Effectiveness of mulching after mechanised construction of firebreaks on the hydrological and erosive response of soil in a Mediterranean forest affected by a severe wildfire

Often, forest wildfires are suppressed by the construction of firebreaks using heavy machinery. This may result in increased runoff and erosion due to the removal of ground cover and soil compaction, especially in Mediterranean environments. To avoid this impact, mulching is adopted immediately after fire suppression, but woodchips – widely available as native residues in forests – are less used compared to the most common straw. The effects of these operations on the hydrological and erosive response of soil on machinery tracks remain, however, poorly quantified. In a semi-arid pine forest (Castilla-La Mancha, Spain), the hydrological and erosive response of soil to a very intense rainstorm has been measured after mechanised construction of firebreaks in unburned areas and soil mulching with woodchips, using a portable rainfall simulator. The effectiveness of this post-fire management action has also been evaluated in burned areas. Water infiltration, surface runoff and soil loss as well as ground cover and soil properties were compared among: (i) unburned soils subjected to disturbance due to machinery (with or without mulching); and (ii) burned soils treated with mulching or not. Water infiltration and surface runoff did not significantly change after machinery use and mulching. In contrast, erosion increased on machinery tracks, even in comparison with burned soils. Mulching using woodchips reduced soil loss due to wildfire and firebreak construction by over 70 %. This increase in soil's erosive response was mainly due to a severe reduction in ground cover on machinery tracks, but the application of woodchips helped to limit this impact on rainsplash erosion. Moreover, the machinery led to an increase in soil compaction. In this case, mulching was not effective at controlling either this effect of firebreak construction or the increase in soil water repellency surveyed in burned areas. Overall, this study suggests caution in firebreak construction using heavy machinery as well as immediate mulching to reduce soil erosion.