A meta-analysis on the effectiveness of post-fire soil erosion mitigation treatments

Wildfires are considered to be one of the main causes of soil erosion and land degradation processes in fire-prone areas [1], which are expected to increase in the future because of fire patterns shifting worldwide as a consequence of changes in climate and land use [2]. To maintain the sustainability of ecosystems and protect the values at risk downstream from the fire-affected areas, it is vital to mitigate the increased hydrological and erosive response after fires. Despite soil erosion mitigation treatments have been widely applied after wildfires, their effectiveness has only been assessed in local and regional-scale studies, so the obtained conclusions might be heavily influenced by site-specific conditions.

To overcome this constraint, a meta-analysis was applied on the scientific literature on post-fire soil erosion mitigation treatments indexed in Scopus. The search resulted in 34 publications from which 53 and 222 pairs of treated/untreated observations on post-fire runoff and erosion, respectively, were obtained. The overall effectiveness of mitigation treatments, expressed as effect size, was determined for the runoff and erosion observations, and further analyzed for four different types of treatments (mulching, barriers, seeding, and chemical). The erosion observations concerning mulches were analyzed for differences in effect size between 3 different types of materials (straw, wood, and hydromulch) as well as between different application rates of straw and wood. The erosion observations were also analyzed for the overall effect size of post-fire year, burn severity, rainfall amount and erosivity, and ground cover.

The results showed that all four types of treatments significantly reduced post-fire soil erosion, but that only the mulch and barrier treatments significantly reduced post-fire runoff. From the 3 different mulch treatments, the straw and wood were significantly more efficient in mitigating erosion than the hydromulch. The different straw and wood mulch application rates also influenced their effectiveness. Straw mulch was less effective at rates below than above 200 g m^-2, while mulching with wood at high rates (1300 to 1750 g m^-2) produced more variable outcomes. Results also suggested that the overall effectiveness of the treatments was greatest shortly after fire, in severely burned sites, providing or promoting the development of ground cover over 70%, and with increasing rainfall erosivity.

It can be concluded that, in overall terms, the application of the studied post-fire erosion mitigation treatments represented a better choice than doing nothing, especially in sites where erosion is high. However, works on this topic are underrepresented outside of the USA, Spain and Portugal. Most of the studies were conducted at hillslope scale and tested mulching and/or barriers, while larger scales and other treatments were neglected. Further efforts are needed in testing, from field and modelling experiments, combinations of existing and/or emerging erosion mitigation treatments to ensure that the most suitable measures are applied after fires.

[1] Shakesby (2011). Earth-Sci. Revs. 105:71-100. DOI: 10.1016/j.earscirev.2011.01.001

[2] Andela et al. (2017). Science 356: 1356-1362. DOI: 10.1126/science.aal4108