Use prescribed fire to control problematic plants: brackish/salt marshes

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

Effectiveness

55%
Certainty

40%
Harms

10%

Calculating overall effectiveness category

Background information and definitions

Prescribed burns can be used to manage problematic plants that may overgrow and outcompete desirable vegetation. By removing above-ground vegetation, fire can also be used to manage the physical vegetation structure (Flores et al. 2011). Prescribed burns may have only temporary effects: many plants can regrow from remaining stumps, roots or rhizomes (underground horizontal stems).

Potential benefits of management by prescribed burning should be weighed up against potential risks. For example, it can be difficult to control the intensity, duration and area of a prescribed burn: burning when the ground is wet and/or cold might be safer (Hackney & de la Cruz 1981). Burning can damage seed banks, and might produce apparently desirable changes in vegetation over the short term followed by a rapid return to a degraded state. Burning can also damage the physical habitat (e.g. by exposing sediments, increasing erosion and reducing accumulation of organic matter; McKee & Grace 2012) and may be harmful to animals like amphibians (Smith & Sutherland 2014) and birds (Flores et al. 2011).

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.

For this action, “vegetation” refers to overall or non-target vegetation. Studies that only report responses of target problematic plants have not been summarized.

^{Flores C., Bounds D.L. & Ruby D.E. (2011) Does prescribed fire benefit wetland vegetation? Wetlands, 31, 35–44.}

^{Hackney C.T. & de la Cruz A.A. (1981) Effects of fire on brackish marsh communities: management implications. Wetlands, 1, 75–86.}

^{McKee K.L. & Grace J.B. (2012) Effects of Prescribed Burning on Marsh Elevation Change and the Risk of Wetland Loss. U.S. Geological Survey Open-File Report 2012-1031.}

^{Smith R.K. & Sutherland W.J. (2014) Amphibian Conservation: Global Evidence for the Effects of Interventions. Pelagic Publishing, Exeter, UK.}

Study locations

Key messages

Four studies evaluated the effects, on vegetation, of using prescribed fire to control problematic plants in brackish/salt marshes. All four studies were in the USA. Two studies were based on the same experimental set-up.

VEGETATION COMMUNITY

Overall richness/diversity (4 studies): Two replicated, randomized, paired, controlled studies in brackish and salt marshes in the USA reported that burned and unburned plots had similar plant species richness over the following 1–3 years. Two studies in saltgrass- or reed-dominated marshes in the USA reported that burned areas had greater plant species richness than unburned areas, after approximately 1–3 years. In one of the studies, burned areas had also been sprayed with herbicide for nine years – and contained more plant species than a nearby natural marsh.

VEGETATION ABUNDANCE

Overall abundance (3 studies): Three replicated, randomized, paired, controlled studies in brackish and salt marshes in the USA evaluated the effect of prescribed burning on vegetation cover. One study found that autumn-burned plots had lower overall vegetation cover than unburned plots after 11 months, but one found that winter-burned plots had similar overall vegetation cover to unburned plots after one year. Two of the studies reported that winter-burned plots had less standing dead vegetation cover than unburned plots in the following summer or winter.
Individual species abundance (4 studies): All four studies quantified the effect of this action on the abundance of individual plant species, other than a species being controlled. For example, three replicated, randomized, paired, controlled studies in brackish and salt marshes in the USA reported mixed effects of burning on cover of saltgrass Distichlis spicata: higher in burned than unburned plots in the following summer, lower in burned than unburned plots in the following winter, or mixed effects amongst marsh types. Two replicated, randomized, paired, controlled studies in brackish and salt marshes in the USA reported that burning did not reduce cover of saltmeadow cordgrass Spartina patens, compared to cover in unburned plots, over the following 1–3 years. One site comparison study of brackish marshes in the USA reported that a marsh that had been burned for three years (and sprayed with herbicide for nine) contained more smooth cordgrass Spartina alterniflora than an unburned and unsprayed marsh, and a similar amount of smooth cordgrass to a nearby natural marsh.

VEGETATION STRUCTURE

Visual obstruction (1 study): One replicated, randomized, paired, controlled study in brackish and salt marshes in the USA found that the visual obstruction caused by vegetation (combination of height and horizontal cover) was similar in burned and unburned plots, after 11 months.
Height (1 study): One site comparison study of brackish marshes in the USA found that in a marsh burned for two years (and sprayed with herbicide for nine), the dominant plant species (smooth cordgrass Spartina alterniflora) grew to a similar height as in a nearby natural marsh.

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 replicated, randomized, paired, controlled study in 1992–1993 in an ephemeral brackish marsh dominated by saltgrass Distichlis spicata in California, USA (De Szalay & Resh 1997) found that burned plots had greater plant species richness than unburned plots, and lower overall vegetation cover, but similar or greater cover of dominant species other than saltgrass. After 11 months, burned plots had greater overall plant species richness than unburned plots (burned: 6.3 species/m²; unburned: 3.1 species/m²) but lower overall vegetation cover (burned: 88%; unburned: >99%). Burned plots had lower saltgrass cover (burned: 65%; unburned: 99%) despite a statistically similar saltgrass density under both treatments (burned: 2,000; unburned: 1,770 stems/m²). Of six other dominant herb species, two had greater cover in burned plots (burned: 9–11%; unburned: 0%) whilst four had statistically similar cover under each treatment (burned 2–17%; unburned: <1–5%). Density of these species was not reported. Methods: Ten pairs of 100-m² plots were established in an impounded brackish marsh, managed for waterfowl (autumn/winter flooding with spring/summer drawdown) but dominated by saltgrass. In September 1992, ten plots were burned (one plot/pair; 50 m²/plot). The other plots were not burned. In August 1993, vegetation was surveyed in two 1-m² quadrats/plot. Cover estimates included live and dead standing plants.
Study and other actions tested
Referenced paper
de Szalay F.A. & Resh V.H. (1997) Responses of wetland invertebrates and plants important in waterfowl diets to burning and mowing of emergent vegetation. Wetlands, 17, 149-156.
A replicated, randomized, paired, controlled study in 1995–1997 in 14 brackish and salt marshes in Louisiana, USA (Gabrey et al. 1999) reported that prescribed winter burning had no significant effect on plant species richness, vegetation structure and overall vegetation cover in the following winter, but increased cover of the two dominant plant species and reduced cover of dead vegetation. Unless specified, statistical significance was not assessed. After one year, a total of 5–8 plant species were recorded across burned plots (vs 6–7 species across unburned plots). Burned and unburned plots created statistically similar visual obstruction (data reported as an index combining plant height and horizontal cover) and had statistically similar overall vegetation cover (burned: 72%; unburned: 76%). However, burned plots had greater live cover of the two dominant plant species (saltmeadow cordgrass Spartina patens: 28–59%; saltgrass Distichlis spicata: 2–11%) and less cover of standing dead vegetation (35–61%) than unburned plots (saltmeadow cordgrass: 19–23%; saltgrass: 1–5%; dead: 75–76%). Vegetation was also surveyed 1–2 months after burning. At this point, all metrics apart from species richness were lower in burned than unburned plots (see original paper for data). Methods: The experiment was carried out in 14 coastal marshes of varying salinity and tidal influence. In winter 1995/1996, when 5 cm of water covered the marshes, one random half of each marsh was burned. In January–February 1996 and 1997, vegetation was surveyed at 40 points in each half of each marsh. This study was based on the same experimental set-up as (3).
Study and other actions tested
Referenced paper
Gabrey S.W., Afton A.D. & Wilson B.C. (1999) Effects of winter burning and structural marsh management on vegetation and winter bird abundance in the Gulf Coast Chenier Plain, USA. Wetlands, 19, 594-603.
A replicated, randomized, paired, controlled study in 1995–1998 in 11 brackish and salt marshes in Louisiana, USA (Gabrey et al. 2001) found that prescribed winter burning typically had no significant effect on summer plant species richness but had mixed effects on the cover of two dominant plant species and caused only a temporary reduction in cover of dead vegetation. Averaged over the three summers following intervention, burned and unburned plots had statistically similar plant species richness in three of four marsh types (for which burned: 4.2–5.7 species/plot; unburned: 5.0–5.5 species/plot; other marsh type higher richness in burned plots). Cover of saltmeadow cordgrass Spartina patens was similar in burned and unburned plots in three of four marsh types, but greater in burned plots in the other marsh type. Cover of saltgrass Distichlis spicata was greater in burned plots in two of four marsh types, lower in burned plots in one marsh type and similar in burned and unburned plots in the other marsh type (see original paper for data). Averaged over all marsh types, cover of standing dead vegetation was lower in burned plots in the first summer (burned: 45%; unburned: 69%) but did not significantly differ between treatments in the following two summers (burned: 64–78%; unburned: 71–72%). Methods: The experiment was carried out in 11 coastal marshes (of four types based on salinity and tidal influence). In winter 1995/1996, one random half of each marsh was burned. The “unburned” half of one marsh experienced a lightning fire in summer 1997. In May–June 1996–1998, vegetation was surveyed at 40 points in each half of each marsh. This study was based on the same experimental set-up as (2).
Study and other actions tested
Referenced paper
Gabrey S.W., Afton A.D. & Wilson B.C. (2001) Effects of structural marsh management and winter burning on plant and bird communities during summer in the Gulf Coast Chenier Plain. Wildlife Society Bulletin, 29, 218-231.
A site comparison study in 2004 of three brackish marshes in an estuary in New Jersey, USA (Hagan et al. 2007) found that prescribed burning (along with applying herbicide) converted a marsh dominated by common reed Phragmites australis to one dominated by smooth cordgrass Spartina alterniflora, with similar cordgrass abundance and height to a natural marsh, but more plant species. After three years of burning and nine years of herbicide application, the treated marsh was statistically similar to a nearby natural marsh in terms of cordgrass dominance (treated: 78%; natural: 83% of stems were smooth cordgrass), density (treated: 286; natural: 360 stems/m²), above-ground biomass (treated: 457; natural: 802 g/m²) and height (treated: 78; natural: 94 cm). However, the treated marsh contained six plant species, including common reed, whilst the natural marsh contained only three. A third, untreated marsh was still dominated by common reed (100% of stems; density: 80 stems/m²; biomass: 2,124 g/m²; height: 317 cm; no other plant species). Methods: In August 2004, vegetation was surveyed in three tidal brackish marshes. One marsh was formerly dominated by common reed, but had been burned in 1996–1998 and sprayed with herbicide in 1996–2004. The study does not distinguish between the effects of these interventions. The second, natural marsh was dominated by smooth cordgrass. The third marsh was dominated by common reed and had not been treated. In each marsh, vegetation was clipped from six 0.25 x 0.25 m quadrats then identified, measured, dried and weighed.
Study and other actions tested
Referenced paper
Hagan S.M., Brown S.A. & Able K.W. (2007) Production of mummichog (Fundulus heteroclitus): response in marshes treated for common reed (Phragmites australis) removal. Wetlands, 27, 54-67.

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.

Where has this evidence come from?

List of journals searched by synopsis

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This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

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Introduce organisms to control problematic plants: freshwater marshes Action Link	Unknown effectiveness (limited evidence)	1	Synopsis Link
Control problematic plants (multiple interventions): brackish/saline marshes or swamps Action Link	No evidence found (no assessment)	0	Synopsis Link
Use prescribed fire to control problematic plants: brackish/salt marshes Action Link	Likely to be beneficial	4	Synopsis Link