Raise water level to restore/create freshwater marshes from other land uses

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

Source countries

Key messages

  • Twenty-six studies evaluated the effects, on vegetation, of raising the water level to restore/create freshwater marshes from other land uses or habitat types. Twenty-one studies were in the USA. There was one study in each of Israel, the UK, China, Luxembourg and Canada. Eight studies used sites from a common set of 62 restored prairie potholes in the Midwest USA. Five studies monitored the effects of one river dechannelization project in Florida.

VEGETATION COMMUNITY

  • Overall extent (5 studies): One replicated, paired, before-and-after, site comparison study in the USA reported that damming a stream reduced the area of emergent vegetation on the floodplain. Two before-and-after studies of a floodplain in the USA reported that after dechannelizing a river to raise the water level, the area of emergent herbaceous vegetation increased. Two studies in the USA and Luxembourg simply quantified coverage of wetland vegetation 1–6 years after raising the water table (sometimes along with other interventions).
  • Community types (9 studies): Nine studies quantified the effect of this action on specific types of marsh vegetation. For example, one before-and-after study of a floodplain in the USA reported greatly increased coverage of wet prairie plant communities after dechannelizing a river to raise the water table, but only slightly increased coverage of mixed herbaceous/shrubby wetland communities. Five studies in the USA and Luxembourg simply quantified the numberabundance or extent of wetland plant communities present 1–6 years after raising the water table (typically along with other interventions).
  • Community composition (8 studies): Three replicated, site comparison studies (two also paired) in the USA evaluated the effects of rewetting farmed depressions (along with planting cover crops in/around them). One of these studies reported that restored wetlands contained a different overall plant community to natural wetlands after 5–7 years. One study reported that the plant community composition differed more between restored and natural wetlands than amongst restored or natural wetlands. The final study found that restoration increased vegetation quality after ≥10 years, but not to the level of natural wetlands. Two site comparison studies in China and the USA reported that the plant community became more similar to natural wetlands over 6–15 years after raising the water level – in terms of species composition or overall wetness. Three replicated studies in the USA simply quantified the plant community composition for up to three years after rewetting farmland (sometimes along with other interventions).
  • Overall richness/diversity (12 studies): Four replicated, site comparison studies (two also paired) of one set of historically farmed depressions in the USA reported that restored wetlands (rewetted, along with planting cover crops in/around the sites) had lower overall plant species richness than nearby natural wetlands, after 1–7 years. Two before-and-after, site comparison studies of historical wetlands on a floodplain in the USA reported that raising the water level reduced overall plant species richness in the following six years. One site comparison study of lakeshore marshes in China reported that the total plant species richness in former paddy fields with breached weirs was similar to a nearby natural marsh, after 2–15 years. Five studies (two replicated) in the USA and Israel simply quantified overall plant species richness and/or diversity between three months and 19 years after raising the water table (sometimes along with other interventions).
  • Characteristic plant richness/diversity (1 study): One before-and-after, site-comparison study of a floodplain in the USA reported that dechannelizing a river to raise the water level had no clear effect on the richness of wetland-characteristic plant species in the following six years.

VEGETATION ABUNDANCE                                                                            

  • Overall abundance (9 studies): Three before-and-after, site-comparison studies of historical wetlands on a floodplain in the USA reported that dechannelizing a river to raise the water level reduced overall vegetation cover in the following 6–9 years. One site comparison study in China reported that vegetation biomass in former paddy fields with breached weirs was similar to a nearby natural marsh, after 2–15 years. In contrast, one replicated, site comparison study in the USA found that vegetation cover in rewetted, formerly farmed depressions (also planted with cover crops) was lower than in nearby natural wetlands, after 5–7 years. Four studies (two replicated) in the USA and the UK simply quantified vegetation abundance between three months and six years after raising the water table (sometimes along with other interventions).
  • Characteristic plant abundance (4 studies): Three before-and-after studies (two also site comparisons) of historical wetlands on a floodplain in the USA reported that dechannelizing a river to raise the water level increased the abundance of habitat- and/or wetland-characteristic plant species in the following 6–9 years. One study in the UK simply quantified the abundance of wet meadow plant species present 3–5 years after rewetting farmland (and introducing grazing).
  • Bryophyte abundance (1 study): One replicated, site comparison study in the USA found that the frequency of bryophytes in (the wettest parts of) marshes rewetted 34 years previously was not significantly different from their frequency in (the wettest parts of) nearby natural marshes.
  • Individual species abundance (11 studies): Eleven studies quantified the effect of this action on the abundance of individual plant species. For example, one replicated, site comparison study of freshwater marshes in the USA reported that Kneiff’s feathermoss Leptodictyum riparium was the most abundant plant species in marshes rewetted 34 years previously and nearby natural marshes. One before-and-after study of historical wetlands on a floodplain in the USA reported that after dechannelizing a river to raise the water level, some plots became dominated by a non-native grass species.

VEGETATION STRUCTURE

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 study in 1982–1984 aiming to create a freshwater marsh on formerly mined land in Florida, USA (Erwin & Best 1985) reported that building a levee to raise the water table (along with reprofiling) allowed marsh vegetation to develop within three months. Three months after intervention, 16 plant species were present with 33% total vegetation cover. After two years, 26 plant species were present with 75% total vegetation cover. During the second year after creation, the most abundant plant species were broadleaf cattail Typha latifolia (17–60% cover) and water pennywort Hydrocotyle sp. (17–35% cover). Methods: The study aimed to create a marsh on surface-mined land (historically a mix of forest and rangeland). In the early 1980s, the water table was raised by building a levee downslope. The area was also landscaped to a gentle slope with shallow depressions. The study does not distinguish between the effects of these interventions. Some ponds were also dug, but no wetland soil was added to this area. The interventions were completed by May 1982. Between autumn 1982 and summer 1984, the cover of every plant species was recorded along three randomly placed permanent transects (crossing zones of emergent and floating/submerged vegetation).

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  2. A replicated, paired, before-and-after, site comparison study in 1958–1989 aiming to create marshes along a stream in Iowa, USA (Weller et al. 1991) reported that damming the stream increased the area of open water at the expense of emergent riparian vegetation. Statistical significance was not assessed. Before damming, the 445-ha study area contained 188 ha of emergent herbaceous vegetation, plus 21 ha of open water and 239 ha of upland/forest. Approximately 5–19 years after damming, there was only 78 ha of emergent herbaceous vegetation, plus 157 ha of open water and 213 ha of upland/forest. Over 14–28 years, three riparian transects immediately upstream of dams experienced an increase in open water coverage (before: 1–10%; after: 58–72%) and a decline in coverage of tall wetland grasses (before: 7–14%; after: 0–7%). In contrast, two riparian transects away from dams had stable open water coverage (before: 2%; after: 1–3%) but experienced an increase in coverage of tall wetland grasses (before: 4–5%; after: 18–41%). Other major changes in vegetation cover were mirrored across both types of transect (e.g. decreased cover of tussock sedge Carex stricta; see original paper for data). Methods: Between 1961 and 1975, three impoundments were created by damming Elk Creek, with the aim of creating marshland for waterbirds. Maps of the riparian zone before (1958) and after (1980) damming were drawn from aerial imagery. Vegetation cover was surveyed along five transects crossing the riparian zone, before (early 1960s) and after (1989) damming.

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  3. A replicated study in 1990–1992 of 11 wetlands created or restored on farmland in Wisconsin, USA (Reinartz & Warne 1993) reported that they had developed vegetation cover, mostly of wetland plants. On average, 3-year-old created wetlands contained 44 plant species with 85% total vegetation cover, and 23 native wetland plant species with 60% cover. The most abundant plant species were cattails Typha spp. (33% cover). Woody plants were present around the wetland margins. The overall plant community composition changed with wetland age (data reported as a graphical analysis; statistical significance of differences not assessed). Similarly, vegetation cover was generally higher in older wetlands, with 3-year-old wetlands having significantly greater vegetation cover than 1-year-old wetlands (total: 28%; native wetland: 6%). However, plant species richness and diversity did not significantly change with wetland age (see original paper for data). Methods: In autumn 1988 or 1989, eleven areas of agricultural land (<2.2 ha) were flooded by blocking or removing drainage channels. Vegetation was surveyed one year after flooding (1990, five wetlands), two years after (1991, eleven wetlands) or three years after (1992, six wetlands). Plant species and cover were recorded in twenty-five 1-m2 quadrats/wetland, and a complete species list was made for each wetland.

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  4. A replicated study in 1989–1991 of 62 prairie potholes in the Midwest USA (Galatowitsch & van der Valk 1995) reported that restored potholes (rewetted and planted with cover crops) developed cover of wetland plant communities within three years of first flooding. This included zones of wet meadow vegetation, emergent wetland vegetation, and aquatic (floating and submerged) vegetation. However, the abundance of each community type depended on how often the potholes were flooded, how they had been drained and for how long they had been drained. For example, potholes flooded in only one of three years were dominated by emergent wetland plants or mudflat annuals. Potholes flooded in three of three years contained these plant communities along with zones of aquatic vegetation. The study also reported data on the abundance of individual plant species (see original chapter). Methods: In 1989, 1990 and 1991, vegetation was surveyed in 62 potholes whose drainage structures had been blocked or removed in 1988, to raise the water table. All potholes had been cultivated for ≥25 years. Some had been planted with upland cover crops before rewetting; the study evaluates the combined effect of these interventions in these potholes. Some of the restored potholes were used in (5), (6), (7) and (8) and were studied at later dates in (11), (12) and (15).

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  5. A replicated, site comparison study in 1989–1991 in the Midwest USA (Galatowitsch & van der Valk 1995) reported that that restored prairie potholes (rewetted and planted with cover crops) developed some zones of wetland vegetation within three years, but contained fewer plant species than naturally occurring potholes. Of the 22 rewetted potholes, 16 contained an emergent vegetation zone and 13 contained a submerged vegetation zone, but only two contained a sedge meadow or wet prairie zone. After three years, 10 restored potholes contained fewer plant species (17–38 species/pothole; average: 27) than 10 contemporary natural potholes (32–56 species/pothole; average: 46) or historical reports for pristine potholes (57–126 species/pothole). More specifically, the restored potholes contained fewer shallow-emergent, sedge-meadow, wet-prairie and floating aquatic species on average (1–5 species/group/pothole) than contemporary natural potholes (4–17 species/group/pothole), but more submerged aquatic species (6 vs 1 species/pothole). Methods: In 1989, 1990 and 1991, vegetation was surveyed in 22 potholes whose drainage structures had been blocked or removed in 1988, to raise the water table. All of these potholes had been cultivated for ≥25 years, but were flooded every summer since rewetting. Some had been planted with upland cover crops before rewetting: the study evaluates the combined effect of these interventions in these potholes. Vegetation was also surveyed in 10 adjacent natural potholes (never drained, but surrounded by farmland). Species richness of seven pristine potholes (unaffected by surrounding agriculture) was obtained from published records from the 1890s–1980s. This study used a subset of the restored potholes from (4) – but not clearly the same 22 potholes as (6).

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  6. A replicated study in 1989–1991 of 22 prairie pothole marshes in the Midwest USA (Galatowitsch & van der Valk 1996a) reported that most restored potholes (rewetted and planted with cover crops) developed some characteristic wetland plant zones within three years. Natural prairie potholes have zones of plant communities, characteristic of different water levels. Of 22 potholes studied three years after restoration, 18 had developed at least one characteristic pothole vegetation zone. Sixteen had a deep water zone with submerged vegetation. Thirteen had an emergent vegetation zone. Two had a sedge meadow zone. None had a wet prairie zone. Methods: In 1989–1991, cover of every plant species was estimated in 22 potholes and used to identify vegetation zones. All potholes had been historically drained and cultivated, but restored in 1987–1989 by rewetting (blocking/removing drainage structures and building water-retaining dikes where necessary) and sometimes planting cover crops. Note that the study evaluates the combined effect of these interventions in some potholes. All potholes had flooded in three consecutive years when surveyed. This study used a subset of the restored potholes from (4) – but not clearly the same 22 potholes as (5).

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  7. A replicated, paired, site comparison study in 1991 of 20 prairie potholes in Iowa, USA (Galatowitsch & van der Valk 1996b) found that restored potholes (rewetted and planted with cover crops) contained a distinct plant community to natural wetlands after three years, with fewer plant species. The plant community composition differed more between restored and natural wetlands (12–44% similarity) than it did amongst restored or natural wetlands (31–63% similarity). The study reported lower richness in restored than natural wetlands of plant species overall, floating, shallow emergent, wet prairie and sedge meadow species, but higher richness of submerged species (data reported in 4b). The study also reported data on the cover of common individual plant species (see original paper). Methods: In April–October 1991, plant species and cover were recorded in 10 pairs of similarly sized pothole marshes (twenty 1-m2 quadrats/pothole). In each pair, one marsh had been restored three years previously (by blocking/breaking drainage structures to raise the water level and, in some sites, planting cover crops; note that the study evaluates the combined effect of these interventions in some potholes). Restored potholes had previously been drained and farmed for ≥25 years. The other marshes were natural (never drained or farmed, but surrounded by farmland). This study used a subset of the restored potholes from (4) and exactly the same potholes as (8). Some of the potholes were also studied in (12).

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  8. A replicated, paired, site comparison study in 1991 of 20 prairie potholes in Iowa, USA (Galatowitsch & van der Valk 1996c) found that restored potholes (rewetted and planted with cover crops) contained fewer plant species than natural wetlands after three years, and that their plant community was composed of different types of species. Statistical significance was not assessed. Fifty-eight different plant species were recorded across 10 restored potholes (vs 96 species across 10 natural potholes). Fewer floating, emergent, wet prairie and sedge meadow species were recorded in restored potholes (total: 4, 13, 4 and 13 species respectively) than natural potholes (total: 5, 20, 19 and 34 species respectively). More submerged species were recorded in restored than natural potholes (total: 9 vs 4 species). The study also reported data on the cover of individual plant species (see original paper). Methods: This study used exactly the same potholes and methods as (7).

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  9. A replicated study in 1991–1992 of rewetted prairie pothole marshes in Iowa, USA (VanRees-Siewert & Dinsmore 1996) reported that they developed 63% coverage of emergent vegetation within four years. For twelve 1-year-old marshes, the average area covered by emergent vegetation stands was only 20%. Emergent vegetation was clustered around the pothole margin in 10 of the marshes. Coverage of emergent vegetation increased with wetland age, to an average of 63% across six 4-year-old marshes. Emergent vegetation was distributed across the whole pothole in three of six cases, clustered around the margin in two cases and clustered in the centre in one case. Methods: In July 1991 and 1992, emergent vegetation stands were mapped in 24 rewetted prairie pothole marshes (<6 ha; drained and farmed for >20 years before rewetting; details of rewetting not reported). The potholes were surveyed one, two, three or four years after rewetting. Sixteen potholes were surveyed in both years. Eight potholes were surveyed in 1992 only. This summary takes some methodological details from VanRees-Siewert (1993).

    Additional Reference: VanRees-Siewert K.L. (1993) The influence of wetland age on bird and aquatic macroinvertebrate use of restored Iowa wetlands. M.S. Thesis, Iowa State University, Ames, IA, USA.

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  10. A study in 1994–1996 of rewetted cropland in Israel (Kaplan et al. 1998) reported that it was colonized by emergent (and aquatic) vegetation within two years. After two years, a total of 74 species had colonized the rewetted area. This included emergent and wet-meadow plants such as papyrus Cyperus papyrus, toad rush Juncus bufonius, purple loosestrife Lythrum salicaria, lakeshore bulrush Scirpus litoralis, southern cattail Typha domingensis and water speedwell Veronica anagallis-aquatica. The total species count also included aquatic and upland species. Methods:  In 1994, part of the Hula Valley was rewetted to create Lake Agmon. The valley had been drained in the 1950s to create cropland. Each month for two years after rewetting, plant species were recorded in twenty-two 5 x 5 m quadrats, placed along transects perpendicular to the lake shoreline. The water depth in quadrats was 0–300 cm.

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  11. A replicated study in 1989–2000 of 41 restored prairie potholes (rewetted and planted with cover crops) in the Midwest USA (Mulhouse & Galatowitsch 2003) reported that they were colonized by plants, including wetland species, with increasing species richness and changes in plant community composition over time. Statistical significance was not assessed. After 1–3 years, all restored potholes contained ≤50 plant species and some contained <11. After 12 years, all potholes contained ≥11 plant species and the richest four potholes contained >90. Considering just wetland plants, the overall community composition became more similar amongst the 41 potholes over time (data reported as a similarity index). After 12 years, the most common emergent/wet meadow species included reed canarygrass Phalaris arundinacea (present in all 41 potholes), asters Aster spp. (36 potholes), water knotweed Polygonum amphibium (35 potholes) and pale spikesedge Eleocharis macrostachya (34 potholes). Methods: This study analyzed data from 41 prairie potholes that had been restored from farmland in 1988 and sampled through to 2000. Restoration involved rewetting by breaking/blocking drainage systems (resulting water levels varied from annual flooding to seasonal saturation), and planting cover crops in adjacent uplands. Note that the study evaluates the combined effect of rewetting and planting cover crops in some potholes. In summer 1989, 1991 and 2000, plant species and cover were recorded across the whole of each pothole (including upland buffer zone). This study used a subset of potholes from (4). Most of the potholes were also studied in (15).

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  12. A replicated, site comparison study in 1993–1994 of 27 prairie pothole marshes in Iowa, USA (Seabloom & van der Valk 2003) found that restored potholes (rewetted and planted with cover crops) had lower vegetation cover and species richness than natural potholes, and were characterized by a different set of species. After 5–7 years, restored potholes had lower overall vegetation cover than natural potholes (see original paper for data and statistical model). Restored potholes also contained fewer plant species: overall (restored: 19; natural: 29 species/pothole) and per quadrat (restored: 3.2; natural: 4.8 species/m2). Finally, restored potholes contained a different plant community to natural potholes (statistical significance not assessed). Of 47 analyzed species, 23 were significantly more common in natural potholes whilst only eight were significantly more common in restored potholes (data reported as statistical model results). Methods: In 1993 or 1994, vegetation was surveyed in 27 prairie potholes. Seventeen potholes had been restored from farmland 5–7 years previously, by breaking/blocking drainage systems, and planting cover crops “along the margins of several sites”. Ten potholes were natural (never drained). Plant species and cover were recorded in 11–42 quadrats, each 1 m2, across each pothole. Note that the study evaluates the combined effect of rewetting and planting cover crops in some potholes. This study used a subset of the restored potholes from (4), and used some of the potholes from (7) and (8).

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  13. A study in 1998–2003 aiming to restore a wet meadow on farmland in England, UK (Lyons & Ausden 2005) reported that following rewetting and the introduction of grazing, the site was colonized by vegetation – including wet meadow species. After three years, overall vegetation cover was 71%, including 23% cover of wet meadow plant species (“typical of lowland pasture regularly flooded with fresh water”) and 42% cover of grassland species. The most abundant plant species was rough meadow grass Poa trivialis (37% cover). After five years, overall vegetation cover had increased to 94% and wet meadow plant cover had increased to 65%, whilst grassland plant cover had decreased to 13%. The most abundant plant species was now creeping bent Agrostis stolonifera (29% cover). Methods: In 1998, eighty-four hectares of arable farmland, next to a wetland nature reserve, were rewetted: by damming existing drainage, installing a wind-driven water pump and digging new water-control ditches to bring the water table to the soil surface. Grazing livestock (sheep and cattle) were also introduced. Note that the study evaluates the combined effect of rewetting and grazing. In August 2001 and 2003, plant species and cover were recorded in twenty-eight 1-m2 quadrats.

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  14. A site comparison study in 2003–2004 of six lakeshore marshes in eastern China (Lu et al. 2007) reported that restored marshes (developing in abandoned rice paddies with breached weirs) developed a similar plant community to a nearby reference marsh, with similar species richness and biomass. Unless specified, statistical significance was not assessed. The overall plant community composition in restored marshes became more similar to a reference marsh over time. A 2-year-old restored marsh was 64% similar to the reference marsh. A 15-year-old restored marsh was 97% similar to the reference marsh, dominated by common reed Phragmites communis, Amur silvergrass Miscanthus sacchariflorus and Manchurian wild rice Zizania latifolia. The restored marshes contained 10–13 plant species (vs reference: 9) and 1,270–2,100 g/m2 plant biomass (vs reference: 1,590 g/m2). Methods: In March–October 2003 and 2004, vegetation was surveyed in 1-m2 quadrats (number not clearly reported) in six lakeshore marshes. Biomass was dried before weighing. Five marshes had been restored from rice paddies: the weirs around the paddies had been damaged 2, 5, 10 or 15 years previously to allow lake water to flow in and out naturally. The other, reference marsh was “less disturbed” and had not been cultivated for >30 years.

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  15. A replicated study in 1991–2007 of 37 restored prairie potholes (rewetted and planted with cover crops) in the Midwest USA (Aronson & Galatowitsch 2008) reported that they were colonized by wetland plants, but not all the species found in nearby natural potholes. Restored potholes contained 138 wetland plant species (22 species/pothole) after three years, 268 wetland plant species (60 species/pothole) after 12 years, and 279 wetland plant species (57 species/pothole) after 19 years. However, some species found in nearby natural potholes never colonized the restored potholes: 22% of common species, 70% of uncommon species and 93% of rare species. Overall, plant community composition across the 37 restored potholes became more similar over time (data reported as a similarity index). After 19 years, vegetation in the wet meadow zone was dominated by reed canary grass Phalaris arundinacea (66% cover), the emergent zone by cattails Typha spp. (56% cover) and the aquatic zone by pondweed Potamogeton spp. (77% cover). Methods: This study analyzed data from 37 prairie potholes that had been restored from farmland in 1988 and sampled through to 2007. Restoration involved rewetting by breaking/blocking drainage systems (resulting water levels varied from annual flooding to seasonal saturation), and planting cover crops in/around some sites. Note that the study evaluates the combined effect of rewetting and planting cover crops in some potholes. In summer 1991, 2000 and 2007, plant species and cover were recorded across the whole of each pothole. including upland buffer zone). This study used a subset of potholes from (4) and (11).

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  16. A study in 2005 of meadows next to an ephemeral stream in California, USA (Hammersmark et al. 2009) reported that after diverting the incised stream into a new shallower channel, some wetland plant communities developed. Six years after intervention, three communities dominated by wetland-characteristic plant species had developed close to the stream where the water table was highest (flooded for >24 days in the growing season, on average). The most abundant species in each community were Nebraska sedge Carex nebracensis (16% cover), Bach’s calicoflower Downingia bacigalupii (16% cover) and pale spikerush Eleocharis macrostachya (39% cover). The communities contained 8–18 plant species/4 m2 and had 56–83% vegetation cover. Areas with a lower water table (flooded for <10 days in the growing season, on average) retained plant communities dominated by the non-native common meadowgrass Poa pratensis and Japanese brome Bromus japonica – which were also abundant before intervention (not quantified). Methods: In summer 2005, vascular plant species and their cover were recorded in 128 plots, each 4 m2, across a floodplain. In 1999, the floodplain water table had been raised by plugging the old channelized and incised stream (creating a series of ponds) and diverting the stream along a new, shallower course.

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  17. A study in 2003–2006 aiming to restore wetland plant communities on a floodplain in Luxembourg (Schaich et al. 2010) reported that following rewetting by redirecting the river, wetland plant communities developed. Before rewetting, the floodplain was dominated by dry grassland (not quantified). After 1–3 years of rewetting, wetland plant communities comprised 53–65% of the vegetated area on the floodplain. These communities included wet grasslands (29–30%), sedge meadows (12–24%) and reedbeds (11%). Methods: In winter 2003, a historically drained valley was rewetted by redirecting the river from a deep, artificial channel at the valley edge to a shallower channel in the valley bottom. At the same time, biannual mowing ceased (H. Schaich pers. comm.) and in August 2004, year-round rotational cattle grazing began. Note that this study evaluates the combined effect of rewetting, cessation of mowing, and grazing. Plant community types were mapped, in the field, in summer 2004–2006.

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  18. A replicated, paired, site comparison study in 2006 of 36 depressions in the Midwest USA (Balas et al. 2012) found that rewetting historically farmed depressions (and re-seeding their catchments to grassland) increased vegetation quality, but not to the same level as in natural depressions. On average, the plant species in restored depressions were more characteristic of undisturbed habitats in the study region (Conservatism Score: 3.3 out of 10) than the plant species in depressions that remained drained and farmed (Conservatism Score: 1.2 out of 10). However, the species in restored depressions remained less characteristic of undisturbed habitats in the study region than the species in contemporary natural wetlands (Conservatism Score: 4.0 out of 10). Methods: In 2006, plant species were recorded within, and in a 10 m buffer around, 36 depressions. Twelve depressions (four at each of three sites) were under each land use: (a) restored wetlands: historically drained and farmed; restored ≥10 years ago by breaking/blocking drainage structures and reseeding surrounding land to grassland; (b) degraded depressions: still drained and farmed; (c) natural wetlands: never drained or farmed. Note that this study evaluates the combined effect of rewetting and revegetating catchments.

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  19. A replicated, site comparison study in 2011 of 15 freshwater marshes in Minnesota, USA (Fuselier et al. 2012) found that rewetted marshes had lower bryophyte species richness than natural marshes after 34 years, but contained a similar frequency of bryophytes. Rewetted marshes contained only 3–6 bryophyte species in total (vs natural marshes: 8–12). Rewetted marshes also contained significantly fewer bryophyte species per quadrat: across the whole marsh (rewetted 1.7; natural: 3.3 species/0.36 m2) and in the wettest areas (rewetted: 0.6; natural: 0.8 species/0.36 m2). However, rewetted marshes supported a statistically similar frequency of bryophytes (occurrence in 44% of subquadrats in the wettest areas) to natural marshes (55%). Kneiff’s feathermoss Leptodictyum riparium was the most abundant bryophyte species in both rewetted and natural marshes (see original paper for full data). Methods: In summer 2011, aquatic and semi-aquatic bryophytes were surveyed in 15 marshes. Six marshes were historically drained and farmed, but had been rewetted 34 years previously by blocking drainage ditches. The other nine marshes were natural (never drained or farmed). Five of the natural marshes burned in April 2011. Bryophyte species were recorded across the whole of each marsh and in twenty-four 0.36-m2 quadrats/marsh (split into 100-cm2 subquadrats, and placed along four transects from wetter to drier areas).

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  20. A before-and-after, site comparison study in 1998–2007 in Florida, USA (Toth 2012) reported that dechannelizing the river to rewet the floodplain reduced overall vegetation cover, forage grass cover and plant species richness in the wet prairie zone, but made the overall plant community more characteristic of wetland conditions. Statistical significance was not assessed. In the year before rewetting, restoration plots had 93–99% overall vegetation cover (forage grasses: 58–71%; typical wet-prairie species: 1–6%). There were 16–18 plant species/100 m2 (7–9 wetland-characteristic). Over six years after rewetting, overall vegetation cover fluctuated between 13% and 84% (forage grasses: 1–42%; typical wet-prairie species: 2–32%). There were 5–26 plant species/100 m2 (3–19 wetland-characteristic). The overall plant community became more characteristic of wetland conditions over time, reaching a similar level to nearby reference wet prairies around three years after rewetting, and becoming significantly more wetland-characteristic than plots that remained drained (data reported as a wetland indicator index). In the drained plots, vegetation cover and richness were relatively stable over time (see original paper for data). Methods: Between October 1999 and February 2001, Section C of the Kissimmee River floodplain was rewetted by dechannelizing the river. Twenty-one 100-m2 plots were established in the historical wet prairie zone of the floodplain: 15 in the dechannelized section and six in an upstream section that remained channelized. Plant species and their cover were surveyed in spring and summer before intervention (1998–1999) and for roughly six years after (until 2007). Vegetation was also surveyed in two nearby, near-natural wet prairies (date not reported). This study used the same floodplain section(s) as (21) and (26), and used a subset of the plots in (22) and (24).

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  21. A before-and-after study in 1954–2008 in Florida, USA (Spencer & Bousquin 2014) reported that after dechannelizing a river to rewet the floodplain, the area of wet prairie and mixed marsh/shrubby wetland vegetation increased. After roughly 3–8 years of rewetting, wet prairie vegetation covered 33–39% of the floodplain (vs 13–15% in a degraded state before rewetting, and 29% in the natural state before degradation). Mixed herbaceous/shrubby wetlands covered 7–18% the floodplain (vs 3–15% before rewetting and 52% before degradation). In total, wetland vegetation (herbaceous, shrubby, forested and submerged) covered 65–83% of the floodplain after rewetting (vs 22–37% before rewetting and 84% before degradation). Methods: Between 1999 and 2001, Section C of the Kissimmee River was dechannelized. This restored its natural meandering course, raised the water table on the adjacent floodplain and allowed for seasonal floods. Floodplain vegetation was mapped from aerial photographs taken before degradation (1954), during degradation (1974, 1996) and after restoration (2003, 2008). This study used the same rewetted floodplain section as (20), (22), (24) and (26).

    Study and other actions tested
  22. A before-and-after, site comparison study in 1998–2007 in Florida, USA (Toth 2015) reported that dechannelizing a river to rewet the floodplain increased variation in plant community composition between plots in the wet prairie zone, but reduced forage grass cover, overall vegetation cover and plant species richness. Results summarized for this study are not based on assessments of statistical significance. Variation in plant community composition amongst restoration plots (i.e. large-scale diversity) was relatively low before rewetting began (59–65% similarity). It was higher in the six years after rewetting was complete (17–49% similarity). This was linked to reduced dominance (cover) of forage grasses (before: 61–72%; after: 0–39%). There were also declines in total vegetation cover (before: 94–99%; after: 14–83%) and richness (before: 17–19; after: 5–26 species/100 m2). Note the increased variability after rewetting, reflecting differences between seasons and years. Meanwhile, the vegetation was relatively stable over time in another part of the floodplain that remained drained: 41–71% community similarity, 42–77% forage grass cover, 82–99% total vegetation cover, 18–25 species/100 m2. Methods: Between October 1999 and February 2001, Section C of the Kissimmee River floodplain was rewetted by dechannelizing the river. Twenty-four 100-m2 plots were established in the historical wet prairie zone (more recently used as upland pasture): 18 in the dechannelized section and six in an upstream section that remained channelized. Plant species and their cover were surveyed in spring and summer before intervention (1998–1999) and for roughly six years after (until 2007). This study used the same floodplain section(s) as (21) and (26), and shared wet prairie plots with (20) and (24).

    Study and other actions tested
  23. A before-and-after study in 2000–2010 along a river in Alberta, Canada (Hillman et al. 2016) found that artificially increasing flow by diverting water from another river had no significant effect on the coverage of broadleaf cattail Typha latifolia beds, but reduced the coverage of grass-like plants and increased coverage of shrubs across the whole riparian zone. For all data and statistical models, see original paper. After six years of increased flows, the proportion of the riparian zone covered by cattail beds was not significantly different to the proportion four years before flows were increased – although there was a trend towards higher cattail coverage. Across the entire riparian zone (i.e. including wetland and upland areas), coverage of other grass-like plants (grasses, sedges and rushes) was significantly lower after flows were increased than before. Coverage of woody plants (true willows Salix spp. and wolf willow Eleagnus commutata) was significantly higher after flows were increased than before. Methods: In 2004, a canal linking the Highwood River to the Little Bow River was upgraded to increase its flow capacity. This increased summer discharge in the Little Bow River. Broad riparian vegetation types were mapped using aerial photographs taken in 2000 and 2010. Cattle grazing was locally intensive.

    Study and other actions tested
  24. A before-and-after study in 1998–2007 in Florida, USA (Toth 2016) reported that after dechannelizing the river to rewet the floodplain, both non-native limpo grass Hemarthria altissima and native wetland-characteristic vegetation increased in abundance in historic wetland zones, although individual plots became dominated by one or the other. Statistical significance was not assessed. Before rewetting began, limpo grass was present in only 3 of 27 marsh and wet prairie plots with only 0–4% cover. Roughly six years after rewetting was complete, limpo grass was present in 15–17 of 27 plots, with an average cover of 22–26%. Further data were presented for the 18 wet prairie plots. Here, there were 14–17 plant species/100 m2 before rewetting, then 17–19 plant species/100 m2 six years after. Cover of three wet-prairie indicator species was <1–3% before rewetting, then 16–25% six years after. However, these averages mask a dichotomy. After rewetting, nine wet prairie plots were dominated by the indicator species (38% cover; limpo grass cover: <12%) whilst nine were dominated by limpo grass (43% cover; indicator species cover: 3%). Methods: Between October 1999 and February 2001, Section C of the Kissimmee River floodplain was rewetted by dechannelizing the river. Plant species and their cover were surveyed in spring and summer before intervention (1998–1999) and for roughly six years after (until 2007), in twenty-seven 100-m2 plots (18 in historic wet prairies and nine in historic marshes). This study used the same rewetted floodplain section as (21) and shared plots with (20), (22) and (26).

    Study and other actions tested
  25. A before-and-after, site comparison study in 1938–2013 on a floodplain in Illinois, USA (Hine et al. 2017) reported that after raising the water table by turning off drainage pumps, the total wetland area and coverage of emergent wetland vegetation increased. Statistical significance was not assessed. In the autumn after rewetting, the total wetland area was 252 ha. Emergent vegetation covered only 114 ha of the site (63 ha permanent marsh and 51 ha temporary mudflat colonizers). Six years later, the total wetland area was 1,944 ha. Emergent vegetation covered 558 ha (450 ha permanent and 108 ha temporary). Over the six years, the average proportion of the rewetted site covered by emergent vegetation (29%) was similar to historical (25%) and contemporary (36%) values for similar wetland sites. However, the proportion covered by permanent vegetation was higher (rewetted: 21%; historical: 12%; contemporary: 4%) and the proportion covered by temporary vegetation was lower (rewetted: 9%; historical: 12%; contemporary: 33%). Methods: In 2007 (precise date not reported), drainage systems were switched off to raise the water table in a historically farmed floodplain area. This created a range of wetland and deepwater habitats. Each autumn between 2007 and 2013, vegetation types were mapped using field surveys and aerial photographs. Vegetation coverage was compared to published records for natural wetland sites in the same river valley, from 1938–1942 and 2005–2006.

    Study and other actions tested
  26. A before-and-after, site comparison study in 1998–2010 in Florida, USA (Toth 2017) reported that dechannelizing the river to rewet the floodplain reduced overall vegetation cover and cover of pasture/upland grasses in the historical marsh zone, but increased the abundance of wetland- and habitat-characteristic herbs. Statistical significance was not assessed. In restoration plots, overall vegetation cover was 96–98% in the year before rewetting began, then 10–90% roughly 1–9 years after rewetting was complete (highly variable between seasons and years). Cover of pasture/upland grasses was 75–79% before rewetting vs 0–1% after. Cover of wetland-characteristic herbs was 10–11% before rewetting vs 15–61% in all but one sample after. Broadleaf-marsh-characteristic species were absent before rewetting but colonized after (present in ≥67% of restoration plots after nine years, with 5% average cover). Over the entire study period (ignoring a year of extreme flooding), vegetation cover was relatively stable in another part of the floodplain that remained drained: 66–98% overall vegetation cover, 34–78% cover of pasture/upland grasses, 12–43% cover of wetland-characteristic herbs, and no broadleaf-marsh-characteristic species. Methods: Between October 1999 and February 2001, Section C of the Kissimmee River floodplain was rewetted by dechannelizing the river. Fifteen 100-m2 plots were established in parts of the floodplain that were historically marshes (more recently used as cattle pasture). There were nine plots in the dechannelized section and six in an upstream section that remained channelized. Plant species and their cover were surveyed in spring and summer before intervention (1998–1999) and for roughly nine years after (until 2010). This study used the same floodplain section(s) as (20), (21) and (22), and shared plots with (24).

    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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