Deposit soil/sediment and introduce vegetation: brackish/salt marshes

Supporting evidence from individual studies

1. A paired, site comparison study in 1978–1979 of four intertidal salt marshes in Texas, USA (Webb & Newling 1984) found that a created marsh – where cordgrasses Spartina spp. were planted and sown after depositing sediment – typically developed similar vegetation biomass and cordgrass of similar height to natural marshes, with mixed results for cordgrass density. Within four years, total above-ground vegetation biomass was statistically similar in created and natural marshes in 19 of 24 comparisons (for which created: 278–982 g/m²; natural: 132–1,102 g/m²). The average height of smooth cordgrass Spartina alterniflora was statistically similar in created and natural marshes in 15 of 21 comparisons (for which created: 34–124 cm; natural: 13–114 cm). In the other eight comparisons, all below mean high tide, cordgrass was taller in created marshes. The density of smooth cordgrass was similar in created and natural marshes in only 10 of 24 comparisons (for which created: 27–489; natural: 12–635 stems/m²). Its density was higher in created marshes in seven comparisons (all above mean high tide; created: 27–156; natural: 0–63 stems/m²) but lower in created marshes in seven comparisons (all below mean high tide; created: 192–489; natural: 396–814 stems/m²). Methods: Between August and November 1978 and 1979, vegetation was surveyed in one created and three natural salt marshes (six 0.5-m² quadrats at each of 3–5 tide levels/marsh). Marsh creation involved depositing sediment in 1975 then, in some areas, planting cordgrass sprigs in summer 1976 and sowing cordgrass seeds in spring 1977. All quadrats were in or near to planted/sown areas (not clearly reported). Biomass was dried before weighing. One site from this study was also included in (3). Some data from this study were included in (5).

   Study and other actions tested

   Referenced paper

   Webb J.W. & Newling C.J. (1984) Comparison of natural and man-made salt marshes in Galveston Bay Complex, Texas. Wetlands, 4, 75-86.

2. A site comparison study in 1989 of four estuarine salt marshes in California, USA (Zedler 1993) found that parts of a marsh created by depositing sediment and planting California cordgrass Spartina foliosa supported a similar cordgrass density to adjacent natural marshes, but with shorter plants. Statistical significance was not assessed. After 7–9 years, four of six transects in the created marsh supported a cordgrass density (100–193 stems/m²) within the range of nearby natural marshes (73–193 stems/m²). The other two transects supported a lower cordgrass density (40–60 stems/m²). However, a greater proportion of stems were in shorter height classes in the created marshes than in the natural marshes (data reported graphically). Methods: In September 1989, California cordgrass was surveyed in 0.1-m² quadrats. Thirty-six quadrats (six transects) were surveyed in a marsh created by depositing dredge spoil (in 1980) then planting California cordgrass (in 1984–1986). Fifty-four quadrats (seven transects) were surveyed in three nearby natural marshes.

   Study and other actions tested

   Referenced paper

   Zedler J.B. (1993) Canopy architecture of natural and planted cordgrass marshes: selecting habitat evaluation criteria. Ecological Applications, 3, 123-138.

3. A replicated, site comparison study in 1990 of 15 coastal salt marshes in Texas, USA (Delaney et al. 2000) found that marshes created by depositing sediment and/or introducing vegetation had similar coverage of emergent plant community types to natural marshes, but contained larger, smoother patches of the most abundant community type. Created and natural marshes had statistically similar coverage of four of four plant communities: three types of emergent herbaceous vegetation (created: 2–47%; natural: 1–45% coverage/type) and one type of shrubby vegetation (created: 6%; natural: 2%). The most abundant community type was dense, regularly flooded smooth cordgrass Spartina alterniflora. In created marshes, this occurred in larger blocks with straighter edges (vs smaller patches with undulating edges in natural marshes; data reported as landscape metrics). Methods: Ten created marshes, and representative sections of five nearby natural marshes, were mapped from aerial photographs taken in October 1990. All marshes were landward of barrier islands. The created marshes were 3–15 years old. Marsh creation involved: planting smooth cordgrass onto deposited sediment (five marshes) or excavated upland (one marsh); planting only (two marshes); or depositing sediment only (two marshes). The study does not report further details of creation methods, or separate results for different means of creation. Some sites from this study were also included in (4).

   Study and other actions tested

   Referenced paper

   Delaney T.P., Webb J.W. & Minello T.J. (2000) Comparison of physical characteristics between created and natural estuarine marshes in Galveston Bay, Texas. Wetlands Ecology and Management, 8, 343-352.

4. A replicated, paired, site comparison study in 1995–1998 of 20 coastal salt marshes in Texas, USA (Shafer & Streever 2000) found that marshes created by depositing sediment and planting smooth cordgrass Spartina alterniflora had similar vegetation coverage to natural marshes after 2–23 years, and also had similar amounts of edge habitat. Created and natural marshes had statistically similar coverage of marsh vegetation (measured as the ratio of open water to vegetation; created: 0–0.7 for 9 of 10 marshes, 3.5 in the other; natural: 0–0.3). Created and natural marshes also had a statistically similar proportion of edge habitat (i.e. where marsh vegetation meets open water, measured as an edge-to-area ratio; created: 123–1,805 m/ha; natural: 239–1,134 m/ha). Methods: Vegetation patches were mapped on 10 pairs of salt marshes, using aerial photographs taken in 1995 or 1998. Each pair contained a created marsh (formed by depositing dredged sediment then planting smooth cordgrass) and a nearby natural marsh (with vegetation composition and exposure as similar as possible to the created marsh). Created marshes were 2–23 years old when photographed. Some sites from this study were also included in (3) and (5).

   Study and other actions tested

   Referenced paper

   Shafer D.J. & Streever W.J. (2000) A comparison of 28 natural and dredged material salt marshes in Texas with an emphasis on geomorphological variables. Wetlands Ecology and Management, 8, 353-366.

5. A 2000 review analyzing coastal salt marshes in nine sites in the southeast USA (Streever 2000) found that marshes created by depositing sediment and planting marsh vegetation contained a similar amount of smooth cordgrass Spartina alterniflora to natural marshes, after 1–9 years. Averaged across all years, smooth cordgrass abundance was statistically similar in created and natural marshes (no significant difference from a 1:1 ratio). This was true for both above-ground biomass and stem density. Biomass in created marshes ranged from 0.5 times to 3.3 times the biomass in natural marshes. Stem density in created marshes ranged from 0.5 times to 1.9 times the density in natural marshes (but was never lower in created than natural marshes >3 years after creation). Methods: The review used published data from Florida, Georgia, North Carolina and Texas. It included data from (1) and several sites from (3) and (4). For each metric, year and site (above-ground biomass in seven sites, density in eight sites), a ratio was calculated to compare smooth cordgrass abundance in created and natural marshes. In all sites, marsh creation involved depositing dredged sediment in coastal waters until the surface was intertidal, then planting smooth cordgrass. Other marsh plant species were planted in some sites.

   Study and other actions tested

   Referenced paper

   Streever W.J. (2000) Spartina alterniflora marshes on dredged material: a critical review of the ongoing debate over success. Wetlands Ecology and Management, 8, 295-316.

6. A replicated, before-and-after, site comparison study in 1991–1995 of seven salt marshes in Texas, USA (Darnell & Smith 2001) reported that areas of deposited sediment planted with marsh plants developed marsh vegetation, but that that the plant community composition differed from natural marshes after 2–4 years. Statistical significance was not assessed. After 2–4 years, 34–63% of each created marsh was covered by salt marsh vegetation; 1–35% was unvegetated salt pans and 16–30% was subtidal open water. The most abundant species was smooth cordgrass Spartina alterniflora (18–45% cover). For comparison, nearby natural marshes were composed entirely of salt marsh vegetation (52–71% of area) and unvegetated salt pans (29–48% of area). The most abundant species were saltwort Batis maritima (31–41% cover) and shoregrass Monanthochloe littoralis (5–47% cover). Smooth cordgrass was absent. Similarity in the overall plant community composition between created and natural marshes ranged from <1 to 36%. Methods: Four marshes were created on the Texas coast between 1991 and 1993. Dredged sediment was pumped into 2–7 ha cells, enclosed by levees. Once the sediment had settled, marsh vegetation was planted into the intertidal parts of each site (three sites: 12 herb and shrub species; one site smooth cordgrass only). The created marshes were designed to contain diverse habitats rather than to replicate the natural marshes exactly; they had a steeper gradient and were flooded more often than the natural marshes. The four created marshes, and three nearby natural marshes, were surveyed in 1995. Habitat coverage was mapped from aerial photographs. Plant species were recorded along transects at 200–303 points/site.

   Study and other actions tested

   Referenced paper

   Darnell T.M. & Smith E.H. (2002) Recommended design for more accurate duplication of natural conditions in salt marsh creation. Environmental Management, 29, 813-823.