Differences in intrinsic water use efficiency and driving mechanisms between life forms along the succession pathway in karst evergreen and deciduous broad-leaved mixed forest

Leaf δ¹³C, as a proxy for intrinsic water-use efficiency (iWUE), is a core physiological indicator connecting the carbon and water cycles in ecosystems. Understanding its dynamics and driving mechanisms along the successional pathway is crucial for revealing the water-forest relationship, especially under water-scarce conditions. This study established a series of plots along the successional pathway in a karst evergreen and deciduous broad-leaved mixed forest and measured leaf δ¹³C as a proxy for iWUE. The results showed significant increases in δ¹³C-based iWUE proxy of evergreen and deciduous trees along the successional pathway. The increase in the δ¹³C-based iWUE proxy of deciduous trees was associated with greater species diversity linked to improved soil physicochemical properties, which is consistent with a resource-complementarity pathway. In the original BSEM, species diversity showed a positive path coefficient for deciduous leaf δ¹³C (β = 0.596, 95% CI: 0.270–0.911), whereas LES PC1 showed a positive path coefficient for evergreen leaf δ¹³C (β = 0.439, 95% CI: 0.086–0.770). In contrast, the δ¹³C-based iWUE proxy of evergreen trees was more closely associated with LES shifts related to topographic heterogeneity. This study highlighted the asynchronous responses of evergreen and deciduous trees to soil condition variations along the succession of extremely heterogeneous karst forests, providing a solid foundation for regional environmental management and active restoration.