Use herbicide to maintain or restore disturbance: freshwater swamps

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
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How is the evidence assessed?

Effectiveness

30%
Certainty

30%
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. Applying herbicide might be one way to do this.

Bear in mind that the effects of herbicide might be highly dependent on the chemical used, how it is applied and local site conditions (e.g. nutrient availability, water levels, presence/density of wild herbivores) (Tobias et al. 2016).

Caution: In many herbicides, the active chemicals are not specific to the problematic species so can cause collateral damage to desirable species. Relying on herbicides as the only tool to manage problematic plants can lead to the development of herbicide resistance in future generations (Powles et al. 1997). Herbicides can have severe negative side effects on biodiversity, the environment and human health (Pimentel et al. 1992). Accordingly, herbicide use – particularly in or near wetlands or water bodies – is limited in many countries.

Related actions: Use herbicide to control problematic plants, whose success is not linked to a change in disturbance regime.

^{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.}

^{Pimentel D., Acquay H., Biltonen M., Rice P., Silva M., Nelson J., Lipner V., Giordano S., Horowitz A. & D’Amore M. (1992) Environmental and economic costs of pesticide use. BioScience, 42, 750–760.}

^{Powles S.B., Preston C., Bryan I.B. & Jutsum A.R. (1997) Herbicide resistance: impact and management. Advances in Agronomy, 58, 57–93.}

^{Tobias V.D., Block G. & Laca E.A. (2016) Controlling perennial pepperweed (Lepidium latifolium) in a brackish tidal marsh. Wetlands Ecology and Management, 24, 411–418.}

Study locations

Key messages

One study evaluated the effects, on vegetation, of using herbicide to maintain or restore disturbance in freshwater swamps. The study was in the USA.

VEGETATION COMMUNITY

VEGETATION ABUNDANCE

Herb abundance (1 study): One replicated, randomized, controlled, before-and-after, site comparison study of freshwater swamps in the USA found that applying herbicide to woody vegetation (after cutting it) had no significant effect on herbaceous ground cover one year later: there were similar changes in treated and untreated swamps.

VEGETATION STRUCTURE

Basal area (1 study): One replicated, randomized, controlled, before-and-after, site comparison study of freshwater swamps in the USA found that applying herbicide to woody vegetation (after cutting it) had no significant effect on the basal area of woody vegetation one year later: there were similar changes in treated and untreated swamps.
Canopy cover (1 study): The same study found that applying herbicide to woody vegetation (after cutting it) reduced canopy cover – to similar levels as in high-quality swamps after one year.

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, controlled, before-and-after, site comparison study in 2009–2011 of 19 ephemeral freshwater swamps in Florida, USA (Gorman et al. 2013) found that cutting and applying herbicide to midstory vegetation reduced canopy cover one year later, but had no significant effect on ground cover or basal area. One year before intervention, treated swamps had higher canopy cover (55%) than untreated high-quality swamps (36%). One year after intervention, canopy cover in treated swamps had declined to 41%: not significantly different from the 37% cover in high-quality swamps. In untreated low-quality swamps, canopy cover was 49–54%. Other vegetation metrics showed statistically similar responses over time (one year before vs one year after intervention) in both treated and untreated swamps. This was true for herbaceous ground cover (treated: 23% vs 17%; high-quality: 48% vs 37%; low-quality: 22% vs 19%) and the basal area of woody vegetation (treated: 14% vs 12%; high-quality: 10% vs 9%; low-quality: 16% vs 15%). Methods: In August–September 2010, excessive woody vegetation – that had grown following suppression of dry season fires – was removed from eight swamps (<6 ha). Midstory vegetation (<12.7 cm trunk diameter) was cut and removed, then herbicide (triclopyr) was applied to stumps. Note that this study evaluates the combined effect of cutting and applying herbicide. Vegetation was not treated in seven additional overgrown swamps (“low-quality habitat” for wildlife) or in four additional swamps without a dense midstory (“high-quality habitat” for wildlife). Vegetation was surveyed in each swamp in autumn 2009 and 2011. Canopy cover included the midstory and overstory. Herb cover was estimated in one 0.1-m² quadrat/swamp.
Study and other actions tested
Referenced paper
Gorman T.A., Haas C.A. & Himes J.G. (2013) Evaluating methods to restore amphibian habitat in fire-suppressed pine flatwoods wetlands. Fire Ecology, 9, 96-109.

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

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Remove plant litter: freshwater swamps Action Link	No evidence found (no assessment)	0	Synopsis Link
Cut large trees/shrubs to maintain or restore disturbance: brackish/saline swamps Action Link	No evidence found (no assessment)	0	Synopsis Link
Use prescribed fire to maintain or restore disturbance: freshwater swamps Action Link	Unknown effectiveness (limited evidence)	2	Synopsis Link