Facilitate tidal exchange to restore degraded brackish/saline swamps

Overall effectiveness category Likely to be beneficial
Number of studies: 4
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How is the evidence assessed?

Effectiveness

70%
Certainty

60%
Harms

0%

Calculating overall effectiveness category

Background information and definitions

This action involves facilitating tidal exchange to degraded swamps (i.e. still recognisable as, or retaining substantial characteristics of, the target habitat). The action could be a single permanent one (e.g. breaching sea walls or embankments, installing or widening culverts, excavating tidal creeks) or a reversible one (e.g. opening sluice gates once per day). Facilitating tidal exchange can affect multiple properties of a site: it can raise moisture levels, raise or reduce salinity, increase physical disturbance, and increase supplies of sediment and wetland plant propagules.

Tidal wetlands may be brackish/saline (e.g. mangroves, coastal marshes) or freshwater (e.g. at the upstream end of estuaries, as in the Mississippi, Yangtze, and Elbe rivers; Baldwin et al. 2009).

Studies of accidental restoration of tidal exchange, such as when coastal defences are breached by a storm, have not been summarized as evidence.

Related actions: Facilitate tidal exchange to restore/create swamps from other land uses; Add salt to control problematic plants; Reprofile/relandscape or Remove surface soil/sediment, both of which can alter patterns of tidal exchange.

^{Baldwin A.H., Barendregt A. & Whigham D. (2009) Tidal Freshwater Wetlands. Backhuys Publishers, Leiden.}

Study locations

Key messages

Four studies evaluated the effects, on vegetation, of facilitating tidal exchange to restore degraded brackish/saline swamps. Three studies were in Mexico and one was in India.

VEGETATION COMMUNITY

Overall extent (2 studies): Two before-and-after studies on the coasts of India and Mexico reported that the area of mangrove forest in each site was greater 5–6 years after restoring tidal exchange (sometimes along with planting mangrove seedlings) than in the years before.
Community composition (1 study): One before-and-after study of a mangrove forest in Mexico reported that the tree community composition was identical before and five years after restoring tidal exchange: the same three tree species were present at both times.
Community types (1 study): One before-and-after study of a mangrove forest in Mexico reported that the relative coverage of stands dominated by each of three tree species was similar before and five years after restoring tidal exchange.
Tree/shrub richness/diversity (2 studies): One site comparison study in Mexico reported that a tidally restored mangrove forest contained a similar number of tree species to nearby natural mangroves, after 10–11 years. One before-and-after study in Mexico reported identical tree species richness in a mangrove forest before and five years after restoring tidal exchange.

VEGETATION ABUNDANCE

Tree/shrub abundance (2 studies): Two site comparison studies in Mexico reported that tidally restored mangrove forests contained a lower density of trees or seedlings than nearby natural mangroves.
Individual species abundance (1 study): One site comparison study in Mexico compared the abundance of three mangrove tree species in a tidally restored area and nearby natural forests (see original paper for data).

VEGETATION STRUCTURE

Height (2 studies): One site comparison study in Mexico reported that trees in a tidally restored mangrove forest were a similar height to trees in nearby natural mangroves, after 10–11 years. Another replicated, site comparison study in Mexico reported that seedlings in a tidally restored mangrove forest were taller than seedlings in a nearby natural mangrove.
Diameter (1 study): One site comparison study in Mexico reported that trees in a tidally restored mangrove forest had a similar diameter to trees in nearby natural mangroves, after 10–11 years.
Basal area (1 study): One site comparison study in Mexico reported that trees in a tidally restored mangrove forest had a smaller basal area than trees in natural mangroves, after 10–11 years.

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

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).
Study and other actions tested
Referenced paper
Selvam V., Ravichandran K.K., Gnanappazham L. & Navamuniyammal M. (2003) Assessment of community-based restoration of Pichavaram mangrove wetland using remote sensing data. Current Science, 85, 794-798.
A site comparison study in 2006–2007 of mangrove forests in northwest Mexico (Vovides et al. 2011) reported that a forest connected to a partially reopened tidal channel contained a different tree community and fewer trees than pristine natural forests, but that both forests contained similarly sized trees. Statistical significance was not assessed. After 10–11 years, the tidally restored area contained two tree species (mostly black mangrove Avicennia germinans with some red mangrove Rhizophora mangle). Pristine forests contained 2–3 species (always including white mangrove Laguncularia racemosa). The tidally restored area contained only 3.3 trees/m² (vs pristine: 4.5–7.9 trees/m²) with a basal area of 34 cm²/m² (vs pristine: 48–68 cm²/m². For data on the abundance of individual species, see original paper. The average size of trees in the tidally restored area (height: 0.9–1.0 m; diameter: 3–5 cm) was within the range of trees in the pristine forest (height: 0.9–1.3 m; diameter: 2–9 cm). Methods: In 2006 or 2007, live trees were counted, identified and measured in 10 plots. Two 5-m² plots were in a forest with partially restored tidal exchange. Its feeder channel had been blocked during road construction in 1995, then reopened one year later in 1996 (but only to 3.5 m diameter, not the pre-construction 5 m). Eight 1-m² plots were in pristine mangrove patches in a nearby lagoon.
Study and other actions tested
Referenced paper
Vovides A.G., Bashan Y., López-Portillo J.A. & Guevara R. (2011) Nitrogen fixation in preserved, reforested, naturally regenerated and impaired mangroves as an indicator of functional restoration in mangroves in an arid region of Mexico. Restoration Ecology, 19, 236-244.
A before-and-after study in 2004–2009 of a mangrove forest in northwest Mexico (Bashan et al. 2013) reported that after excavating a channel to restore tidal exchange, the area of live mangrove trees increased. Before excavation, the site contained only 2,890 m² of live mangrove trees. Approximately five years after excavation, the site contained 11,830 m² of live mangrove trees. Mangroves recolonized the site and expanded into surrounding sand dunes. The same three tree species were present before and after restoration, in similar proportions (% of total mangrove area): 49% red mangrove Rhizophora mangle, 23–24% black mangrove Avicennia germinans and 27–28% white mangrove Laguncularia racemosa. Methods: In April–June 2004, tidal exchange was restored to a degraded mangrove forest by excavating a stepped outlet channel through a sandbar (deposited by a hurricane three years previously). The channel had to be cleared of sediment twice after the initial excavation. The area covered by live mangrove trees was measured from satellite images, and verified with field surveys, before excavation (early 2004) and approximately five years after (2009).
Study and other actions tested
Referenced paper
Bashan Y., Moreno M., Salazar B.G. & Alvarez L. (2013) Restoration and recovery of hurricane-damaged mangroves using the knickpoint retreat effect and tides as dredging tools. Journal of Environmental Management, 116, 196-203.
A before-and-after, site comparison study in southeast Mexico (Zaldívar-Jiménez et al. 2017) reported that following dredging of tidal channels to restore more natural tidal exchange to a degraded mangrove forest, mangrove seedlings colonized. Before dredging, there were no mangrove seedlings present in the study site. After dredging, there were 82 seedlings/100 m². They were 56 cm tall on average. In a nearby undisturbed mangrove, there were 3,400 seedlings/100 m². These were 40 cm tall on average. Methods: Around 2010, tidal channels were dredged in the 1,300-ha degraded mangrove forest on the edge of Términos Lagoon. This increased the flooding frequency and reduced flooding duration towards levels in undisturbed mangroves, but reduced sediment salinity below levels in undisturbed mangroves. Local communities were also engaged in restoration activities and decision-making. The study does not report details of vegetation monitoring.
Study and other actions tested
Referenced paper
Zaldívar-Jiménez A., Ladrón-de-Guevara-Porras P., Pérez-Ceballos R., Díaz-Mondragón S. & Rosado-Solórzano R. (2017) US-Mexico joint Gulf of Mexico large marine ecosystem based assessment and management: experience in community involvement and mangrove wetland restoration in Términos lagoon, Mexico. Environmental Development, 22, 206-213.

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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This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

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Lower water level to restore degraded brackish/saline swamps Action Link	No evidence found (no assessment)	Synopsis Link