Soil-bioengineering to stabilize gravel roadside slopes in the steep Hyrcanian Forests of Northern Iran

Soil-bioengineering techniques for stabilizing road cut and fill slopes based on indicator herbaceous and woody species in the Hyrcanian Forests of Northern Iran effectively reduce shallow landslide occurrences and improve overall slope stability. This study assessed the role of roots in stabilizing shallow soil layers by measuring the root biomechanical properties and root distribution of 13 indicator species, from cut and fill slopes. Our study identified Athyrium filix-femina and Pteris cretica as the most effective species for slope stabilization on cut slopes, providing root reinforcement values of 4538 Pa and 4513 Pa, respectively, and reducing slope instability by up to 18 %. On fill slopes, Sambucus ebulus and Phyllitis scolopendrium showed significant root reinforcement potential, reducing slope instability by up to 17 % and 18 %, respectively. Root systems of these species were primarily concentrated in the upper 0.1 m to 0.2 m of soil, providing simultaneously soil reinforcement and erosion control. Findings indicate that in the Hyrcanian Forest, where favorable climate conditions prevail, native pioneer species are particularly effective for slope stabilization, especially in areas without natural vegetation. Over time, this approach can restore disturbed areas, enhance biodiversity, and improve forest health. As a sustainable alternative to traditional engineering methods, soil-bioengineering offers forest managers a practical solution for reducing landslide risks while fostering ecosystem resilience. While these results highlighted the potential of herbaceous vegetation in mitigating slope failures, soil type and local climate may influence their effectiveness. Consequently, site-specific applications and further research needed to optimize vegetation selection for long-term slope stability. This study provided a framework for integrating native plants into soil-bioengineering techniques for forest road management, promoting environmental sustainability and ecosystem resilience.