Abstract
Forest wildfires typically increase runoff and associated soil and organic matter losses. Both ploughing and mulching with forest residues have been applied in recently burnt areas in Portugal to mitigate these effects in soil erosion, but their effectiveness has never been compared directly. To this end, soil and organic matter losses by water after a wildfire were studied in two eucalypt plantations in central Portugal that had been affected by the same wildfire (August 2015). One of the sites was instrumented with six erosion plots (2 m by 8 m), divided over two blocks with one treatment per block: control (doing nothing) and ploughing to 0.2 m depth with a tracked excavator. The other site was instrumented with nine erosion plots, divided over three blocks with three treatments in each block: control (doing nothing) and mulching with forest logging residues at reduced (2.6 Mg ha-1) and standard application rates (8 Mg ha-1). Mulching was performed one month after the wildfire, whereas ploughing took place one year after the wildfire. For this study, soil and organic matter losses were monitored at 12 occasions from July 2016 to May 2017, roughly coinciding with the second post-fire year. Over this relatively dry period sediment losses at the control plots of both ploughed and mulched sites averaged 1.6 and 0.6 Mg ha-1 respectively. The corresponding losses of the ploughed plots were 19% lower, whereas those of the mulched plots were 67 and 93% lower at the reduced and standard mulch rates, respectively. The organic matter content of the eroded sediments was 22% in the unploughed plots, and ploughing reduced this figure in half, which could be explained by the inversion of the topsoil horizons by the excavator. Mulching at the standard application rate seemed to produce a clear enrichment in organic matter content compared to mulching at the reduced rate as well as doing nothing (25 vs. 16 and 14%). The two main findings of this research were that i) erosion rates exceeded the 1 Mg ha-1 tolerable soil loss during the second post-fire year, indicating that mitigation measures have to be implemented, ii) ploughing was clearly less suited for mitigating post-fire erosion than mulching with forest logging residues, even at application rates as low as that typically used in operational post-fire emergency stabilization with straw mulching.
Highlights
The effects of wildfires on vegetation and litter combustion and on soil properties have been exhaustively studied (Shakesby and Doerr, 2006; Shakesby, 2011; Francos et al, 2016)
Rainfall intensities were different between sites, with the highest intensities occurring in the ploughing site on October 2016 (32 and 36 mm h-1), while in the mulching site occurred in November 2016 (21 mm h-1) (Fig. 3)
Soil and organic matter losses after a wildfire were measured and compared on two hillslopes, one ploughed and the other treated with mulching
Summary
The effects of wildfires on vegetation and litter combustion and on soil properties have been exhaustively studied (Shakesby and Doerr, 2006; Shakesby, 2011; Francos et al, 2016). In addition to the direct and indirect effects of wildfires, forest management practices can increase runoff and soil erosion in burned areas (Shakesby et al, 1996; Malvar et al, 2016), highlighting the need for application of effective post-fire erosion mitigation measures. Ploughing has been considered as an effective technique to reduce compaction resulting from the forestry heavy machinery (Robichaud et al, 2000), since it increases the macropores volume, surface roughness and water infiltration capacity (Shakesby, 2011; Malvar et al, 2016). When applied at larger scales or in areas with slopes higher than 15o, it can potentially increase soil losses (Robichaud et al, 2000)
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