Physically remove problematic plants: freshwater marshes

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

Study locations

Key messages

  • Five studies evaluated the effects, on vegetation, of physically removing problematic plants from freshwater marshes. Three studies were in the USA, one was in India and one was in France. Two of the studies in the USA were in the same site and shared some plots.

VEGETATION COMMUNITY

  • Community composition (1 study): One replicated, randomized, paired, controlled, before-and-after study in the USA found that physically removing all vegetation from a cattail-invaded marsh altered the overall plant community composition, over the following two years.
  • Overall richness/diversity (3 studies): One replicated, randomized, paired, controlled, before-and-after study in the USA found that removing all vegetation from a cattail-invaded marsh increased overall plant species richness 1–2 years later. Two replicated, randomized, paired, controlled, before-and-after studies in wet meadows in the USA found that physically removing vegetation had no significant effect on overall plant species richness or diversity three years later. One of the studies removed all vegetation, whilst the other controlled regrowth of the invasive species (by physical removal along with herbicide application).
  • Characteristic plant richness/diversity (1 study): One controlled, before-and-after study in a temporary marsh in France reported that stripping all vegetation increased the number of habitat-characteristic plant species present in the following two years.

VEGETATION ABUNDANCE

  • Overall abundance (3 studies): Three before-and-after studies (two also replicated, randomized, paired, controlled) in freshwater marshes/wet meadows in India and the USA found that physically removing vegetation had no clear or significant effect on overall vegetation cover, nine months or three years later. Two of the studies removed all vegetation, whilst one controlled regrowth of the invasive species (by physical removal along with herbicide application).
  • Herb abundance (2 studies): Of two replicated, randomized, paired, controlled, before-and-after studies in loosestrife-invaded wet meadows in the USA, one reported that removing all vegetation increased the cover of grass-like plants, and reduced the cover of forbs, three years later. The other study found that controlling regrowth of the invasive species – by physical removal and applying herbicide – had no significant effect on cover of grass-like plants or forbs after three years.
  • Algae/phytoplankton abundance (1 study): One before-and-after, site comparison study in India reported that removing all vegetation from a knotgrass-invaded marsh increased the cover of algae nine months later.
  • Individual species abundance (3 studies): Three studies quantified the effect of this action on the abundance of individual plant species, other than the target problematic species. For example, one before-and-after, site comparison study in India reported that removing all vegetation from a knotgrass-invaded marsh increased the cover of some other common herb species nine months later.

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 before-and-after, site comparison study in 1984–1986 in an ephemeral freshwater marsh invaded by knotgrass Paspalum distichum in northwest India (Middleton et al. 1991) reported that an area cleared of vegetation developed similar vegetation cover to uncleared areas within nine months, but with different dominant species. Statistical significance was not assessed. Before intervention, the marsh had 69–70% total vegetation cover, 49–51% cover of knotgrass, <1% cover of water snowflake Nymphoides indicum and 2–4% cover of algae. After nine months, and at the same time of year, cleared areas had developed 68% total vegetation cover. This included <1% knotgrass cover, 29% water snowflake cover and 24% algal cover. Meanwhile, uncleared areas had 64% total vegetation cover, 49% knotgrass cover, 1% water snowflake cover and 6% algae cover. Methods: In June 1985, knotgrass-invaded vegetation was cleared, using bulldozers, from a marshy area in Keoladeo National Park. Comparable estimates of vegetation cover were made before clearance (March 1984 and 1985; ≥638 quadrats across whole marsh in each survey) and after clearance (March 1986; 55 quadrats in cleared area and ≥638 quadrats across rest of marsh). All quadrats were 1 m2.

    Study and other actions tested
  2. A replicated, randomized, paired, controlled, before-and-after study in 1988–1991 in two wet meadows invaded by purple loosestrife Lythrum salicaria in New York State, USA (Morrison 2002) found that physically removing all vegetation had no significant effect on vegetation richness, diversity or overall cover three years later, but increased cover of grass-like plants and reduced cover of forbs. After three years and in both meadows, cleared and uncleared plots had statistically similar plant species richness (cleared: 8; uncleared: 7–8 species/m2), plant diversity (data reported as a diversity index) and overall vegetation cover (cleared: 79%; uncleared: 78–117%). However, cleared plots were dominated by grass-like plants (73–74% cover) and had little cover of forbs (overall: 6–9%; purple loosestrife: 2%), whereas uncleared plots had little cover of grass-like plants (26–39%) and had high cover of forbs (overall: 41–92%; purple loosestrife: 31–78%). Note that these differences were only statistically significant in one of the two meadows. For data on the cover of other individual plant species, see original paper. Before intervention and within each meadow, plots destined for each treatment had statistically similar total vegetation cover (99–153%), plant species richness (8–10 species/m2). plant diversity, grass-like plant cover (11–67%) and loosestrife cover (18–82%). In one meadow, overall forb cover was lower in plots destined for clearance (25%) than plots not destined for clearance (121%). Methods: In 1988, six pairs of 1-m2 plots were established across two loosestrife-invaded wet meadows. In September, all vegetation was dug up and removed from one random plot in each pair. These plots were also used in (3). Vegetation was not removed from the other plots. Plant species and their cover were surveyed before removal (August 1988) and three years after (September 1991).

    Study and other actions tested
  3. A replicated, randomized, paired, controlled, before-and-after study in 1988–1991 in two wet meadows that had been cleared of vegetation in New York State, USA (Morrison 2002) found that controlling regrowth of invasive purple loosestrife Lythrum salicaria (by pulling up seedlings and applying herbicide to large shoots) had no significant effect on plant species richness, diversity or vegetation cover. After three years, plots with and without control of loosestrife regrowth had statistically similar plant species richness (control: 7; no control: 8 species/m2), plant diversity (data reported as a diversity index), total vegetation cover (control: 67–82%; no control: 79%), grass-like plant cover (control: 60–75%; no control: 70–73%) and forb cover (control: 5–20%; no control: 8–10%). Purple loosestrife cover was 0% in plots where regrowth had been controlled, but still only 2% in plots where regrowth had not been controlled. For data on the cover of other individual plant species, see original paper. Before intervention and within each meadow, plots destined for each treatment had statistically similar plant species richness (8–9 species/m2), plant diversity, total vegetation cover (103–143%), grass-like plant cover (16–58%), forb cover (25–56%) and purple loosestrife cover (23–63%). Methods: In 1988, six pairs of 1-m2 plots were established across two loosestrife-invaded wet meadows. In September, all vegetation was dug up and removed from the plots. In six of the plots (one random plot/pair), loosestrife regrowth was controlled twice/year thereafter (pulling up seedlings and painting large shoots with glyphosate; the study does not distinguish between the effects of these interventions). In the other plots loosestrife regrowth was not controlled. These plots were also used in (2). Plant species and their cover were surveyed before initial removal (August 1988) and three years after (September 1991).

    Study and other actions tested
  4. A controlled, before-and-after study in 2001–2003 in an ephemeral freshwater wetland dominated by compact rush Juncus conglomeratus in southern France (Félisiak et al. 2004) reported that removing the vegetation increased the number of plant species characteristic of Mediterranean temporary marshes over the following two years. Statistical significance was not assessed. The number of characteristic plant species increased in stripped plots, from zero in the year before intervention to 3–4 in the two years after (units not reported). The number of characteristic plant species was relatively stable in unmanaged plots (before: 2–4; after: 3–6). Methods: Four plots were established in rush-dominated vegetation near a reservoir. In autumn 2001, one plot was stripped of vegetation (including the root mat), exposing bare soil. The other three plots were left undisturbed. Plant species were recorded in the year before intervention (2001) and for two years after (2002 and 2003).An example of management by removal 

    Study and other actions tested
  5. A replicated, randomized, paired, controlled, before-and-after study in 2011–2013 in a freshwater marsh invaded by hybrid cattail Typha x glauca in Michigan, USA (Lishawa et al. 2015) found that physically removing the cattail-dominated vegetation changed the plant community composition and increased plant species richness and diversity. In the two years following vegetation removal, the overall plant community composition significantly differed between cleared and uncleared plots (data reported as a graphical analysis). Cleared plots had lower relative cover of hybrid cattail (cleared: 21–26%; uncleared: 87% of total cover). They also contained less hybrid cattail biomass (cleared: 29–51; uncleared: 500–700 g/m2). In both years, cleared plots contained more plant species (cleared: 13–14; uncleared: 8 species/16 m2) and had greater plant diversity (reported as a diversity index). Before intervention, plots destined for each treatment contained statistically similar plant communities with similar relative cover of cattail (84–87%), cattail biomass (data not reported), species richness (5–7 species/16 m2) and diversity. Methods: Sixteen 4-m2 plots were established in two areas of a freshwater marsh that had been invaded by hybrid cattail (one for >30 years, one for <20 years). In August 2011, vegetation was removed from eight plots (four random plots/area): vegetation was cut and removed, then rhizomes (underground horizontal stems) were dug up and removed. No vegetation was removed from the other eight plots. Roots and rhizomes were cut around the edge of each plot. Vegetation was surveyed in July before (2011) and for two years after (2012–2013) intervention. Dry above-ground biomass was estimated, after intervention only, from the height of cattail stems.

    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

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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