Deciphering the degradation mechanisms and complex drivers of ecosystem services (ESs) in ecological transition zones under climate change and human disturbance is a key scientific challenge. In this study, the Lvliang Mountains, a typical semiarid and semihumid transition zone, were investigated using the Integrated Valuation of Ecosystem Services and Tradeoffs model to quantify the spatiotemporal dynamics (2000-2023) of 4 ESs-water yield, carbon storage, habitat quality, and soil conservation-and the ecosystem service composite index (ESCI). The extreme gradient boosting algorithm and partial least squares structural equation modeling were then applied to identify key drivers, quantify their direct effects, and analyze vegetation-mediated indirect pathways. The results showed that (a) high-value ESCI areas were spatially aligned with the 4 ESs and clustered in high-vegetation mountains; water yield and soil conservation varied synchronously, whereas carbon storage and habitat quality followed a rise-then-fall pattern; (b) from 2000 to 2023, ESCI spatial patterns were mainly dominated by topographic and meteorological factors, while vegetation dynamics mediated indirect environmental effects with stronger contributions in the middle period, and anthropogenic influences shifted from early negative disturbance to later positive ecological effects; (c) vegetation served as a critical mediator, with the elevation range of 1,100 to 2,400 m acting as an optimal elevation zone that stabilized a multifactor mechanism involving water vapor, vegetation, and elevation; and (d) ecological management should integrate topography, water conditions, and vegetation restoration. These findings may provide useful insight into ES formation in similar semihumid to semiarid transition zone mountain systems.