Analyzing the influence of mechanical root properties of commonly planted agroforestry trees on Mt. Elgon. A suitability assessment of the best eco-engineering tree species for shallow seated landslide control

Abstract

Landslides continue to occur in the Elgon region despite interventions such as tree planting initiatives aimed at restraining them. The current study assessed the mechanical properties of six selected agroforestry tree roots on slope stability with a keen focus on root tensile strength, soil shear strength, and index of root binding. A standard deviation ellipse method was applied to model the spatial distribution patterns of selected agroforestry trees. Tree-landslide relationship was tested using the Pearson correlation method while root tensile and soil shear strength with a one-way (ANOVA) and descriptive statistics respectively. Species distribution results indicate a high dispersion rate of Croton macrostachyus and Markhamia luteaacross the study area and high concentration of Albizia coriaria downstream. A weak negative correlation (r = -0.20 < 0.01) was reported between diameter at breast height and landslide size. Tensile strength results observed a significant difference among species with (F (5, 573) = [18.161], p < 0.001) and Grevillea robusta (3.02±1.217kg/mm²), Albizia coriaria (2.53±1.382kg/mm²), and Markhamia lutea (2.28±1.01kg/mm²) as the best performers. The best shearing species was Albizia coriaria with average shear strength (52.46±10.24) kpa followed by Markhamia lutea (50.70±15.47) kpa. The Eucalyptus spp. on the other hand underperformed with average shear strength of (46.75±12.92) kpa. In conclusion, the presence of trees reduces landslide risk in an area and DBH is a very important guiding factor. Grevillea robusta, Albizia coriaria, andMarkhamia lutea emerged as best performers in terms of root tensile strength and soil shear strength hence their suitability for enhancing slope stability. However, Eucalyptus Spp., which is widely favoured in the region for its rapid growth was the worst performer with very low shear strength. Therefore, careful consideration of the tree characteristics is essential during promotion campaigns for slope stability in fragile environments.