Use prescribed fire to maintain or restore disturbance: brackish/salt marshes

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

Study locations

Key messages

  • Ten studies evaluated the effects, on vegetation, of using prescribed fire to maintain or restore disturbance in brackish/salt marshes. Seven studies were in the USA. Two studies were in Argentina but based on the same experimental set-up. One study was in Guadeloupe.

VEGETATION COMMUNITY

  • Community composition (1 study): One replicated, paired, controlled study in a salt marsh in Argentina reported that burned plots had a different overall plant community composition to unburned plots, five months after burning. The same was true in one of two comparisons 17 months after burning.
  • Overall richness/diversity (5 studies): Three studies (including one replicated, paired, controlled) in brackish marshes in the USA and Guadeloupe reported that burning had no significant effect on overall plant species richness, measured approximately 10 weeks to 2 years after the latest burn. In one of the studies, the effects of burning and legal protection were not separated. One replicated, randomized, paired, controlled, before-and-after study in brackish marshes in the USA reported that burning typically had no significant effect on changes in plant species richness over two years. One replicated, paired, controlled study in a salt marsh in Argentina reported that burned plots had greater overall plant species richness and diversity than unburned plots, 5–17 months after burning.
  • Characteristic plant richness/diversity (1 study): One study of a coastal marsh in the USA reported that over three years after restoration – involving a prescribed burn along with restoration of tidal exchange – the number of salt-tolerant plant species increased, whilst the number of freshwater plant species decreased.

VEGETATION ABUNDANCE

  • Overall abundance (5 studies): Three replicated studies (one also randomized, paired, controlled) in brackish marshes in the USA found that overall vegetation biomass was lower in burned than unburned plots, 10 weeks or 1 year after the latest burn. One replicated, randomized, paired, controlled, before-and-after study in alkali marshes in the USA found that a single prescribed burn had no significant effect on overall vegetation biomass: there was a similar change over two years in burned and unburned plots. One replicated, paired, controlled study in a salt marsh in Argentina found that the effect of a single prescribed burn on the frequency of seedlings depended on the time since burning, but that seedlings were more frequent in burned than unburned plots after 9–12 months.
  • Characteristic plant abundance (1 study): One study of a coastal marsh in the USA found that over three years after restoration – involving a prescribed burn along with restoration of tidal exchange – the cover of salt-tolerant plant species increased, whilst the cover of freshwater plant species decreased.
  • Individual species abundance (7 studies): Seven studies quantified the effect of this action on the abundance of individual plant species. For example, five studies quantified the effects of prescribed burning on the abundance of dominant cordgrasses Spartina sp. in brackish and salt marshes in the USA and Argentina. Two replicated, paired, controlled studies found that cordgrass abundance (biomass or cover) was lower in burned than unburned plots, between 10 weeks and 17 months after the latest burn. However, one replicated, paired, site comparison study found that burning typically had no significant effect on cordgrass biomass or density after 2–8 months. One replicated, before-and-after study found that cordgrass biomass was lower, but cover greater, one year after burning than before. One study reported mixed effects on cordgrass cover across two marshes.

VEGETATION STRUCTURE

  • Height (2 studies): Two studies (one controlled, one site comparison) in brackish marshes in the USA and Guadeloupe reported that the height of dominant grass-like plants was lower in burned than unburned areas for up to 1–2 years after the latest burn. The study in the USA reported recovery, to a slightly greater height than in unburned areas, after three years. The study in Guadeloupe also reported that the tallest trees in burned marshes were shorter than the tallest trees in unburned marshes.

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 controlled, before-and-after study in 1977–1979 in a brackish marsh in Mississippi, USA (Hackney & de la Cruz 1981) reported that a prescribed burn temporarily reduced the biomass and height of black rush Juncus roemerianus, but persistently reduced dominance of black rush and big cordgrass Spartina cynosuroides. Statistical significance was not assessed. One study area was initially dominated by black rush. Before burning, above-ground rush biomass was 520 g/m2 (live) and 1,080 g/m2 (dead). In the first six months after burning, black rush biomass was depressed (live: 5–360; dead: 0–84 g/m2). Over the following 30 months, live black rush biomass recovered (290–820 g/m2) whilst dead biomass remained depressed (43–740 g/m2). The maximum height of black rush was 153, 182 and 214 cm respectively in plots one- two- and three- years after burning, compared to 203 cm in unburned plots. Across these plots, black rush comprised only 56–87% of the plant biomass in burned plots (vs 62–94% in unburned plots). Another study area was initially dominated by big cordgrass. It comprised only 1–97% of the plant biomass in burned plots (vs 62–99% in unburned plots). Methods: In early 1977, 1978 or 1979, some plots in rush- or cordgrass-dominated areas of a tidal brackish marsh were burned once. Some additional plots were left unburned. The marsh was historically burned, but not since 1973. Vegetation was surveyed until November 1979. The study does not report further methodological details.

    Study and other actions tested
  2. A replicated, before-and-after study in 1988–1989 in two brackish marshes in Florida, USA (Schmalzer et al. 1991) found that a prescribed burn reduced vegetation biomass, increased cover of tall vegetation and increased species richness of short vegetation. In both marshes, above-ground vegetation biomass was lower one year after burning (530 g/m2) than before (1,730–1,810 g/m2). The same was true for live and dead biomass separately, but the ratio of live to dead biomass increased after burning (see original paper for data). Cover of plants >50 cm tall was greater one year after burning than before (before: 107–108%; after: 120–131%). Richness of plants <50 cm tall increased in both marshes (before: 0.5–1 species/transect; after: 3–4 species/transect). There were no significant changes in cover of shorter plants, richness of taller plants, or total richness (see original paper for data). Results for the dominant species in each marsh (black rush Juncus roemerianus and sand cordgrass Spartina bakeri) mirrored overall results: lower biomass after burning (rush: 465 g/m2; cordgrass: 400 g/m2) than before (rush: 1,576 g/m2; cordgrass: 1,312 g/m2), but greater cover after burning (rush: 99%; cordgrass: 92%) than before (rush: 92%; cordgrass: 70%). Statistical significance of these dominant species results was not assessed. Methods: Two marshes, one rush-dominated and one cordgrass-dominated, were burned in November 1988. The marshes had “long been exposed to fire” but had last burned in 1985. Plant species and their cover were recorded immediately before and one year after the prescribed burn, along four or five 15-m-long transects/marsh. Vegetation was cut from twenty-five 0.25-m2 quadrats/marsh, then dried and weighed.

    Study and other actions tested
  3. A replicated, paired, controlled study in 1991 in a brackish marsh in Louisiana, USA (Taylor et al. 1994) found that burned plots contained less plant biomass than unburned plots, but had similar plant species richness. Ten weeks after a single burn, above-ground vegetation biomass was lower in burned plots (565 g/m2) than in unburned plots (947 g/m2). For five of six common plant species, biomass was statistically similar in burned and unburned plots. For the sixth species, saltmeadow cordgrass Spartina patens, burned plots contained significantly less biomass (311 g/m2) than unburned plots (645 g/m2). Burned and unburned plots contained a statistically similar number of plant species (data not reported). Methods: Twenty 1-m2 plots were established, in five sets of four, in a coastal brackish marsh. The marsh had probably been historically burned: burning is a traditional management technique in the area. Ten plots (two plots/set) were burned in June 1991. The other plots were not burned. Half of the plots in each treatment were also fenced to exclude herbivores. In September 1991, vegetation was cut from each plot then identified, dried and weighed.

    Study and other actions tested
  4. A before-and-after study in 1993–1996 of a coastal marsh in Florida, USA (Brockmeyer et al. 1996) found that following a prescribed burn along with restoration of tidal exchange, species richness and cover of salt-tolerant vegetation increased, whilst species richness and cover of freshwater vegetation decreased. Within three years of tidal restoration, the number of salt-tolerant plant species in the marsh increased from seven to eight. Cover of salt-tolerant vegetation significantly increased (by 1,056%). The number of freshwater plant species decreased from thirteen to one. Cover of freshwater vegetation significantly decreased (by 74%). There was a non-significant 56% decline in southern cattail Typha domingensis cover. Methods: In 1993, thirteen culverts were built to restore tidal exchange to a degraded, impounded, cattail-invaded marsh. In February 1995, the marsh was burned. The study does not distinguish between the effects of these interventions. Vegetation was surveyed along fifteen 15-m transects in October 1993 (before culverts were built) and March 1996.

    Study and other actions tested
  5. A replicated, randomized, paired, controlled, before-and-after study in 1992–1994 in two brackish marshes in Louisiana, USA (Ford & Grace 1998) found that burning reduced vegetation biomass and affected the cover of dominant plant species, but had mixed effects on the cover of dominant plant species and plant species richness. One year after the latest burn, above-ground vegetation biomass was lower in burned areas (280–770 g/m2) than in unburned areas (450–1,200 g/m2). Burning significantly affected the cover of all three dominant plant species in one marsh (e.g. saltmeadow cordgrass Spartina patens cover was 27–35% in burned areas, vs 56–78% in unburned areas) but had no significant effect on cover of both dominant plant species in the other marsh (see original paper for data). Burning had no significant effect on plant species richness in three of four comparisons: there were statistically similar changes over two years in burned and unburned areas (see original paper for data). In the other comparison, involving subplots fenced to exclude wild mammals, plant species richness increased in burned areas (by 3.8 species/m2) but did not significantly change in unburned areas (non-significant decline of 0.4 species/m2). Methods: Ten pairs of 100-m2 plots were established across two brackish marshes (regularly burned for at least 100 years). One random plot in each pair was burned in autumn 1992 and 1993. The other plots were not burned. Each plot contained two 4-m2 subplots, one of which was fenced. Plant species and their cover were recorded in autumn 1992 (before intervention) and 1994. Vegetation was cut from one 0.25-m2 quadrat/subplot, then dried and weighed, in autumn 1994.

    Study and other actions tested
  6. A replicated, paired, site comparison study in 1989 in two brackish marshes in Louisiana, USA (Flynn et al. 1999) found that a single prescribed burn typically had no significant effect on density or biomass of saltmeadow cordgrass Spartina patens. Between two and eight months after intervention, burned and unburned plots contained a statistically similar density of cordgrass stems (data not reported) and similar cordgrass biomass in five of six statistically tested comparisons (for which burned: 420–2,750 g/m2; unburned: 680–2,480 g/m2). In the other comparison, cordgrass biomass was lower in burned plots (1,970 g/m2) than in unburned plots (2,650 g/m2). Methods: Vegetation was sampled in May, August, October and November 1989, from 1–10 plots/marsh burned in March and 1–10 plots/marsh not burned that year. It is not clear whether the marshes had been burned before 1989, but burning is a traditional management technique in the area. Each sample involved cutting vegetation from one 0.1-m2 quadrat/plot then counting stems, and drying and weighing cordgrass plants.

    Study and other actions tested
  7. A replicated, paired, controlled study in 1999–2000 in an ephemeral inland salt marsh in northeast Argentina (Feldman & Lewis 2005) found that burned plots contained a different plant community to unburned plots for up to 17 months, with higher plant diversity and richness, and lower cover of the dominant grass species. Five months after a prescribed burn, the overall plant community composition differed between burned and unburned plots in two of two comparisons. After 17 months, clear differences persisted in only one of two comparisons (data reported as graphical analyses; statistical significance of differences not assessed). At both times, burned plots had significantly higher plant species richness than unburned plots (burned: 11–15 species/16 m2; unburned: 6–10 species/16 m2), significantly higher plant diversity (data not reported), and significantly lower cover of gulf cordgrass Spartina argentinensis (burned: 24–53%; unburned: 61–73%). Methods: Two pairs of 100 x 150 m plots were established in a cordgrass-dominated ephemeral marsh. The plots had not burned for ≥3 years, although fire is usually a common disturbance in these wetlands. In August 1999, one plot in each pair was deliberately burned. Plant species and their cover were recorded in December 1999 and 2000, in twelve 4 x 4 m quadrats/plot. This study was based on the same experimental set-up as (10).

    Study and other actions tested
  8. A site comparison study in 2003 of three ephemeral brackish marshes in Guadeloupe (Imbert & Delbé 2006) found that a marsh where traditional burning was maintained had similar plant species richness to marshes where burning had ceased, but supported a greater relative abundance of herbaceous vegetation. The burned marsh had statistically similar plant species richness (27 species/320 m2) to the unburned marshes (32 species/480 m2). However, the burned marsh was dominated more by short herbs (45% of all individual plants; unburned: 21%) and less by trees/woody lianas (14% of all individual plants; unburned: 27%). The dominant herb, sawgrass Cladium jamaicense, was significantly shorter in burned than unburned marshes (see original paper for data). The tallest tree stems in burned marshes were only 1–2 m, compared to 8 m in the unburned marsh. Methods: In March–April 2003, plant species, cover and height were recorded in three coastal brackish marshes. One marsh was still burned under a traditional management regime (last burned in 2001). In the other two marshes, within a nature reserve, traditional burning had ceased around 1998. Vegetation was surveyed in 16–24 plots, each 20 m2, in each marsh.

    Study and other actions tested
  9. A replicated, randomized, paired, controlled, before-and-after study in 1998–2000 in ephemeral alkali marshes around one lake in Idaho, USA (Austin et al. 2007) found that a single prescribed burn had no significant effect on vegetation biomass. After both one and two years, changes in live above-ground plant biomass were statistically similar in burned plots (non-significant change of <40 g/m2 from before to after intervention) and unburned plots (non-significant change of <100 g/m2 from before to after intervention). Methods: Three pairs of fields with similar neighbouring vegetation were studied. Each field contained a range of wetland habitats, including alkali flats (seasonally flooded; developed a salt crust each summer). All fields had been historically grazed and cut, but were undisturbed from 1996. In October 1998 (when vegetation was dormant) one random field per pair was burned. Vegetation was surveyed in June–July before intervention (1998) and for two years after (1999, 2000).

    Study and other actions tested
  10. A replicated, paired, controlled study in 1999–2000 in an ephemeral inland salt marsh in northeast Argentina (Feldman & Lewis 2007) found that the effect of a single prescribed burn on seedling frequency varied according to the time since burning. In two of two comparisons after one month, the frequency of plant seedlings was statistically similar in burned plots (2% of quadrats contained ≥1 seedling) and unburned plots (6% of quadrats contained ≥1 seedling). After six months, seedlings were less frequent in burned plots in two of two comparisons (burned: 0%; unburned: 16–17%). After 9–12 months, seedlings were more frequent in burned plots in six of six comparisons (burned: 9–81%; unburned: 0–31%). Methods: Two pairs of 100 x 150 m plots were established in a cordgrass-dominated ephemeral marsh. The plots had not burned for ≥3 years, although fire is usually a common disturbance in these wetlands. In August 1999, one plot in each pair was deliberately burned. Seedlings of all plant species were counted between September 1999 and August 2000, in one hundred 50 x 50 cm quadrats/plot. This study was based on the same experimental set-up as (7).

    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.

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Marsh and Swamp Conservation

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Marsh and Swamp Conservation
Marsh and Swamp Conservation

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