Post-fire hillslope stabilization treatments

Post-fire hillslope stabilization treatments

Post-fire hillslope stabilization treatments are treatments aimed at stabilizing fire-affected slopes by counteracting the negative impact of fire on vegetation and soil properties. The final objective of these treatments is reducing the risk of catastrophic runoff and erosion events and protecting valued resources downhill . Post-fire hillslope stabilization treatments are also called post-fire mitigation treatments, post-fire risk reduction treatments, and emergency stabilization treatments.

Vegetation fires usually partially or totally consume the canopy, above-ground organic residues, and soil organic compounds, reducing soil protection, enhancing soil water repellency, and compromising soil stability . The combined effect of fire and the occurrence of heavy storm in the post-fire season can lead to catastrophic hydrologic and erosion events with severe impacts on population and valued resources downhill such as housing, infrastructures, water supply systems, and critical habitats. For example, heavy rains after a devastating fire season in California (USA) led to catastrophic floods and mudflows in 2018 with at least 13 people dead, housing destroyed, and critical infrastructures severely affected. Stabilization treatments are part of the Burned Area Emergency Response programme implemented by the US Forest Service (USA) and similar programmes conducted in many other fire-prone regions.

Classification of post-fire hillslope stabilization treatments

The design of the hillslope stabilization treatments can be aimed at:

• Counteracting the effect of fire on the soil protection provided by the vegetation: treatments such us seeding and mulching are aimed at reducing runoff and erosion by providing protection to the soil surface and reducing runoff. They mimick the effect of the canopy and aboveground organic layers that protect the soil against the raindrop impact, hold rainfall water decreasing runoff, and reduce runoff speed.
• Counteracting the effect of fire on specific soil properties involved in the runoff and erosion processes: other treatments seek for offsetting fire effects on soils that enhance runoff and erosion such as enhanced soil water repellency (surfactants) or decreased soil aggregation (flocculants). These products can be applied alone or combined with other stabilization treatments to increase their effectiveness.
• Reducing slope length: treatments such as the erosion barriers are intended to reduce the amount of sediments that reach values at risk downhill by reducing the slope length and, thus, runoff speed and erosion potential. They also create small dams that hold the runoff and promote infiltration and sediments settling.

Types, selection, and implementation of post-fire treatments

The selection of hillslope stabilization treatments should consider three key elements: (i) the effectiveness of the treatment, (iii) its cost of production and transport, and (i) the cost of the values-at-risk to be protected . The most cost-effective treatment might not be adequate to protect critical values-at-risk whereas the treatment with the highest effectiveness and cost could not be suitable to treat large areas if the cost of repairing or replacing values-at-risk is low. Since the maximum risk of catastrophic events happens during the first year after the fire, the implementation of this emergency treatments must be conducted as soon as possible after the fire.

• Seeding: Seeds of native or non-native grasses (usually non-reproducing annuals to avoid negative interactions with native flora and alteration of local biodiversity) are manually or aerially dispersed over the burned terrain to promote a rapid vegetation establishment that provides cover and increase soil stability. Seeding have been the post-fire hillslope mitigation treatment most frequently used in the world and mainly in the USA (in the 1970s and 1980s 75% of the burned areas treated in the USA were seeded ). However, negative side-effects such us the potential introduction of non-native species, negative interactions with the recover of native vegetation, and its low efficiency during the first year after the fire before their germination contributed to the reduction of its application (in the 2000s 30% of the burned areas treated in the USA were seeded ).
• Log erosion barriers: Also called contour-felled logs, this treatment became popular in the USA during the 1990s coinciding with the decrease of seeded areas . Log barriers are felled-logs in parallel to the contour lines that aim to reduce runoff speed and create areas where the runoff water can infiltrate, and the sediments settle. Recent studies however, have showed the low effectiveness of this technique for heavy rainfall, when the holding capacity is easily exceeded, or in the mid-term, when their holding capacity is reduced by sediments from previous erosion events. Additionally, a poor design and installation (gaps between the log and the soil surface, heterogeneous shaped or not completely levelled logs, etc.) can led to enhanced erosion potential due to the concentration of the runoff in lowest point of the barrier. Despite these recent insights, log barriers are still extensively applied in USA and Mediterranean countries.
• Mulching: involves covering the soil surface with a material that provides protection against raindrop impact, retains water, and reduce runoff speed. The material most commonly used is straw, whose application reached in the 2000s 18% of the burned areas treated in the USA . This extensive use is due to its high effectiveness but also the development of its aerial application that reduced the application costs and made viable its application in inaccessible areas . An alternative to straw mulching is the wood-based mulching using wood chips, shreds, and strand but also pine needles and forest residues . Although marginally used before the 2010s, wood-based mulching is becoming popular since it prevents side-effects of straw mulching such us the introduction of non-native species and weeds or its longer longevity and stability to wind . Additionally, the material for the wood-based mulches can be produced locally from burned or green trees, reducing transport costs.
• Chemical treatments: tackifiers, fertilizers, and flocculants have been used as stand-alone emergency treatments or in combination with other methods to provide increased soil stability and reduce runoff and erosion . Anionic polyacrylamide (PAM), a synthetic petroleum by-product, is the chemical treatment most frequently used on fire-affected areas, although its application is not as frequent as that for the previous treatments. PAM, is a flocculant that applied as pellets or in solution to the soil surface potentially connect small soil particles, increase soil stability and infiltration, and reduce soil erosion. However, studies on their effectiveness in fire-affected areas are inconclusive or show no significant effect on runoff and erosion .
• Combined treatments: Seeding has been frequently combined with fertilizers to increase the viability of the seedlings . Seeds have been coated with surfactant to reduce fire-induced soil water repellency and increase water availability in the post-fire period . Organic fibres (wood shreds, paper, cotton and flax) have been mixed with seeds, fertilizers and tackifiers to produce hydromulches . Although combined treatments can have marginally higher effectiveness, the associated increase in production or transport costs can make their application less cost-effective and only viable to provide additional protection to critical values-at-risk.