Directly plant trees/shrubs: freshwater wetlands

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

Study locations

Key messages

  • Seventeen studies evaluated the effects, on vegetation, of directly planting trees/shrubs in freshwater wetlands. Fifteen studies were in the USA. Two were in Australia. Two of the studies took place in the same site, but used different experimental set-ups.

VEGETATION COMMUNITY

  • Community composition (2 studies): Two replicated studies of freshwater wetlands in the USA found that planting trees/shrubs (sometimes along with other interventions) had no significant effect on aspects of plant community composition, after 1–11 years. Specifically, planted and unplanted wetlands had a similar proportion of species in different plant groups and relative abundance of different plant groups.
  • Overall richness/diversity (1 study): One replicated, randomized, controlled, before-and-after study in depressional wetlands in the USA found that wetlands sparsely planted with tree seedlings contained a similar number of plant species, after 1–4 years, to unplanted wetlands.

VEGETATION ABUNDANCE

  • Overall abundance (2 studies): Two replicated studies (one site comparison; one randomized, controlled, before-and-after) of freshwater wetlands in the USA found that planting trees/shrubs (sometimes along with other interventions) had no significant effect on overall vegetation cover (both ground and canopy, separately or combined) after 1–11 years.
  • Herb abundance (1 study): One study in a former firing range in the USA simply 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 USA simply 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 planting trees/shrubs (after reprofiling) had less horizontal vegetation cover, after 7–11 years, than nearby swamps recovering naturally from logging.
  • Height (6 studies): One replicated, site comparison study in the USA found that swamps created by planting trees/shrubs (after reprofiling) contained shorter woody vegetation, after 7–11 years, than nearby swamps recovering naturally from logging. Herbaceous vegetation, however, was of similar height. Five studies (four replicated) in freshwater wetlands in the USA simply quantified the height of trees and shrubs over 1–6 growing seasons after they were planted; in four of these studies, the average height typically increased over time.
  • Diameter (1 study): One study in a freshwater wetland in the USA reported an increase in the diameter of surviving trees over the year after they were planted.
  • Basal area (1 study): One replicated, site comparison study in the USA found that swamps created by planting trees/shrubs (after reprofiling) had a lower vegetation basal area, after 7–11 years, than nearby swamps recovering naturally from logging.

OTHER

  • Survival (15 studies): Fifteen studies (including eight replicated) in the USA and Australia quantified survival of individual trees/shrubs planted in freshwater wetlands. Survival rates ranged from 0% to 100% after 4–66 months. Seven of the studies (including six replicated) in the USA and Australia reported 0% survival of planted vegetation in at least some cases, after 1–6 growing seasons. Proposed factors affecting survival included elevation/water levels, the season of planting, protection from herbivores, root pruning, extreme weather, and if/how invasive vegetation was removed before planting.
  • Growth (2 studies): Two studies monitored true growth of individual trees/shrubs (rather than changes in average height of survivors). The two studies, in freshwater wetlands in the USA, reported that planted trees grew in diameter and/or height over their first 1–2 growing seasons.

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 study in 1979 of a swamp restoration project in Florida, USA (Gilbert et al. 1980) reported that 7–85% of planted tree seedlings survived over 4–5 months. Of the 16 planted species, survival was lowest for longleaf pine Pinus palustris (7%) and highest for green ash Fraxinus pennsylvanica and baldcypress Taxodium distichum (both 85%). For all species, the survival rate varied between plots at different elevations. Statistical significance was not assessed. Methods: In January and February 1979, seedlings of native Florida tree species were planted into a wetland undergoing restoration. The site was historically a swamp, then mined for phosphate. It was modified before planting by levelling spoil piles, creating wet basins and installing inflow/outfall pipes. Most of the 10,400 seedlings were planted in 26 multispecies plots (4–5 species/plot; seedlings 1.5 metres apart) but some were planted randomly throughout the site. Survival of planted seedlings was recorded in June 1979.

    Study and other actions tested
  2. A study in 1980–1981 of a swamp restoration project in Florida, USA (Clewell 1981) reported that 46–91% of planted tree seedlings survived over one year. Eight species were planted, but the study only reports results for three. The survival rate was 46% for slash pine Pinus elliotti, 85% for red maple Acer rubrum and 91% for sweetgum Liquidamar styraciflua. Methods: In early 1980, five 0.19-ha plots were planted with approximately 3,000 tree seedlings (600 seedlings/plot; mixed species). The site was historically a swamp, then mined for phosphate. Existing grass-like plants were burned before planting trees. Survival of planted seedlings was recorded in early 1981.

    Study and other actions tested
  3. A replicated study in 1985–1987 in a floodplain swamp in Louisiana, USA (Conner & Flynn 1989) reported variable survival and changes in average height of planted baldcypress Taxodium distichum seedlings, depending on protection from herbivores, water levels and season of planting. Statistical significance was not assessed. Overall, 0–88% of seedlings survived over 2–3 growing seasons. In five of seven cases with data, the average height of surviving seedlings was greater after 2–3 growing seasons (76–130 cm) than it had been after one growing season (60–102 cm). Seedlings protected from herbivores had higher survival than unprotected seedlings when planted in the spring, but not when planted in autumn (see Action: Use fences or barriers to protect planted areas). Amongst protected seedlings, survival was higher in drier plots after a wet year (1986; driest plot: 84–88%; wettest plot: 52–70%) but higher in wetter plots after a dry year (1987; driest plot: 20–40%; wettest plot: 52–70%). Methods: In 1985, three plots (flooded at different depths and for different durations) were each planted with 250 baldcypress seedlings: 200 in February/March and 50 in September. Chickenwire fences protected 75 seedlings/plot from herbivores (especially nutria Myocastor coypus). Seedlings were root-pruned and stored cold (4°C) before planting. Plots contained other trees (330–590 stems/ha) and saplings/shrubs (1,000–3,500 stems/ha). Baldcypress seedling survival and height were recorded in October 1985, 1986 and 1987.

    Study and other actions tested
  4. A replicated study in 1988–1990 in up to five created freshwater wetlands in eastern Massachusetts, USA (Jarman et al. 1991) reported that survival of planted red maple Acer rubrum saplings depended on whether their roots were pruned. Statistical significance was not assessed. After approximately 1–2 years, the survival rate was >75% for saplings whose roots had been pruned “several months” before planting, but <25% for saplings whose roots had not been pruned. Methods: In the late 1980s, red maple saplings saved from destroyed wetlands were planted in up to five newly created wetlands (excavated from uplands, connected to natural wetlands, planted with herbs and shrubs as well as red maple). The roots of some saplings were pruned before planting. The study does not report the number of saplings planted, the precise number of wetlands planted with red maple, or precise dates of planting and monitoring.

    Study and other actions tested
  5. A study in 1992–1994 in a freshwater marsh in Louisiana, USA (Myers et al. 1995) reported that planted baldcypress Taxodium distichum seedlings grew, but that seedlings exposed to herbivores all died within two years. Statistical significance was not assessed. Over one growing season, seedlings protected from herbivores grew thicker by 0.32–0.85 cm. Unprotected seedlings grew thicker by 0.28 cm. Half of the unprotected seedlings survived the first growing season, but none survived the winter following the second growing season. The study does not report survival rates for the protected seedlings. Methods: In January 1992, four hundred baldcypress seedlings were planted into a marsh (historically a swamp, but logged around 80 years previously). Of these, 320 were protected from herbivores (240 with plastic sleeves, 80 with sticky insect-trapping oil) and 80 were left unprotected. Some protected and unprotected seedlings received additional treatments: fertilization and/or removal of competing vines. Nutria Myocastor coypus were “intensively trapped” in the month before planting. Seedling diameter was measured at planting (January 1992) and after one growing season (October 1992). Survival was monitored until after the second growing season (early 1994).

    Study and other actions tested
  6. A study in 1992–1993 around the margins of an excavated freshwater wetland in Ohio, USA (Niswander & Mitsch 1995) reported that 96% of planted trees survived for 16 months. In contrast, almost all planted shrubby cinquefoil Potentilla fruticosa had died or was “very unhealthy”. Methods: In spring 1992, approximately 757 trees/shrubs were planted (4.6 m apart) into intermittently flooded land around two connected basins (6.1 ha total area) which had been excavated in autumn 1991. Ten different species were planted: two maples Acer spp., river birch Betula nigra, green hawthorn Crateagus viridis, green ash Fraxinus pennsylvanica, sweetgum Liquidambar styraciflua, black gum Nyssa sylvatica, pin oak Quercus palustris, arrowwood Viburnum recognitum and shrubby cinquefoil.

    Study and other actions tested
  7. A study in 1994–1995 a reprofiled and planted freshwater wetland in Maryland, USA (Perry et al. 1996) reported that the majority of planted trees/shrubs survived for one year, but there was little other change in vegetation cover and structure. Statistical significance was not assessed. After approximately one year, 83% of planted shrubs and 91% of planted trees were still alive. Survival varied between species, but was never lower than 69% (for highbush blueberry Vaccinium corymbosum). Over the year after planting, the average diameter of surviving trees increased from 11 to 15 mm. There was little other change in vegetation cover (grasses: 67–69%; other herbs: 17–19%; woody plants: 1%) or structure (tree height: 147–149 cm; tree canopy diameter: 29–33 cm; shrub height: 101 cm; shrub canopy diameter: 31–36 cm). Methods: In spring/summer 1994, a mixture of tree and shrub species (6,327 individuals) were planted into 5.5 ha of a former firing range, which had been reprofiled to manage water levels. Vegetation was surveyed in August 1994 and 1995. Tree/shrub survival, and diameter of surviving trees/shrubs, were monitored in twelve 25 x 25 m plots. Cover of all plant species was recorded in 120 quadrats, each 1 m2. The study does not distinguish between the effect of planting trees/shrubs and reprofiling on non-planted vegetation.

    Study and other actions tested
  8. A replicated study in 1994–1995 in a wet meadow in New South Wales, Australia (de Jong 2000) reported 95–100% survival of planted tree/shrub seedlings after nine months, but 0–90% survival after 50 months. After nine months, >95% of planted seedlings were still alive: for all five sown species, in both drier and wetter plots, and whether or not vegetation was cleared before planting. After 50 months, and following extremes of both flooding and drought, survival was more variable. Some seedlings survived in 9 of 10 cases, with a survival rate of 10–90%. In the other case, the survival rate was 0% for prickly tea tree Leptospermum juniperinum seedlings in lower (wetter) plots. Methods: In October–December 1994, five tree/shrub species present in local wetlands were planted into a wet meadow, with the aim of restoring a swamp. Three hundred nursery-reared seedlings of each species were planted, at least 1 m apart. Of the 300 seedlings/species, 150 were planted in a drier area (vs 150 in a wetter area) and 200 were planted in plots cleared of vegetation (vs 100 in intact vegetation). Survival was monitored after nine months (all seedlings) and 50 months (10 seedlings/species/water level).

    Study and other actions tested
  9. A replicated study in 1993–1996 in a degraded freshwater swamp in South Carolina, USA (McLeod et al. 2000) reported that 14–87% of planted tree seedlings survived over four growing seasons, and that the average height of seedlings increased. Statistical significance was not assessed. Six tree species were planted. After four growing seasons, survival rates were 14% for cherrybark oak Quercus falcata var. pagodaefolia, 54% for willow oak Quercus phellos, 62% for water tupelo Nyssa aquatica and Nuttall oak Quercus nuttallii, 83% for overcup oak Quercus lyrata and 87% for baldcypress Taxodium distichum. When planted, seedlings were 42–89 cm tall on average. After four growing seasons, survivors were 153–285 cm tall on average. The study also reported that survival and height change varied with elevation/wetness for some species, but found that clearing competing vegetation typically had no significant effect on survival or growth (see Action: Remove vegetation that could compete with planted vegetation and original paper). Methods: In April 1993, tree seedlings were planted (25 plots; 6 seedlings/species/plot; seedlings 2 m apart) into a degraded swamp. Heated effluent had killed existing trees between 1955 and 1985. All seedlings were protected with tree guards. In 20 plots, competing vegetation was cleared in summer 1993 and 1994, by mowing or applying herbicide. Seedling survival and height were recorded at planting, then each autumn until 1996. This study used the same swamp as (10), but a different experimental set-up.

    Study and other actions tested
  10. A replicated study in 1994–1996 in a degraded freshwater swamp in South Carolina, USA (McLeod et al. 2001) reported that 0–95% of planted tree seedlings survived over three growing seasons, but that the average height of seedlings increased. Statistical significance was not assessed. Four tree species were planted. After three growing seasons, survival rates were 0% for laurel oak Quercus laurifolia, 73–90% for water hickory Carya aquatica, 78–90% for overcup oak Quercus lyrata and 70–95% for baldcypress Taxodium distichum. When planted, seedlings of the last three species were 47–85 cm tall on average. After three growing seasons, survivors were 104–192 cm tall on average. The study also reported that survival and height change varied with elevation/wetness for all species, and with site conditions (presence of tree canopy or grasses) for baldcypress (see Action: Remove vegetation that could compete with planted vegetation and original paper). Methods: Fifteen 180-m2 plots were established in a degraded swamp (where heated effluent had existing trees between 1955 and 1985). Five plots contained black willow Salix nigra, five were cleared of willow and five were dominated by grasses. In February 1994, four hundred and eighty seedlings (120 seedlings/species) were planted, 2 m apart, into the 15 plots (8 seedlings/species/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 (9), but a different experimental set-up.

    Study and other actions tested
  11. A replicated, site comparison study in 2000 of 11 freshwater swamps in Virginia, USA (Snell-Rood & Cristol 2003) found that created swamps – planted with trees/shrubs after reprofiling – 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 planting a mix of wetland trees/shrubs after reprofiling upland sites to increase soil moisture (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
  12. A replicated study in 2002–2004 in three freshwater wetlands in Wisconsin, USA (Hovick & Reinartz 2007) reported variable survival of 23 planted tree/shrub species after 1–2 growing seasons, depending on numerous factors. Overall, survival rates ranged from 0% after one growing season to 100% after two growing seasons. Survival rates depended on the combination of species, site, time after planting, plot elevation/wetness, and whether/how invasive reed canarygrass Phalaris arundinacea was removed before planting. After two growing seasons, three species had 100% survival under one treatment: blackcurrant Ribes americanum, elderberry Sambucus canadensis and highbush cranberry Viburnum opulus var. americanum, planted where canarygrass had been sprayed with herbicide and the soil had been ploughed. Twelve species failed to survive under at least one treatment. Yellow birch Betula alleganiensis failed to survive under any treatment. Methods: In spring 2003 or 2004, seedlings of 11 tree and 12 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 treatments not clear). Removal treatments involved spraying with herbicide, herbicide then ploughing, herbicide then burning, or mowing then herbicide. Survival of all seedlings was monitored in September 2003 and 2004.

    Study and other actions tested
  13. A replicated, randomized, controlled, before-and-after study in 2000–2005 in 16 ephemeral freshwater wetlands undergoing restoration in South Carolina, USA (De Steven et al. 2010) found that sparsely planting wetland tree seedlings had no significant effect on plant species richness or cover. Over four years after planting, there was no significant difference in any measured vegetation metric between planted and unplanted wetlands. Metrics included: total plant species richness; total vegetation cover; proportion of wetland-characteristic, herbaceous and woody plant species; and relative cover of wetland-characteristic, herbaceous and woody plants. The study does not report data for planted and unplanted wetlands separately (see Action: Restore/create marshes or swamps (multiple interventions) for combined data). Methods: In 2001, baldcypress Taxodium distichum and swamp tupelo Nyssa biflora seedlings were planted (≥5 m apart) into eight depressional wetlands. Eight nearby wetlands were not planted. Earlier that year, all 16 wetlands received the following interventions: plugging drainage ditches, cutting and removing non-wetland trees, and treating tree regrowth with herbicide. Vegetation was surveyed before (2000) and for four years after (2001–2005) planting, in 0.1-ha plots (3–5/wetland) and 4-m2 quadrats (8–12/wetland).

    Study and other actions tested
  14. A replicated study in 2014–2015 in two degraded floodplain swamps in Victoria, Australia (Greet et al. 2016) reported 0100% survival of planted swamp gum Eucalyptus camphora seedlings over one year, largely depending on whether herbivores were excluded or not. In plots fenced to exclude browsing and grazing mammals, 98–100% of seedlings survived. In unfenced plots, only 0–4% of seedlings survived. Methods: In March 2014, swamp gum seedlings were planted into eighteen 100-m2 plots across two floodplain wetlands (50 seedlings/plot). In each wetland, eight plots had been fenced and one was left open. All plots had been recently cut and sprayed with herbicide (to control reed canarygrass Phalaris arundinacea or common reed Phragmites australis), and planted with native shrubs and herbs along with swamp gum. Some fenced plots were also covered with matting or woodchips. Seedling survival was monitored in March 2015.

    Study and other actions tested
  15. A replicated study in 2008–2013 in two created freshwater swamps in Michigan, USA (Kangas et al. 2016) reported 0–94% survival of planted white cedar Thuja occidentalis seedlings after five years, and a change in average height of −2 cm/year to +39 cm/year between two and five years after planting. These results depended on seedling elevation, site and whether plots were fenced to exclude white-tailed deer Odocoileus virginianus. For example, the highest survival rate (94%) was for seedlings planted on mounds in the drier site, and within deer-exclusion fencing. In the wetter site, ≤1% of seedlings survived when planted in lower flats, whether or not they were protected from deer browsing. For full details, see Actions: Create mounds or hollows (before planting) and Use fences or barriers to protect planted areas. Methods: In spring 2008, one-year-old white cedar seedlings were planted into 37 plots (of varying size) on two recently excavated wetlands. Each plot was planted with 5–106 seedlings, approximately 2.8 m apart. There were 2–6 plots/site for each of two elevation treatments (mounded/never flooded or flat/sometimes flooded) and two fencing treatments (fenced or open). Surviving trees were monitored in April 2010 and October 2013.

    Study and other actions tested
  16. A study in 2002–2005 aiming to restore a forested wetland in Arkansas, USA (Sleeper & Ficklin 2016) reported 50% survival of planted tree seedlings after three years. An average of 377 trees/ha were still alive in 2005, compared to the 748 trees/ha planted in 2002. Methods: In 2002, bare-root tree seedlings were planted into flats (seasonally wet areas, intermediate in elevation between created mounds and hollows) on a floodplain wetland. The site been used for agriculture since the 1960s. Wetland restoration activities (details not reported) began in 2001. The species planted were baldcypress Taxodium distichum, water oak Quercus nigra, overcup oak Quercus lyrata, Nuttall’s oak Quercus texana, and green ash Fraxinus pennsylvanica (data reported for all species combined).

    Study and other actions tested
  17. A study in 2013–2015 in an ephemeral freshwater marsh in Florida, USA (Dreschel et al. 2017) reported that 89–100% of planted tree saplings survived over two years, and that survivors typically grew. Statistical significance was not assessed. Two years after planting in floating peat bags, survival rates were 89% for strangler fig Ficus aurea saplings, 97% for red maple Acer rubrum saplings and 100% for pond apple Annona glabra saplings. Average growth rates were positive in 27 of 28 reported cases (height: 0.2–1.4 mm/day; diameter: 0.01–0.06 mm/day; variation depending on species and planting method). In the other case, the average growth rate of red maple planted in unfertilized, upright peat bags was −0.01 mm/day. Methods: In October 2013, thirty-five nursery-reared saplings/species were planted into peat bags (punctured with multiple holes; 1–2 saplings/bag). Fertilizer or additional floatation aids were added to some bags. The planted bags were then floated on the marsh, flat or upright. All saplings were measured at planting. Survivors were recorded and measured for up to two years.

    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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