Restore/create brackish/saline marshes or swamps (multiple actions)

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

Study locations

Key messages

  • Eight studies evaluated the effects, on vegetation, of using >3 combined actions to restore/create brackish/saline marshes or swamps. Six studies were in the USA. One was in Singapore. One was in Indonesia. Three studies were based on the same experimental set-up.

VEGETATION COMMUNITY

  • Overall extent (1 study): One study of a coastal site in the USA reported that the coverage of mangrove vegetation increased, and the coverage of herbaceous vegetation declined, over five years after intervention (intended to restore mangrove forest).
  • Overall richness/diversity (3 studies): Three studies of one salt marsh restoration site in the USA simply quantified plant species richness for up to 13 growing seasons after intervention.
  • Tree/shrub richness/diversity (1 study): One site comparison study in Indonesia reported that a restored aquaculture pond contained a similar number of mangrove species to nearby reference forests, just 6–7 months after intervention. Some trees may have been present before intervention.

VEGETATION ABUNDANCE

  • Overall abundance (4 studies): One replicated, paired, site comparison study of salt marshes in the USA found that restored marshes had similar overall vegetation cover to natural marshes after 9–20 years. Three studies of one salt marsh restoration site in the USA simply quantified overall vegetation abundance for up to 13 growing seasons after intervention.
  • Tree/shrub abundance (3 studies): One replicated, paired, site comparison study of salt marshes in the USA found that restored marshes had similar, limited shrub cover to natural marshes after 9–20 years. One site comparison study of mangrove forests in Singapore reported that a created mangrove forest supported lower above-ground biomass than mature natural forests after ≥15 years. One study in Indonesia simply counted the number of mangrove trees present 6–7 months after intervention.
  • Individual species abundance (4 studies): Four studies in estuaries in the USA simply quantified the abundance of individual plant species for up to 13 growing seasons after intervention.

VEGETATION STRUCTURE

  • Overall structure (1 study): One replicated, paired, site comparison study of salt marshes in the USA found that restored marshes had less cover of short vegetation and greater cover of medium-height vegetation than natural marshes after 9–20 years. Restored and natural marshes had similar cover of tall vegetation.
  • Height (2 studies): One study of a created mangrove forest in Singapore reported that the average height of surviving mangrove saplings increased over five years. One study of a salt marsh restoration site in the USA reported that maximum vegetation height did not clearly increase between the third and twelfth/thirteenth growing seasons after intervention.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A study in 1981–1982 in an estuary in Maryland, USA (Earhart & Garbisch 1983) reported that 53% of a site prepared with multiple interventions contained smooth cordgrass Spartina alterniflora. After approximately one year, smooth cordgrass stands covered 4.5 ha of an 8.5 ha prepared area. Methods: In November 1981, fine-grained dredge sediment was deposited in Tar Bay. In April–May 1982, an 8.5-ha area 20–50 cm above mean low water was sown with a mix of smooth cordgrass seeds and cat litter (as a drying agent; approximately 96 seeds/m2) and harrowed with spikes or chains. In June and August, the area was fertilized (NPK fertilizer; 110 kg/ha). The area covered by smooth cordgrass stands, both “dense” and “sparse”, was recorded in December 1982.

    Study and other actions tested
  2. A study in 1996–2000 of a salt marsh restoration site in California, USA (Lindig-Cisneros & Zedler 2002) reported that over four growing seasons after multiple interventions, unplanted seedlings colonized and vegetation cover developed. Over the first growing season after intervention 35,507 unplanted seedlings of eight species were recorded across eighty-five 4-m2 plots. At least 98% of unplanted seedlings were pickleweed Salicornia virginica, dwarf saltwort Salicornia bigelovii or estuary seablite Suaeda esteroa. For these species, the number of unplanted seedlings/plot typically depended on elevation and the identity and number of planted species (see original paper). After four growing seasons, plots contained 3.2–5.3 plant species on average. There was 94% total vegetation cover, dominated by pickleweed and dwarf saltwort. Methods: In 1996/1997, an upland area was lowered to intertidal elevations and graded into a slope. In this area, eighty-five 4-m2 study plots were amended with fine sediment, tilled and levelled. Seventy plots were then planted with salt marsh herbs/succulents (90 seedlings/plot; 1–6 species/plot; eight species total). Non-planted seedlings were counted (and removed) throughout the growing season in 1998, and at the end of the growing season in 1999. Plant species and their cover were surveyed, along two transects/plot, until autumn 2000. This study was based on the same experimental set-up as (3) and (6).

    Study and other actions tested
  3. A study in 1996–2000 of a salt marsh restoration site in California, USA (Callaway et al. 2003) reported that three growing seasons after multiple interventions, the site contained both planted and unplanted vegetation. On average, plots contained 94 g/m2 standing above-ground biomass if not planted, and 372–431 g/m2 standing above-ground biomass if planted (see Action: Directly plant whole plants). Unplanted species colonized all plots, but only five species were found across 15 unplanted plots. Three species comprised 97% of the above-ground biomass in unplanted plots: dwarf saltwort Salicornia bigelovii (59 g/m2), pickleweed Salicornia virginica (29 g/m2) and estuary seablite Suaeda esteroa (5 g/m2). Methods: In 1996/1997, an upland area was lowered to intertidal elevations and graded into a slope. In this area, eighty-five 4-m2 study plots were amended with fine sediment, tilled and levelled. Seventy plots were then planted with salt marsh herbs/succulents (90 seedlings/plot; 1–6 species/plot; eight species total). Non-planted vegetation was cleared from all plots in 1997 and 1998, but was left to grow from 1999. In January 2000, standing vegetation was cut from a 20 x 120 cm quadrat in each plot, then dried and weighed. This study was based on the same experimental set-up as (2) and (6).

    Study and other actions tested
  4. A study in 1999–2004 of a coastal site in Florida, USA (Lewis 2005) reported that following multiple interventions, mangrove trees spontaneously colonized. Mangrove vegetation covered 4% of the site immediately after intervention, then 95% after five years. Mangrove seedlings were observed growing three months after intervention (174 seedlings/m2) and five years after intervention (40 seedlings/m2). After five years, white mangroves Laguncularia racemosa were 1.6 m tall on average and black mangroves Avicennia germinans were 0.9 m tall on average. Herbaceous Marsh vegetation coverage declined from 32% of one year after intervention to 5% after five years. Methods: In spring/summer 1999, a degraded coastal site was subjected to multiple interventions intended to expand mangrove forest habitat: clearing invasive shrubs/trees with chainsaws and herbicide, reprofiling to intertidal elevations (similar to nearby mangroves), excavating tidal creeks, and planting smooth cordgrass Spartina alterniflora on bare ground (to trap mangrove propagules). Vegetation was surveyed immediately after intervention was complete (September 1999) and five years later (September 2004). This summary takes some methodological details from Mauseth et al. (2001).

    Additional Reference: Mauseth G.S., Urquhart-Donnelly J.S. & Lewis R.R. (2001) Compensatory restoration of mangrove habitat following the Tampa Bay oil spill. International Oil Spill Conference Proceedings 2001, 761–767.

    Study and other actions tested
  5. A replicated, paired, site comparison study in 2001–2002 of 22 coastal salt marshes in Virginia, USA (Desrochers et al. 2008) found that marshes created using multiple interventions had similar plant species richness, overall vegetation cover and shrub cover to natural marshes, but that the created marshes had lower cover of short vegetation than the natural marshes. After 9–20 years, there was no significant difference between created and natural marshes in plant species richness (created: 4.1; natural: 5.7 species/marsh), overall vegetation cover (created: 83%; natural: 80%) and shrub cover (created: 2%; natural: 3%). Both marsh types had statistically similar cover of tall vegetation (created: 9%; natural: 13%). However, created marshes had lower cover of short vegetation (created: 9%; natural: 27%) and greater cover of medium-height vegetation (created: 63%; natural: 35%). Seven plant species found in the natural marshes were absent from the created marshes. Methods: In May–July 2001 and 2002, vegetation was surveyed in 11 pairs of marshes (matched by size, shape and surrounding land use). In each pair, one marsh had been created 9–20 years previously and one was natural. Marsh creation involved removing upland soil, reprofiling to a suitable slope, creating a connection to a tidal creek, and planting (mostly grasses/rushes, sometimes shrubs; 6 of 11 marshes planted with only one species). In each of six surveys, the cover of every plant species and bare mud were recorded along 2–6 transects/marsh (transects 100 m long).

    Study and other actions tested
  6. A study in 1996–2009 of a salt marsh restoration site in California, USA (Doherty et al. 2011) reported that 12–13 growing seasons after multiple interventions, the site contained salt marsh vegetation dominated by pickleweed Salicornia virginica and salt marsh daisy Jaumea carnosa. Pickleweed was present in 100% of plots, with 110 g/0.25 m2 above-ground biomass. Salt marsh daisy was present in 87% of plots, with 75 g/0.25 m2 above-ground biomass. Total above-ground biomass was 210 g/0.25 m2 (vs 79 after 1–3 growing seasons). There were 4.0 plant species/plot (vs 3.0–4.5), 3.5 canopy layers (vs 1.9–2.7) and a maximum vegetation height of 33–37 cm (vs 20–38). Relationships between these outcomes and the number of species planted in restoration plots that were significant after 1–3 growing seasons were no longer significant after 11–12 years (see original paper). Methods: In 1996/1997, an upland area was lowered to intertidal elevations and graded into a slope. In this area, eighty-five 4-m2 plots were amended with fine sediment, tilled and levelled. Most were then planted with salt marsh herbs/succulents (90 seedlings/plot; 1–6 species/plot; eight species total). Non-planted vegetation was cleared from all plots in 1997 and 1998, but was left to grow from 1999. Vegetation was surveyed in 45 planted plots in 1997–2000 and 2008–2009. Biomass included standing vegetation only and was dried before weighing. This study used a subset of the plots from (2) and (3).

    Study and other actions tested
  7. A site comparison study involving one mangrove creation site in Singapore (Friess 2017) reported that the average height of surviving trees increased over five years, but that above-ground biomass remained lower than in nearby natural mangrove forests after ≥15 years. Statistical significance was not assessed. After five years, surviving trees were 1.5–2.0 m tall (vs <0.45 m tall when sown or planted). After ≥15 years, the above-ground biomass in the created mangrove (36 t C/ha) was lower than in mature natural mangroves in the rest of Singapore (105–227 t C/ha). Methods: In 1996, a mangrove creation project was established on Pulau Semakau Island. Creation involved depositing ash and other waste materials between granite bunds, adding a 0.5–1.0 m thick layer of mangrove mud, planting propagules, planting nursery-reared seedlings, exposing acid soil to seawater to raise its pH, and removing barnacles and seaweed growing on seedlings. Both loop-root mangrove Rhizophora mucronata and tall-stilt mangrove Rhizophora apiculata were planted, at the elevations they occupied in nearby natural forests. This summary takes some methodological details from Tanaka et al. (2003). The date of biomass monitoring is not clear, but was likely in 2011 or later.

    Additional Reference: Tanaka Y, Arita K, Yauchi E. 2003. A mangrove mitigation project in Singapore. Asia and Pacific Coasts 2003: Proceedings of the 2nd International Conference, Makuhari, Japan, 256–257.

    Study and other actions tested
  8. A site comparison study in 2013–2014 in disused aquaculture ponds in South Sulawesi, Indonesia (Oh et al. 2017) reported that 6–7 months after carrying out restoration interventions, the ponds contained 651 mangrove plants and more mangrove species than nearby reference forests. The restored ponds contained 471 mangrove seedlings/saplings and 180 mangrove trees. Of these, only 137 (21%) were found on reprofiled areas (so definitely colonized after intervention). In total, the restored ponds contained 13 mangrove species (vs 11 in nearby reference forests). The most common genera in both restored and reference forests were Rhizophora spp. (54–65% of seedlings/saplings; 29% of trees) and Avicennia spp. (21–23% of seedlings/saplings; 31–42% of trees). Methods: In November/December 2013, twenty-nine disused aquaculture ponds (21.5 ha) were subjected to multiple restoration interventions: breaching pond walls to improve tidal exchange, reprofiling some walls to more suitable elevations for mangroves (details not reported), adding a pile of broken branches to trap propagules, and releasing >218,000 propagules (>7 species) at high tide. In June 2014, vegetation was surveyed in the restored ponds and two nearby reference mangrove forests (the least disturbed local forests; area surveyed not clearly reported).

    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.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

Marsh and Swamp Conservation

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation
Marsh and Swamp Conservation

Marsh and Swamp Conservation - Published 2021

Marsh and Swamp Synopsis

What Works 2021 cover

What Works in Conservation

What Works in Conservation provides expert assessments of the effectiveness of actions, based on summarised evidence, in synopses. Subjects covered so far include amphibians, birds, mammals, forests, peatland and control of freshwater invasive species. More are in progress.

More about What Works in Conservation

Download free PDF or purchase
The Conservation Evidence Journal

The Conservation Evidence Journal

An online, free to publish in, open-access journal publishing results from research and projects that test the effectiveness of conservation actions.

Read the latest volume: Volume 18

Go to the CE Journal

Discover more on our blog

Our blog contains the latest news and updates from the Conservation Evidence team, the Conservation Evidence Journal, and our global partners in evidence-based conservation.


Who uses Conservation Evidence?

Meet some of the evidence champions

Endangered Landscape Programme Red List Champion - Arc Kent Wildlife Trust The Rufford Foundation Save the Frogs - Ghana Bern wood Supporting Conservation Leaders National Biodiversity Network Sustainability Dashboard Frog Life The international journey of Conservation - Oryx British trust for ornithology Cool Farm Alliance UNEP AWFA Butterfly Conservation People trust for endangered species Vincet Wildlife Trust