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Abstract:The purpose of this study was to assess how terracing affected overland flow and associated sediment losses, at the micro-plot scale (0.25 m 2 ), in recently burnt stands of the two principal forest types in north-central Portugal, i.e. mono-specific stands of Maritime Pine and Eucalypt. Terracing is an increasingly common practice of slope engineering in the study region but its impacts on runoff and erosion are poorly studied. Non-terraced plots at the Eucalypt and the Pine site revealed similar median runoff coefficients (rc: 20-30%) as well as comparable median sediment losses (15-25 g m -2 ) during the first seven months following wildfire. During the ensuing, slightly wetter 18-month period, however, non-terraced plots at the Pine site lost noticeably more sediments (in median, 90 vs. 18 g m -2 ), in spite the runoff response had remained basically the same (median rc: 33 vs. 28%). By contrast, terraced plots at the same Pine site lost hugely more sediments (in median, 1,200 g m -2 ) during this 18-month period. Terraced plots at the Eucalypt site even lost three times more sediments (in median, 3,600 g m -2 ). Ground cover and resistance to shear stress seemed to be key factors in the observed/inferred impacts of terracing.

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Besides direct damage, wildfires can produce striking direct and indirect impacts, including extreme runoff-erosion responses with serious negative consequences for land-use sustainability and downstream values at risk. The best way to reduce post-fire runoff is the rapid application of mulch: a protective cover of straw, needles, wood-based material, or other ground cover. In this study, we assessed the efficacy of forest residue mulching for reducing post-fire runoff across spatial scales on a logged and burnt eucalypt hillslope. Additionally, we assessed the underlying processes involved in the hydrologic responses of burnt areas in central Portugal during the first 2 years after a wildfire, with special focus on soil moisture, soil water repellency, and rainfall. Forest residue mulch reduced post-fire runoff by 50% in both micro-plots (0·25 m 2 ) and hillslope scale silt fence plots (100 m 2 ) during the first year after the wildfire. Runoff in the micro-plots was tenfold higher than the larger plots, regardless of mulching. The main controlling factors for the volume of runoff were rainfall amount (direct correlation) and soil moisture (inverse correlation). Soil water repellency and soil moisture were not affected by the mulch but played a role in the hydrologic response. In contrast, there was no decrease in runoff rate within the first two post-fire years. Forest residue mulching can be an effective postfire treatment for reducing runoff rates, and the consistent decrease in runoff rates between the two spatial scales suggests that the scaling of runoff was not affected by the mulch.

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