylor et al. , 2021). If it is considered necessary to add materials, such as lime or fertiliser, these should generally be added when the site is not flooded to reduce the risk of it dissolving or being washed away. Additives can also be mixed into the sediment before planting. One study had success with kelp compost, mixing 40 L/1.5 m 2 of compost (two parts soil, one part kelp) into the top 30 cm of soil (O’Brien & Zedler, 2006), while another study found success ploughing 2 kg/m 2 of reed debris to a depth of 20 cm (Guan et al. , 2011). The addition of fertiliser alongside planting can speed the growth of new vegetation by balancing the nutrients available in the sediment. It should be noted that not all additives have proven to be successful; adding too much fertiliser, such as nitrogen, can do more harm than good by causing eutrophication, which is a great threat to coastal systems globally (Albornoz, 2016; Malone & Newton, 2020).
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