Directly plant trees/shrubs: brackish/saline wetlands

How is the evidence assessed?
  • Effectiveness
    65%
  • Certainty
    65%
  • Harms
    5%

Study locations

Key messages

  • Forty-seven studies evaluated the effects, on vegetation, of directly planting trees/shrubs in brackish/saline wetlands. Forty-four studies involved planting mangroves or other coastal swamp trees: 20 in Asia, seven in Central America, six in Africa, four in North America, four in South America, two in Oceania and one globally. Three studies involved planting shrubs in the USA or Spain. There was overlap in the sites used in two studies. One systematic review included several of the other summarized studies.

VEGETATION COMMUNITY

  • Overall extent (3 studies): Two before-and-after studies in India and South Africa reported that the area of mangrove forest was greater 6–42 years after planting mangrove trees (sometimes along with other interventions) than in the years before. One study in Sri Lanka simply quantified the area of mangrove vegetation present 8–10 years after planting seedlings (and propagules).
  • Tree/shrub richness/diversity (6 studies): Three site comparison studies in the USA, Mexico and Brazil reported that where mangrove forests developed after planting trees (sometimes along with other interventions), they contained a similar number of tree species to mature and/or naturally regenerating forests after 10–30 years. One site comparison study in Vietnam reported that after 14–34 years, a planted mangrove forest contained more tree species than a (slightly older) naturally regenerated forest. One replicated, paired, before-and-after, site comparison study in Kenya reported that planted mangrove forest contained fewer adult tree species than mature natural forest after five years, but more species of seedling. One study in a former shrimp pond in Thailand simply reported the number of unplanted tree species that had colonized six years after planting (along with other interventions).

VEGETATION ABUNDANCE

  • Tree/shrub abundance (9 studies): Three replicated, site comparison studies of coastal sites in the Philippines, the USA and Brazil reported that where mangrove forests developed after planting trees (sometimes along with other interventions), woody vegetation was typically more dense than in mature natural forests and/or naturally regenerating forests. Two site comparison studies in Kenya and Vietnam found that tree abundance (density and biomass) was similar in planted and natural mangroves after 5–34 years. One site comparison study in Mexico reported that a planted mangrove forest contained fewer trees than pristine natural forests after 12 years. Two site comparison studies in the Philippines reported mixed results according to time since planting and site. One study in Thailand simply quantified the abundance of mangrove trees six years after planting (along with other interventions).
  • Algae/phytoplankton abundance (1 study): One site comparison study in Kenya found that mangrove forests restored by planting contained a similar algal biomass, after eight years, to mature natural forests. However, mangrove forests created by planting into bare sediment contained less algal biomass than mature natural forests.
  • Individual species abundance (7 studies): Seven studies quantified the effect of this action on the abundance of individual plant species. Four of the studies compared the abundance of woody vegetation or algae in planted mangrove forests and mature natural forests – and sometimes naturally regenerating forests (see original papers for data). One replicated, paired, controlled study in a brackish wetland in the USA reported that there were fewer common reed Phragmites australis stems in plots planted with wetland shrubs (and herbs) than in unplanted plots, after 1–3 years. One before-and-after study of an intertidal site in the USA reported greater abundance of red mangrove Rhizophora mangle over five years after planting (along with other interventions) than before.

VEGETATION STRUCTURE

  • Overall structure (3 studies): Three replicated, site comparison studies of coastal sites in the Kenya, the USA and the Philippines reported that where mangrove forests developed after planting trees (sometimes along with other interventions), their overall structure differed from mature natural forests for up to 50 years.
  • Height (18 studies): Four site comparison studies (three replicated, three paired) of coastal sites in Kenya, the USA, Brazil and the Philippines reported that where mangrove forests developed after planting trees (sometimes along with other interventions), the vegetation was shorter than in mature and naturally regenerating forests after 5–30 years. One site comparison study in Mexico reported that planted mangrove forests contained taller trees than pristine natural forests after 12 years. Fourteen studies (four replicated) in Asia, Central/South America, Africa and North America simply quantified the height of mangrove trees for up to six years after they were planted; in 13 of these studies, the average height increased over time.
  • Diameter (7 studies): Two site comparison studies in Mexio and Vietnam reported that tree diameters were similar in planted and natural mangroves after 12–34 years. In contrast, two site comparison studies in Brazil and the Philippines reported that planted mangroves contained thinner tree stems than mature natural mangroves after 7–12 years. The study in Brazil also reported that stem diameters were thinner than in naturally regenerating areas. Three studies in India and Nigeria simply quantified the diameter of mangrove trees for up to three years after they were planted; in all three studies, the average stem diameter increased over time.
  • Basal area (3 studies): Two site comparison studies (one also replicated, paired, before-and-after) in Kenya and Mexico reported that planted mangrove forests had a smaller basal area than mature natural forests after 5–12 years. One replicated, site comparison study in the USA reported that where mangrove forests developed after planting trees (along with other interventions), their basal area was similar to mature natural forests after 17–30 years.

OTHER

  • Survival (37 studies): Thirty-six studies (including one review and one systematic review) quantified survival rates of individual trees/shrubs planted in brackish/saline wetlands. Survival rates ranged from 0% to 100% after 15 days to 21 years. The studies were of mangroves in North America, Central/South AmericaAsia, Africa, Oceania or globally, and of shrubs in the USA or Spain. Six studies reported 100% survival in some cases. Eleven studies reported 0% survival or absence of planted species in some cases. In six studies, survival of planted seedlings was not distinguished from survival of seeds or propagules. Proposed factors affecting survival included elevation/water levels, exposure to wind/waves, soil properties, sediment deposition, oyster/barnacle colonization, salinity, use of guidance and post-planting care.
  • Growth (9 studies): Nine studies monitored true growth of individual trees/shrubs (rather than changes in average height of survivors). The nine studies, in Colombia, the USA, the Philippines, Brazil and China, reported that planted trees/shrubs typically grew, over periods from 40 days to 50 years. One replicated study in the USA reported that planted seedlings grew less quickly than naturally colonizing seedlings. One replicated, site comparison study in the Philippines found that growth rates of trees in planted mangroves became more similar to those in mature natural mangroves over time.

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 1977 review of mangrove plantings in Florida, USA (Teas 1977) reported 0–100% survival of planted seedlings or trees over six months to 32 years. Experiments yielding high survival rates included: planting seedlings in sheltered coastal sites (85–90% survival after 1–4 years); planting >4-year-old trees, with roots wrapped in burlap, at or above mid-tide level (80–100% survival after 13 months); and planting trees, each 0.3–3.6 m tall, alongside sheltered canals (100% survival after six months). Experiments yielding low survival rates included: planting seedlings in exposed east-coast sites (0–2% survival after 7–10 months); planting >4-year-old trees below mid-tide level (0% survival after 13 months; insect damage noted); transplanting fourteen trees, each 4.6–6.1 m tall (0% survival after six months); and planting young seedlings in the Dry Tortugas Islands (80% survival after one year but 0% survival after 32 years). Methods: The review reported results from several experiments planting mangrove seedlings or trees under a range of conditions. Most experiments involved planting red mangrove Rhizophora mangle; some also included black mangrove Avicennia germinans and white mangrove Laguncularia racemosa. Between 14 and 60,000 plants were planted in each experiment. Some were nursery-reared and some were transplanted from wild populations.

    Study and other actions tested
  2. A replicated study in 1984–1985 on chalky coastal sediments around three islands in Colombia (Bóhorquez & Prada 1988) reported that only 20% of transplanted red mangrove Rhizophora mangle seedlings survived over 247 days, but that survivors grew. Statistical significance was not assessed. Overall, 26 of 130 planted seedlings were still in place and alive after 247 days. Seedling survival was 0% and 5% on the two most exposed islands, but 35% on the least exposed island. Seedling survival was 0% at the highest elevations, 60% in moderate elevations and 100% at the lowest elevations. On average, surviving seedlings grew 6 cm taller and four new leaves at moderate elevations, but grew 32 cm taller and six new leaves at the lowest elevations. Methods: In November 1984, a total of 130 red mangrove seedlings (<70 cm tall) were transplanted to three islands, at three different elevations (“high beach”, “intertidal” and “low beach”). The study does not report the number of seedlings on each island or at each elevation, and does not quantify elevation. Seedlings were spaced at 9/m2, watered every two weeks with fresh water, and cleaned of dust and debris. Seedlings in “poor condition” were removed. Seedling survival, height and leaf number were monitored until July 1985.

    Study and other actions tested
  3. A study in a marshy, estuarine site in northeast India (Das et al. 1997) reported 25–83% survival of planted mangrove saplings after one year, and that the average size of saplings typically increased over two years. Statistical significance was not assessed. Ten species were planted. One year after planting, mangrove apple Sonneratia apetala had the highest survival rate (76–83%) and Ceriopsis decandra the lowest (25–33%). Between one and two years after planting, the average size of surviving trees typically increased: height in 20 of 20 cases, trunk diameter in 16 of 20 cases, number of branches in 15 of 20 cases and canopy diameter in 14 of 20 cases (see original paper for data). Nine of 10 species had higher survival in mixed plantations than monocultures, but size metrics were more likely to increase over time in monocultures (40 of 40 cases) than in mixed plantations (25 of 40 cases). Methods: At an unspecified time, 1-year-old mangrove saplings (reared in a nursery from cuttings) were planted 2 m apart over 10-ha degraded salt marsh within the Mahanadi Delta. Ten species were planted in single-species or mixed-species stands (further details of layout not reported). Survival was monitored after one year. Surviving saplings were measured after one and two years.

    Study and other actions tested
  4. A replicated study in 1995–1997 in two degraded mangroves in Colombia (Elster 2000) reported that 0–100% of planted trees survived over 15 months – depending on species, age and environmental conditions – but that survivors grew. For example, white mangrove Laguncularia racemosa seedlings had significantly higher survival rates (0–90%) than black mangrove Avicennia germinans seedlings (0–11%). For these species, seedlings had lower survival rates (0–90%) than saplings (20–100%; statistical significance not assessed). Surviving plants grew over 13 months (see original paper). The study suggests that variation in survival and growth was related to dust, winds, soil moisture, soil firmness and/or caterpillar damage. Methods: In 1995 (start of the dry season), seedlings and/or saplings of three mangrove species were planted into two degraded mangrove sites. In both sites, channels had been unblocked (in 1989 or earlier in 1995) to restore freshwater inputs and reduce the salinity that killed the existing mangrove trees (around 1965 or 1975). Sets of 10–30 trees were planted in a range of soil conditions (1–2 sets/species/site; see original paper for full details). Survival and plant height were monitored at planting and for up to 15 months.

    Study and other actions tested
  5. A replicated study in 1992–1997 of two mangrove plantations in Kuwait (Abo El-Nil 2001) reported that most grey mangrove Avicennia marina seedlings established when planted below average high tide level, and that their average height and stem number increased over time. Statistical significance was not assessed. No seedlings survived when planted above the average high tide level. Of the seedlings planted at or below average high tide level, 85–92% survived for at least one year. When planted, seedlings had 1 main stem and were 33–63 cm tall on average. After five years, surviving seedlings had 3–8 main stems and were 128–288 cm tall on average. Flowering, fruiting and seeding were also observed. Methods: In June 1992 or 1994, mangrove seedlings (number not reported) were planted into two mudflats. At each site, five rows were planted at varying tidal heights. The seedlings were grown in a greenhouse from wild seeds collected the previous year (from two separate mangroves). Measurements were taken at planting and for at least five years afterwards.

    Study and other actions tested
  6. A study in 1994–1998 on a mudflat in northwest Mexico (Toledo et al. 2001) reported that 74% of planted black mangrove Avicennia germinans seedlings survived for two years, and that the average height of seedlings increased over time. After six months, surviving seedlings were 13 cm tall on average. After two years, surviving seedlings were 62 cm tall on average. Statistical significance was not assessed. Methods: In December 1994, nursery-grown black mangrove seedlings were planted into an intertidal mudflat (where the previous mangrove forest had been cut down three years earlier). A total of 555 seedlings were planted, in 111 clusters of five. Clusters were 1 m apart and at least 60 cm from naturally colonizing trees. The plastic bag containing each cluster was slit to allow the roots to grow. Seedling survival and height were monitored for two years. This site was also studied in (32).

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  7. A study in 2000–2001 in a lagoon in southern Mexico (Reyes Chargoy & Tovilla Hernández 2002) reported 95% survival of planted red mangrove Rhizophora mangle seedlings after seven months, and that the average height of surviving seedlings increased. Statistical significance was not assessed. When planted, seedlings were 32 cm tall on average. Seven months later, surviving seedlings were 72 cm tall on average. Methods: In late 2000, a total of 550 nursery-reared red mangrove seedlings (90 days old) were planted at the edge of Pozuelos lagoon (elevation not reported). This site was flooded by two tides/day throughout the year. Surviving seedlings were surveyed for up to seven months after planting.

    Study and other actions tested
  8. A replicated, paired, before-and-after, site comparison study in 1994–1999 involving three areas of planted mangroves in southeast Kenya (Bosire et al. 2003) reported that the planted areas had a similar density of trees to mature natural forests after five years, but contained fewer adult tree species and differed in other structural metrics. Unless specified, statistical significance was not assessed. The three planted areas were initially bare sediment. After five years, they contained 3,330–7,640 trees/ha (vs natural: 3,770–4,300 trees/ha; see original paper for on individual species density). Each planted area contained only one species of adult tree (i.e. the planted species), whereas natural areas contained 1–4 species of adult tree (but were always dominated by a single species, comprising 69–100% of individuals). Planted areas contained 4–5 species of seedling (vs natural: only 3 species). Vegetation in planted areas was less structurally complex than in natural areas (reported as a complexity index), was only 3–5 m tall on average (vs natural: 6–8 m) and had a basal area of only 3–12 m2/ha (vs natural: 27–42 m2/ha). In two of three comparisons, planted areas contained significantly fewer seedlings than natural areas (but more in the other comparison). After five years, denuded areas that were not planted remained unvegetated. Methods: In 1994, mangrove saplings were planted into three areas (0.3–6.7 ha) of bare, tidal sediment (historically logged mangrove forest). Each area was planted with one species: grey mangrove Avicennia marina, mangrove apple Sonneratia alba or loop-root mangrove Rhizophora mucronata. In 1999, vegetation was surveyed in the planted areas (three 100-m2 plots/area). For each planted area, an area of natural forest and denuded but unplanted sediment were also surveyed.

    Study and other actions tested
  9. A replicated study in 2000 in a salt marsh in Louisiana, USA (Egerova et al. 2003) reported 11–45% survival of planted groundsel Baccharis halimifolia seedlings after four months, but found that surviving seedlings grew. Four months after planting, 11% of groundsel seedlings planted into bare sediment were still alive. These seedlings were 49 cm tall on average, and had grown 4.4 cm taller since planting. For groundsel seedlings planted within patches of smooth cordgrass Spartina alterniflora, the survival rate was 45%. These seedlings were 68 cm tall on average, and had grown 7.2 cm taller since planting. Survival, final height and growth rate were all significantly greater for seedlings planted within cordgrass patches than bare sediment. Methods: In May–June 2000, a total of 160 groundsel seedlings were planted into 20 plots in the high intertidal zone of a salt marsh (constructed four years previously). The groundsel seedlings were 15–55 cm tall, transplanted from another area in the marsh, and planted approximately 25 cm apart within each plot. Ten plots were in the centre of smooth cordgrass patches (where most cordgrass stems were dead). Ten plots were on adjacent bare or sparsely vegetated sediment. Groundsel survival and height were monitored for up to four months after planting.

    Study and other actions tested
  10. A before-and-after study in 1986–2002 of a coastal wetland in southern India (Selvam et al. 2003) reported that after excavating channels to restore tidal exchange and planting mangrove seedlings, the area of mangrove forest increased. Before intervention, the site contained only 325 ha of mangrove forest (all mature) and 375 ha of degraded mangrove. Approximately six years after intervention began, the site contained 618 ha of mangrove forest (411 ha mature; 297 ha developing) and only 65 ha of degraded mangrove. Methods: Large scale restoration of a degraded mangrove forest began in 1996. Tidal exchange was restored to subsided, stagnant areas by excavating tidal channels. Then, mangrove seedlings were planted (details not reported). The study does not distinguish between the effects, on naturally colonizing vegetation, of planting and restoring tidal exchange. The local community was engaged in restoration and long-term management of the mangroves (e.g. de-silting tidal channels). The area covered by mangrove vegetation was measured from satellite images, and verified with field surveys, before intervention (1982) and approximately six years after it began (2002).

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  11. A replicated study in the early 2000s on five coastal mudflats in Kuwait (Bhat et al. 2004) reported 16–81% survival of planted grey mangrove Avicennia marina seedlings after nine months, and that the number of branches/seedling typically increased over time but their height typically did not. Statistical significance was not assessed. On average, surviving seedlings had 1–2 branches three months after planting, then 3–7 branches nine months after planting. When planted, the average height of seedlings was 20–25 cm. After nine months, the average height of surviving seedlings was 19–27 cm in four of five sites (46–47 cm in the other site). The study suggests that survival and growth were affected by physical factors such as soil texture, salinity, elevation and the presence of algae. Methods: Grey mangrove seedlings were planted in five tidal, coastal mudflats (1,500–2,000 seedlings/site, 1 m apart). The seedlings had been reared in a nursery from propagules collected in the United Arab Emirates and acclimatized to local high salinities before planting. Surviving seedlings were recorded and measured for up to nine months after planting.

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  12. A study in 1998–2003 in the United Arab Emirates (Tamaei 2005) reported that 57% of planted grey mangrove Avicennia marina seedlings survived for five years, and that the average size of seedlings increased over time. Seedling mortality occurred in patches. The study suggests the following causes: erosion at the water’s edge, burial with sand from a collapsed road, sandstorms, insect herbivory, and weak root systems unable to support the seedlings. After five years, surviving seedlings were 48 cm tall and had a stem diameter of 82 mm. When planted, seedlings were 27 cm tall and had a stem diameter of 48 mm. Statistical significance was not assessed. Methods: In March–May 1998, grey mangrove seedlings were planted (2 seedlings/m2, 40–50 cm above low tide level) around the edge of an excavated, oval, tidal canal. The 79,580 planted seedlings had been reared in a nearby nursery for six months. Survival (all seedlings) and size (100 seedlings) were monitored in April 2003.

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  13. A site comparison study in 2002 of three mangrove forests in southeast Kenya (Crona et al. 2006) reported that planting non-native mangrove apple Sonneratia alba into degraded forest generally restored habitat structure, algal richness and algal biomass to near natural levels, but replanting clear-cut forest did not. Unless specified, statistical significance was not assessed. After eight years, sites where mangrove apple had been planted into degraded forest did not clearly differ from natural forests in terms of canopy cover (planted: 50–75%; natural: 50–75%), the basal area of aerial roots (planted: 0.4–0.6 m2 roots/m2 forest; natural: 0.3–0.6 m2 roots/m2 forest) and algal richness (planted: 23 taxa/5 m2; natural: 18 taxa/5 m2), and did not significantly differ in terms of algal biomass (planted; 962–4,519 g/m2; natural: 681–2,963 g/m2). In contrast, sites where mangrove apple had been planted after clear-cutting had 100% canopy cover, only 0.2 m2 of aerial roots/m2 forest, only 10 algal species and only 5–167 g/m2 of algal biomass. Both types of planted mangroves contained more aerial roots (degraded: 322–424/m2; clear-cut: 380–400/m2) than natural mangroves (174–280/m2). For data on the biomass of individual algal species, see original paper. Methods: In early 2002, three mangrove forests were surveyed: two planted with mangrove apple trees in 1994 (amongst remnant forest, or in a site clear-cut in the 1970s) and one natural (mature). Twenty 0.25-m2 quadrats were surveyed in each mangrove. Aerial roots were counted and measured. Algae were identified, collected, dried and weighed.

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  14. A replicated study in 2005 on a mudflat in Florida, USA (Milbrandt & Tinsley 2006) reported that only 6–34% of planted black mangrove Avicennia germinans seedlings survived over seven weeks, and that planted seedlings grew less quickly than naturally colonizing seedlings. Statistical significance was not assessed. Survival was 34% for seedlings planted into established stands of saltwort Batis maritima, 11% for seedlings planted into bare mudflat and 6% for seedlings planted into freshly created saltwort stands. In established saltwort stands, planted seedlings grew 20 mm/week, compared to 50 mm/week for naturally colonizing seedlings. Methods: In June 2005, fifty-four nursery-reared black mangrove seedlings (43 cm tall) were planted into a mudflat where mangrove forest had died off. This area was lower than an adjacent area with healthy forest. Eighteen seedlings were planted in each habitat type: established saltwort stands, saltwort stands planted <5 days earlier, and bare mudflat. Survival and height were measured after seven weeks. The initial and final heights of 36 naturally colonizing seedlings were also recorded.

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  15. A replicated, paired, controlled study in 2000–2004 in a degraded brackish marsh in New Jersey, USA (Wang et al. 2006) reported 38–100% survival of three planted shrub species over two years, and that survivors grew in six of seven cases. Statistical significance was not assessed. Survival rates were 38–73% for southern wax myrtle Myrica cerifera, 92–100% for sea myrtle Baccharis halimifolia and 100% for Jesuit’s bark Iva frutescens. In six of seven cases, surviving plants grew in height (8–252% increase) and circumference (9–233% increase). In the other case, southern wax myrtle grew in height by <1% and shrunk in circumference by 3%. The study also reported that areas planted with the herbs (and some shrubs) contained fewer common reed stems (7–25 stems/m2) than adjacent unplanted areas (66–149 stems/m2). Methods: In summer–autumn 2000–2002, three shrub and five herb species were planted in three areas on the edge of a marsh (4–7 species/area; 4–48 plants/species/area; individual plants 60–100 cm apart). All planted shrubs had been collected from local marshes. Invasive common reed Phragmites australis had been cleared <1 year before planting, by applying herbicide and cutting. Plant survival and size were recorded 1–2 years after planting. Common reed stems were counted in the planted areas and three adjacent unplanted areas, 2–4 years after reed clearance.

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  16. A replicated study in 2004–2005 in a coastal brackish/saline marsh in Victoria, Australia (Raulings et al. 2007) reported that 0–93% of planted swamp paperbark Melaleuca ericifolia seedlings survived over 5–8 months. When planted into mounds, 93% of seedlings survived over five months. When not planted into mounds, 0–12% of seedlings survived over 5–8 months. Amongst these, the survival rate was higher for older seedlings and seedlings planted in drier areas, but was not affected by planting method (dug or cored planting holes; see original paper for data). Seedling height was also reported, but is difficult to interpret owing to the high mortality. Methods: In March and November 2004, a total of 890 swamp paperbark seedlings were planted into 35 plots in a brackish/saline marsh. Seedlings had been grown in a nursery for 4–6 months. Plots varied in elevation: they were at different heights relative to the shoreline, or were pairs of mounds and hollows. However, all plots experienced extreme water levels during the study (some seedlings submerged, some with no standing water). Survival and seedling height were monitored for 5–8 months from planting.

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  17. A study in 2007–2008 in a degraded mangrove forest in Cuba (Baigorría Montero et al. 2008) reported 100% survival of planted black mangrove Avicennia germinans seedlings after 15 days, and that the average size of surviving seedlings increased over two months. All 125 surveyed seedlings were alive 15 days after planting. Seedlings planted amongst saltwort Batis maritima were 5 cm tall 15 days after planting, 10 cm tall after one month, and 21 cm tall after two months (with 3 branches and 6 leaves/plant). Seedlings planted into bare sediment were only 18 cm tall after two months (with <1 branch and 3 leaves/plant). Methods: In November 2007, five thousand nursery-reared black mangrove seedlings were planted in a degraded mangrove forest (damaged by storms and sediment deposition in 2002–2004). Seedlings were planted 1.5–2.0 m apart and 15–20 cm deep. Some seedlings were planted within patches of saltwort, and some into bare sediment. Seedlings were monitored until January 2008, but survival rates beyond 15 days were not clearly reported.

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  18. A replicated study in 2003–2005 in two historically logged mangrove areas in southeast Kenya (Kirui et al. 2008) reported that 29–87% of planted mangrove saplings survived over 13–25 months. Four species were planted. In one area, 35–55% of planted mangrove apple Sonneratia alba saplings survived for 25 months. In another area, the survival rate after 13 months was 29% for large-leafed mangrove Bruguiera gymnorhiza, 71% for spurred mangrove Ceriops tagal and 87% for grey mangrove Avicennia marina. For these species, survival was negatively related to salinity and positively related to height above the shoreline, but was not significantly affected by the number of species planted within plots or whether saplings were at the edge or middle of plots. Methods: Two historically clear-felled areas within Gazi Bay were planted with nursery-raised saplings. In July 2003, mangrove apple was planted in one area flooded by tides every day (697 saplings, mostly 4–5 months old, 0.5–1m apart). In August 2004, the other three species were planted in another area flooded only during spring high tides (3,390 saplings, 6 months old, 0.6 m apart, in 32 single or mixed-species plots). Saplings that died within one month were replaced. Survival was then recorded after 25 months (mangrove apple) or 13 months (other species).

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  19. A study in 2006–2008 in the Philippines (Primavera & Esteban 2008) reported approximately 50% survival of planted grey mangrove Avicennia marina seedlings after six months, but <10% survival after 18 months. The study suggests mortality was mainly due to frequent tidal flooding, with most surviving plants located at the highest elevations. Other contributing factors were garbage, trampling by fishers, and people digging in the sediment. Methods: In 2006, >400 nursery-reared grey mangrove seedlings were planted at various elevations along the banks of the Iloilo River (further details not reported). Survival was monitored over 18 months.

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  20. A study in 2006 in the Philippines (Primavera & Esteban 2008) reported that all planted mangrove seedlings died within three months. The study suggests mortality was mainly due to prolonged flooding, evidenced by rotting stems. Seedlings were also damaged by barnacles, algae and sediment deposition. Methods: Approximately 20,000 mangrove seedlings were planted in the lower intertidal to subtidal zone of a coastal site at Dumangas. The seedlings were mostly (90%) nursery-reared grey mangrove Avicennia marina. The other 10% included mangrove apple Sonneratia alba and Rhizophora spp. Survival was monitored over three months.

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  21. A study of mangrove planting projects in the Philippines (Samson & Rollon 2008) reported <5% survival of planted mangrove seedlings/propagules, but growth of surviving seedlings. Plantings almost exclusively involved Rhizophora spp. In two sites where survival was quantified, <5% of planted individuals survived (over nine months in one site; timescale not reported for other site). The study suggests that seedlings were killed by mechanical stress, substrate erosion, and oysters growing on their stems. Growth of surviving seedlings was quantified in eight sites. “Young individuals” grew by 3–13 cm over approximately 40 days (equivalent to 30–75 cm/year). Growth rates significantly differed between elevations: lowest in the low intertidal zone, and highest in the upper intertidal zone. Methods: The study reported results from various mangrove planting projects initiated since the 1980s: both afforestation (planting in mudflats, sandflats or seagrass beds) and reforestation (re-planting cleared mangroves, mostly fishponds). Seedlings and/or propagules were generally planted 1 m apart, following national guidelines, but often with 2–5 individuals at each planting spot.

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  22. A replicated, paired, site comparison study of six coastal sites in the Philippines (Samson & Rollon 2008) reported that planted mangrove forests typically contained a higher density of trees and greater canopy cover than natural mangrove forests. Statistical significance was not assessed. After “several years”, planted forests contained a greater density of trees than natural forests in 9 of 10 comparisons (for which planted: 27–93 trees/100 m2; natural: 22–42 trees/100 m2). Planted forests had greater canopy cover than natural forests in 5 of 9 comparisons (data reported as a canopy index; other comparisons lower in planted forests). Methods: The study surveyed planted and natural mangrove forests at six sites (1–22 plots/forest type/site; dates not reported). Plantings had taken place since the 1980s (precise dates not reported) and almost exclusively involved Rhizophora spp. seedlings and/or propagules. These were generally planted 1 m apart, following national guidelines, but often with 2–5 individuals at each planting spot. Some plantings involved afforestation (planting in mudflats, sandflats or seagrass beds) and some involved reforestation (re-planting cleared mangroves, mostly fishponds).

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  23. A replicated, site comparison study in 2005 in Florida, USA (Shafer & Roberts 2008) reported that 12 of 17 sites planted with mangroves (along with other interventions) contained mangrove forests after 17–30 years – but that these differed from mature natural forests in overall complexity, tree density and canopy height. Statistical significance was not assessed. After 17–30 years, mangrove forests had developed in 12 of the 17 sites. Mangrove forests had not persisted in four sites and been deliberately removed from one. Nine of the sites that developed forests were surveyed in detail. The created/restored forests had a different overall structure to natural forests (data reported as a complexity index and graphical analysis). Created/restored forests contained 16,925 trees/ha (vs natural: only 6,594 trees/ha) and had a canopy height of only 4.0 m (vs natural: 6.4 m). Both created/restored and natural forests had an average basal area of 31 m2/ha, and contained 1–3 tree species. Methods: In 2005, vegetation was surveyed in 17 sites (three 2 x 2 m plots/site). All of these sites had been planted with red mangrove Rhizophora mangle between 1975 and 1987 (either seedlings or propagules; precise numbers not reported). Some sites had also been planted with smooth cordgrass Spartina alterniflora. All but one site was planted after levelling upland areas. The study does not distinguish between the effects, on unplanted trees, of planting mangroves, planting cordgrass and reprofiling. Comparisons were made with previously published data from seven nearby natural forests.

    Study and other actions tested
  24. A replicated study in 2006–2009 of 47 mangrove restoration projects in Sumatra, Indonesia (Wibisono & Sualia 2008) reported 0–99% survival of planted seedlings/propagules after <15 months. Some planted individuals survived in 45 of the 47 projects. Survival rates ranged from 17% to 99% per project. The study suggests that survival was influenced by factors such as elevation, sediment deposition, flash floods, grazing by crabs, smothering by algae, soaking propagules before planting, and prior planting experience of communities (effects not quantified). Methods: Between February 2006 and September 2008, approximately 1.6 million mangrove seedlings and/or propagules were planted across 47 projects (mostly in separate sites). The study does not distinguish between the effects of planting seedlings and propagules. Eight species were planted (mostly Rhizophora spp.) on mudflats, in degraded mangroves, in former aquaculture ponds, and along water channels. Individuals were generally planted 0.3–1.0 m apart, but sometimes with double plantings at a single point. At some time within 15 months of planting (not clearly reported), survival rates were checked for 20% of the planted individuals in each project.

    Study and other actions tested
  25. A replicated, before-and-after study in 2006–2008 on estuarine mudflats in southern Spain (Castillo & Figueroa 2009) reported 90% survival of sea purslane Atriplex portulacoides, one year after planting. Methods: Between November 2006 and January 2007, nursery-reared sea purslane was planted around the edge of some polluted, unvegetated, tidal mudflats in the Odiel Estuary (number of plants and sites not reported). The main area of each site was planted with clumps of herbaceous plants. Survival was monitored one year after planting.

    Study and other actions tested
  26. A replicated study in 2008 in two coastal sites in the Cayman Islands (Krumholz & Jadot 2009) reported 48–84% survival of planted red mangrove Rhizophora mangle saplings after 10 months, and that the average height of surviving trees increased. After five months, 94% of the planted saplings were still alive in both sites. After 10 months (including hurricane season), survival rates had dropped to 84% in the sheltered site and 48% in the exposed site. The average height of surviving seedlings was similar in both sites: 39 cm when planted, 42 cm after five months, and 51–52 cm after 10 months (statistical significance not assessed). Methods: In early 2008, approximately 400 containers of 2–3 red mangrove saplings were transplanted into shallow water across two coastal sites. The containers were specially designed concrete pots: 25 cm tall, 40–45 cm diameter, 16 kg when empty, holes in the sides to allow water exchange and the bottom to allow root growth. Saplings had been grown in the containers in a nursery for 15 months. Sapling survival and height (tallest sapling in each container) were monitored in January, June and December 2008.

    Study and other actions tested
  27. A site comparison study in 2009 on the coast of Peninsular Malaysia (Affandi et al. 2010) reported that only 7% of planted Avicennia alba seedlings survived for four months, but that survivors had grown in height by 2.5 mm/cm. For comparison, seedlings growing naturally in a nearby established mangrove had 92% survival over four months, and survivors had grown in height by 1.5 mm/cm. Methods: In April 2009, Avicennia alba seedlings were planted on a bare intertidal area with clay/loam soils. The 314 seedlings had been grown in coconut-fibre logs in a nursery for six months (5 seedlings/3 m log). Then, the coir logs were placed directly onto the intertidal area. A breakwater had been built to shelter the seedlings from waves, but it had the unintended effect of encouraging sediment deposition around the seedlings. Seedling survival and growth (relative to initial height) were monitored for four months: for the planted seedlings and 80 seedlings growing spontaneously in a nearby mangrove forest.

    Study and other actions tested
  28. A study in 2006–2009 in an aquaculture pond undergoing restoration in southern India (Anon 2010) reported 100% survival of planted mangrove saplings after 45 months. Methods: A total of 2,050 mangrove saplings were planted along embankments in an abandoned fishpond: 1,723 Rhizophora spp. saplings in one lower row, and 327 grey mangrove Avicennia marina saplings in one upper row (precise water levels not clear). Within each row, saplings were 5 m apart. Survival was recorded in November 2009. The mangroves were part of a system to allow sustainable farming of fish and salt marsh vegetation. 

    Study and other actions tested
  29. A study in 2008–2009 on a mudflat in Peninsular Malaysia (Hashim et al. 2010) reported that only 30% of planted grey mangrove Avicennia marina seedlings survived for seven months, but that the average height of seedlings increased over time. When planted, the seedlings were 41 cm tall on average. After seven months, surviving seedlings were 54 cm tall on average. Statistical significance was not assessed. The study suggests that seedlings were killed by barnacle growth, sediment deposition and disturbance from fishermen. Methods: In July 2008, coconut-fibre “logs” containing a total of 5,780 grey mangrove seedlings were transferred to an intertidal mudflat. The planting site was on an exposed shore, but situated behind a breakwater and next to an existing mangrove forest. Seedling survival and height were monitored until February 2009.

    Study and other actions tested
  30. A study in 1999–2005 in a reprofiled shrimp pond in Thailand (Matsui et al. 2010) reported 44–83% survival of planted mangrove seedlings over one year, that the average height of planted seedlings increased, and that additional seedlings colonized naturally. Over one year, survival rates were: 44% for spurred mangrove Ceriops tagal; 70% for loop-root mangrove Rhizophora mucronata; 72% for tall-stilt mangrove Rhizophora apiculata; and 83% for Bruguiera cylindrica. After six years, surviving trees were 190–430 cm tall on average (vs 23–75 cm three months after planting). Also after six years, a 300-m2 section of the pond contained 1,797 unplanted trees of 15 different species (see original paper for data on individual species abundance). Methods: In September 1999, seedlings of four mangrove species were planted in a 6,525-m2 former shrimp pond (500–800 seedlings/species, 1.5 m apart, at elevations matching their natural habitat). Three months previously, the pond had been reprofiled and tidal exchange restored by levelling the banks. The study does not distinguish between the effects, on naturally colonizing vegetation, of planting, reprofiling and restoring tidal exchange. Survival and height of 50–80 seedlings/species were recorded between three months and six years after planting.

    Study and other actions tested
  31. A study in 2002–2005 in a degraded coastal swamp in southeast Brazil (Zamith & Scarano 2010) reported that 0–93% of planted tree/shrub seedlings survived over three years, and found that survivors typically grew. Nine species were planted. For five species, most planted individuals survived over three years. Survival rates ranged from 57% for Myrcia multiflora to 93% for Tabaebuia cassinoides. For the other four species, survival rates were 2% (two species) or 0% (two species). Seedlings grew significantly larger in 54 of 69 comparisons (involving stem diameter, height or canopy cover of the first five species). Seedlings shrunk in seven of the other comparisons. The study found that survival and growth varied according to species, growth metric, initial seedling height (see original paper), addition of organic matter (see Action: Add below-ground organic matter before/after planting) and whether seedlings were planted into mounds or at ground level (see Action: Create mounds or hollows before planting). Methods: In May 2002, a total of 1,230 nursery-grown tree and shrub seedlings were planted into a degraded coastal swamp. There were 90–150 seedlings/species. Seedlings were planted 1.5 m apart, in mounds or at ground level, and with or without added manure. Invasive trees and grasses were removed from the swamp before planting. Seedling survival was monitored until May 2005. Seedling diameter, height and canopy area were measured in August 2002 and August 2005.

    Study and other actions tested
  32. A site comparison study in 2006–2007 of mangrove forests in northwest Mexico (Vovides et al. 2011) reported that a planted forest contained a similar tree community to pristine natural forests after 12 years, but contained fewer and taller trees. Unless specified, statistical significance was not assessed. The planted forest contained three tree species (planted black mangrove Avicennia germinans and two others). Pristine forests contained 2–3 species (black mangrove and 1–2 others). The overall tree density was significantly lower in the planted forest (1.4 trees/m2) than pristine forests (4.5–7.9 trees/m2). For data on the abundance of individual species, see original paper. In the planted forest, trees were 1.3–1.8 m tall on average (vs pristine: only 0.9–1.3 m), had stems 3–7 cm thick on average (vs pristine: 2–9 cm), and had an average basal area of 27 cm2/m2 (vs pristine: 48–68 cm2/m2). Methods: In 2006 or 2007, trees were counted, identified and measured in 10 plots. Two 5-m2 plots were in a replanted forest, where 111 clusters of nursery-reared black mangrove seedlings had been planted, 1 m apart, in December 1994. Eight 1-m2 plots were in pristine mangrove patches in the same lagoon. This study included the area restored in (6).

    Study and other actions tested
  33. A replicated, paired, site comparison study in 2011 in three mangrove forests in southern Brazil (Rovai et al. 2012) reported that planted areas typically contained more, thinner, shorter woody stems than natural forests (both mature and regenerating) – but a similar number of tree species. Statistical significance was not assessed. Planted areas contained 4,500–22,037 woody stems/ha, with a basal area of 4–10 m2/ha, an average diameter of 3 cm, and average height of 2–3 m. Stem density and basal area were greater than in mature forests in at least two of three sites (mature density: 512–861 stems/m2; basal area: 4–7 m2/ha). Diameter and height were less than in mature forests in three of three sites (mature diameter: 9–15 cm; height: 6–9 m). The pattern of results was similar for comparisons with naturally regenerating forests. Planted and natural forests all contained 2–3 tree species. However, in two of three sites, planted areas were dominated by white mangrove Laguncularia racemosa (90–99% of stems) whereas natural areas were co-dominated by white mangrove (36–45% of stems) and siriúba Avicennia schaueriana (54–63% of stems). For data on abundance and structure of individual species, see original paper. Methods: In 2011, trees were counted, identified and measured in three areas in each of three sites: one area planted 10–12 years previously (details not reported), one area naturally regenerating for 10 years, and one mature stand. The planted and regenerating areas had, historically, been damaged by sediment excavation or pollution from a landfill site.

    Study and other actions tested
  34. A replicated, site comparison study in 2008–2010 in 11 mangrove forests in the Philippines (Salmo et al. 2013) found that overall vegetation structure, tree density, biomass, leaf cover and growth rates in planted forests became more similar to mature natural forests over time. For example, a 6-year-old planted mangrove contained 7,780 trees/ha (vs 18-year-old: 1,358 trees/ha; natural: 1,442–1,499 trees/ha). Above-ground tree biomass was only 116 T/ha in the 18-year-old planted mangrove (vs 50-year-old: 132 T/ha; natural: 148–151 T/ha). Mangrove seedlings were only observed in the 50-year-old planted mangrove (20 seedlings/100 m2) and the natural mangrove (12 seedlings/100 m2). The overall vegetation structure in all ages of planted mangrove remained significantly different from natural mangroves (data reported as a graphical analysis). Methods: Between 2008 and 2010, vegetation was surveyed in (a) eight monospecific mangrove forests planted with loop-root mangrove Rhizophora mucronata 6–50 years ago, and (b) three natural, mature mangrove forests. All trees were counted and measured in three 3–5 m radius plots/mangrove. Above-ground biomass was estimated from diameter-mass relationships. Leaf cover was estimated from photographs.

    Study and other actions tested
  35. A replicated study in 2006–2007 on the coast of southeast Mexico (Tsuruda 2013) reported 10–60% survival of planted black mangrove Avicennia germinans seedlings over 5–11 months. In one planted area on a sandy ridge and with restored tidal flushing, survival was 60% after 11 months. In the other planted area, survival was 10–30% after five months. The conditions in this area were not clearly reported, but the sediment was probably more clayey and tidal flushing less frequent. Methods: Nursery-reared mangrove seedlings (number not reported) were planted in two coastal areas in March or September/October 2006. Survival was monitored in February 2006.

    Study and other actions tested
  36. A study in 2009–2010 on the coast of Peninsular Malaysia (Motamedi et al. 2014) reported that no more than 5% of planted mangrove seedlings survived for one year. The study suggests that seedlings were toppled by waves and tidal flows, and were probably planted in water that was too deep. Methods: In early 2009, a mixture of grey mangrove Avicennia marina and tall-stilt mangrove Rhizophora apiculata seedlings (number not reported) were planted behind a constructed breakwater, at the edge of an existing mangrove forest. The seedlings had been raised in a nursery (some in coconut-fibre logs, which were transferred to the field site) and were 20 cm tall when planted. Survival was recorded “within one year” after planting.

    Study and other actions tested
  37. A replicated study on a saltflat in western Mexico (Flores-Verdugo et al. 2015) reported that only 3–50% of planted black mangrove Avicennia germinans seedlings survived over six months, but reported that the average height of seedlings increased over this period. Seedlings were planted alongside excavated tidal channels. After six months, only 3–5% of seedlings survived when planted 1 m away from the channels. However, 40–50% of seedlings survived when planted ≤50 cm from the channels. Surviving seedlings were 7.8–11.0 cm tall on average, compared to 6.5–7.2 cm for all planted seedlings. Methods: In August–September (year not reported), 600 nursery-reared black mangrove seedlings were planted alongside four excavated channels on a bare saltflat. The channels were designed to increase tidal flushing and mitigate hypersaline conditions. Fifty seedlings were planted along the edge of each channel, 50 seedlings were planted 50 cm away and 30 seedlings were planted 100 cm away. Within rows, seedlings were 50–100 cm apart. Seedlings along one channel were shaded with black mesh (see Action: Add cover other than mulch before/after planting). Seedling survival and height were recorded for approximately six months.

    Study and other actions tested
  38. A before-and-after study in 1969–2011 in an estuary in South Africa (Hoppe-Speer et al. 2015) reported that over 42 years after planting mangrove trees, the area of mangrove vegetation increased. Before planting, there were no mangroves present in the estuary. In the year after planting (1970), mangrove forests could not be identified on aerial photographs. Forty-two years after planting (2011), mangrove forests had established and covered 1.6 ha. Although mangroves encroached into and replaced existing salt marshes, the area of salt marsh in the estuary actually increased slightly over time (1970: 2.9 ha; 2011: 3.1 ha). Salt marshes developed on newly deposited sediment. Methods: In 1969, twenty-five grey mangrove Avicennia marina trees (age unclear) were planted into salt marsh in the Nahoon Estuary. This site is 60 km south of naturally occurring mangrove forests in South Africa. “A few” mangrove trees of other species were planted “a few” years later. The area of mangrove forest and salt marsh in the estuary was determined from aerial photographs (taken 1970–2004), satellite images (taken 2004–2010) and field surveys (2011).

    Study and other actions tested
  39. A replicated study in 2012–2014 on the coast of Manus Island, Papua New Guinea (Arihafa 2016) reported that planted mangrove trees survived in 19 of 33 cases (species x site combinations). In these cases, the number of trees present was 4–102% of the number known to be planted (additional undocumented planting by local communities explains values >100%). Some planted propagules or saplings survived in seven of nine sites. All five planted species survived in at least one site. Methods: Between June 2012 and April 2014, more than 8,300 seedlings and propagules of five mangrove species were planted in nine sites around Manus Island (1–9 sites/species). The study does not distinguish between the effects of planting seedlings and propagules. The number of seedlings or propagules introduced was recorded for about half of the area planted (where local communities were guided by NGO staff) but not for the other half (where local communities planted independently). Six of the nine sites had recently contained mangrove forests, but the other three had never been forested. Seedlings originating from planting efforts were counted in April 2014.

    Study and other actions tested
  40. A 2016 systematic review of mangrove restoration studies around the world (Bayraktarov et al. 2016) reported a 51% average survival rate of planted mangrove trees and sown mangrove propagules. Survival ranged from 0% (17 of 106 cases) to ≥95% (15 of 106 cases). The average survival rate was 56% in developed countries and 45% in developing countries. Methods: The review was based on 106 cases (e.g. different species, environments or intervention methods) from 28 publications and at least 17 countries, 104 of which involved planting or sowing mangroves (see Appendix to original paper). Literature searches were carried out in 2014. Planting and sowing were sometimes into environments thought to be suitable (but sometimes into hostile environments) and were sometimes preceded by site preparation (but sometimes not). Study duration ranged from one month to 21 years. Survival was sometimes estimated from other metrics, such as cover. The review does not separate results for survival of planted seedlings vs sown propagules. The review includes studies (1), (6), (29), (30) and (36) summarized above.

    Study and other actions tested
  41. A paired, site comparison study in 2014–2015 of mangrove forests in the Philippines (Duncan et al. 2016) reported that replanted mangroves had a smaller basal area with shorter, thinner trees than mature natural mangroves, but had a similar or greater stem density and similar canopy closure. Statistical significance was not assessed. After 7–9 years, planted mangroves had a basal area of 28–33 m2/ha (vs natural: 11–17 m2/ha). On average, trees in planted mangroves were only 3.8–4.6 m tall (vs natural: 6.2–6.7 m) and had a stem diameter of only 2.7–6.9 cm (vs natural: 5.8–18.0 cm). In one site, stem density was similar in planted and natural mangroves (planted: 1,916; natural: 2,152 stems/ha), but in the other site, stem density was greater in the planted mangrove (planted: 11,839; natural: 6,496 stems/ha). Canopy closure was 84–87% in planted mangroves (vs natural: 85–88%). Methods: In 2014–2015, vegetation was surveyed in a replanted and natural mangrove forest at each of two sites on Panay Island (eight 7-m radius plots/forest). One replanted forest (Bakhawan) had been planted with tall-stilt mangrove Rhizophora apiculata in 2006, then colonized by other species. The other replanted forest (Ermita) had been planted with mixed mangrove species in 2007, although only white mangrove Sonneratia alba survived.

    Study and other actions tested
  42. A site comparison study in 2012 of two mangrove forests in southern Vietnam (Nam et al. 2016) reported that a planted forest contained more tree species than a naturally recolonized forest, but found that both forests had similar tree density, diameter, basal area and biomass. After 14–34 years, 15 true mangrove tree species were recorded in the planted forest (vs 12 in the recolonized forest; statistical significance of difference not assessed). The most common species in both forests was tall-stilt mangrove Rhizophora apiculata (planted: 80%; recolonized: 73% of all trees). There was no significant difference between the forests in tree density (planted: 1,963; recolonized: 2,548 trees/ha), diameter (planted: 11; recolonized: 10 cm), basal area (planted: 22; recolonized: 23 m2/ha) or above-ground biomass (planted: 131; recolonized: 147 Mg/ha). Methods: In June 2012, forest structure was surveyed along 15 transects in recovering mangroves (degraded in the 1960s–1970s by wartime herbicide spraying and deforestation). Six transects were in a forest replanted in 1978–1998 (35 tree species, mostly tall-stilt mangrove) then thinned at five year intervals. Nine transects were in a forest where trees had grown without any human intervention. Each transect contained six 150-m2 plots and ran perpendicular to creeks/coastlines. Above-ground biomass was estimated using diameter data. Both live and dead trees were surveyed.

    Study and other actions tested
  43. A replicated, before-and-after study in 2012–2013 in a brackish/saline estuarine site with mudflats and existing mangroves in southeast China (Peng et al. 2016) reported 0–80% survival of planted seedlings over 12 months, and that surviving seedlings grew. The lowest survival rate (0%) was exhibited by three of four species, including non-native mangrove apple Sonneratia apetala, in a strongly shaded, 8-year-old mangrove plantation. The highest survival rate (74%) was for river mangrove Aegiceras corniculatum in a lightly shaded, 2-year-old mangrove plantation. The study reported increases in the biomass, height and basal area of surviving seedlings (statistical significance not assessed). Growth rates depended on the combination of species and the habitat in which it was planted (see original paper). Methods: In June 2012, seedlings of four mangrove tree species were planted into four habitats: a tidal mudflat, and 2-, 4- and 8-year-old mangrove plantations. Twelve sets of 50 seedlings were sown for each species (3 sets/habitat). Seedlings were monitored every three months for a year.

    Study and other actions tested
  44. A study in 2005–2008 on a mudflat alongside a brackish creek in southern Nigeria (Zabbey & Tanee 2016) reported that 72% of planted red mangrove Rhizophora racemosa seedlings survived for three years, and that the average size of survivors increased over time. After three years, surviving individuals were 3.1 m tall and had a stem diameter of 2.8 cm (compared to 0.6 m tall and 1.4 cm diameter one month after planting). These results are not based on assessments of statistical significance. Methods: In November 2005, four hundred red mangrove seedlings were planted alongside Bodo Creek, in a former mangrove swamp that had been killed by an oil spill in 2003. The nursery-reared seedlings were planted 1 m apart. “Some” wilting seedlings were replaced one month after initial planting, and were not included in the analysis. Before planting, dead stumps were removed from the study site (but left at the margins to prevent erosion) and the sediment was tilled. For a year from July 2005, the site was also fertilized (weekly or bi-weekly; 1.2 kg NPK fertilizer/0.17 ha/application). Surviving seedlings were monitored one month and approximately three years after planting.

    Study and other actions tested
  45. A study in 2010–2013 alongside a brackish creek in southern Nigeria (Zabbey & Tanee 2016) reported that only 12% of planted red mangrove Rhizophora racemosa seedlings survived for three years, but that the average size of survivors increased over time. After three years, surviving individuals were 1.3 m tall and had a stem diameter of 2.1 cm (compared to 0.5 m tall and 1.3 cm diameter immediately after planting). These results are not based on assessments of statistical significance. Methods: In April 2010, four hundred red mangrove seedlings were planted alongside Kono Creek, in an area cleared of invasive nipa palm Nypa fruticans (clear cut, and rhizomes removed). The site was weakly brackish (<4 ppt). The nursery-reared seedlings were planted early in the morning, 1 m apart. Seedlings were measured immediately after planting and three years later.

    Study and other actions tested
  46. A before-and-after study in 2011–2016 of an intertidal site in Florida, USA (Donnelly et al. 2017) reported 62% survival of planted red mangrove Rhizophora mangle seedlings after two years, and increases in red mangrove abundance and height over five years. Before planting, the upper intertidal zone was sparsely vegetated (<1 mangrove stem/m2; 3% cover). Five years after planting red mangroves into this zone, their density had increased to 3.5 stems/m2 and their cover had increased to 81%. Most of this vegetation had been planted: limited natural recruitment (0.2 seedlings/m2) was only observed from the fourth year of the study. The average height of surviving seedlings increased, from 36 cm when planted to 92 cm after five years (statistical significance not assessed). Methods: The study aimed to stabilize a 200 m stretch of shoreline, on the edge of an ancient shell waste dump. In April/May 2011, nursery-reared red mangrove seedlings were planted in the high intertidal zone (450 seedlings; 2 seedlings/m). Smooth cordgrass Spartina alterniflora was planted in the mid-intertidal zone and oyster-shell mats were placed in the lower intertidal zone. The study does not distinguish between the effects of these interventions on non-planted mangroves. Mangrove vegetation in the upper intertidal zone was surveyed before intervention (presumably April 2011) and for five years after (2011–2016).

    Study and other actions tested
  47. A replicated study in 2012–2014 of 23 coastal sites in Sri Lanka (Kodikara et al. 2017) reported 0–78% survival of planted mangrove seedlings and propagules after ≥5 years, and that only 18–20% the area planted with mangroves was forested after 8–10 years. In 9 of the 23 sites, no mangrove trees were alive five or more years after planting. In 7 of the 14 sites with some surviving trees, survival rates were <10%. Only three sites supported >50% survival. Average survival rates were higher in sites where technical guidance was used (46%) than where it was not used (0%), and in sites with post-planting care of seedlings (13%) than without (0%). The study suggests that mangroves were planted into unsuitable environments in many sites. Finally, the study reports that of 1,000–1,200 ha of mangrove forest planted in these sites since 2004, only 200–220 ha was present 8–10 years later. Methods: Between 2012 and 2014, the number of surviving, healthy mangrove trees was counted or estimated in 23 coastal sites around Sri Lanka. In eight sites, the tidal influence was “negligible”. Mangrove propagules and seedlings (97% of which were Rhizophora spp.) were planted between 1996 and 2009, with multiple planting attempts in all sites. In 10 sites, mangroves were cared for after planting. The study does not distinguish between the effects of planting seedlings and propagules.

    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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