Soil Reinforcement by Woody Plant Roots: A Systematic Review and Quantitative Synthesis

Woody plant roots enhance slope stability by increasing soil shear strength, although reinforcement by roots varies with forest type, growth form, root diameter, and disturbance history. Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, we reviewed 253 publications and extracted data relevant to the Wu and Waldron model, including mean root tensile strength, root diameter–tensile strength parameters, root area ratio (RAR), and post-disturbance declines in root strength or cohesion. Broadleaf species had higher mean tensile strength and RAR than conifers. Shrubs showed higher mean tensile strength than trees; however, when adjusted for root diameter using α and β1 parameters, trees exhibited greater strength. This highlights the importance of accounting for root diameter when comparing root tensile strength. The vertical distribution of RAR decreased with soil depth, with most roots concentrated in the upper 30 cm of soil. After disturbances, 71.4% of reported half-lives of root strength or cohesion were five years or less, although variation depended on disturbance type and intensity. These findings emphasize that assessments of root reinforcement must consider forest type, growth form, root diameter, RAR, and strength decline after disturbances.