Introduce tree/shrub seeds or propagules: brackish/saline wetlands
Overall effectiveness category Likely to be beneficial
Number of studies: 19
Background information and definitions
This action involves introducing seeds or propagules of emergent plants to restore/create marshes or swamps. “Propagules” is the term used to describe the seed-like, usually leafless structures produced by mangrove trees to allow them to reproduce and disperse. Seeds or propagules may be collected from plants in greenhouses/laboratories, or from natural sites. They may be sown directly into the soil, scattered over the surface, or carried to suitable sites by water (e.g. after dropping them into the sea during an incoming tide).
Introduction of target vegetation might be useful in severely degraded or bare sites – which may lack remnant plants or seed banks to kick start revegetation with desirable species, and may be at risk of being taken over by undesirable species (Brown & Bedford 1997). It might also be useful in isolated wetlands, far from sources of marsh or swamp plant propagules. Seeds and propagules are easier to handle than plants, and can be a cost-effective way to introduce vegetation to large areas – but they can be more susceptible to herbivory or being washed away (e.g. Schoenholz et al. 2001).
The effects of sowing may be highly dependent on the environmental conditions in each study. Questions you might ask when interpreting the evidence include: Is the study site degraded? Where and when were seeds/propagules introduced? Was there any intervention to improve conditions before planting? What were the conditions over the duration of the study?
The scope of this action does not include sowing nurse plants; sowing submerged or floating plants; sowing to restore bogs, fens, fen meadows or peat swamp forests (see Taylor et al. 2018); or sowing facultative wetland plants in upland sites. In contrast, the scope does include sowing non-native species to conserve marshes or swamps – whilst acknowledging that this is often considered ethically unacceptable due to the risk of invasion (e.g. Ren et al. 2009).
Related actions: Directly plant whole plants; Introduce vegetation fragments; Transplant or replace wetland soil; Restore/create marshes or swamps using multiple interventions, often including planting.
Brown S.C. & Bedford B.L. (1997) Restoration of wetland vegetation with transplanted wetland soil: an experimental study. Wetlands, 17, 424–437.
Ren H., Lu H., Shen W., Huang C., Guo Q., Li Z. & Jian S. (2009) Sonneratia apetala Buch.Ham in the mangrove ecosystems of China: an invasive species or restoration species? Ecological Engineering, 35, 1243–1248.
Schoenholz S.H., James J.P., Kaminski R.M., Leopold B.D. & Ezell A.W. (2001) Afforestation of bottomland hardwoods in the Lower Mississippi Alluvial Valley: status and trends. Wetlands, 21, 602–613.
Taylor N.G., Grillas P. & Sutherland W.J. (2018) Peatland Conservation: Global Evidence for the Effects of Interventions to Conserve Peatland Vegetation. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge.
Supporting evidence from individual studies
A replicated study in 1986–1987 in an experimentally degraded mangrove forest in Guadeloupe (Scherrer et al. 1989) reported >70% survival of planted red mangrove Rhizophora mangle propagules after approximately one year, and that the average height of saplings increased. Statistical significance was not assessed. Mangrove propagules were planted in oiled and non-oiled plots in March and July. After one year, the survival rate of propagules planted in oiled plots in July was 72–81% (data not reported for non-oiled plots). Across all treatments, healthy surviving saplings were 150–200 cm tall (vs 0 cm when planted as propagules). Saplings were 200–220 cm tall in non-oiled plots and 200–210 cm in plots sprayed with pure oil, compared to 150–180 cm in plots sprayed with oil and additives. Methods: In March and July 1986, mature, field-collected, red mangrove propagules were planted in four 2-m2 plots (72 propagules/plot) in a tidal mangrove forest that had been cleared for the study. Six plots (three plots/month) had been sprayed with 5L/m2 crude oil in early March. In four plots (two plots/month), the oil contained an additive (chemical dispersant, or a bioactivator to stimulate microbial activity). The height of healthy saplings (i.e. alive and not damaged by insects, crabs or falling branches) was measured every 50 days for approximately one year after planting.Study and other actions tested
A study in 1989–1993 in a historically mined mangrove in Thailand (Komiyama et al. 1996) reported that 53% of planted tall-stilt mangrove Rhizophora apiculata seedlings survived for four years, and that the average height of surviving seedlings increased over time. Statistical significance was not assessed. Seedling survival rates were 70% after one year, 55% after three years and 53% after four years. The most common seedling height was 50–75 cm after one year, then 100–125 cm after four years. The most common seedling diameter was 0.5–1.0 cm after one year, then 2.0–2.5 cm after four years. The study also suggested that survival and changes in height were affected by elevation and firmness of sediment (see original paper for data). Methods: In August 1989, tall-stilt mangrove propagules were planted into 1,000 m2 of tidal coastal land (3,950 wild-collected propagules, 50 cm apart). The area had been mined for tin in the previous decade. Data were collected immediately before planting (August 1989) and for up to four years after (November 1990, August 1992 and August 1993).Study and other actions tested
A replicated study in 1988–1990 on the coast of Louisiana, USA (Patterson et al. 1997) reported 40–55% survival of planted black mangrove Avicennia germinans propagules after 15 months, and that the average height of surviving seedlings increased. After four months, 89–92% of propagules were still alive (31–33% of which were rooted and upright). After 15 months, 44–55% of propagules were still alive (all of these were rooted and upright). Surviving seedlings were 25–28 cm tall on average. The survival rate was statistically similar for propagules introduced to mangroves or salt marshes, but seedlings grew significantly more within salt marshes. Methods: In November 1998, twenty field-collected black mangrove propagules were placed loose within each of 32 cages (0.1 m2, 6 mm mesh, 0.6 m tall). Half of the cages were within existing black mangrove vegetation and half were in salt marsh vegetation (dominated by smooth cordgrass Spartina alterniflora) further inland. Propagules/seedlings were monitored in March 1989 and February 1990.Study and other actions tested
A replicated study in 1995–1997 on a tidal mudflat in New South Wales, Australia (Day et al. 1999) reported 0–53% survival of planted grey mangrove Avicennia marina propagules after two growing seasons, but that survivors grew. In one of three planted areas, there were no mangrove seedlings present after two weeks. In the other two areas, there were 3.5–8.4 mangrove seedlings/plot present after two growing seasons (vs 16 planted propagules/plot). Seedlings were 30–49 cm tall, on average. There were more surviving seedlings in sand or natural substrate (6.7–8.4 seedlings/plot) than in slag (3.5 seedlings/plot), but the average height of seedlings was lower in sand (30 cm) than in all other substrates (45–49 cm). Methods: In December 1995, sixty 1-m2 plots were established (in three sets of 20) on a tidal mudflat in the Hunter River estuary. The plots were excavated to 20 cm depth and refilled with sand, the local natural substrate (sand/silt/clay; ploughed or unploughed) or slag (a waste product from iron production). Sixteen locally collected grey mangrove propagules were planted into each plot. Seedlings were counted in each set after approximately two weeks, then counted and measured in two of the sets after 15 months.Study and other actions tested
A replicated study in 1997–1998 in four sandy coastal sites in Florida, USA (Salgado Kent & Lin 1999) reported 0–100% survival of planted red mangrove Rhizophora mangle propagules after 4–8 months, but that surviving propagules developed stems and leaves in most cases. Statistical significance was not assessed. Survival rates were highest (76–100%) for propagules sheltered within translucent plastic pipes that extended above and below ground. Survival rates were lower (0–6%) for propagules sheltered by bamboo pipes, sheltered by plastic pipes below ground only, or not sheltered. Some propagules developed stems in 9 of 12 cases and leaves in 10 of 12 cases (location x shelter combinations). The rate of stem and leaf development depended on shelter treatment, site, and when seedlings were collected and planted (see original paper). Methods: In August and November 1997, a total of 796 red mangrove propagules were planted in four areas of coastal, sandy sediment (13–35 propagules/site/season for each of the four shelter treatments; see above). The sites experienced “moderate to high” wave energy. Propagules were collected locally then rooted in a nursery. Healthy propagules were planted near the high tide level. Propagules (or the seedlings they became) were monitored twice a month for up to eight months after planting.Study and other actions tested
A replicated study in 1995–1997 in a degraded mangrove forest in Colombia (Elster 2000) reported that 0–100% of planted propagules survived over 15 months, depending on species and environmental conditions, but that survivors grew. When planted and secured in flooded soils, survival rates were 71% for red mangrove Rhizophora mangle, 28% for white mangrove Laguncularia racemosa and 10% for black mangrove Avicennia germinans. Caterpillars ate most of the black mangroves. When planted into saturated soils, only red mangroves survived (100%). When planted into dry soils, no species survived for longer than 10 months. Surviving plants grew by 20–110 cm over 13 months. Methods: In November–December 1995 (start of the dry season), field-collected propagules were planted into a former mangrove site. The site had been reconnected to the main lagoon system (in 1989) to reduce the salinity that killed the former mangroves (around 1965). Sets of 15–100 propagules were planted (1–3 sets/species) in a range of soil conditions: flooded (5–10 cm deep), saturated (water table at soil surface) or dry (water table 2 cm below surface). After planting, water depths varied seasonally. Shade was provided for some propagules. Survival and plant height were monitored for up to 15 months.Study and other actions tested
A replicated, paired, site comparison study in 1996–1997 involving two sites planted with red mangrove Rhizophora mangle propagules (after reprofiling) in Florida, USA (McKee & Faulkner 2000) reported that they supported a different tree density, structure and community to mature natural mangrove forests after 7–15 years. Statistical significance was not assessed. Restored sites contained 6,830–27,700 trees/ha (vs natural: only 1,840–2,131 trees/ha) but had a basal area of only 3–18 m2/ha (vs natural: 26–28 m2/ha). Accordingly, trees in restored sites were all <10 cm in diameter (average: 2.1–2.7 cm) whereas natural sites contained trees both <10 cm and ≥10 cm in diameter. Restored sites contained two or three tree species (vs natural: three), but in different proportions (e.g. 48–75% of trees in restored sites were white mangrove Laguncularia racemosa, vs natural: 17–26%; similar pattern for relative density, dominance and importance). Methods: Between November 1996 and December 1997, trees were surveyed in two pairs of restored and natural mangrove forests. Restoration, completed in 1982 or 1990, involved removing previously dumped sediment and excavating tidal channels, then planting red mangrove propagules. The study does not distinguish between the effects, on unplanted trees, of planting and reprofiling. Trees ≥2 m tall and ≥2 cm in diameter were recorded at 21 points/site. One pair of sites in this study was also used in (10).Study and other actions tested
A replicated study in 2000–2001 in two lagoons in southern Mexico (Reyes Chargoy & Tovilla Hernández 2002) reported 39–100% survival of planted red mangrove Rhizophora mangle propagules after 8–11 months, but that the average height of surviving seedlings increased. Statistical significance was not assessed. Pozuelos Lagoon was flooded by two tides/day throughout the year. Here, all 3,019 planted propagules apparently produced surviving seedlings eight months later. These were 65 cm tall on average (vs 11 cm one month after planting). However, the study also reported extensive natural colonization by mangrove seedlings (> the number of propagules planted), and it is unclear if/how planted and unplanted seedlings were distinguished. In Cabildo Lagoon, only 39% of 19,345 planted propagules produced surviving seedlings after 8–11 months. These were 60–75 cm tall on average (vs 12–18 cm one month after planting). The study suggests that seedlings in this site were killed by low water levels and trampling by people and animals. Methods: Between June and September 2000, field-collected red mangrove propagules were planted in the intertidal zone of two lagoons (from 20-cm-deep water to the “limit of the wet zone”). Surviving seedlings were surveyed for up to eleven months after planting.Study and other actions tested
A replicated study in 1985–2001 in five coastal sites in the United Arab Emirates (Tamaei 2004) reported that most sown grey mangrove Avicennia marina seeds germinated, and that the average height of the few surviving trees increased over time. Statistical significance was not assessed. Across the five sites, the germination rate ranged from 65% to 92%. Survival rates of germinated seedlings ranged from <1% to 27% after 5–8 years. The study suggests grazing by gazelles Gazella sp. and vandalism as causes of mortality. Surviving trees reached a height of 98–287 cm after 5–8 years, and 5.7–5.8 m after 13–16 years (one site only). Methods: Between 1985 and 1996, grey mangrove seeds were sown into five coastal sites (including one in the wastewater channel of a research centre). The seeds were collected from local mangrove trees, then buried 3 cm deep and 25–150 cm apart in the middle of the intertidal zone. Salinity was 40–45 ppt. At least 1,420 seeds were sown in each site. Germination rates were recorded 35–40 days after sowing. Surviving plants were counted, and height of 20 healthy seedlings measured, for up to 16 years.Study and other actions tested
A site comparison study in 1989–2000 in Florida, USA (Proffitt & Devlin 2005) reported that an area planted with red mangrove Rhizophora mangle propagules (after reprofiling) developed mangrove forest stands, but that these contained more trees with a greater basal area than natural forest after 18 years. Unless specified, statistical significance was not assessed. Tall mangrove stands occupied 74% of the restored area after six years, then 95% after 14 years. After 18 years, 60–87% of planted red mangrove trees were still alive. Survivors had grown, from 0.5–3 m tall six years after planting to 2–5 m tall 18 years after planting. Two of three mangrove species present in nearby natural forest had colonized the restored site: black mangrove Avicennia germinans and white mangrove Laguncularia racemosa. Overall, trees in the restored site were thinner (restored: 3 cm; natural: 13 cm diameter) but had a greater basal area (restored: 43 m2/ha; natural: 16–19 m2/ha). Methods: Between 1989 and 2000, vegetation was surveyed in a restored area and adjacent natural mangrove. Restoration, in the early 1980s, involved removing previously dumped sediment and excavating a tidal channel, then planting red mangrove propagules (in pairs 1 m apart). The study does not distinguish between the effects of these interventions on non-planted trees. Surveys involved monitoring individual marked trees over time, counting/measuring trees within 25-m2 plots or 1-m2 quadrats, and taking aerial photographs to estimate overall mangrove area (see original paper for details). This study monitored one of the sites from (7).Study and other actions tested
A replicated study in 2001–2003 on coastal salt marsh and sediment in Belize (McKee et al. 2007) reported 80–100% survival of planted red mangrove Rhizophora mangle propagules over two years, and that the average number of leaves per seedling increased over time. Statistical significance was not assessed. After two years, 80–100% of propagules planted amongst existing vegetation and 92% of propagules planted into bare sediment survived as seedlings. Surviving seedlings had 29–43 leaves on average (35–43 leaves/seedling developing amongst existing vegetation; 29 leaves/seedling developing on bare sediment). In comparison, seedlings had only four leaves on average after six months. Methods: In August 2001, nine plots were established in an intertidal area that used to be mangrove forest (clear-cut in 1991). Three plots were dominated by sea purslane Sesuvium portulacastrum, three were dominated by saltgrass Distichlis spicata, and three were bare sediment. Ten red mangrove propagules (collected from the surrounding forest) were planted into each plot. Between 6 and 24 months after planting the propagules, surviving seedlings were recorded and their leaves were counted.Study and other actions tested
A study of mangrove planting projects in the Philippines (Samson & Rollon 2008) reported <5% survival of planted mangrove propagules/seedlings, 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). Propagules and/or seedlings were generally planted 1 m apart, following national guidelines, but often with 2–5 individuals at each planting spot.Study and other actions tested
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. propagules and/or seedlings. 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).Study and other actions tested
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 propagules or seedlings; 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. This study included the sites in (7) and (10).Study and other actions tested
A replicated study in 2006–2009 of 47 mangrove restoration projects in Sumatra, Indonesia (Wibisono & Sualia 2008) reported 0–99% survival of planted propagules/seedlings 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 propagules and seedlings. 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
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 ungerminated 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 propagules and seedlings. The number of propagules or seedlings 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
A 2016 systematic review of mangrove restoration studies around the world (Bayraktarov et al. 2016) reported a 51% average survival rate of sown mangrove propagules and planted mangrove trees. 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 sowing or planting mangroves (see Appendix to original paper). Literature searches were carried out in 2014. Sowing and planting 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 sown propagules vs planted seedlings. The review includes studies (4) and (10) summarized above.Study and other actions tested
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 38–100% germination of planted mangrove propagules. Germination rates depended on the combination of species and the habitat in which it was planted. For example, the germination rate of non-native mangrove apple Sonneratia alba ranged from 67% amongst the oldest, darkest forest to 100% on bare mudflats. Three native mangrove species had germination rates of 38–90%, but only one (river mangrove Aegiceras corniculatum) was clearly affected by the habitat. Methods: In June 2012, propagules of four mangrove tree species were planted into four habitats: a tidal mudflat, a 2-year-old mangrove apple plantation, a 4-year-old mangrove apple plantation, and an 8-year-old mangrove apple plantation. Twelve sets of 30 propagules were sown for each species (3 sets/species/habitat). Germination was monitored daily for 20 days.Study and other actions tested
A replicated study in 2012–2014 of 23 coastal sites in Sri Lanka (Kodikara et al. 2017) reported 0–78% survival of planted mangrove propagules and seedlings after ≥5 years, and that only 17–20% of 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 propagules and seedlings.Study and other actions tested