Transplant or replace wetland soil: freshwater marshes

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

Source countries

Key messages

  • Ten studies evaluated the effects, on vegetation, of transplanting wetland soil to restore or create freshwater marshes. Nine studies were in the USA. One study was in Guam. Two studies were in the same region but used different sites.

VEGETATION COMMUNITY

  • Community composition (3 studies): Two replicated, controlled studies in rewetted marshes in the USA found that areas amended with wetland soil contained a plant community characteristic of wetter conditions than unamended plots after one growing season – but not after two. One replicated, randomized, controlled study in a recently excavated marsh in the USA found that amended and unamended plots contained a plant community of similar overall wetness after both one and two growing seasons.
  • Overall richness/diversity (10 studies): Eight studies (including four at least replicated and controlled) in freshwater marshes in the USA reported that areas amended with wetland soil had greater plant richness and/or diversity than unamended areas and/or nearby natural marshes. One replicated, paired, controlled study in rewetted freshwater marshes in the USA found that plots amended with sieved marsh soil contained a similar number of wetland plant species to unamended plots, after 1–2 growing seasons. One before-and-after study of freshwater pool in Guam simply quantified plant species richness one year after adding wetland soil (along with other interventions).
  • Characteristic plant richness/diversity (1 study): One replicated, randomized, paired, controlled study in a freshwater marsh in the USA reported that plots amended with wetland soil developed a greater richness of wetland-characteristic plant species than unamended plots, at the end of the growing season.

VEGETATION ABUNDANCE

  • Overall abundance (6 studies): Six controlled studies in freshwater marshes in the USA reported that plots amended with wetland soil typically contained more vegetation overall than unamended plots, after 1–2 growing seasons. This was true for cover and biomass, but not stem density.
  • Individual species abundance (7 studies): Seven studies (including one replicated, randomized, paired, controlled, site comparison) in freshwater marshes, meadows and pools in the USA and Guam quantified the effect of this action (sometimes along with others) on the abundance of individual plant species. Results were mixed and likely depended on the composition of the donor wetland.

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 controlled, site comparison study in 1978–1980 of four freshwater marshes in Florida, USA (Swanson & Shuey 1980) reported that an excavated marsh amended with wetland soil contained more plant species than unamended and natural marshes, and more marsh-characteristic plant species than the unamended marsh. Statistical significance was not assessed. After two years, the amended marsh contained 95 vascular plant species (vs 70 in the unamended marsh and 76–88 in natural marshes). The amended marsh also contained 17 marsh plant species (i.e. present in at least one natural marsh) that were not present in the unamended marsh. Methods: In summer 1978, two 0.16-ha depressions were excavated in rangeland. Topsoil and vegetation from a nearby natural marsh was added to one depression (30 cm depth) but not the other. The whole site was seeded with pioneer herbs before topsoil addition (to prevent erosion) and limed and fertilized after. In summer 1980, plant species were recorded in each excavated marsh and two natural marshes, along a transect extending from the centre to the edge of each.

    Study and other actions tested
  2. A before-and-after study in the 1980s in a marsh developing on reclaimed mining land in Florida, USA (Clewell 1981) reported that following the addition of soil from a natural marsh, the number of plant species increased. Before soil was added, the marsh contained 45 plant species. One year after soil was added, the marsh contained 88 plant species, “many” of which occurred in the donor sites. Methods: In the early 1980s, a 5-cm-thick layer of topsoil from natural marshes was added to a developing marsh. The site had been planted with dry pasture grasses four years earlier, but had since developed marsh vegetation because it was kept wet by seepage from a settling pond. The study does not report precise dates and details of monitoring.

    Study and other actions tested
  3. A controlled study in 1982–1984 in a marsh undergoing restoration in Florida, USA (Erwin & Best 1985) reported that an area amended with wetland soil typically had higher plant species richness and total vegetation cover than an unamended area, and that the areas were dominated by different plant species. Statistical significance was not assessed. Over two years, more plant species were recorded in the amended area than the unamended area in six of six comparisons (amended: 34–48 species/392 m; unamended: 14–30 species/276 m; note different transect lengths). Total vegetation cover was higher in the amended area in four of six comparisons (for which amended: 84–105%; unamended: 33–62%; other comparisons lower in amended area). From the second year after intervention, pickerelweed Pontederia cordata dominated the amended marsh (32–64% cover) but broadleaf cattail Typha latifolia was typically the most abundant species in the unamended marsh (17–60% cover). Methods: The study site was a surface-mined marsh undergoing restoration through rewetting, reprofiling and pool excavation. Part of the site was topped with a 2–10 cm layer of soil from a nearby wetland. Restoration was completed in May 1982. Between autumn 1982 and summer 1984, plant species and their cover were recorded along transects (crossing zones of emergent and floating/submerged vegetation, but otherwise randomly placed). There were three transects (total length 392 m) in the amended part of the site and three (total length 276 m) in the unamended part.

    Study and other actions tested
  4. A replicated, randomized, paired, controlled, site comparison study in 1989–1990 in a created freshwater marsh in Texas, USA (McKnight 1992) reported that plots amended with soil from a donor marsh contained more plant species, more wetland-characteristic plant species and typically more plant biomass than unamended plots. Unless specified, statistical significance was not assessed. Mature vegetation in amended plots contained 17–28 plant species/0.25 m2 (vs unamended: 6–14) and 8–13 plant species/0.25 m2 that “prefer wet or semi-wet soils” (vs unamended: 1–3). In three of four comparisons, amended plots contained significantly more above-ground vegetation biomass (for which amended: 99–769 g/m2; unamended: 5–83 g/m2; other comparison no significant difference). Only 12 of the 20 plant taxa present in the donor site were present in amended plots, but they comprised 96% of the biomass in amended plots (see original paper for data on biomass of individual plant species). Methods: In February 1990, one hundred and twenty 0.25-m2 plots were established in a created marsh (formerly grassland), in four blocks of 30 according to moisture level. In eighty plots (20 plots/block), the top 6–7 cm of soil were removed and replaced with soil from the top 10–15 cm of a nearby donor marsh. The donor soil had been stockpiled over winter. The other 40 plots (10 plots/block) were left undisturbed. Later in 1990, when emergent vegetation was “mature”, it was cut from each plot then identified, dried and weighed. Vegetation in the donor marsh was surveyed along transects perpendicular to the shoreline in October 1989.

    Study and other actions tested
  5. A before-and-after study in 1992–1993 on a tourist resort in Guam (Ritter & Sweet 1993) reported that a freshwater pool created by excavation, lining with wetland soil and planting herb species contained two of the four planted species after one year, and four additional species. The two planted species present after one year were spikerush Eleocharis dulcis (60% cover) and rusty flatsedge Cyperus oderatus (<1% cover). All planted taro Colocasioa esculenta died; the study suggests it was “excessively flooded”. Planted water lettuce Pistia stratioides was deliberately removed after five months, when it had reached 20% cover. Four additional species were present after one year: two rushes, one grass and one forb (<1–10% cover). Methods: In January 1992, a 600-m2 wetland was excavated on a natural valley slope, lined with wetland soil (30 cm deep) and planted with four herbaceous species (120 spikerush, an unclear number of rusty flatsedge, 20 taro, 5% cover of water lettuce). The study does not distinguish between the effects of these interventions on non-planted vegetation. The wetland was fed by ground and surface water, and had a stable 20–60 cm water depth. Final vegetation cover was estimated in January 1993.

    Study and other actions tested
  6. A replicated, site comparison study in 1991 of three mixed sedge meadows/freshwater marshes in Wisconsin, USA (Ashworth 1997) found that wetlands amended with peat from a donor meadow/marsh had higher plant diversity than a natural wetland, and different cover of some key plant species. Statistical significance was not assessed. After five years, both amended wetlands had higher plant diversity than a nearby natural wetland (data reported as a diversity index). Taxa with different cover in amended and natural wetlands included Canadian reedgrass Calamagrostis canadensis (amended: 5–13%; natural: 34%), sedges Carex spp. (amended: 2–12%; natural: 25%) and cattails Typha spp. (amended: 5–13%; natural: 2%). The study reported differences between the amended and natural wetlands in peat depth and water levels, which may have been related to differences in vegetation. Methods: In 1986, sand was removed from two former wetlands and replaced with peat from the surface of a nearby meadow/marsh. The deposited peat formed a layer 5–180 cm thick. In 1991, plant species and their cover were estimated in 1-m2 quadrats along transects: approximately 50 quadrats in each of the two amended wetlands (0.2 ha), and 19 quadrats in an adjacent, undisturbed wetland (<0.01 ha).

    Study and other actions tested
  7. A replicated, controlled study in 1992–1993 in five freshwater marshes undergoing restoration in New York State, USA (Brown & Bedford 1997) found that plots amended with sieved marsh soil had greater cover of wetland plant species over two growing seasons than unamended plots, and a more wetland-characteristic plant community in the first growing season. Over two growing seasons after intervention, amended plots had greater total cover of wetland plant species (28–96%) than unamended plots (19–54%). However, the number of wetland plant species never significantly differed between amended plots (3.0–3.9 species/plot) and unamended plots (2.0–2.8 species/plot). The overall plant community was more characteristic of wetland conditions in amended plots than unamended plots after one growing season, but there was no significant difference between treatments after two (data reported as a wetland indicator index). Methods: In May 1992, twenty-one 0.25-m2 plots were established across five recently rewetted sites (drained for ≥40 years previously). In six plots (three plots in each of two sites), 15 cm of topsoil was removed and replaced with sieved soil (1 cm mesh) from nearby remnant marshes. The other 15 plots (three plots/site) were left undisturbed. Plant species and cover were recorded in autumn 1992 and 1993.

    Study and other actions tested
  8. A replicated, paired, controlled study in 1993–1995 in five freshwater marshes undergoing restoration in New York State, USA (Brown & Bedford 1997) found that plots amended with wetland soil typically contained more and greater cover of wetland plant species than unamended plots over two years – and contained a more wetland-characteristic plant community after one. Over two years after intervention, amended plots contained more wetland plant species than unamended plots in six of six comparisons (amended: 6.7–9.1; unamended: 1.4–4.7 species/plot). Amended plots had greater total cover of wetland plants in five of six comparisons (for which amended: 80–193%; unamended: 5–96%; other comparison no significant difference). The overall plant community was more characteristic of wetland conditions in amended plots than unamended plots after one year, but there was no significant difference between treatments after two (data reported as a wetland indicator index). After two years, cover of cattails Typha spp. was low, and statistically similar, in amended plots (1–10%) and unamended plots (0–2%). Methods: In summer 1993, soil from remnant marshes in drainage ditches was spread onto five degraded wetlands (drained for ≥40 years). In autumn 1993, all five sites were rewetted. Plant species and cover were recorded in 1994 and 1995 (precise date not reported), in 54 quadrats in areas amended with wetland soil and 39 quadrats in nearby unamended areas. Quadrats spanned a range of elevations.

    Study and other actions tested
  9. A replicated, randomized, controlled study in 1991–1992 in an excavated freshwater wetland in Pennsylvania, USA (Stauffer & Brooks 1997) found that plots amended with wetland soil contained a different plant community to unamended plots with more wetland-characteristic plants, greater overall vegetation cover and higher plant richness and diversity. After both one and two growing seasons, amended and unamended plots shared <14% of plant species. The plant community was more characteristic of wetland conditions in amended plots, although not significantly so (data reported as a wetland indicator index). Cover of wetland-characteristic plants was higher in amended plots (40–45%) than unamended plots (3–5%). Amended plots also had greater overall vegetation cover (amended: 83–96%; unamended: 27–45%), contained more plant species (amended: 15–19; unamended: 7 species/3 m2) and had higher plant diversity (data reported as a diversity index). Total stem density did not significantly differ between treatments (amended: 97–133; unamended: 78–86 stems/0.25 m2). For data on the frequency of individual species, see original paper. Methods: In May 1991, soil from the top 15 cm of a mature marsh was mixed into the surface of four 6 x 6 m plots in a recently excavated wetland. Four additional plots were not amended with wetland soil. Vegetation was surveyed in August 1991 and 1992, in twelve 0.25-m2 quadrats/plot.

    Study and other actions tested
  10. A replicated, controlled study in 1999–2001 of 12 excavated wetlands in Wyoming, USA (McKinstry & Anderson 2005) found that wetlands amended with marsh soil developed vegetation cover, whilst unamended wetlands did not. Amended wetlands contained three plant species after one year and eight plant species after two years. Of 40 quadrats surveyed in each amended wetland, 3–6 contained vegetation after one year and 1–22 contained vegetation after two years. At this point, plant biomass was mostly alkali bulrush Scirpus maritimus (142g; 51% of total) or cattails Typha spp. (96g; 34% of total). No plants were recorded in unamended wetlands. Methods: In late 1999, twelve wetlands (<1 ha each) were excavated in clay soils. A 10–15 cm thick layer of soil from a nearby marsh was spread around the edge of six wetlands (water depth: 0–100 cm). The other six wetlands did not receive soil. In September 2000 and 2001, all vegetation was collected from forty 0.25-m2 quadrats/wetland then identified, dried and weighed. Quadrats were placed along transects perpendicular to the shoreline.

    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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