Use prescribed fire to maintain or restore disturbance: brackish/salt marshes
Overall effectiveness category Likely to be beneficial
Number of studies: 10
Background information and definitions
Disturbance can clear dominant plants, maintain light availability and control nutrient levels – and may maintain vegetation in a desirable and/or species-rich state (Hall et al. 2008; Middleton 2013). Therefore, conservationists sometimes want to actively restore disturbance where it has ceased, or maintain disturbance at a site where it would otherwise be lost. Prescribed burns are one way to do this.
Fire itself may be the historic or traditional disturbance that maintains wetlands in a desirable state. Some wetlands, especially ones that dry out in summer, burn naturally every few years (Sutter & Kral 1994). In other wetlands, prescribed burns have been used by humans to manage vegetation (Middleton 2013). Reduced disturbance from fire in these systems – whether through abandonment or deliberate control of fire (e.g. via fire breaks or legislation) – can be detrimental to vegetation diversity, composition and structure (e.g. Clark & Wilson 2001).
Caution: Disturbance, and fire in particular, is not a natural feature of all wetlands. For example, even within the southeast USA, the natural fire frequency can vary from once per year to once per century (Sutter & Kral 1994). It can be difficult to control the intensity, duration and area of prescribed burns. Burns in winter or wet season, might be easier to control than burns in the summer or dry season. Smoke from prescribed burns could be detrimental to human health, especially near urban areas (Agee 1996). Also note potential impacts on animals within wetlands – but that some taxa might be unaffected or be able to avoid fire (e.g. Ditlhogo et al. 1992).
The timing and duration of monitoring might be particularly important when evaluating the effects of this action. Burning might produce apparently desirable changes in vegetation over the short term, followed by a rapid return to a degraded state.
Related actions: Use prescribed fire to control problematic plants, whose success is not linked to a change in disturbance regime; Reduce frequency of prescribed burning; Reduce intensity of prescribed burning; Change season/timing of prescribed burning.
Agee J. (1996) Achieving conservation biology objectives with fire in the Pacific Northwest. Weed Technology, 10, 417–421.
Clark D.L. & Wilson M.V. (2001) Fire, mowing, and hand-removal of woody species in restoring a native wetland prairie in the Willamette Valley of Oregon. Wetlands, 21, 135–144.
Ditlhogo M.K.M., James R., Laurance B.R. & Sutherland W.J. (1992) The effects of conservation management of reed beds. I. The invertebrates." Journal of Applied Ecology, 29, 265–276.
Hall S.J., Lindig-Cisneros R. & Zedler J.B. (2008) Does harvesting sustain plant diversity in Central Mexican wetlands? Wetlands, 28, 776–792.
Middleton B.A. (2013) Rediscovering traditional vegetation management in preserves: trading experiences between cultures and continents. Biological Conservation, 158, 750–760.
Sutter R.D. & Kral R. (1994) The ecology, status, and conservation of two non-alluvial wetland communities in the South Atlantic and Eastern Gulf coastal plain, USA. Biological Conservation, 235–243.
Supporting evidence from individual studies
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
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
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
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
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
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
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
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
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
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