Cut or burn oil-contaminated vegetation: brackish/salt marshes
Overall effectiveness category Likely to be ineffective or harmful
Number of studies: 2
Background information and definitions
Oil spills can damage vegetation in marshes. Lighter oils can kill plants through their toxic effects, whilst heavier oils coat and smother vegetation (Michel & Rutherford 2013). Coastal sites are vulnerable to offshore tanker spillages. For example, an estimated 5.5 million tonnes of oil has been released into mangrove-lined coastal waters around the world since 1958, killing at least 126,000 ha of mangrove vegetation (Duke 2016). Oil pipelines crossing inland wetlands (e.g. the Russia-China Oil Pipeline; Yu et al. 2010) pose a threat from leaks and malfunctions. Loss of vegetation can increase the risk of erosion and permanent habitat loss (Beland et al. 2017; https://youtu.be/UkATPicHIo4).
This action involves cutting or burning above-ground plant parts from oil-contaminated marshes. This may increase oxygen supply to the root zone (oil smothering leaves may interfere with oxygen diffusion) and prevent complete death of the oiled plants (Zengel & Michel 1996).
Caution: Activity on the wetland surface (e.g. foot traffic) could churn oil further into the sediment. Removing above-ground vegetation could increase rates of erosion.
Beland M., Biggs T.W., Roberts D.A., Peterson S.H., Kokaly R.F. & Piazza S. (2017) Oiling accelerates loss of salt marshes, southeastern Louisiana. PLoS ONE, 12, e0181197.
Michel J. & Rutherford N. (2013) Oil Spills in Marshes: Planning & Response Considerations. US Department of Commerce.
Michel J. & Rutherford N. (2014) Impacts, recovery rates and treatment options for spilled oil in marshes. Marine Pollution Bulletin, 82, 19–25.
Yu X., Wang G., Zou Y., Wang Q., Zhao H. & Lu X. (2010) Effects of pipeline construction on wetland ecosystems: Russia-China Oil Pipeline Project (Mohe-Daqing Section). Ambio, 39, 447–450.
Zengel S.A. & Michel J. (1996) Vegetation cutting as a clean-up method for salt and brackish marshes impacted by oil spills: a review and case history of the effects on plant recovery. Marine Pollution Bulletin, 32, 876–885.
Supporting evidence from individual studies
A 1996 review of studies in brackish/salt marshes in the UK and the USA (Zengel & Michel 1996) reported mixed effects of cutting oil-contaminated vegetation on its recovery. Statistical significance was not assessed. Considering the eight cases that quantitatively compared cut and uncut areas in the field, the review suggests that cutting had no clear effect on vegetation abundance (density, biomass or cover) in four cases (50%) and a negative effect on vegetation abundance (biomass or cover) in four cases (50%). Across all 21 cases, the review suggests that “vegetation recovery” was positively affected by cutting in seven cases (33%), negatively affected by cutting in nine cases (43%), and not clearly affected by cutting in five cases (24%). These results should be interpreted carefully: the review does not report effect sizes, which may be more important than the number of studies reporting effects in a particular direction. Methods: The review included 21 cases, from 14 publications and at least 13 different marshes, in which oil-damaged vegetation in brackish/salt marshes was cut. Vegetation abundance, height or “recovery” (not clearly defined) were monitored between 14 weeks and 29 months after cutting (8–29 months after cutting for the eight quantitative studies).Study and other actions tested
A replicated, paired, controlled, site comparison study in 2007–2008 in two brackish/salt marshes in southern Brazil (Wolinski et al. 2011) found that cutting and removing smooth cordgrass Spartina alterniflora from oiled plots did not increase cordgrass biomass, density or height. Over the nine months following intervention, cut and uncut plots contained a similar above-ground cordgrass biomass in 5 of 8 comparisons (for which cut: 32–127; uncut: 61–159 g/m2), similar cordgrass densities in 7 of 9 comparisons (for which cut: 32–382; uncut: 35–372 plants/m2), and cordgrass of similar maximum height in 11 of 18 comparisons (for which cut: 43–102; uncut: 39–102 cm). In the other comparisons, cut plots contained less cordgrass biomass and fewer, shorter cordgrass plants. In all comparisons at least six months after intervention, all metrics (biomass, density and height) were statistically similar in cut plots, uncut plots and natural (undisturbed) plots (see original paper for data). Methods: Eighteen 2.5 x 2.5 m plots were established (in six sets of three) across two estuarine marshes (salinity: 12–34 ppt) dominated by smooth cordgrass. In December 2007, twelve plots (two plots/set) were sprayed with oil (ship fuel; 6 L/11 m2). One week later, vegetation was cut and removed from six of the oiled plots (one plot/set). The final six plots (one plot/set) were neither oiled nor cut. Smooth cordgrass within the plots was surveyed monthly until September 2008. To sample biomass, live cordgrass was cut, dried and weighed.Study and other actions tested