Use herbicide to maintain or restore disturbance: freshwater swamps
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
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.
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