Introduce tree/shrub seeds or propagules: freshwater wetlands
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 2
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 1995 in a wet meadow in New South Wales, Australia (de Jong 2000) reported 0–18% germination of sown tree/shrub seeds after two months, depending on the species and whether vegetation was cleared before sowing, but 0% survival after eight months. In plots that had been cleared of vegetation before sowing, all five sown species germinated. The number of seedlings present after two months was 1–18% of the number of seeds sown. In plots that had not been cleared of vegetation, only two of five species germinated. For these species, the number of seedlings present after two months was ≤1% of the number of seeds sown. After eight months, after prolonged saturation or flooding, no seedlings were present in any plot. Methods: In January–February 1995, seeds of five tree/shrub species present in local wetlands were sown on to a wet meadow, with the aim of restoring a swamp. For each species, three hundred 25 x 25 cm plots were sown with approximately 50 seeds. Of the 300 plots, 200 were cleared of vegetation before sowing. Half of the plots/species were higher (and drier) than the others. Seedlings of the planted species were counted in every plot after two and eight months.Study and other actions tested
A study in 2006–2009 in a floodplain swamp restoration site in Wisconsin, USA (Thomsen et al. 2012) reported that seedlings of only three of five sown tree species were present. Neither black ash Fraxinus nigra nor river birch Betula nigra seedlings were present in the site within three years of sowing seeds. Seedlings of the other three sown species were present (green ash Fraxinus pennsylvanica, American elm Ulmus americana and silver maple Acer saccharinum; abundance data reported graphically) but the study does not distinguish seedlings originating from sown vs naturally arriving seeds. Methods: Between November 2006 and May 2009, seeds of five tree species (numbers not clearly reported) were broadcast across 16 plots in a floodplain swamp restoration site (a clearing created by a storm). All plots had been cleared of invasive reed canarygrass Phalaris arundinacea and disked in November 2006 (before first sowing). Herbicide was then applied regularly through to November 2008). Tree seedlings were counted in August 2007–2009.Study and other actions tested