Introduce seeds of non-woody plants: brackish/saline wetlands
Overall effectiveness category Likely to be beneficial
Number of studies: 8
Background information and definitions
This action involves introducing seeds of emergent plants to restore/create marshes or swamps. Seeds 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 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 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 blocks of vegetation; 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 1979–1981 on reprofiled borrow pits in North Carolina, USA (Broome et al. 1982) reported that smooth cordgrass Spartina alterniflora biomass developed in plots where sown cordgrass seeds germinated. Cordgrass seeds only germinated in some plots, which the authors suggested were those with suitable moisture levels (data not reported). In these plots, there was 304–1,163 g/m2 above-ground biomass of smooth cordgrass after one growing season. Methods: In spring 1979 and 1981, smooth cordgrass seeds were mixed into the surface of “several” plots on reprofiled coastal land (30 cm below to 60 cm above mean sea level; salinity <20 ppt). The seeds had been stored at 2–4°C, first dry, then in artificial sea water. The plots were dry during sowing but rewetted after. All plots were fertilized before or after sowing. In October 1979 and 1981, live vegetation was cut from the plots, then dried and weighed.Study and other actions tested
A replicated study in 1981–1982 on a mudflat in the Netherlands (Groenendijk 1986) reported that 1–23% of sown common cordgrass Spartina anglica seeds germinated within one month, and that the average height of surviving plants increased over one growing season. Initial germination rates were highest in plots at higher elevations (97–110 cm above mean sea level) and for seeds sown at intermediate depths (1.5 cm; statistical significance not assessed). The average height of surviving cordgrass plants was 1–2 cm one month after planting, then 8–15 cm six months after planting (with taller plants at the higher elevations). After two growing seasons, sown cordgrass only persisted at the highest elevation, and here in only 50% of plots. These plots had also been colonized by new cordgrasses and saltworts Salicornia spp. (not quantified). Methods: In April 1981, field-collected common cordgrass seeds were sown into one hundred and eighty 0.25-m2 plots (20 seeds/plot). The plots were on a mudflat, below a cordgrass-dominated salt marsh. They were arranged in five groups of 36 plots, with each group at a different elevation (42–110 cm above mean sea level). One third of the seeds (12 plots/group) were sown at each of three depths (0.5, 1.5 or 3.0 cm). The presence and height of cordgrass plants were monitored between May and November 1981. Plot-level survival was monitored in July 1982.Study and other actions tested
A replicated study in 1989 in a salt marsh in the Netherlands (Bakker & de Vries 1992) reported that 0–19% of salt marsh plant species’ seeds germinated after sowing, and that 0–83% of seedlings survived their first growing season. Germination and survival rates varied between sown species, the plant community in which they were sown, and whether vegetation was grazed or mown (see original paper for details). The species with the highest overall germination rate was sea aster Aster tripolium: 98 seedlings found during the first growing season (vs 750 seeds sown). The species with the highest overall survival rate was spear-leaved orache Atriplex prostrata: 35 seedlings alive at the end of August (vs 80 seedlings germinated). Only one Danish scurvygrass Cochlearia danica seedling survived until the end of August (vs 63 germinated). Methods: In March 1989, at total of 9,000 seeds were sown into a coastal salt marsh. Five batches of 50 seeds were sown for each of six species, in three recipient plant communities, and for each of two disturbance regimes (summer grazing or annual mowing; date not reported). Germination and survival were monitored until the end of August.Study and other actions tested
A replicated study in 2001 in an estuary in California, USA (Zedler 2003) reported that after sowing 21,600 seeds of two salt marsh species, only 17 seedlings grew. These 17 seedlings were all dwarf saltwort Salicornia bigelovii. No seedlings of arrowgrass Triglochin concinna were found. Methods: In March 2001, a total of 10,800 seeds/species were sown onto an area of recently reprofiled intertidal sediment. Sets of 50 seeds (25 seeds/species) were sown under single adult herbs/succulents (144 sets), under clusters of adult herbs/succulents (144 sets) or onto bare sediment (144 sets). All seed sets were covered with burlap fabric after sowing. Seedlings were counted over the 2001 growing season.Study and other actions tested
A replicated study in 2006 in an estuarine salt marsh in California, USA (Varty & Zedler 2008) reported that plots sown with dwarf saltwort Salicornia bigelovii seeds contained dwarf saltwort seedlings two months later. A total of 650 seedlings were counted across seventy-two 0.25-m2 plots. There were 3–14 seedlings/0.25 m2 depending on plot elevation and location in the marsh (see Action: Create mounds or hollows before planting). Thinning the dominant pickleweed Salicornia virginica had no significant effect on seedling density. The study notes that there were probably lots of dwarf saltwort seeds already in the soil, and many of the seedlings probably germinated from these seeds. Methods: In March 2006, dwarf saltwort seeds were sown onto seventy-two 0.25-m2 plots in a pickleweed-dominated salt marsh (approximately 68 seeds/plot). Four dwarf saltwort seedlings were also planted in each plot. In some of the plots, the surface was lowered by 5–10 cm and/or pickleweed stems were cut and removed before planting. Seedlings were counted in May 2006.Study and other actions tested
A replicated, before-and-after study in an alkaline, estuarine wetland in eastern China (Guan et al. 2011) reported that five months after sowing seeds of seablite Suaeda salsa onto bare prepared plots, seablite was present. Seeds were sown May. In October, sown plots contained 292–532 seablite plants/m2, with an above-ground biomass of 396–771 g/m2. Seablite plants were 59–63 cm tall, on average. Variation in density and biomass were related to the method used to prepare plots for sowing (see Actions: Add inorganic fertilizer before/after planting and Add below-ground organic matter before/after planting). Methods: In May 2009, three pairs of 6-m2 plots were established in a degraded, unvegetated, hypersaline/alkaline wetland in the Yellow River estuary. Approximately 5,000 seablite seeds were sown onto each plot, then watered. Three plots had been prepared by ploughing (to 20 cm depth), three by ploughing and mixing in urea (130 kg N/ha), and three by ploughing and mixing in reed debris (2 kg/m2). Vegetation was sampled in five 1-m2 quadrats/plot until October 2009. Biomass measurements involved samples of approximately 100 plants/plot. Details of height measurements were not reported.Study and other actions tested
A 2016 systematic review of salt marsh restoration studies around the world (Bayraktarov et al. 2016) reported a 65% average survival rate of sown and planted vegetation. Survival ranged from 0% (2 of 64 cases) to ≥95% (7 of 64 cases). Methods: These results are based on 64 cases (e.g. different species, environments or intervention methods) from 16 publications and five countries, 63 of which involved sowing or planting salt marsh vegetation (mostly herbs and succulents, sometimes shrubs; 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 sometimes preceded by site preparation (but sometimes not). Study duration ranged from 20 days to 13 years. Survival was sometimes estimated from other metrics, such as cover. The review does not separate results for sowing vs planting. The review does not include any of the other studies summarized for this action.Study and other actions tested
A replicated study in 2014 on a recently deposited tidal flat in eastern China (Hu et al. 2016) reported that sown bulrush Scirpus mariqueter seeds successfully germinated. After one growing season, 0.6–1.1% of sown bulrush seeds had germinated and emerged as seedlings. There were 0.06–0.50 bulrush shoots/m2. Neither the germination rate nor shoot density significantly differed between different sowing densities. Methods: In March–April 2015, field-collected bulrush seeds were sown into a recent accumulation of intertidal sediment in the Yangtze estuary. Three 200-m2 plots were each sown with a different density of seeds: 1,000, 2,000 or 4,000 seeds/m2. Seeds were sown 5 cm deep, but the study noted that substantial sediment deposition over the rest of the growing season (>15 cm depth). Bulrush seedlings and shoots were counted twice each month until October, in ten 4-m2 quadrats/plot.Study and other actions tested