Rainfall-induced instabilities in highly permeable earthen slopes typically originate at the slope toe; however, the triggering mechanism remains unclear. In this study, we captured the initial microscopic deformations and the overall macroscopic progressive damage of slope instability, extracted the stress paths and contact force chains of soil particles in different parts of the slope before and after rainfall, and revealed the triggering mechanism of soil slope instability induced by rainfall by conducting model tests and utilizing CFD-DEM (computational fluid dynamics-discrete element method) fluid-structure coupling numerical simulations. Our findings revealed that the slope toe exhibits stress concentration prior to rainfall and is a sensitive area of the entire slope before rainfall. After rainfall, rainwater infiltrates, and the seepage rate is the highest near the slope toe. The force-chain arch formed by the large particles at the slope toe, which play the role of the skeleton, is gradually weakened. The essence of rainfall-induced soil slope failure lies in the gradual erosion of the stable contact force chains between soil particles at the slope toe by seepage forces, leading to a progressive weakening, fracture, and disappearance from the outside inward in a collective movement. Once the failure of the slope toe is triggered, the damage area of the inter-granular contact force chains is significantly larger than the displacement plastic zone (or shear band), and the stress in the soil near the slope rapidly transitions from high to low. Subsequently, as the soil particles continue to slip and roll, the soil stress fluctuates and gradually increases, forming a stress-concentrated force chain arch at the rear edge of the slip surface highlighting the slope's certain self-stabilizing capability after failure. Throughout the process, the stress path at the foot of the slope is the longest.

Trees in degraded forest areas are generally exposed to water stress due to harsh environmental conditions, threatening their survival. This study simulated the environmental conditions of a degraded forest area by constructing an artificial rainfall slope and observing the physiological responses of Pinus densiflora to control, mulching, and waterbag treatments. P. densiflora exhibited distinct isohydric plant characteristics of reducing net photosynthetic rate and stomatal transpiration rate through regulating stomatal conductance in response to decreased soil moisture, particularly in the control and waterbag treatments. Additionally, the trees increased photochemical quenching, such as Y(NPQ), to dissipate excess energy as heat and minimize damage to the photosynthetic apparatus. However, these adaptive mechanisms have temporal limitations, necessitating appropriate measures. Under extreme drought stress (DS45), mulching treatment showed 4.5 times and 2.2 times higher in PIabs and SFIabs than in the control, and after the recovery period (R30), waterbag and mulching treatment showed similar levels, while PIabs and SFIabs in the control were only 45% and 75% of those in the mulching and waterbag treatments, respectively. Specifically, mulching extended the physiological mechanisms supporting survival by more than a week, making it the most effective method for enhancing the planting ground in degraded forest areas. Although the waterbag treatment was less effective than mulching treatment, it still significantly contributed to forming better growth conditions compared to the control. These findings highlight the potential for mulching and waterbag treatments to enhance forest restoration efforts, suggesting future research and application could lead to more resilient reforested areas capable of withstanding climate change-induced drought conditions.