Introducing a methodology to quantify the hydro-mechanical protective effects of forests on slope stability: application to a deciduous forest in Western Norway

Trees enhance slope stability through hydrological and mechanical processes, reducing the likelihood of shallow landslides. In this study, a methodology for quantifying the hydro-mechanical effects of forests on slope stability is developed and applied to a deciduous forest in Western Norway. The approach uses an infinite slope stability model for unsaturated conditions to estimate the factor of safety ( FS ), incorporating forest effects through suction stress and basal root cohesion. The “Forest Factor” ( FF ) is introduced as the ratio between the minimum FS inside and outside a forest after a precipitation event, and its lower quartile ( FF Q 1 ) across all events is proposed as a design value for protection forest management. For the Norwegian case, hydrological data from May 2023 to September 2025 from a forest and a neighbouring hayfield, along with previously published estimations of root cohesion are used as inputs to the methodology. The resulting FF Q 1 ranged between 1.18 and 2.27, depending on season and soil depth. The hydrological reinforcement effects of the forest were strongest in summer but present year-round, and generally exceeding the mechanical root reinforcement. Analysis of selected precipitation events highlights the importance of antecedent hydraulic conditions in soil response to rainfall. These results emphasize the need to account for both hydrological and mechanical vegetation effects in landslide risk assessments. The methodology application represents the first attempt to quantify the hydro-mechanical effects of forests based on long-term monitoring in Scandinavia, and the estimated FF Q 1 provide practical guidance for managing and planning protection forests in similar geo-climatological settings.