Contrasting contributions of vertical water transport processes to WUE variability across vegetation types in the karst critical zone of Southwest China

Study region Southwest China. Study focus Although WUE is widely used to characterize ecosystem water–carbon coupling, the mechanisms by which water transport across different interfaces of the Earth's Critical Zone constrains WUE remain unclear in karst ecosystems. The study developed a vertical water transport processes (VWTPs) framework that decomposes water transport into four sequential components and integrated variance decomposition, random forest analysis, and structural equation modeling to quantify their contributions to WUE variability across major vegetation types during 2002–2022. New Hydrological Insights for the Region Results revealed that WUE in karst ecosystems is governed by vegetation-specific vertical water transport processes rather than by a single ecohydrological pathway. Variance decomposition of the complete VWTPs chain (GPP/TWI) showed that RWSE had the largest main-effect contribution across vegetation types, while TWCE contributed relatively more in grasslands. Pairwise covariance effects accounted for 39.30% of the total contribution, indicating that joint variability among VWTPs components represented an important component of the overall variability. Vegetation structure consistently played a prominent role in regulating VWTPs, whereas climatic regulation shifted from precipitation-dominated water-supply pathways in croplands and grasslands to atmospheric demand-driven physiological pathways in forests and shrublands. These findings provide a process-based understanding of vegetation-specific carbon–water coupling in karst critical zones.