and/or diversity of benthic macroinvertebrates after removal of mangroves. These invertebrates included crabs, snails and bivalves.
Problematic vegetation: We found few studies that quantified the effectiveness – on the vegetation itself – of vegetation control on tidal flats (other than for cordgrasses; see Cutts et al., 2024a-c). Truman (1961) reported complete mortality of grey mangroves Avicennia marina var. australasica when a sufficient dose of herbicide was applied (4% 2,4-D or 2,4,5-T) – but note that use of these herbicides, especially the latter, is now prohibited or restricted in many countries. In Mangawhai Harbour, New Zealand, mangrove removal (specific method unclear) was associated with an increased density of aerial roots (pneumatophores) over the following two years. Mangrove seedlings also gradually colonised the removal site (Alfaro, 2010). In the Waikareao Estuary, New Zealand, anoxic sediment and nutrient release resulting in algal blooms were also observed following mechanical mulching of mangroves (Lundquist et al ., 2012). Schlosser et al . (2010) suggest that eelgrass Zostera japonica on tidal flats can be removed by excavation and killed by covering with opaque material such as burlap fabric, but that flame heat treatment is not an effective control method.
3. Factors that can affect outcomes
Adjacent habitats: Upstream or up-current habitats can act as a source of organisms to colonise any cleared tidal flat. This colonisation might be desirable in the case of characteristic tidal flat organisms (e.g. benthic invertebrates). However, connected patches of undesirable vegetation can hinder the success and longevity of vegetation control in a focal site (Rauzon & Drigot, 2002; Wolters et al ., 2008). Decisions to manage vegetation in a focal site might also be influenced by the presence of tidal flats nearby. For example, these might temper the value of creating a new area of tidal flat or justify vegetation control to prevent colonisation of existing flats. Physical conditions: Local tide patterns, currents, waves and sediment characteristics can affect the physical consequences of vegetation removal. For example, erosion of muddy sediments is more likely in exposed sandy sites than in sheltered muddy sites (Lundquist et al ., 2017). It may sometimes be prudent to retain vegetation in areas where removal could lead to significant erosion and instability of tidal wetlands (Qiang He, pers. comm.).
4. Implementation
Prevention: Consider managing the ultimate cause of problematic vegetation encroachment; this will often be more successful and cost-effective in the long term. For example, if algal blooms are linked to eutrophication from direct discharge of effluents and sewage outfalls onto tidal flats, consider managing this discharge. Active planting of mangrove propagules or seedlings on tidal flats, as previously widely practised across South East Asia, is now strongly discouraged due to low survival (inappropriate site selection) and adverse impacts on the tidal
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