Implications of hornbeam and beech root systems on slope stability : from field and laboratory measurements to modelling methods

Purpose: Root reinforcement is a key parameter in slope stability analysis, but is difficult to be effectively included at the hillslope-scale due to the complexity of root systems. As a result, hillslope-scale analysis of root reinforcement still requires high levels of field validation to account for variability in root properties as a function of topography, ecology, and soil properties. This study investigated root distributions and estimated root reinforcement at an unprecedent scale of field and laboratory measurement, using this to understand differences among species (Carpinus betulus and Fagus orientalis), diameter at breast height (DBH), slope position, altitude, vertical and horizontal distances from trees in Hyrcanian temperate forests, Iran. Method: We excavated 1080 profile trenches 0.5 m wide, 1.0 m length, and 1.0 m deep upslope and downslope from trunks of C. betulus and F. orientalis with a range of DBH (7.5–82.5 cm) at three different altitudes (400, 950, and 1300 m a.s.l.). We assessed the effects of different forest coverage on slope stability via a 3-D limit equilibrium-based slope stability model where parameter uncertainties are explicitly accounted for using Monte Carlo Simulation. Results: The Root Area Ratio (RAR) of C. betulus is always higher than F. orientalis. RAR of F. orientalis is higher in upslope, whereas RAR of C. betulus is similar in both positions. Higher RAR contributed to higher root reinforcements for C. betulus when comparted with F. orientalis. Additionally, after accounting for DBH influences, altitude significantly affects the root reinforcement of C. betulus. The results of slope stability analysis showed that the most stabilizing species is C. betulus in a mature growth condition, maintaining an instability probability of ~ 18.3%. Conclusion: C. betulus is preferable to F. orientalis for increasing slope stability. Forest managers should consider this outcome when developing strategies for silvicultural treatment and reforestation projects in mountainous areas of temperate regions.