Remove vegetation that could compete with planted trees/shrubs: freshwater wetlands

Overall effectiveness category Likely to be beneficial
Number of studies: 5
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How is the evidence assessed?

Effectiveness

40%
Certainty

50%
Harms

10%

Calculating overall effectiveness category

Background information and definitions

Removing other plants before or after planting desirable marsh or swamp plants could reduce competition for space, light and nutrients. Survival and growth of planted vegetation may be improved. Note that abundant competitors, and/or the absence of the vegetation to be introduced, could be symptoms of inappropriate physical conditions that may also need to be managed. Also note that existing vegetation may help to protect planted vegetation from extreme temperatures and sunlight, and protect the wetland surface from erosion.

To be clear, this action includes various specific actions that remove undesirable plants (e.g. physical removal, mowing, herbicide application) or kill undesirable seeds (e.g. burning, covering the soil with black plastic) in areas planted with desirable marsh or swamp plants. Management might be one-off or continuous. Evidence summarized for this action focuses on responses of the planted vegetation; studies that report responses of other vegetation are included in separate interventions elsewhere on this site.

Related actions: Introduce nurse plants before/after planting target marsh or swamp vegetation.

Study locations

Key messages

Five studies evaluated the effects, on trees/shrubs planted in freshwater wetlands, of removing competing plants. Four studies were in the USA. Two of these took place in the same swamp, but with different experimental set-ups. One study was in Australia.

VEGETATION COMMUNITY

VEGETATION ABUNDANCE

VEGETATION STRUCTURE

Height (3 studies): Three replicated, controlled studies (two also randomized, two also paired) in a wet meadow in Australia and a degraded swamp in the USA found that clearing vegetation before planting tree/shrub seedlings typically had no clear or significant effect on their height, after 1–4 growing seasons. However, one of the studies in the USA found that planted baldcypress Taxodium distichum seedlings were taller, after three growing seasons, when planted amongst cut woody vegetation than below an uncleared canopy.
Diameter/perimeter/area (1 study): One replicated, randomized, paired, controlled study in a wet meadow in Australia found that clearing vegetation, before planting tree/shrub seedlings, typically had no significant effect on the diameter of these seedlings nine months later.

OTHER

Germination/emergence (1 study): One replicated, randomized, paired, controlled study in a wet meadow in Australia found that there were more seedlings in plots that had been cleared of vegetation before sowing tree/shrub seeds, than in plots that had not been cleared before sowing. Seedlings were counted two months after sowing.
Survival (4 studies): Three replicated, controlled studies (two also randomized, two also paired) in a wet meadow in Australia and a degraded swamp in the USA found that clearing vegetation before planting tree/shrub seedlings typically had no clear or significant effect on their survival, after 1–4 growing seasons. However, one of the studies in the USA found that planted baldcypress Taxodium distichum seedlings had a lower survival rate, after three growing seasons, when planted amongst cut woody vegetation than below an uncleared canopy. One replicated, randomized, paired, controlled study in degraded swamps in the USA found that removing reed canarygrass Phalaris arundinacea before planting tree/shrub seedlings never significantly reduced their survival rate over 1–2 growing seasons, and often increased it.
Growth (1 study): One replicated, randomized, controlled study in the USA found that baldcypress Taxodium distichum seedlings planted into a marsh grew more in diameter, but less in height, when planted into plots cleared of vines than when planted into uncleared plots.

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 replicated, randomized, controlled study in 1992 in a freshwater marsh in Louisiana, USA (Myers et al. 1995) found that where competing vines were cleared, planted baldcypress Taxodium distichum seedlings grew more in diameter and less in height than where vines were not cleared. Over one growing season, seedlings in cleared plots grew more in diameter (0.74 cm) than seedlings surrounded by vines (0.43 cm). However, cleared seedlings grew less in height (5.4 cm) than surrounded seedlings (8.7 cm). Clearing vines had a bigger effect on seedlings on diameter growth for seedlings within plastic guards than without (data not reported), but had a similar effect on seedlings whether fertilized (cleared seedlings grew 0.26 cm more than uncleared) or not (cleared seedlings grew 0.28 cm more than uncleared). Methods: In January 1992, four hundred baldcypress seedlings were planted into a marsh – with the aim of restoring the swamp that was logged around 80 years previously. Vines were cleared from around 200 random seedlings every 3–4 weeks after planting, but left to grow around the other 200 seedlings. An equal number of cleared and uncleared seedlings received additional treatments: protection from herbivores and/or fertilization. Seedling diameter and height were measured at planting (January 1992) and after one growing season (October 1992).
Study and other actions tested
Referenced paper
Myers R.S., Shaffer G.P. & Llewellyn D.W. (1995) Baldcypress (Taxodium distichum (L.) Rich.) restoration in southeast Louisiana: the relative effects of herbivory, flooding, competition, and macronutrients. Wetlands, 15, 141-148.
A replicated, randomized, paired, controlled study in 1994–1995 in a wet meadow in New South Wales, Australia (de Jong 2000) found that clearing vegetation increased germination of sown tree/shrub seeds, but typically had no clear or significant effect on the survival or size of planted seedlings. Statistical significance of survival results was not assessed. Two months after sowing seeds, there were more seedlings in cleared than uncleared plots in 10 of 10 cases (cleared: 1–9 seedlings/plot; uncleared: <1 seedling/plot). Nine months after planting, seedlings in cleared and uncleared plots had similar survival rates in 20 of 20 comparisons (cleared: 96–100%; uncleared: 96–100%), statistically similar heights in 20 of 20 comparisons (cleared: 69–127 cm; uncleared: 68–121 cm), and statistically similar stem diameters in 17 of 20 comparisons (for which cleared: 7–14 mm; uncleared: 7–16 mm). Bi-monthly weeding after initial clearance had no clear effect on seedling survival, and no significant effect on seedling size in 19 of 20 comparisons (see original paper for data). Methods: In spring/summer 1994/1995, five tree and shrub species were planted into a wet meadow, with the aim of restoring a swamp. For each species, 300 plots (25 x 25 cm) were sown with seeds (50 seeds/plot) and 300 plots were sown with nursery-reared seedlings (1 seedling/plot). A random 400 plots/species were cleared of vegetation before planting (details of clearing not reported). Of these, a random 200 plots were also weeded every two months. Seedlings in sown plots were counted after two months. Seedlings in planted plots were counted and measured after nine months.
Study and other actions tested
Referenced paper
de J.N.H. (2000) Woody plant restoration and natural regeneration in wet meadow at Coomonderry Swamp on the south coast of New South Wales. Marine and Freshwater Research, 51, 81.
A replicated, paired, controlled study in 1993–1996 in a degraded swamp in South Carolina, USA (McLeod et al. 2000) found that clearing competing vegetation after planting tree seedlings typically had no significant effect on their survival or average height. After four growing seasons and averaged across species, seedling survival rates did not significantly differ between plots where vegetation was cleared (57–63%) and plots where vegetation not cleared (66%), regardless of the clearance method. For five of six planted species, the average height of surviving seedlings was statistically similar in cleared plots (100–311 cm) and uncleared plots (145–265 cm), regardless of the clearance method. For the other species, baldcypress Taxodium distichum, seedlings were taller in cleared plots (253–285 cm) than uncleared plots (213 cm): significantly so for three of four clearance methods. At planting, seedlings were 40–94 cm tall and did not significantly differ in height within each species and clearance method. Methods: In April 1993, tree seedlings were planted into 25 plots (6 seedlings/species/plot; seedlings 2 m apart) in a degraded swamp (natural forest killed by heated effluent between 1955 and 1985). In spring/summer 1993 and 1994, five plots received each vegetation clearance treatment: mowing whole plot; mowing 1 m strips in which seedlings were planted; applying herbicide (Accord®) to whole plot; applying herbicide to 1 m strips in which seedlings were planted; no clearance. All seedlings were protected with tree guards. Seedling survival and height were recorded at planting, then each autumn until 1996. This study used the same swamp as (4), but a different experimental set-up.
Study and other actions tested
Referenced paper
McLeod K.W., Reed M.R. & Wike L.D. (2000) Elevation, competition control, and species affect bottomland forest restoration. Wetlands, 20, 162-168.
A replicated, randomized, controlled study in 1994–1996 in a degraded swamp in South Carolina, USA (McLeod et al. 2001) found that clearing black willows Salix nigra before planting tree seedlings reduced survival and increased the average height for one of three planted species, but had no significant effect on the other two. After three growing seasons, baldcypress Taxodium distichum seedlings had a lower survival rate, but survivors were taller, when planted amongst cut willows (survival: 75%; height: 192 cm) than when planted under a willow canopy (survival: 95%; height: 134 cm). For two other planted species, overcup oak Quercus lyrata and water hickory Carya aquatica, survival rates and the height of survivors did not significantly differ between seedlings planted amongst cut willows (survival: 73–78%; height: 112–148 cm) and seedlings planted under a willow canopy (survival: 90%; height: 104–134 cm). At planting, seedlings were 47–85 cm tall and did not significantly differ in height within each species and clearance treatment. Methods: In winter 1993/1994, ten 180-m² plots were established in a degraded swamp (natural forest killed by heated effluent between 1955 and 1985). In five random plots, all willow trees were cut to within 15 cm of the ground. In five other plots, the willow canopy was left intact. In February 1994, eight seedlings of each species were planted, 2 m apart, into each plot. All seedlings were protected with tree guards. Seedling survival and height were recorded at planting, then each autumn until 1996. This study used the same swamp as (3), but a different experimental set-up.
Study and other actions tested
Referenced paper
McLeod K.W., Reed M.R. & Nelson E.A. (2001) Influence of a willow canopy on tree seedling establishment for wetland restoration. Wetlands, 21, 395-402.
A replicated, randomized, paired, controlled study in 2002–2004 in three freshwater wetlands in Wisconsin, USA (Hovick & Reinartz 2007) reported that removing invasive reed canarygrass Phalaris arundinacea either increased or had no significant effect on survival of planted trees/shrubs over 1–2 growing seasons. In 68 of 136 comparisons, plots from which canarygrass had been removed supported significantly higher survival rates of planted trees/shrubs (13–100%) than plots where canarygrass remained (0–73%). In the other 68 comparisons, plots from which canarygrass had been removed supported statistically similar survival rates (0–100%) to plots where canarygrass remained (0–100%), although there was a trend for higher survival in cleared plots in 46 of these comparisons. The effect of canarygrass removal depended on the tree/shrub species, site and removal treatment. In other words, most species responded significantly to canarygrass removal only as a result of certain methods or in certain sites (see original paper). Plots from which canarygrass had been removed before planting also typically had higher overall plant species richness and diversity, and contained more non-planted tree seedlings, than plots where canarygrass was not removed (see Actions: Physically damage problematic plants, Use cutting/mowing to control problematic herbaceous plants, Use prescribed fire to control problematic plants and Use herbicide to control problematic plants). Methods: In spring 2003 or 2004, seedlings of 23 tree and shrub species were planted into three degraded wetlands (roughly 1 seedling/m²). Reed canarygrass had been removed from some planted areas, but left in others (distribution of seedlings amongst areas not clear). Removal treatments involved spraying with herbicide in all three wetlands, and in one wetland additional burning, mowing or ploughing. Survival of all seedlings was monitored in September 2003 and 2004. Other plant species and their cover were recorded in August 2004.
Study and other actions tested
Referenced paper
Hovick S.M. & Reinartz J.A. (2007) Restoring forest in wetlands dominated by reed canarygrass: the effects of pre-planting treatments on early survival of planted stock. Wetlands, 27, 24-39.

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

Related Actions

Actions	Effectiveness	Studies	Category
Remove vegetation that could compete with planted trees/shrubs: brackish/saline wetlands Action Link	No evidence found (no assessment)	0	Synopsis Link
Disturb soil/sediment surface before planting trees/shrubs: brackish/saline wetlands Action Link	Unknown effectiveness (limited evidence)	1	Synopsis Link
Reprofile/relandscape (before planting) Action Link	No evidence found (no assessment)	0	Synopsis Link