Introduction: Nature-based solutions, particularly the use of vegetation and soil bioengineering techniques, are widely recognized as effective approaches for controlling soil erosion and improving slope stability. Among the different components of vegetation, including the canopy, stem, and root system, roots play a particularly important role in slope stabilization by mechanically reinforcing the soil. Numerous factors influence the reinforcing capacity of root systems, among which seasonal variation has received relatively little attention. Therefore, this study aimed to investigate the effect of season on root mechanical properties and root-induced soil reinforcement in a 45-year-old oak (Quercus castaneifolia) plantation located in the Kheyrud Forest near Nowshahr, northern Iran. The study site was previously occupied by a degraded hornbeam (Carpinus betulus L.) stand that had been clear-cut and subsequently replanted with oak.
Materials and Methods: Four oak trees were randomly selected for sampling, with a mean diameter at breast height (DBH) of 16.5 cm and a mean height of 10.0 m. To investigate the lateral distribution of the root system, one soil profile was excavated downslope of each tree at a horizontal distance of 0.5 m from the stem. The excavation depth corresponded to the maximum depth of lateral root occurrence, and root distribution was recorded across different soil horizons on the exposed profile wall. Subsequently, the mechanical properties of roots with diameters ranging from 0.3 to 23.8 mm were determined using both an in situ pull-out device under field conditions and a universal testing machine under laboratory conditions during the autumn (86 samples) and growing (90 samples) seasons. Finally, root-induced soil reinforcement was estimated by integrating root distribution and mechanical property data using the modified Wu model and the RBMw model.
Results: The results showed that both root number and Root Area Ratio (RAR) decreased logarithmically with increasing soil depth, with the highest values occurring in the upper soil layers. A positive power-law relationship was observed between root diameter and tensile force for both the universal testing machine and the in situ pull-out device, whereas root tensile strength exhibited a negative power-law relationship with root diameter. The mean ± standard error of root tensile force during the growing and autumn seasons was 358.96 ± 22.51 N and 250.12 ± 34.95 N, respectively. The corresponding mean ± standard error of root tensile strength measured by the universal testing machine was 12.23 ± 0.69 MPa in the growing season and 18.54 ± 0.82 MPa in autumn. Analysis of covariance (ANCOVA) revealed significant seasonal differences in root tensile strength, with significantly higher values in autumn, whereas tensile force did not differ significantly between seasons. Both reinforcement models indicated that soil reinforcement decreased with increasing soil depth, with maximum reinforcement occurring in the upper soil layers where root density was greatest. The mean soil reinforcement estimated by the modified Wu model was 62.52 Pa and 45.29 Pa during the autumn and growing seasons, respectively, whereas the corresponding values estimated by the RBMw model were 53.92 Pa and 49.47 Pa. The Wilcoxon signed-rank test applied to the modified Wu model outputs showed significantly greater soil reinforcement in autumn, whereas the paired t-test applied to the RBMw model outputs indicated no significant seasonal difference in mean soil reinforcement.
Conclusion: Seasonal variation significantly influenced the tensile strength of Quercus castaneifolia roots, with higher tensile strength observed during the autumn season, whereas tensile force remained unaffected by season. In addition, increasing soil depth resulted in lower Root Area Ratio (RAR) values and consequently reduced soil reinforcement in both models. Although the modified Wu and RBMw models exhibited similar trends in predicting soil reinforcement, the modified Wu model was more sensitive to seasonal variation and detected significantly greater soil reinforcement during autumn, whereas the RBMw model did not reveal significant seasonal differences. By integrating laboratory tensile tests with in situ pull-out measurements, this study demonstrated that model selection can substantially influence the evaluation of seasonal effects on root reinforcement and soil reinforcement capacity. These findings highlight the importance of incorporating seasonal variation into slope stability analyses to improve the reliability of soil bioengineering design and the sustainable management of vegetated slopes.