Correlation of tree stem diameter with root architecture, mechanical properties, and slope stability in a mountainous Cryptomeria plantation

Slope instability, worsened by climate change and deforestation, is a major hazard in mountainous regions. While vegetation-based bioengineering offers a sustainable solution, limited research exists on how tree stem diameter is related to root architecture and slope stabilization potential. This study integrates field investigation, laboratory testing, and numerical modeling to assess the contribution of Cryptomeria D. Don trees of varying diameters to slope stability in the Longchi Forest, Sichuan Province, China. Four trees with diameters ranging from 230 mm to 430 mm were analyzed to examine the relationship between tree stem diameter and root architectural indices, i.e., Root area ratio (RAR), root density (RD), and root biomass (RB). A continuous profiling method was used for the roots zone excavation. The roots tensile tests were conducted to evaluate the biomechanical properties of the roots of the selected trees. The additional root cohesion was estimated using the most commonly used Wu and Waldron model and the Fiber bundle model, while the shear strength of the bare soil was evaluated using direct shear tests. The results revealed that as stem diameter increased, RAR in the top 10 cm of soil increased from 0.54% to 0.82%, RD increased from 0.00168 to 0.00292 roots/cm³, and RB from 0.01245 to 0.02041 g/cm³. Average root tensile strength increased from 15.51 MPa (230 mm tree) to 22.17 MPa (430 mm tree), while root cohesion in the topsoil increased from 28.04 kPa to 58.2 kPa. Slope stability simulations showed that vegetation enhanced the Factor of Safety (FoS) by 35.6% for the smallest tree and 70.8% for the largest, compared to bare slopes. These findings underscore the relationship between tree stem diameter, root reinforcement, and slope stability, offering a practical basis for integrating tree size into eco-engineering design and landslide mitigation efforts. This study advances our understanding of the biomechanical contributions of vegetation to slope stabilization and offers valuable insights for forest management and bioengineering practices in geohazard-prone regions.