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

Authors: KOYANAGI, KENTA - Free University Of Bozen-Bolzano; ANDREOLI, ANDREA - Free University Of Bozen-Bolzano; NORDIO, GIOVANNA - Free University Of Bozen-Bolzano; ANACHE, JAMIL - Universidade De Sao Paulo; PÖPPL, RONALD - University Of Vienna; Renschler, Christian; FLANAGAN, DENNIS - Purdue University; COMITI, FRANCESCO - Free University Of Bozen-Bolzano

Submitted to: Physical Geography
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/28/2026
Publication Date: 6/5/2026
Citation: Koyanagi, K., Andreoli, A., Nordio, G., Anache, J.A.A., Pöppl, R., Renschler, C.S., Flanagan, D.C., Comiti, F. 2026. Runoff and erosion responses to deadwood cover dynamics in windthrown mountain forests: Insights from WEPP simulations. Physical Geography. https://doi.org/10.1080/02723646.2026.2683692.
DOI: https://doi.org/10.1080/02723646.2026.2683692

Interpretive Summary: After a damaging windstorm, a mountain forest can turn into a large deadwood cover. Little is known how disintegrating deadwood cover impacts runoff, and soil erosion over time. This study uses field and model data to assess deadwood cover impact on runoff and erosion over time. The Water Erosion Prediction Project (WEPP) model is used in this study for a windthrown mountain forest in the Italian Alps. Summertime runoff and sediment yield monitoring data sets were collected for four years. The runoff plots for deadwood were compared to grass covered plot of the same size and slope. Three model scenarios with varied optimized parameter sets were analyzed: uncalibrated (Sim #1), baseline hydraulic conductivity plus baseline inter-rill erodibility (Sim #2), and baseline hydraulic conductivity plus baseline inter-rill erodibility and added residue cover (Sim #3). For Sim #1, only 18% predicted runoff and none of the predicted sediment yield fell within the range of monthly observations. For Sim #2, the predicted runoff increased to 82% and sediment yield to 27%. This shows the critical need for monitoring-based validation works in extrapolating existing runoff-erosion models to mountain forests. For Sim #3 runoff prediction increased by 90.9% and there was no improvement for the sediment yield prediction. This justifies the deadwood parameterization approach, at least from a hydrological point of view. The calibrated WEPP predicted surprisingly small increases in median monthly runoff and sediment yield (<1%) along simulated deadwood cover reductions in our grass-covered plot. Monthly runoff and sediment yield increased by 13 to 263% and 139 to 3,931% with bare soil setting, respectively. This shows the importance of vegetation recovery in preventing accelerated runoff and soil erosion by salvage logging. The results show the important impact of hydrological and erosional effects of windthrows in mountain forests moderated by deadwood. The results can help with similar situations in mountain forests in the United States.

Technical Abstract: Mountain forests can be extensively replaced with deadwood cover by windstorms, but little knowledge exists about how decreases in deadwood cover over time influence runoff and sediment dynamics on windthrown hillslopes. In order to gain insights into runoff and erosion responses to decreasing deadwood cover using calibrated and validated Water Erosion Prediction Project (WEPP), we utilized a novel 4-year summer runoff and sediment yield monitoring data at a 4.5-m-wide × 6.0-m-long ‘deadwood + grass’ covered plot in a recently windthrown mountain forest of the Italian Alps. Three simulations were separately run with varied optimized parameter sets: uncalibrated (Sim #1), baseline hydraulic conductivity + baseline inter-rill erodibility (Sim #2), and baseline hydraulic conductivity + baseline inter-rill erodibility + additional residue cover (Sim #3). Whereas only 18.2% of predicted runoff and none of the predicted sediment yield fell within monthly observation ± 95% confidence intervals under Sim #1, the fractions respectively increased to 81.8% and 27.3% in Sim #2, highlighting the critical need for monitoring-based validation works in extrapolating existing runoff-erosion models to mountain forests. While the percentage further improved to 90.9% for runoff prediction in Sim #3, no improvement was observed for sediment yield prediction, justifying our deadwood parameterization approach, at least from a hydrological point of view. Our calibrated WEPP predicted surprisingly small increases in median monthly runoff and sediment yield (<1%) along simulated deadwood cover reductions in our grass-covered plot. However, monthly runoff and sediment yield were contrastingly predicted to increase by 13-263% and 139-3931% with bare soil setting, quantitatively indicating the importance of the vegetation recovery in preventing accelerated runoff and soil erosion by salvage logging. Our findings shed light on the persisting hydrological and erosional effects of windthrows in mountain forests mediated by the presence of deadwood.