Reprofile/relandscape: freshwater swamps

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 2
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How is the evidence assessed?

Effectiveness

50%
Certainty

32%
Harms

10%

Calculating overall effectiveness category

Background information and definitions

This action involves large-scale reprofiling or landscaping, aiming to restore or create swamps. This includes excavating large basins (>8 ha or >280 m diameter), moving soil/sediment from the site into levees/berms/impoundments, removing unnatural hills or levees, filling in deep depressions and altering the elevation/slope of coastal areas. In other words, this action aims to restore wetland hydrology (how wet the soil is and when it is wet/flooded) by adjusting the ground surface relative to the water table or sea. Soil from existing marshes or swamps might be imported as part of relandscaping efforts, but we consider this as a separate action.

Caution: Heavy machinery is usually needed for this intervention. Heavy vehicles can churn and compress wetland soils (Campbell et al. 2002).

^{Campbell D.A., Cole C.A. & Brooks R.P. (2002) A comparison of created and natural wetlands in Pennsylvania, USA. Wetlands Ecology and Management, 10, 41–49.}

Study locations

Key messages

Two studies evaluated the effects, on vegetation, of reprofiling or relandscaping to restore or create freshwater swamps. Both studies were in the USA.

VEGETATION COMMUNITY

Community composition (1 study): One replicated, site comparison study in the USA found that swamps created by reprofiling uplands (along with planting trees/shrubs) contained a similar proportion of tree species in different plant groups, after 7–11 years, to nearby swamps recovering naturally from logging.

VEGETATION ABUNDANCE

Overall abundance (1 study): One replicated, site comparison study in the USA found that swamps created by reprofiling uplands (along with planting trees/shrubs) had similar ground and canopy cover, after 7–11 years, to nearby swamps recovering naturally from logging.
Herb abundance (1 study): One study in a former firing range in the USAsimply quantified herb cover approximately 1–2 years after reprofiling the site (and planting trees/shrubs).
Tree/shrub abundance (1 study): One study in a former firing range in the USAsimply quantified woody plant cover approximately 1–2 years after reprofiling the site (and planting trees/shrubs).

VEGETATION STRUCTURE

Visual obstruction (1 study): One replicated, site comparison study in the USA found that swamps created by reprofiling uplands (along with planting trees/shrubs) had less horizontal vegetation cover, after 7–11 years, than nearby swamps recovering naturally from logging.
Height (1 study): The same study found that swamps created by reprofiling uplands (along with planting trees/shrubs) contained shorter woody vegetation, after 7–11 years, than nearby swamps recovering naturally from logging. Herbaceous vegetation, however, was of similar height in both created and naturally recovering swamps.
Basal area (1 study): The same study found that swamps created by reprofiling uplands (along with planting trees/shrubs) had a lower vegetation basal area, after 7–11 years, than nearby swamps recovering naturally from logging.

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

A study in 1994–1995 in Maryland, USA (Perry et al. 1996) reported that approximately 1–2 years after reprofiling and planting trees/shrubs, the site contained mostly herbaceous vegetation. The created wetland had 67–69% grass cover, 17–19% cover of other herbs, and 1% cover of woody plants. Methods: In winter 1993/1994, around 5.5 ha of a former firing range was reprofiled to wetland elevations. In spring/summer 1994, a mixture of tree and shrub species (6,327 individuals) were planted into the reprofiled site. Vegetation was surveyed in August 1994 and 1995. Cover of all plant species was recorded in 120 quadrats, each 1 m². The study does not distinguish between the effect of reprofiling and planting on non-planted vegetation.
Study and other actions tested
Referenced paper
Perry M.C., Sibrel C.B. & Gough G.A. (1996) Wetlands mitigation: partnership between an electric power company and a federal wildlife refuge. Environmental Management, 20, 933-939.
A replicated, site comparison study in 2000 of 11 freshwater swamps in Virginia, USA (Snell-Rood & Cristol 2003) found that created swamps – reprofiled then planted with trees/shrubs – had a similar proportion of habitat-characteristic vegetation and similar horizontal vegetation cover to similar-aged swamps recovering naturally from logging, but contained shorter woody vegetation with a lower basal area and density. After 7–11 years, created and naturally recovering swamps contained statistically similar proportions of tree species characteristic of four soil moisture classes (from “highly saturated” to “partially saturated”), had statistically similar vegetation cover (both ground and canopy) and contained herbs of statistically similar height (data not reported). However, woody vegetation in created swamps was shorter (created: 2.0 m; natural: 4.4 m) and had a lower basal area (created: 59 cm²/100 m²; natural: 519 cm²/100 m²). Finally, created swamps had lower horizontal vegetation cover, both 1 m and 2 m above the ground (created: 26–45%; natural: 83–92%). Methods: In summer 2000, vegetation was surveyed in 11 swamps of similar age, water level and surrounding land use. Six swamps had been created by reprofiling upland sites to increase soil moisture, then planting a mix of wetland trees/shrubs (and in one case, adding wetland soil). The study does not distinguish between the effects of these interventions on non-planted vegetation. Five swamps were recovering naturally after clearcut logging.
Study and other actions tested
Referenced paper
Snell-Rood E.C. & Cristol D.A. (2003) Avian communities of created and natural wetlands: bottomland forests in Virginia. The Condor, 105, 303-315.

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?

List of journals searched by synopsis

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

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