Abstract: Plant roots undergo a life cycle of growth, maturation, and decay, yet the effect of dynamic process of this cycle on the mechanical behavior of the root–soil composite have been largely overlooked. Long-term deterioration of root–soil mechanical properties caused by root decay is a critical consideration in assessing the durability of ecological slope-protection structures. Using Masson pine–covered slopes in Wuping County, Fujian Province as the study subject, this paper employs single-root tensile tests and large-scale direct shear tests on root–soil composites to systematically investigate the evolution of root mechanical properties during mortality dynamics and the corresponding changes in the shear strength of the root–soil composite. The results show that the tensile strength and ultimate tensile force of Masson pine roots decrease exponentially with decay duration. In the early stage of decay, tensile strength declines rapidly by 30%–50%, after which the degradation rate gradually levels off. Root decay significantly affected the shear strength of the root–soil composite: the root-conferred additional cohesion decreases progressively with decay duration, whereas the internal friction angle remains essentially unchanged. Based on the experimental data, we developed a power-law model for root tensile strength and an exponential-decay model for composite shear strength. We further modified the Wu model by introducing a cohesion-decay coefficient to quantify the decay-duration-dependent reduction in shear strength. These findings clarify the mechanism by which root decay undermines soil reinforcement and provide a theoretical basis for long-term stability assessment and maintenance planning of ecological slope-protection projects.

Key words: root decay, root-soil composite, single-root tensile test, large-scale direct shear test, shallow landslide