Reprofile/relandscape: brackish/salt marshes

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

Study locations

Key messages

  • Nine studies evaluated the effects, on vegetation, of reprofiling/relandscaping to restore or create brackish/salt marshes. Seven studies were in the USA. One was in Belgium. One was in Italy. Two of the studies were based on the same marsh.

VEGETATION COMMUNITY

  • Overall extent (2 studies): One paired, site comparison study in an estuary in the USA reported that vegetation coverage on reprofiled sediment, after 2–3 years, did not clearly differ from natural marsh areas in two of three comparisons. One replicated, paired, site comparison study in the USA reported that reprofiled coastal areas, where submerged sediment had been pushed into ridges, contained a smaller proportion of salt marsh habitat than nearby natural areas.
  • Overall richness/diversity (2 studies): Two studies in Belgium and Italy simply quantified plant species richness in marshy areas that had been reprofiled or excavated (sometimes along with other interventions), for up to 23 years after intervention began.
  • Characteristic plant richness/diversity (1 study): One study in an estuary in the USA simply reported the number of salt marsh plant species that colonized an area of reprofiled sediment over seven years.

VEGETATION ABUNDANCE

  • Overall abundance (2 studies): One site comparison study of salt marshes in the USA reported that a marsh created by reprofiling sediment (along with other interventions, including planting) had lower overall vegetation cover than a nearby natural marsh, after three growing seasons. One study in an estuary in Belgium simply quantified the cover of vegetation that colonized an area of reprofiled sediment over five years.
  • Individual species abundance (6 studies): Six studies quantified the effect of this action on the abundance of individual plant species. Of four site comparison studies in the USA, three reported that the dominant herb species was typically less abundant – in terms of cover or biomass – in marshes that had been reprofiled (sometimes along with other interventions) than in natural areas, after 2–5 years. The other study reported that density of the dominant herb species in a reprofiled (and planted) marsh was within the range of nearby natural marshes, after five years. Two studies in the USA and Belgium simply quantified cover of individual plant species over five years after reprofiling (sometimes along with other interventions).

VEGETATION STRUCTURE

  • Overall structure (1 study): One replicated, paired, site comparison study in the USA found that the layout of salt marsh habitat (e.g. patch size and complexity) differed between reprofiled coastal areas, where submerged sediment had been pushed into ridges, and nearby natural areas.
  • Height (1 study): One site comparison study in the USA reported that California cordgrass Spartina foliosa was shorter in a 5-year-old reprofiled marsh (also planted with cordgrass) than in nearby natural marshes.

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 paired, site comparison study in 1981 in four brackish and salt marshes in Texas, USA (Pitre & Anthamatten 1981) reported mixed recovery of vegetation cover in reprofiled areas, relative to natural areas, after 2–3 years. Unless specified, statistical significance was not assessed. The density and cover of 1–2 dominant grass/succulent species was reported for each marsh (see original paper for data). The density of these species was statistically similar in reprofiled and natural areas in four of seven comparisons, lower in the reprofiled area in two comparisons and higher in the reprofiled area in the other comparison. Their cover was lower in reprofiled areas in four of seven comparisons, higher in two comparisons and similar in reprofiled and natural areas in the other comparison. Overall vegetation coverage was reported for three of the four marshes. There was no clear difference between reprofiled and natural areas in two of three comparisons (reprofiled: 50–85%; natural: 50–85%) but lower coverage in the reprofiled area in the other comparison (reprofiled: 75%; natural: 95%). Methods: In spring 1981, vegetation was surveyed in 25 x 25 cm plots (number not reported), randomly placed in reprofiled and natural (undisturbed) areas of four marshes. Excess sediment had been deposited on parts of each marsh during construction activities. Two to three years before surveying, this sediment was removed (and any depressions filled) to return these areas to their natural elevation.

    Study and other actions tested
  2. A site comparison study in 1989 of two estuarine salt marshes in California, USA (Langis et al. 1991) found that a marsh created by reprofiling, planting California cordgrass Spartina foliosa and fertilizing contained less cordgrass biomass, after 4–5 years, than an adjacent natural marsh. The created marsh contained 192 g/m2 above-ground California cordgrass biomass: significantly lower than the 454 g/m2 in the natural marsh. Methods: In July 1989, California cordgrass was cut from 9–12 quadrats at a similar elevation in the two marshes, then dried and weighed. One marsh (same marsh as in Study 3) had been created by reprofiling into islands and creeks (autumn 1984), planting California cordgrass along creek banks (March 1985) and fertilizing with urea (25 g/m2; four times 1985–1986). This study evaluates the combined effect of these interventions on any non-planted cordgrass. A nearby natural marsh, exposed to similar tides, was chosen for comparison.

    Study and other actions tested
  3. A site comparison study in 1989 of four estuarine salt marshes in California, USA (Zedler 1993) found that a marsh created by reprofiling, planting California cordgrass Spartina foliosa and fertilizing supported a similar cordgrass density to adjacent natural marshes, but with shorter plants. Statistical significance was not assessed. Five years after reprofiling, four of four transects in the created marsh supported a cordgrass density (133–173 stems/m2) within the range of nearby natural marshes (73–193 stems/m2). However, cordgrass was shorter in the created than natural marshes, with a greater proportion of stems in shorter height classes (see original paper for data). Methods: In September 1989, California cordgrass was surveyed in 0.1-m2 quadrats. Twelve quadrats (four transects) were surveyed in a created marsh (reprofiled into islands and creeks in 1984, planted with California cordgrass in 1985, fertilized with urea in 1985–1986; same marsh as in Study 2). This study evaluates the combined effect of these interventions on any non-planted cordgrass. Fifty-four quadrats (seven transects) were surveyed in three nearby natural marshes.

    Study and other actions tested
  4. A replicated study in 1989–1991 in an estuary in California, USA (Gibson et al. 1994) reported that after excavating a salt marsh and planting California cordgrass Spartina foliosa, there were increases in California cordgrass density and biomass. Statistical significance was not assessed. After one growing season, there were 25 cordgrass stems/m2 and 60 g/m2 dry above-ground biomass. After two growing seasons, there were 50 cordgrass stems/m2 and 220 g/m2 dry above-ground biomass. Methods: Between 1989 and March 1990, dredge spoil that had been deposited in San Diego Bay was excavated to elevations suitable for California cordgrass. In March 1990, California cordgrass was planted into four 5-m2 plots in the marsh (ten 4-L pots of cordgrass/plot). None of these four plots received any additional treatment. California cordgrass stems were counted and measured until October 1991. Note that this study does not distinguish between the effects of excavation and planting on any non-planted cordgrass.

    Study and other actions tested
  5. A site comparison study in 1998–2002 of two salt marshes in California, USA (Armitage et al. 2006) reported that a reprofiled, planted and fenced marsh had lower vegetation cover than a nearby natural marsh after three growing seasons, but that both marshes were dominated by pickleweed Salicornia virginica. Statistical significance was not assessed. After three growing seasons, the created marsh had 62% total vegetation cover (compared to 87% in a nearby natural marsh). The most abundant species in both marshes was pickleweed (created: 39%; natural: 62% cover). Four plant species colonized plots where they had not been planted. In these plots, pickleweed cover was 23–27%. Where they colonized, the other species had <1% cover. Methods: In autumn 1997, an upland area was reprofiled to form an intertidal mudflat. In March 1998, rooted cuttings of four salt marsh herb/succulent species were planted into fifty-five 4-m2 plots around the edge of the mudflat (25–81 plants/plot; combinations of 1 or 2 species/plot). After one growing season, the plots were protected with rabbit-proof fencing. Debris and colonizing vegetation were regularly removed during the first two growing seasons, but left in place thereafter. The study does not distinguish between the effects of reprofiling, planting and fencing on the non-planted vegetation. Total vegetation cover was measured in 0.25-m2 quadrats: in the created marsh (1 quadrat/plot/year until October 2000) and a nearby natural marsh of similar elevation (10 quadrats in July 1999).

    Study and other actions tested
  6. A replicated, paired, site comparison study in 2004 in two salt marshes in Texas, USA (Feagin & Wu 2006) found that relandscaped plots, where sediment had been pushed into ridges, contained less and more patchy marsh habitat than natural reference plots. Relandscaped plots contained less marsh habitat (low marsh: 6–7%; high marsh: <1–3%) than natural areas (low marsh: 27–37%; high marsh: <1–10%). Accordingly, relandscaped plots contained more open water (76–91%) than natural areas (20–53%). Five of seven landscape structural metrics also significantly differed between relandscaped and natural plots. Relandscaped plots were dominated by multiple small patches of low marsh with relatively complex outlines, whereas natural plots contained fewer, larger, clumped patches of low marsh with relatively simple outlines (see original paper for data). Methods: Vegetation was mapped from aerial photographs taken in 2004 (ground-truthed in May 2005). Vegetation was compared in three pairs of 4-ha plots/marsh. In each pair, one plot contained reprofiled marsh (submerged sediment pushed into ridges, in a grid pattern or arcs; date of intervention not clearly reported). The other contained natural, undisturbed marsh.

    Study and other actions tested
  7. A study in 1999–2007 aiming to create a salt marsh in an estuary in Belgium (Pétillon et al. 2010) reported that an area cleared and reprofiled to allow tidal inundation was colonized by vegetation, including salt marsh species, within one year. Immediately after reprofiling the area was bare sediment. After one year, the site had 16% total vegetation cover and 5 plant species/4 m2. After five years, the site had 75% total vegetation cover, with 10 plant species/4 m2 and a total of 119 plant species across the site. The most abundant species were annual seablite Suaeda maritima (20% cover) and glasswort Salicornia europaea (8% cover). Between one and five years after reprofiling, the plant community composition changed significantly, especially in higher areas flooded less often (data reported as a turnover index). Methods: Between 1999 and 2002, a site on the edge of an estuary was reprofiled to facilitate tidal inundation. Buildings and fill material were removed to create a slope with varying flooding frequencies. Plant species and their cover were recorded in 2003, 2005 and 2007 in 119 permanent 4-m2 quadrats, placed along transects perpendicular to the shoreline.

    Study and other actions tested
  8. A study in 2003–2010 aiming to restore a salt marsh in an estuary in Florida, USA (Taylor 2012) reported than an area where excess fill material was removed to restore tidal influx was colonized by salt marsh plants within one year. After one year, 18 plant species “appropriate for salt marsh habitats” had colonized the reprofiled sediment. These included smooth cordgrass Spartina alterniflora, annual seablite Suaeda linearis and three species of mangrove tree. After seven years, 27 salt marsh plant species were present. A “few” upland species were also recorded, but not Brazilian pepper Schinus terebinthifolius that dominated the site prior to relandscaping. Methods: In 2003, excess material (sand/shell mix from a historic dredging operation) was removed from the surface of a former salt marsh. The elevation was restored to within the range of adjacent healthy marshes. Plant species present were recorded one and seven years after relandscaping.

    Study and other actions tested
  9. A study in 2005–2013 of an excavated, planted and harvested water treatment marsh in Sardinia, Italy (De Martis et al. 2016) reported that it supported 275 plant taxa. This included 201 plant species in 161 genera. Approximately 63% of the taxa were Mediterranean (found predominantly or solely in this region) and approximately 16% were known non-natives in Italy. As expected in the study area, 56% of the taxa were annual plants that complete their life cycle rapidly in favourable conditions (“thereophytes”). Only 2% of taxa had underwater resting buds (“hydrophytes”). Methods: Between 2005 and 2013, plant taxa were recorded in the 37-ha EcoSistema Filtro marsh, which had been constructed with the dual aims of habitat creation and water treatment. There were monthly surveys (a) across the whole site, including banks and upland areas, and (b) in three 16-m2 plots, each April–July and September–December. The wetland had been constructed by excavating basins of varying salinity and levees (including removal of all existing vegetation; beginning 1990) and planting bundles of 2-m-tall common reed Phragmites australis (2004). Some “plant biomass” was mechanically removed between 2005 and 2007. Note that this study evaluates the combined effect of these interventions, and does not separate results from fresh, brackish and saline areas.

    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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