Facilitate tidal exchange to restore degraded brackish/salt marshes

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 facilitating tidal exchange to restore degraded brackish/salt marshes. Six studies were in the USA. One study included sites in both the USA and Canada.

VEGETATION COMMUNITY

  • Overall extent (1 study): One before-and-after study in the USA reported that over 10 years after improving tidal exchange in a degraded marsh, the area of salt marsh vegetation increased – but not quite to historical, pre-degradation levels.
  • Community types (1 study): One before-and-after study in the USA reported that 3–10 years after improving tidal exchange in a degraded marsh, the area of salt marsh community types differed from historical, pre-degradation levels.
  • Community composition (3 studies): Three before-and-after studies in the USA found that in the four years after improving tidal exchange in degraded brackish/salt marshes, the overall plant community composition significantly differed to that present before intervention. However, in one of the studies this was only true in one of two marshes (the most degraded before intervention). One of the studies also reported that the overall plant community composition became more similar to adjacent natural brackish/salt marshes over two growing seasons after intervention.
  • Overall richness/diversity (1 study): One replicated, before-and-after, site comparison study in the USA/Canada found that overall plant species richness was similar in ≥3-year-old tidally restored salt marshes and nearby natural salt marshes. However, there was also no significant difference between degraded marshes (before tidal restoration) and the natural marshes.
  • Characteristic plant richness/diversity (1 study): One study of a coastal marsh in the USA reported that over three years after restoring tidal exchange (along with a prescribed burn), the number of salt-tolerant plant species increased, whilst the number of freshwater plant species decreased.

VEGETATION ABUNDANCE

  • Overall abundance (1 study): One replicated, site comparison study of salt marshes in the USA found that tidally restored areas had a lower overall plant stem density, after 13–54 years, than natural salt marshes.
  • Characteristic plant abundance (2 studies): Two before-and-after studies of coastal marshes in North America reported that within three years of restoring tidal exchange (sometimes along with other interventions), total cover of fresh/brackish plant species decreased. In one study the total cover of salt-tolerant plant species increased, but in the other study it did not. One of the studies also found that tidally restored marshes had lower cover of salt-tolerant plants than nearby natural marshes.
  • Individual species abundance (5 studies): Five studies quantified the effect of this action on the abundance of individual plant species. All five studies were in brackish/salt marshes in the USA. Three before-and-after studies reported increases in cover or frequency of smooth cordgrass Spartina alterniflora in the four years after improving tidal exchange. One replicated, site comparison study found that smooth cordgrass cover was lower in tidally restored areas than in natural salt marshes, 13–54 years after tidal restoration. Two before-and-after studies reported no clear change in frequency or cover of saltmeadow cordgrass Spartina patens in the four years after improving tidal exchange, but one before-and-after study reported an increase in saltmeadow cordgrass cover over two growing seasons after improving tidal exchange. Four studies reported declines in cover or frequency of less salt-tolerant species such as common reed Phragmites australis and cattails Typha spp. in the four years after improving tidal exchange (sometimes along with other interventions). One replicated, site comparison study found that common reed cover was similarly low (<1%) in tidally restored areas and natural salt marshes, 13–54 years after tidal restoration.

VEGETATION STRUCTURE

  • Vegetation height (3 studies): Two before-and-after studies of brackish/salt marshes in the USA found that common reed was shorter 1–4 years after improving tidal exchange than before. One replicated, site comparison study in the USA found that the maximum vegetation height was similar in tidally restored salt marshes and natural salt marshes, 13–54 years after tidal restoration.

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 before-and-after study in 1946–1988 of a coastal salt marsh in Connecticut, USA (Barrett & Niering 1993) reported that after installing culverts across a dyke to restore tidal exchange, coverage of salt marsh plant communities increased and coverage of fresh/brackish plant communities decreased. Statistical significance was not assessed. Salt marsh plant communities covered 88% of the site in 1946 (before tidal exchange was blocked), 4% of the site in 1976 (after 30 years without tidal exchange) and then 63% of the site in 1988 (after 10 years with restored tidal exchange). Twenty-eight percent of the site was covered by the same salt marsh community type in 1988 as it was in 1946. Vegetation dominated by smooth cordgrass Spartina alterniflora was more abundant in 1988 (51% coverage) than in 1946 (40% coverage). Mixed saltgrass Distichlis spicata and saltmeadow cordgrass Spartina patens was less abundant in 1988 (<1% coverage) than in 1946 (37% coverage). Stands dominated by narrowleaf cattail Typha angustifolia and common reed Phragmites australis (36% coverage in total) persisted in 1988 after restoration of tidal exchange (vs 80% coverage in 1976 and 0% coverage in 1946). Methods: The study compared three vegetation maps: before, during and after tidal restriction. A dyke built across the mouth of the marsh in 1946 stopped tidal exchange. Culverts built in 1978 and 1982 restored it.

    Study and other actions tested
  2. A before-and-after study in 1993–1996 of a coastal marsh in Florida, USA (Brockmeyer et al. 1996) found that following restoration of tidal exchange and prescribed burning, species richness and cover of salt-tolerant vegetation increased, whilst species richness and cover of freshwater vegetation decreased. Within three years of tidal restoration, the number of salt-tolerant plant species in the marsh increased from seven to eight. Cover of salt-tolerant vegetation significantly increased (by 1,056%). The number of freshwater plant species decreased from thirteen to one. Cover of freshwater vegetation significantly decreased (by 74%). There was a non-significant 56% decline in southern cattail Typha domingensis cover. Methods: In 1993, thirteen culverts were built to restore tidal exchange to a degraded, impounded, cattail-invaded marsh. In February 1995, the marsh was burned. The study does not distinguish between the effects of these interventions. Vegetation was surveyed along fifteen 15-m transects in October 1993 (before culverts were built) and March 1996.

    Study and other actions tested
  3. A before-and-after, site comparison study in 1996–2000 of two brackish/salt marshes in an estuary in Rhode Island, USA (Roman et al. 2002) found that after installing culverts to improve tidal exchange to a degraded marsh, its vegetation became more like a natural marsh. Over two growing seasons following intervention, the overall plant community composition in the tidally restored marsh became more like that in an adjacent natural marsh. Cover of salt marsh plant taxa such as cordgrasses Spartina spp. and glasswort Salicornia europaea increased, whilst cover of common reed Phragmites australis and narrowleaf cattail Typha angustifolia decreased (data reported as cover categories; statistical significance not assessed). After two growing seasons, the overall plant community in the tidally restored marsh remained significantly different from the natural marsh – but was also significantly different from the composition before intervention (data not reported). In the tidally restored marsh, common reed was significantly shorter over three growing seasons following intervention (84–107 cm) than it had been before intervention (136 cm). Methods: The study involved two brackish/salt marshes either side of a road causeway and narrow culvert. In March 1998, full tidal exchange was restored to the degraded, reed-dominated marsh above the road by installing two wider culverts. The marsh below the road remained relatively undisturbed, with full tidal exchange and dominated by salt marsh vegetation. Tidal creeks and pools were excavated in both marshes. Plant species and their cover were surveyed in late summer before (1996) and after (1998, 1999) intervention. Surveys involved 22–28 quadrats/marsh/year, each 1 m2, placed along transects. Common reed stems were also measured in the degraded/restored marsh until late summer 2000.

    Study and other actions tested
  4. A before-and-after, site comparison study in 1997–2002 of three salt marshes in Massachusetts, USA (Buschbaum et al. 2006) found that widening a culvert to improve tidal exchange altered the plant community composition and reduced the height of common reed Phragmites australis. In one marsh, the overall plant community composition was significantly different in the four years after tidal restoration than before (data not reported). Species experiencing large changes in frequency included common reed (present at only 24% of sampled points after restoration, vs 40% before), narrowleaf cattail Typha angustifolia (6% after vs 18% before) and Spartina alterniflora (28% after vs 17% before). The frequency of dominant saltmeadow cordgrass Spartina patens did not clearly change (50% after vs 46% before). These frequency results are not based on assessments of statistical significance. Common reed was significantly shorter after tidal restoration (60–85 cm) than before (140–155 cm). In two adjacent natural marshes, plant community composition, species frequencies and common reed height were stable over time (see original paper). Methods: In 1998, a small culvert was replaced with a bigger one to increase tidal exchange in a degraded, reed-invaded, coastal marsh. Plant species and reed height were recorded for two years before and four years after intervention, along 4–9 transects in the restored marsh and two natural marsh areas.

    Study and other actions tested
  5. A replicated, before-and-after, site comparison study including up to 36 salt marsh restoration projects in the Gulf of Maine, North America (Konisky et al. 2006) found that after improving tidal exchange, cover of salt-loving species did not increase, cover of fresh/brackish species decreased, and plant species richness remained stable. Before intervention, tidally restricted marshes had lower cover of salt-loving species than natural marshes (degraded: 48%; natural: 64%) and greater cover of fresh/brackish species (degraded: 10%; natural: 3%) but contained a statistically similar number of plant species (degraded: 6.9; natural: 6.6 species/marsh). After three or more years, tidally restored marshes still had lower cover of salt-loving species (47%) than natural marshes, but now had statistically similar cover of fresh/brackish species (6%) and retained statistically similar plant species richness (6.6 species/marsh). There was a temporary dip in cover of salt-loving species (33% after two years). The pattern of results was similar for each of three restoration methods considered (see original paper for data). Methods: The study collated data on vegetation cover and species richness from up to 36 coastal salt marsh restoration projects (7–25 marshes with data for each metric in a given year). The projects were completed, ongoing or pending between 1995 and 2003. They involved restoring tidal hydrology to tidally restricted marshes by (a) removing culverts or tide gates, (b) plugging drainage ditches, or (c) excavating tidal channels or raised areas. Data were averaged (a) for the last year before intervention, (b) for 1, 2 and ≥3 years after intervention, and (c) for natural reference marshes.

    Study and other actions tested
  6. A replicated, before-and-after study in 2003–2008 of two brackish/salt marshes in an estuary in New York State, USA (Rochlin et al. 2012) found that after blocking drainage ditches and excavating tidal channels/pools to improve tidal exchange, one of two marshes experienced changes in plant community composition including a reduction in cover of common reed Phragmites australis. Before intervention, Marsh 1 was highly degraded. In all four years after intervention, the overall plant community composition along transects in this marsh was significantly different to the composition before (data reported as a graphical analysis). Species whose average cover increased included saltmarsh bulrush Schoenoplectus robustus (before: 0.0%; after: 9.0%) and smooth cordgrass Spartina alterniflora (before: 0.2%; after: 2.0%). Cover of both live and dead common reed declined (live: from 25% to 8%; dead: from 21% to 5%). There was no clear change in cover of saltmeadow cordgrass Spartina patens (before: 80%; after: 82%). Statistical significance of these cover results was not assessed. Marsh 2 was less degraded before intervention. It experienced no significant change in overall plant community composition after intervention (data not reported). Methods: Between 2004 and 2006, tidal exchange was restored in two degraded (ditched, tidally restricted and reed-invaded) 16–19 ha brackish/salt marshes. Most existing drainage ditches were filled and new tidal channels/pools were excavated. Additionally, to reduce habitat for mosquito breeding, some depressions in the high marsh were filled in. Vegetation was surveyed each autumn for 2–3 years before and 3–4 years after intervention, along 4–5 transects spanning each marsh.

    Study and other actions tested
  7. A replicated, site comparison study in 2007–2008 across 15 salt marshes in Connecticut, USA (Elphick et al. 2015) found that plots in which tidal exchange had been restored had lower cover of saltmeadow cordgrass Spartina patens and a lower plant stem density than natural marshes, but had statistically similar cover of two reed/rush species and vegetation height. After 13–54 years, tidally restored plots had lower cordgrass cover than natural marshes (2 vs 20%) and a lower density of plant stems overall (3 vs 35 stems/100 cm2). However, there was no significant difference between tidally restored and natural areas in cover of common reed Phragmites australis (both <1% on average), saltmarsh rush Juncus gerardii (both <1% on average) or maximum vegetation height (restored: 45 cm; natural: 40 cm). Methods: Across summer 2007 and 2008, vegetation was surveyed in 33 plots (each 1 ha) spread across 15 salt marshes. Tidal exchange had been restored to 14 plots 13–54 years previously (no further details reported). The other 19 plots contained natural salt marsh vegetation. Vegetation cover was estimated in nine 1-m2 quadrats/plot, stem density in forty-five 100 cm quadrats/plot and vegetation height at 36 points/plot.

    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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