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RESEARCH PRODUCT

The effect of ash and needle cover on surface runoff and erosion in the immediate post-fire period

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description

Abstract Hillslopes are thought to be most susceptible to enhanced surface runoff and erosion immediately following wildfire due to removal of protective vegetation and litter cover, and in many cases a fire-induced reduction in soil wettability. This enhanced susceptibility declines as vegetation and litter layer recover. For logistical reasons, however, few studies have been able to examine the responses of burnt terrain immediately following burning and little is therefore known about the effect of the wettable ash layer that often covers the ground until it is redistributed or removed by wind or water erosion. Here we quantify the effects of ash and needle cast ground cover on surface runoff generation and soil erodibility in the immediate post-fire period (2–10 days) following a severe wildfire in an Aleppo pine forest over calcareous soils in Mediterranean eastern Spain. A homogenous vegetation cover and lack of wind during the fire resulted in 100% ash cover (36 mm mean depth) across the study area. The ground fire did not consume the tree crowns, but its high intensity caused complete tree mortality, leading to subsequent needle fall. Hydrological and erosional responses were measured for fifteen 1-hour rainfall simulations (55 mm h− 1) on 0.25 m2 circular plots for each of three terrain types: (i) ash and needle covered, (ii) ash-only covered and (iii) bare. Furthermore, ash and needle cast cover were monitored over an eleven-week period on otherwise undisturbed terrain to examine its evolution during the first critical months following burning. The soils exhibited no or slight post-fire water repellency. The overlying wettable ash layer had a porosity of 83%, providing a water storage capacity of ∼ 30 mm. A third of the plots with ground cover did not generate any surface runoff. Ash-needle and ash-only plots showed 25- and 18-fold reductions in surface runoff respectively compared to bare terrain. Mean sediment yield from covered plots was more than two orders of magnitudes lower than from bare plots. The ash cover remained unchanged during the first two weeks while overlying needle cast more than doubled. Heavy rainfall (153 mm) over a subsequent six-day period removed the ash almost entirely where no needle cast was present, whereas on ash and needle covered ground only a limited redistribution and accumulation into litter dams occurred. The results quantify the potential importance of ash deposits on the susceptibility of hillslopes to surface runoff and erosion immediately following fire. The combination of ash and needle cast appears particularly effective in reducing hillslope responses. The outcomes demonstrate clearly that the widely held notion of landscapes being most susceptible to surface runoff and erosion directly after burning is not correct where a fire has resulted in substantial ash deposition. Instead, the susceptibility will be greatest following a storm event of sufficient severity to remove this protective layer and before the onset of vegetation recovery.