Restore/create brackish/saline marshes or swamps (specific action unclear)

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

  • Seven studies evaluated the effects, on vegetation, of restoring/creating brackish/saline marshes or swamps using unclear or incompletely described actions. Four studies were in the USA. There was one study in each of Australia, Canada and Indonesia.

VEGETATION COMMUNITY

  • Community composition (4 studies): Three replicated, site comparison studies in the USA and Australia reported that the overall plant or algal community composition in restored/created marshes typically became more like natural reference marshes over time. One replicated, site comparison study of fresh/brackish wetlands in Canada reported that the overall plant community was lower quality in restored/created sites than natural sites, after ≥3 years.
  • Overall richness/diversity (1 study): One replicated, site comparison study of salt marshes in the USA found that created marshes had similar overall plant diversity, after 1–14 years, to natural marshes. Created marshes had lower plant species richness than natural marshes on average, but richness became more similar to natural marshes with time since creation.
  • Algae/phytoplankton richness/diversity (1 study): One replicated, paired, site comparison study of brackish/saline marshes in the USA reported that restored and natural marshes contained a similar number of algal species, and found that they had similar algal diversity, after 1–28 years.

VEGETATION ABUNDANCE

  • Overall abundance (2 studies): One replicated, site comparison study of salt marshes in the USA found that created marshes contained less overall plant biomass, after 1–14 years, than natural marshes – but that biomass increased with time since creation. One replicated, site comparison study of fresh/brackish/saline marshes in the USA found that created (but not restored) marshes had similar overall vegetation cover to natural marshes. Both created and restored marshes had similar cover of wetland vegetation to natural marshes.
  • Herb abundance (2 studies): One replicated, paired, site comparison study of brackish/saline marshes in the USA reported that restored marshes contained a greater density of cordgrasses Spartina than natural marshes in six of eight comparisons. Vegetation was surveyed 1–28 years after restoration, which involved planting cordgrasses. One replicated, paired site comparison study in the USA reported that created intertidal wetlands contained more smooth cordgrass Spartina alterniflora than nearby natural mangrove forests for around 13 years.
  • Tree/shrub abundance (2 studies): One replicated, paired site comparison study in the USA reported that created intertidal wetlands contained fewer adult mangrove trees than nearby natural mangrove forests for up to 20 years – but predicted equivalence within 55 years. One replicated study in Indonesia simply quantified the density of tree seedlings three years after restoration of former mangrove ponds.
  • Algae/phytoplankton abundance (1 study): One paired, site comparison study of brackish/saline marshes in the USA reported that older restored marshes (≥26 years old) contained a similar or greater abundance of algae to natural marshes, whereas younger restored marshes (<13 years old) contained less algae than natural marshes.

VEGETATION STRUCTURE

  • Diameter/perimeter/area (1 study): One replicated, paired site comparison study in the USA reported that created intertidal wetlands contained thinner adult mangrove trees than nearby natural mangrove forests for up to 20 years – but predicted equivalence within 25 years.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A replicated, site comparison study of 17 estuarine salt marshes in the northeast USA (Morgan & Short 2002) found that created marshes contained a different plant community, fewer plant species and less plant biomass than nearby natural marshes, on average – but reported that the oldest created marsh was similar to the natural marshes. The created marshes were 1–14 years old. They typically had a different overall plant community composition to natural marshes (data reported as a graphical analysis; statistical significance not assessed) with less overall cover of high marsh species (created: 0–37%; natural: 1–84%). On average, the created marshes contained less above-ground plant biomass (160 g/m2; vs natural: 350) and fewer plant species (4.4 species/marsh; vs natural: 8.4), although average plant diversity did not significantly differ between created and natural marshes (data reported as a diversity index). However, the oldest created marsh was similar to the natural marshes. This was true for overall community composition, cover of high marsh species (37%), biomass (310 g/m2) and richness (9.1 species/marsh). Methods: In an unspecified year, vegetation was surveyed in six created salt marshes and 11 nearby natural marshes in similar physical environments. Creation involved planting, but the study does not report details of this or prior earthworks. Plant species and their cover were surveyed in 1-m2 quadrats. Live vegetation was cut from 0.5-m2 quadrats, then dried and weighed.

    Study and other actions tested
  2. A replicated, paired, site comparison study in 1998 of 16 brackish/saline marshes in North Carolina, USA (Zheng et al. 2004) reported that restored marshes had similar algal richness and diversity to natural marshes, and that older restored marshes contained a similar amount of algae to natural marshes. Unless specified, statistical significance was not assessed. Both restored and natural marshes contained algal communities dominated by diatoms, filamentous algae and blue-green algae. The similarity in community composition between restored and natural marshes varied between 12 and 96%, depending on the habitat, season and time since restoration. Restored and natural marshes supported a similar number of algal species (restored: 131–204; natural: 118–218 species/habitat/season) and statistically similar algal diversity (data reported as a diversity index). Algal abundance generally increased with time since restoration, such that older restored marshes (≥26 years old) contained a similar or greater amount of algae to natural marshes whereas younger restored marshes (<13 years old) contained less (see original paper for data and statistical models). Finally, restored marshes contained more cordgrass Spartina spp. than natural marshes in six of eight comparisons (for which restored: 297–498 stems/m2; natural: 201–316 stems/m2). Methods: In spring and summer 1998, algae were surveyed in eight pairs of coastal brackish/saline marshes. In each pair, one marsh had been restored 1–28 years previously (restoration methods not reported, but included cordgrass planting) and the other, nearby marsh was natural. Algae were collected from cordgrass stems and the top 1 cm of sediment. Abundance was measured as biovolume or estimated from chlorophyll concentrations. Cordgrass stems were counted in each marsh in October.

    Study and other actions tested
  3. A replicated, site comparison study in 2007 in 35 freshwater, brackish and saline marshes in Hawaii, USA (Bantilan-Smith et al. 2009) found that restored/created marshes had statistically similar overall plant species richness, cover of wetland plant species and cover of exotic plant species to natural marshes, but that restored marshes had lower overall vegetation cover. Data were not reported for most outcomes. Overall vegetation cover was only 59% in restored marshes: significantly lower than the 74% cover in created wetlands and 76% cover in natural wetlands. The study suggests this may have been driven by management of restored wetlands for waterbirds, which require open water and mudflats. The study also reported differences in soil properties between restored/created and natural marshes. Methods: In March/April 2007, plant species and their cover were recorded in 35 coastal lowland marshes: 11 restored, 7 created and 17 natural. Six 1-m2 quadrats were surveyed in each marsh, across flooded, saturated and upland areas. Twenty-seven marshes brackish, saline or hypersaline. The other eight marshes were freshwater. The study does not report details of restoration/creation methods (including dates), or separate the effects of restoration/creation in marshes of different salinity.

    Study and other actions tested
  4. A replicated, controlled, site comparison study in 2003–2007 in an estuarine salt marsh in New South Wales, Australia (Green et al. 2009) reported that restored areas developed a plant community more like natural reference areas over 3–4 years. All four restored areas were colonized by saltwater couch Sporobolus virginicus: the dominant plant species in the reference areas. The overall plant community composition in all four restored areas became more similar to the reference areas (data reported as a graphical analysis). However, it remained distinct from the reference marshes (similarity <50%) in three of four cases. Two additional degraded areas that received no or less intervention (details not clear) also developed a plant community more like reference marshes over time. These results are not based on assessments of statistical significance. Methods: Between July 2004 and April 2007, plant species and cover were surveyed in eight areas of tidal salt marsh around a lagoon (fifty 1-m2 quadrats/area/survey). Two areas contained natural, undisturbed salt marsh. The other six areas had been degraded by vehicle use, mining, rubbish dumping and weed encroachment. Four of these areas were restored between 2003 and mid-2004, with interventions including fencing to exclude vehicles, filling eroded patches with sediment, restoring the surrounding forest and transplanting sods of saltwater couch. The study does not clearly report what interventions, if any, were done in the other two degraded areas.

    Study and other actions tested
  5. A replicated, paired, site comparison study in 2010 of 18 intertidal wetlands on the coast of Florida, USA (Osland et al. 2012) reported that created wetlands were initially dominated by salt marsh vegetation but began to develop mangrove forest vegetation within 20 years. All data in this summary have been taken from statistical models. Young created wetlands (2–5 years old) were dominated by salt marsh vegetation, most of which was smooth cordgrass Spartina alterniflora (above-ground biomass: >270 g/m2; density: >200 stems/m2). The oldest studied created wetlands (19.5 years old) contained a mixture of juvenile (2 trees/m2; 85 cm tall) and adult (98 trees/m2; 4.2 cm diameter) mangrove trees, and no smooth cordgrass. Statistical models predicted that the abundance of smooth cordgrass became equivalent to nearby natural mangrove forests within 13 years. The diameter and density of adult mangrove trees would become equivalent to nearby natural mangrove forests after 25 and 55 years, respectively. Methods: In summer 2010, vegetation was surveyed in nine pairs of intertidal wetlands in Tampa Bay. In each pair, one wetland had been created from upland 2–20 years previously (precise creation methods not reported, but likely involved reprofiling to intertidal elevations and planting with salt marsh herbs) and one wetland was a natural, mature mangrove forest. Surveys were carried out in three 100-m2 plots/site (and subplots of various size within).

    Study and other actions tested
  6. A replicated, paired, site comparison study in 2007–2011 of 133 fresh and brackish wetlands in Alberta, Canada (Wilson et al. 2013) found that restored/created wetlands typically contained higher quality wet meadow vegetation than natural wetlands. All data were reported as a floristic quality index, with higher quality vegetation containing species more characteristic of undisturbed wet meadows in the study area, and a greater proportion of native to non-native species. In six of eight comparisons, the quality of wet meadow vegetation was lower in restored/created wetlands than natural wetlands. This was true for wetlands restored on historically mined land (whether still affected by pollution or not) and for “improved” stormwater ponds (with attempts to make them more like natural wetlands using interventions such as reprofiling, adding wetland soil and planting). In the final two comparisons, the quality of wet meadow vegetation was similar in wetlands restored on farmland and in natural wetlands. Methods: Between 2007 and 2011, plant species were recorded in the wet meadow zone of 133 wetlands (six 1-m2 quadrats/wetland). There were 47 restored or created wetlands (≥ 3 years old) and 86 naturally occurring wetlands (some surrounded mostly by forest, some mostly by agriculture) across two distinct regions. The study does not report full details of restoration/creation methods, or separate results for brackish and fresh wetlands.

    Study and other actions tested
  7. A replicated study in 2010–2016 of former aquaculture ponds undergoing restoration in South Sulawesi, Indonesia (Wetlands International 2016) reported that natural recruitment of mangrove seedlings occurred within one year, creating densities of over 2,500 seedlings/ha after three years. Methods: Between 2010 and 2015, aquaculture ponds in seven villages on Tanakeke Island were subjected to “Ecological Mangrove Rehabilitation”. The study does not completely describe this process, but notes that it involves breaching dikes, re-creating tidal creeks, periodic addition of mangrove propagules, and a “minimal amount” of planting.

    Study and other actions tested
Please cite as:

Taylor N.G., Grillas P., Smith R.K. & Sutherland W.J. (2021) Marsh and Swamp Conservation: Global Evidence for the Effects of Interventions to Conserve Marsh and Swamp Vegetation. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

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Marsh and Swamp Conservation

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Marsh and Swamp Conservation
Marsh and Swamp Conservation

Marsh and Swamp Conservation - Published 2021

Marsh and Swamp Synopsis

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