Influence of the temporal dynamic of root reinforcement on the disposition of shallow landslides

Root reinforcement is a variable factor that influences the disposition of shallow landslides over different time scales. Natural or anthropogenic forest disturbances, such as forest fires or clear cuts, may modify considerably the vegetation cover on a short time scale, with major consequences on several ecosystem services, including the mitigation of risks due to shallow landslides. After catastrophic forest disturbances, it is of primary importance for decision makers to assess how risks will change in order to evaluate the most appropriate protection measures. Therefore, the quantification of the effect of the temporal dynamic of root reinforcement is of fundamental importance to estimate the occurrence probability of shallow landslides.

Data on root distribution and pullout tests for spruce (Picea abies) and beech (Fagus silvatyca) trees are used to upscale the basal and lateral root reinforcement at the stand scale (Schwarz et al., 2012). The decay of root reinforcement is calculated based on data collected in a burnt (Vergani et al., 2017) and a clear-cut area (Vergani et al., 2016). The recovery of root reinforcement after disturbances is estimated considering the growing conditions of the stands (Flepp et al., 2021). The quantification of the dynamic of the forest stands and the derived root reinforcement at stand scale is based on the analysis of four Swiss National Forest Inventory (NFI 1-4). The estimated time-dependent variation of root reinforcement is implemented in the SlideforNET model (ecorisq.org) to calculate the occurrence probability of shallow landslide after disturbances.

The results show that the recovery of root reinforcement after disturbance is effective to reduce the hazards of shallow landslide only for a narrow range of disposition factors. Given a defined rainfall statistic, slope inclination is the factor that most influence the effectiveness of root reinforcement recovery, within a range of inclination variations of 4-8°. Further relevant factors are soil thickness and runoff contributing area.

The extended version of SlideforNET quantifies how effective is the recovery of root reinforcement in stabilizing shallow landslides after stand replacing forest disturbances. This information is fundamental to evaluate if additional temporal or permanent technical measures are needed to keep an acceptable level of risk after forest disturbances.