Wildfires are landscape scale disturbances that can significantly impact hydrologic processes such as surface runoff, sediment yield, and sediment and nutrient transport to streams. Recent syntheses project that drought will increase wildfire risk in Eastern oak forests and lead to undesirable impacts on forest structure and function. In Oct and Nov 2016, unprecedented, large, drought-related wildfires (ranging from 1200 to 9700 hectares) of mixed-severity burns occurred across the southern Appalachian Mountains. We established sites in three burned and three nearby unburned, reference watersheds in western NC, with the former having a mosaic of moderate and high severity fires. Our objective was to evaluate the impact of fire severity (moderate and high) on tree mortality, stream water quality (temperature, chemistry and sediment) and quantity (yield, peak flows, and base flows). We hypothesized that wildfires would result in tree mortality, soil O-horizon consumption, greater stormflow and nutrient and sediment export compared to reference watersheds. We measured immediate and delayed tree mortality in permanent plots, stream stage and water temperature, and NO3, NH4+, and sediment as total suspended solids (TSS) during baseflow and storm events.
Results/Conclusions
Tree mortality due to the wildfires averaged 27, 27.5, and 34 % across the three burned watersheds. Plots ranged in burn severity with up to 100% tree mortality and 100% soil O-horizon consumption. Soil inorganic nitrogen (NH4 + NO3) concentrations increased with increasing burn severity (R2 = 0.29, P < 0.001). Stream nitrate (NO3-N) concentrations were elevated in burned watersheds (mean 0.07 mg L-1) relative to unburned watersheds (mean 0.02 mg L-1); mean monthly NO3-N in the most severely burned watershed reached 0.27 mg L-1, well above the maximum monthly NO3-N in unburned watersheds (0.06 mg L-1). During storm events, stream NO3-N concentrations in burned watersheds increased up to 300%, while unburned watersheds were less flow dependent. The flow-dependent stream NO3-N concentrations in burned watersheds will result in greater NO3-N export relative to unburned watersheds. Mean stream TSS concentrations were lower in burned (17.5 mg L-1) than unburned (28.6 mg L-1) watersheds under baseflow conditions; however, TSS concentrations collected during storm events in burned watersheds (max 9353 mg L-1) greatly exceeded concentrations in unburned watersheds (max 787 mg L-1). The 2016 wildfires have thus degraded forest condition and water quality particularly during storm events. Effects will likely be transient if the canopy and forest floor recover to a pre-disturbance state.