the number of species was similar to a control site where no material was added (Wolters et al. , 2017).
Planting can work well when combined with fertiliser (Taylor et al. , 2021), for example kelp compost (O’Brien & Zedler, 2006) or reed debris (Guan et al., 2011). Another study found that rock phosphate increased overall salt marsh vegetation cover but that this was only effective when also introducing vegetation (Emond et al. , 2016). Using urea was found to increase Seablite Suaeda salsa biomass in China (Guan et al. , 2011).
3. Factors that can affect outcomes
Water levels: Plants have specific tolerances to flooding or waterlogging, which is influenced by the elevation of a site. Older plants may be better able to withstand waterlogging than young seedlings (Cao et al. , 2022). Therefore, consider planting species or individuals at appropriate elevations. Modifying the (micro)topography before planting may also increase survival (see Cutts et al., 2024). Evidence suggests that seed retention of pioneer salt marsh plants is higher in depressions in the soil (Wang et al. , 2018). Salinity: Coastal vegetation is salt-tolerant, but even the most tolerant species will struggle in hypersaline areas (Zedler, 2003). Equally, prolonged periods of low salinity, for example due to inputs of rainwater or runoff from urban areas, are not suitable for salt marsh vegetation and can favour invasion by undesirable species such as, in North America, cattail Typha domingensis (Beare & Zedler 1987) or Perennial Pepperweed Lepidium latifolium (Wiggington et al. , 2020). Exposure: Physical disturbance from waves and currents may limit initial establishment and long-term persistence of vegetation. Attempting to establish vegetation in the highest energy sites will often be a losing battle. Moderate energy levels can be mitigated using barriers such as breakwaters. Species and life-stages appropriate to local energy levels should be chosen. Animals: Establishing salt marsh vegetation may be consumed and/or trampled by animals such as mammals (Wasson et al. , 2021), birds (Zedler, 1993) or crabs (Liu et al. , 2020). High animal densities might prevent establishment. Cages or other exclosures can be used to protect young vegetation (Taylor et al. , 2021; Wasson et al. , 2021). Animals can also have positive effects. For example, planted Scirpus mariqueter suffered less grazing and reached higher densities in areas closer to Giant Mud Crab Scylla serrata burrows, because the crabs preyed upon grazers (Wu & He, 2023).
4. Implementation
Planting: Seedlings/plants can be transplanted from nearby marshes or can be reared in a nursery. Plants are often placed in depressions 5 – 10 cm deep in the soil, but the optimum depth depends on the species (Varty & Zedler, 2008; Guan et al. , 2011; Hu et al. , 2016). Studies where planting has been successful have typically planted 45 – 100 cm apart and planting usually occurs in spring to early summer (Taylor et al. , 2021). In several cases, planting was into fine-grained dredged sediment and sometimes the existing vegetation was removed. In the Yangtze Estuary, China, Zhang & Li (2023) tested different methods of planting
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