Southwest China's karst region has emerged as a global hotspot for vegetation restoration over recent decades. As the world's largest terrestrial carbonate reservoir, this area raises critical questions concerning the impacts of vegetation restoration on hydrological regimes and its potential to enhance karst carbon sinks, challenges that remain difficult to quantify. In this study, we examine an experimental site with over 40 years of spontaneous vegetation restoration to evaluate how changes in spring alkalinity reflect shifts in hydrology and bicarbonatemediated carbon export. Results indicate that spring discharge remains predominantly rainfall-controlled, exhibiting similar annual flow patterns before and after restoration. Rainfall events increase flow while transiently diluting ion concentrations, indicative of unsaturated flow influenced by vegetation cover and surface morphology. End-member mixing analysis reveals that the contributions of unsaturated flow varies between 3 and 36 % and between 4 and 51 % within the Ca2+ concentrations range of 20-60 mg L- 1. Long-term monitoring demonstrates that HCO3- and Ca2+ concentrations have increased by 24.7 % and 14.5 % since the 1980 s, following a slow-fast-stabilizing trend. Concurrently, the variability in these concentrations and the proportion of unsaturated flow have declined, underscoring the growing ecohydrological influence of vegetation. Although carbon export via unsaturated flow is diminishing, overall carbon flux is projected to increase with ongoing vegetation restoration under stable precipitation conditions.