Create mounds or hollows before planting trees/shrubs: freshwater wetlands

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

Study locations

Key messages

  • Three studies evaluated the effects, on vegetation, of creating mounds or hollows in freshwater wetlands before planting trees/shrubs. All three studies were in the USA.

VEGETATION COMMUNITY

  • Community composition (1 study): One replicated, site comparison study of 10-year-old restored/created freshwater wetlands in the USA reported that adding coarse woody debris to wetlands before planting trees/shrubs affected the composition of the ground vegetation layer, but not the tree layer.
  • Overall richness/diversity (2 studies): Two studies in freshwater wetlands in the USA reported that creating mounds or hollows before planting trees/shrubs had no clear or significant effect on plant species richness and diversity 10–12 years later. In one of the studies, the same was true for bryophyte, herb and woody plants richness separately.

VEGETATION ABUNDANCE

 

VEGETATION STRUCTURE

  • Height (1 study): One replicated, paired, controlled study in created freshwater wetlands in the USA found that the average height of white cedar Thuja occidentalis saplings typically increased more, between two and five years after planting, in created mounds than on lower (occasionally flooded) ground.

OTHER                                        

  • Survival (1 study): One replicated, paired, controlled study in created freshwater wetlands in the USA found that white cedar Thuja occidentalis seedlings had higher survival rates when planted into created mounds than on lower (occasionally flooded) ground.

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 study in 1988–2000 in a freshwater swamp in Michigan, USA (Anderson et al. 2007) reported that creating ridges and ditches before re-planting harvested trees had no clear effect on plant species richness, and no significant effect on overall plant diversity, after 11–12 years. Amongst plots that were harvested then re-planted, those with created ridges and ditches had similar plant species richness (36–44 species/2 m2) to those with natural, unmodified ridges and ditches (39 species/2 m2; statistical significance not assessed). The same was true separately for richness of bryophytes (created: 18–20; natural: 18 species/2 m2), Sphagnum mosses (created: 6–7; natural: 7 species/2 m2), herbs (created: 11–12; natural: 10 species/2 m2) and woody plants (created: 7–12; natural: 11 species/2 m2). For comparison, unharvested plots – where trees were planted amongst natural ridges and ditches – contained 47 plant species/2 m2 (including 25 bryophytes, 12 Sphagnum mosses, 8 herbs and 14 woody species). Overall plant diversity was statistically similar in harvested/re-planted plots with created ridges, harvested/re-planted plots with natural ridges, and unharvested/planted plots (data reported as a diversity index). Methods: In 1988, all trees were cut and removed from three plots in a forested swamp. In two plots, microtopography (trenches and adjacent mounds) was created after harvesting, using a disc trencher or a plough. In the third plot, natural pits and mounds remaining after harvesting were not altered. An additional plot was not harvested and the natural microtopography was not altered. All plots were subsequently planted with tree seedlings. In 1999 and 2000, understory vegetation (<1 m tall) was surveyed in twenty 1,000-cm2 quadrats/area. Each quadrat contained a pit or trench, a mound and the slope between them.

    Study and other actions tested
  2. A replicated, paired, controlled study in 2008–2013 in two created freshwater swamps in Michigan, USA (Kangas et al. 2016) reported that white cedar Thuja occidentalis seedlings had higher survival rates when planted into created mounds than on flat ground, and that the average height of survivors increased more on mounds than on flats. After five years and in four of four comparisons, cedar seedlings planted on elevated mounds had a higher survival rate (54–94%) than seedlings planted on lower flats (0–41%). Between two and five years after planting, the average height of surviving trees increased more on mounds than on flats in three of four comparisons (mounds: 11–39 cm/year; flats : 0–23 cm/year). In the other comparison, there was no significant difference between treatments (mounds: 1 cm/year increase; flats: 2 cm/year decrease). Methods: In spring 2008, one-year-old white cedar seedlings were planted into 37 plots on two recently excavated wetlands (5–106 seedlings/plot, approximately 2.8 m apart). The seedlings were planted on created mounds in 20 plots (1.0–1.5 m diameter; 13–25 cm tall) and on a flat surface in the other 17 plots. Mound tops were never flooded. Flats were sometimes flooded. Some mounded and flat plots were also fenced to exclude deer. Surviving trees were monitored in April 2010 and October 2013.

    Study and other actions tested
  3. A replicated, site comparison study in 2013 of eight 10-year-old restored/created freshwater wetlands in Maryland, USA (Russell & Beauchamp 2017) found that adding coarse woody debris to wetlands before planting trees/shrubs generally had no significant effect on plant community composition, richness or diversity – but did affect the ground layer community composition. The amount of coarse woody debris added to wetlands (none, low density, high density) was not significantly related to plant community composition, richness or diversity. This was true for both the ground vegetation layer (<1 m tall) and the tree layer (>1 m tall; data not reported). However, the effect on community composition was also analyzed for wetlands with vs without added coarse woody debris. In this analysis, ground layer community composition significantly differed between treatments (data reported as a graphical analysis). Methods: In June–August 2013, vegetation was surveyed along transects in eight restored/created depressional wetlands (4–6 transects/wetland, extending from the centre to the surrounding upland). The wetlands had been restored or created on farmland in 2003–2004, by: rewetting, adding wheat/barley straw, and planting trees/shrubs in wetland and upland areas. Logs, from trees felled on site, were added to pools/pool margins in six of the wetlands (three low density: 15–50 logs/ha; three high density: 136–333 logs/ha).

    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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