Runoff and erosion responses to deadwood cover dynamics in windthrown mountain forests: insights from WEPP simulations

The hydrological and erosional impacts of dynamically decreasing deadwood cover in disturbed mountain forests are poorly understood. This study investigated hydrological-erosional responses to decreasing deadwood cover using the Water Erosion Prediction Project (WEPP) model, which was calibrated and validated with 4-year monitoring data from a 4.5 × 6.0 m plot in the Italian Alps. Three simulations were separately performed: uncalibrated (Sim #1); hydraulic conductivity and interrill erodibility calibrated (Sim #2); hydraulic conductivity, interrill erodibility, and residue cover parameter calibrated (Sim #3). Sim #1 significantly underpredicted runoff and sediment yield, highlighting the necessity of validation when extrapolating existing models to mountain forests. Sim #2 notably improved the prediction accuracy of both runoff (from 18.2% to 81.8%) and sediment yield (from 0% to 27.3%), whereas Sim #3 further improved runoff prediction accuracy to 90.9%, confirming the hydrological soundness of deadwood parameterization. Decreases in deadwood cover only marginally increased runoff and sediment yield (<1%) in grass-covered scenarios. Contrastingly, bare-soil conditions revealed dramatic increases (runoff: 13–263%, sediment yield: 139–3931%), emphasizing the protective role of vegetation cover. These results suggest that salvage logging can significantly accelerate runoff and erosion unless vegetation is restored, and properly calibrated models can inform low-impact deadwood management and post-windthrow recovery.