Reprofile/relandscape: brackish/saline swamps
Overall effectiveness category Likely to be beneficial
Number of studies: 5
Background information and definitions
This action involves large-scale reprofiling or landscaping, aiming to restore or create swamps. This includes excavating large basins (>8 ha or >280 m diameter), moving soil/sediment from the site into levees/berms/impoundments, removing unnatural hills or levees, filling in deep depressions and altering the elevation/slope of coastal areas. In other words, this intervention aims to restore wetland hydrology (how wet the soil is and when it is wet/flooded) by adjusting the ground surface relative to the water table or sea. Soil from existing marshes or swamps might be imported as part of relandscaping efforts, but we consider this as a separate action.
Caution: Heavy machinery is usually needed for this action. Heavy vehicles can churn and compress wetland soils (Campbell et al. 2002).
Related actions: Raise water level to restore degraded swamps; Raise water level to restore/create swamps from other land uses; Facilitate tidal exchange to restore degraded swamps; Facilitate tidal exchange to restore/create swamps from other land uses; Excavate pools; Create mounds or hollows; Remove surface soil/sediment; Deposit soil/sediment to form physical habitat structure; Reprofile/relandscape before planting.
Campbell D.A., Cole C.A. & Brooks R.P. (2002) A comparison of created and natural wetlands in Pennsylvania, USA. Wetlands Ecology and Management, 10, 41–49.
Supporting evidence from individual studies
A replicated, paired, site comparison study in 1996–1997 involving two reprofiled sites (also planted with mangrove propagules) 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 non-planted trees, of reprofiling and planting. 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 (3).Study and other actions tested
A site comparison study in the early 1990s on the coast of Singapore (Lee et al. 1996) reported that an area reprofiled to the same elevation as a neighbouring remnant mangrove forest was colonized by mangrove vegetation within 42 months, but with greater plant species richness and fewer, shorter plants than the remnant mangrove. Statistical significance was not assessed. After 42 months, the reprofiled area contained 9 plant species and 241 individual plants along an 80-m2 transect (vs natural: 7 species and 487 individuals). Most plants in the reprofiled area were ≤2 m tall (75%) and the tallest were ≤6 m. In the remnant mangrove, most plants were also <2 m tall (77%) but some were >8 m. The reprofiled area was dominated numerically by smallflower bruguiera Bruguiera parviflora (50% of individuals, but mostly saplings) whereas the remnant mangrove was dominated numerically by Avicennia alba (67% of individuals, but mostly saplings). Methods: In 1988, a 1-ha plot between a remnant patch of mangrove forest and a tidal river was reprofiled to allow tidal inundation around 40–50 times/month (as in the remnant mangrove). Forty-two months later, vegetation was surveyed along a 2 x 40 m transect in the reprofiled plot and the remnant mangrove. All individual plants were identified and measured.Study and other actions tested
A site comparison study in 1989–2000 in Florida, USA (Proffitt & Devlin 2005) reported that after reprofiling a coastal site (and planting mangrove propagules) mangrove forest stands developed, but that these contained more trees with a greater basal area than natural forest after 18 years. Tall mangrove stands occupied 74% of the restored area after six years, then 95% after 14 years. 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). Statistical significance was not assessed. 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. The study does not distinguish between the effects, on non-planted trees, of reprofiling and planting. Surveys involved taking aerial photographs to estimate overall mangrove area, and counting/measuring trees within 25-m2 plots or 1-m2 quadrats (see original paper for details). This study monitored one of the sites from (1).Study and other actions tested
A replicated, site comparison study in 2005 in Florida, USA (Shafer & Roberts 2008) reported that 12 of 17 mangrove creation/restoration sites (all reprofiled, 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 16,370 trees/ha on average (vs natural: only 6,594 trees/ha) and had a canopy height of only 3.7 m (vs natural: 6.4 m). Both created/restored and natural forests had a similar average basal area (28–31 m2/ha, and contained 1–3 tree species. Methods: In 2005, vegetation was surveyed in 17 sites (three 2 x 2 m plots/site). Between 1975 and 1987, all of these sites had been reprofiled to appropriate elevations for mangroves. All but one had also been planted with red mangrove Rhizophora mangle seedlings or propagules, and some (precise number not reported) had been planted with smooth cordgrass Spartina alterniflora. The study does not distinguish between the effects, on unplanted trees, of reprofiling, planting mangroves and planting cordgrass. Comparisons were made with previously published data from seven nearby natural forests.Study and other actions tested
A study in 1999–2005 in a former shrimp pond in Thailand (Matsui et al. 2010) reported that six years after reprofiling (along with restoring tidal exchange and planting mangrove seedlings), 1,797 unplanted trees of 15 different species were present. The most abundant species were grey mangrove Avicennia marina (842 trees), Bruguiera cylindrica (486 trees) and Ceriops decandra (267 trees). Four species were represented by a single tree. Methods: In June 1999, an abandoned 6,525-m2 shrimp pond was filled in, and tidal exchange was restored by levelling the banks. In September 1999, seedlings of four mangrove species were planted in the pond (500–800 seedlings/species, 1.5 m apart). The study does not distinguish between the effects, on naturally colonizing vegetation, of reprofiling, restoring tidal exchange and planting. In October 2005, mangrove trees that had spontaneously colonized were recorded in a 300-m2 section of the site.Study and other actions tested