Monitoring soil erosion in karst regions poses significant challenges because of the complex geological structures, particularly in forestland characterised by frequent root activity where rainfall-runoff processes remain poorly understood. In this study, the mechanisms underlying the control of underground fissure development on runoff generation and sediment transport in karst forests were investigated. Indoor simulated rainfall experiments were conducted on custom-designed karst slope systems configured with varying degrees of underground fissure development. The results revealed a distinct threshold effect for the rainfall intensity: at intensities <= 60 mm h-1, compared with bare land, surface runoff and the sediment yield were reduced by 12.33% and 19.32%, respectively, in forestland. However, this regulatory function was absent under extreme rainfall (90 mm h-1), with surface runoff on forested slopes surpassing that on bare slopes. Furthermore, erosion pathways shifted from subsurface-dominated (30 mm h-1) to surface-dominated regimes under extreme rainfall. Notably, the relationship between the degree of fissure development and leakage followed a nonlinear pattern, indicating the existence of a critical fissure threshold that triggered a surge in underground water and soil loss. In conclusion, the rainfall intensity, vegetation cover and degree of fissure development interactively drive a surface-underground synergistic response. This study contributes to the theoretical framework of karst erosion and provides a systematic basis for curbing rocky desertification.