Effect of fire frequency on runoff, soil erosion, and loss of organic matter at the micro-plot scale in north-central Portugal

• Runoff and erosion can be seen also after drought phases in later years after fire.
• Repeated wildfires strongly increase the risk of spilling over in downstream regions.
• OM loss was quite high by repeated fire due to the runoff related ash transport.
• Cover and rainfall intensity have the strongest effect on runoff and erosion.

Wildfire is a natural phenomenon that is a common ecological factor in Mediterranean ecosystems. The increase in occurrence in recent decades has raised widespread concern about the impact of repeated wildfires on runoff and erosion, a topic that has not been widely studied. We addressed these concerns in an area of north-central Portugal by comparing runoff at the micro-plot scale and the associated transport of sediments and organic matter (OM) in unburnt, once burnt, and repeatedly burnt plantations of Maritime Pine. We selected nine sites following a large wildfire in September 2012 that affected roughly 3000 ha of the Viseu municipality. Three of the sites had not been burnt since 1975 and acted as controls, with covers of pine trees, shrubs, and annual vegetation; three sites had burnt only in 2012 and contained burnt pines but no shrubs or annual vegetation; and three degraded sites had suffered from three wildfires prior to 2012 and contained no vegetation. We established nine micro-plots (0.25 m2) at each site and collected runoff, eroded soil, and OM losses in tanks after each rain from October 2012 to September 2014. The repeated wildfires strongly increased the runoff coefficient and the risk of downstream flooding after heavy rains. OM losses were nearly half the volume of the eroded soil in the degraded sites due to the transport of ash in the runoff. Runoff and soil losses occurred not only after erosive rainstorms following a fire but also after a subsequent period of drought. Soil cover, rain intensity, and soil moisture were key factors in the amount of runoff and erosion. The insights provided by this study can contribute to pre- and post-fire activities and management in protect areas and can thus improve post-fire recovery.