Retain/restore/create vegetation around freshwater marshes

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

Study locations

Key messages

  • Four studies evaluated the effects, on vegetation in freshwater marshes, of retaining/restoring/creating vegetation around them. Three studies were in the USA and one was in China. Two studies were largely based on the same sites.

VEGETATION COMMUNITY

  • Community composition (2 studies): Two replicated, site comparison studies in the USA reported that freshwater marshes surrounded by restored upland vegetation contained a different overall plant community, after 1–20 years, to nearby marshes surrounded by natural vegetation. One of the studies also reported differences between marshes in restored vs degraded catchments.
  • Overall richness/diversity (3 studies): One replicated, paired, site comparison study in the USA found that marshes surrounded by restored upland vegetation had greater overall plant species richness than marshes within cropland, and similar richness to marshes within natural grassland. One replicated, site comparison study in the USA reported that freshwater marshes surrounded by restored upland vegetation contained fewer wetland plant species, after 1–20 years, than nearby marshes surrounded by natural vegetation. One before-and-after study of a lakeshore marsh in China reported that after revegetating a polluted input river (along with planting directly into the marsh), overall plant species richness increased.

VEGETATION ABUNDANCE

  • Overall abundance (1 study): One replicated, paired, site comparison study in the USA found that marshes surrounded by restored upland vegetation contained more plant biomass than marshes within cropland, but also more plant biomass than marshes within natural grassland.
  • Characteristic plant abundance (1 study): One replicated, paired, site comparison study in the USA found that marshes surrounded by restored upland vegetation typically had greater cover of wetland-characteristic plants than marshes within cropland, and similar cover of these species to marshes within natural grassland.
  • Individual species abundance (1 study): One replicated, site comparison study of pothole wetlands the USA found that wetlands surrounded by restored upland vegetation had greater cover of hybrid cattail Typha x glauca, after 2–7 years, than nearby natural wetlands.

VEGETATION STRUCTURE

  • Visual obstruction (1 study): One replicated, site comparison study of pothole wetlands in the USA found that parts of wetlands surrounded by restored upland vegetation created more visual obstruction, after 2–7 years, than the corresponding zone of nearby natural wetlands.

About key messages

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

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

Supporting evidence from individual studies

  1. A replicated, paired, site comparison study of 261 ephemeral freshwater marshes (playas) in the Great Plains of the USA (O’Connell et al. 2012) found that marshes within revegetated cropland had greater plant species richness, plant biomass and cover of wetland-characteristic plants than marshes within current cropland, and similar richness and cover of wetland-characteristic plants to marshes within natural grassland. Compared to marshes within current cropland, restored-catchment marshes had greater plant species richness (reported as statistical model results), greater above-ground plant biomass (restored: 420; cropland: 200 g/m2) and typically greater cover of wetland-characteristic plant species (two of three comparisons, for which restored: 22–27%; cropland: 11–15%). Compared to marshes within natural (never ploughed) grassland, restored-catchment marshes had similar plant species richness (reported as statistical model results) and typically similar cover of wetland-characteristic plant species (two of three comparisons, for which restored: 22–27%; natural: 22–26%). However, restored-catchment marshes had greater above-ground plant biomass (420 g/m2) than marshes within natural grassland (240 g/m2). The study also reported that restored-catchment marshes were dominated by Great-Plains-native perennial plants, like natural marshes, but had greater cover of non-native plants than both natural and cropland marshes (see original paper for data). Methods: In summer (year not reported), vegetation was surveyed within 261 playa wetlands. These were arranged in 87 sets of three. In each set, one wetland was within former cropland now planted with a perennial cover crop, one was within extant cropland, and one was within natural grassland. Surveys included crop plants within wetlands. Biomass was dried before weighing. Most of the sites in this study were also studied in (2).

    Study and other actions tested
  2. A replicated, site comparison study of 258 ephemeral freshwater marshes in central USA (O’Connell et al. 2013) reported that marshes within revegetated cropland contained a different plant community to natural marshes (surrounded by permanent grassland) and degraded marshes (surrounded by cropland), with lower cover of perennial wetland plants and fewer perennial wetland species than the natural marshes. Results summarized for this study are not based on assessments of statistical significance. After 1–20 years, the overall plant community composition differed between restored-catchment, natural and degraded marshes (data reported as a graphical analysis). Perennial wetland species were underrepresented in restored-catchment marshes (30% cover; 3.5 species/marsh) compared to natural marshes (47% cover; 5.0 species/marsh). However, restored-catchment marshes had greater cover of these species than degraded marshes (7% cover; species richness not reported). Annual wetland species were overrepresented in restored-catchment marshes compared to natural marshes in terms of abundance (data reported as a graphical analysis only). However, there was a similar number of these species in restored-catchment marshes (5.2 species/marsh) and natural marshes (5.4 species/marsh). Methods: Around 2010, vegetation was surveyed in 258 ephemeral playa marshes (along two transects crossing each marsh, in both the cool and warm seasons). Of these marshes, 86 were undergoing restoration under the Conservation Reserve Program (former cropland in catchment replanted to grassland 1–20 years previously; no intervention within the marshes), 86 were in natural catchments, and 86 were in degraded, farmed catchments. This study used a subset of the sites from (1).

    Study and other actions tested
  3. A before-and-after study in 2008–2014 of a lakeshore freshwater marsh in southern China (Liu et al. 2016) found that after planting herbs into a polluted river feeding it (and planting directly into the marsh), plant species richness increased. Statistical significance was not assessed. The marsh contained 14 plant species before planting but 26 plant species five years after. Methods: In May 2009, the river feeding a lake was planted with pollution-reducing vegetation: bur-reed Sparganium simplex, mare’s tail Hippuris vulgaris and yellow floating heart Nymphoides peltatum. The river water quality had recently declined, due to inputs of nutrients and domestic sewage. Some herbs were also planted directly into the lakeshore marsh (number of species not reported). The study does not distinguish between the effects of these interventions on any non-planted vegetation. Lakeshore vegetation (emergent, floating and submerged) was surveyed before (July 2008) and for approximately five years after (July 2009–2014) planting (details not fully reported).

    Study and other actions tested
  4. A replicated, site comparison study in 2010 of 20 prairie pothole wetlands in North Dakota, USA (Smith et al. 2016) found that potholes amongst restored perennial vegetation contained a different marsh and wet meadow plant community to nearby natural marshes, with greater cattail cover and sometimes greater horizontal vegetation cover. The overall plant community composition in both the marsh and wet meadow zones significantly differed between potholes surrounded by restored perennial upland vegetation and nearby natural potholes (data reported as a graphical analysis). Across both zones, the potholes in restored areas had greater cover of hybrid cattail Typha x glauca (19%) than natural potholes (5%). In the marsh zone – but not the wet meadow zone – visual obstruction was greater in potholes in restored areas than in natural potholes (data reported as a visual obstruction index). Methods: In summer 2010, vegetation was surveyed in the marsh (seasonally flooded) and wet meadow (occasionally flooded) zones of 20 prairie potholes (10 quadrats/zone/pothole). Eleven potholes used to be surrounded by cropland, but this had been restored to perennial vegetation cover (details and dates not reported, but probably around 2–7 years previously). However, these potholes likely contained excess sediment that had washed off the cropland. The other nine potholes were surrounded by land that was not, and had never been, cultivated.

    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.

Where has this evidence come from?

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Marsh and Swamp Conservation

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation
Marsh and Swamp Conservation

Marsh and Swamp Conservation - Published 2021

Marsh and Swamp Synopsis

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