Forest protection and protection forest: Tree root degradation over hydrological shallow landslides triggering

The potential use of protection forests to combat shallow slope instabilities is becoming increasingly important and considerable, especially in the light of the recent landslides and debris/mud flows in regions triggered by rainfalls with increased intensity. Tree vegetation has been constantly subjected to silvicultural activity both in exclusively productive forest areas and in more conservative ones meant to contrast hydrogeological risk. It is important to quantify the root system dynamics in order to correctly evaluate the impact of wood felling or plants death on slope stability. Based on field investigation (on experimental plots and 29 occurred landslides) and numerical modelling (on slope stability and root distribution), the aim of this work is to determine the effects of the evolution of the mechanical characteristics of root systems (and consequently on landslide probability). The paper investigates variations over time in the hazard of rainfall-triggered landslides as a result of root degradation after forest cutting (or death). The case under study is related to experimental investigations aimed at determining the tensile strength and elasticity of root samples of trees dead within a decade, which correspond to decreasing values of soil cohesion (root reinforcement). Two kinds of samples were taken into account: living beech roots from protected wood areas to determine the current characteristics and roots from dead beeches (felled in previous years and at present in degradation) to analyse the evolution of root mechanical characteristics. To analyse the stability of representative slopes, we calculated the return time associated with a rainfall event, which in saturated conditions would lead to the attainment of the limit value of the safety factor and the associated hazard for different rainfall durations during a fixed period of time. Information about the increasing risk of collapse with the degradation of root system was obtained and compared with landslides occurrence in forested slopes of the study area. The results of the present paper show that such slopes may remain stable if they are covered with intact protective vegetation, but they will become unstable if the conditions of the forest deteriorate or after a wooded area dies off: within a decade of tree death the root system of protection forests loses most of its soil-stabilising function.