Deforestation Effects on Rainfall-Induced Shallow Landslides: Remote Sensing and Physically-Based Modelling

Deforestation of steep slopes may temporarily reduce evapotranspiration and lessen root reinforcement thus potentially enhancing landslide susceptibility. Quantifying the effects of deforestation and associated perturbations on landslide characteristics remains a challenge, especially for predictions in remote areas with limited information. We applied the STEP-TRAMM model that uses publicly available climatic and landscape information to assess effects of forest alteration on hydro-mechanical processes. The model considers two types of forest alterations: (i) removal of root reinforcement following permanent forest conversion, and (ii) time dependent root decay and regrowth following clear-cut timber harvesting. The model was applied to four study areas in different climatic regions (New Zealand, Oregon, Sumatra and Cambodia). We compared model predictions of landslide metrics with satellite-imaging of landslides following deforestation. Although we observe a higher propensity and larger landslides in deforested areas, effects were sensitive to deforestation practices and patterns. The largest increase in landslide area was associated with large and interconnected deforested tracts within a few years after deforestation as determined by competition between root decay and forest regrowth. For patchy small-scale forest conversion, the landslide areas were smaller but could occur many years after deforestation (> 10 years). The modeling framework offers ability to evaluate forest alteration scenarios through their potential impact on landslide hazard in specific regions of the landscape.