Use prescribed fire to maintain or restore disturbance: freshwater swamps

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 2
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How is the evidence assessed?

Effectiveness

43%
Certainty

32%
Harms

20%

Calculating overall effectiveness category

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.}

Study locations

Key messages

Two studies evaluated the effects, on vegetation, of using prescribed fire to maintain or restore disturbance in freshwater swamps. Both studies were in the USA.

VEGETATION COMMUNITY

Tree/shrub richness/diversity (1 study): One replicated, site comparison study in the USA found that shrub-dominated wetlands burned every three years contained fewer species of mature tree than unburned wetlands, but a similar number of shrub and sapling species.

VEGETATION ABUNDANCE

Overall abundance (1 study): One replicated, site comparison study of bottomland swamps in the USA found that swamps burned every 2–3 years had a similar overall density of midstory and understory vegetation to unburned swamps.
Herb abundance (2 studies): One replicated, site comparison study of shrub-dominated wetlands in the USA found that wetlands burned every three years had greater cover of grasses than unburned wetlands, but statistically similar cover of forbs and ferns. Another replicated, site comparison study of bottomland swamps in the USA found that swamps burned every 2–3 years had a similar density of understory grasses to unburned swamps.
Tree/shrub abundance (2 studies): One replicated, site comparison study of shrub-dominated wetlands in the USA found that wetlands burned every three years had greater cover of shrubs than unburned wetlands. Another replicated, site comparison study of bottomland swamps in the USA found that swamps burned every 2–3 years had a similar density of shrubs, vines and other woody plants to unburned swamps.

VEGETATION STRUCTURE

Height (2 studies): One replicated, site comparison study of bottomland swamps in the USA found that swamps burned every 2–3 years had a shorter tree canopy than unburned swamps – but a similar-height midstory and understory. Another replicated, site comparison study of shrub-dominated wetlands in the USA found that the tree canopy was a similar height in wetlands burned every three years and unburned wetlands.
Basal area (1 study): One replicated, site comparison study of bottomland swamps in the USA found that swamps burned every 2–3 years had a similar basal area of trees to unburned swamps.
Canopy cover (1 study): One replicated, site comparison study of shrub-dominated wetlands in the USA found that wetlands burned every three years had less canopy cover than unburned wetlands.

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, site comparison study in 2001 of six bottomland swamp stands in Georgia, USA (Moseley et al. 2003) found that burned stands had a shorter tree canopy than unburned stands, but there were no other significant differences in vegetation structure or abundance. Burned stands had a shorter canopy (21 m) than unburned stands (24 m), but a statistically similar midstory height (burned: 13 m; unburned: 12 m) and understory height (burned: 26 cm; unburned: 26 cm). The treatments also had a statistically similar basal area (burned: 31.6; unburned: 30.3 m²/ha), midstory density (burned: 1,342; unburned: 2,370 stems/ha), understory density (burned: 170; unburned: 98 stems/6 m²). The same was true separately for understory grasses (37 vs 1 stems/6 m²), vines (30 vs 38 stems/6 m²), shrubs (75 vs 26 stems/6 m²) and other woody plants (26 vs 32 stems/6 m²). Methods: In summer 2001, vegetation was surveyed in six stands of poorly drained, bottomland hardwood forest. Three stands had been burned every 2–3 years for the past nine years (final burn January 2001). The other three stands had not been burned for at least nine years. Canopy and midstory vegetation were surveyed in two 0.04-ha plots/stand. Understory vegetation was surveyed in six 1-m² quadrats/plot.
Study and other actions tested
Referenced paper
Moseley K.R., Castleberry S.B. & Schweitzer S.H. (2003) Effects of prescribed fire on herpetofauna in bottomland hardwood forests. Southeastern Naturalist, 2, 475-486.
A replicated, site comparison study in 1996 of 48 pocosins (shrub-dominated, freshwater wetlands) within pine forest in North Carolina, USA (Allen et al. 2006) found that triennial prescribed burning increased shrub and grass cover, but reduced canopy cover and tree species richness, and had no significant effect on fern cover, forb cover, canopy height or shrub/sapling species richness. Compared to pocosins that had not burned during any growing season, pocosins burned every three growing seasons had greater shrub cover (burned: 50%; unburned: 40%) and greater grass cover (burned: 14%; unburned: 6%). However, burned pocosins had lower canopy cover (burned: 75%; unburned: 89%) and contained fewer mature tree species (burned: 6 species/site; unburned: 10 species/site). Burned and unburned pocosins had statistically similar fern cover (10% vs 6%), forb cover (3% vs 2%), tree canopy height (22 vs 19 m), shrub species richness (13 vs 12 species/site) and sapling species richness (12 species/site). Methods: In 2006, vegetation was surveyed at 19 sites within pocosins burned every three growing seasons since 1989, and at 29 sites within pocosins that had not burned during the growing season in this period. The pocosins were historically disturbed by fire, but this was suppressed after European settlement. Mature trees were surveyed in four 11-m-radius plots/site. Other vegetation was surveyed in four 5-m-radius plots/site.
Study and other actions tested
Referenced paper
Allen J.C., Krieger S.M., Walters J.R. & Collazo J.A. (2006) Associations of breeding birds with fire-influenced and riparian-upland gradients in a longleaf pine ecosystem. The Auk, 123, 1110-1128.

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?

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This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

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