Ecohydrological niches and water-table thresholds of riparian vegetation associations under long-term environmental flows

Abstract. Ecohydrological assessments of regulated dryland rivers have largely focused on aggregate vegetation responses, such as vegetation cover or greenness, whereas the spatial organization of vegetation at the association level and its coupling with hydrological gradients remain poorly resolved. Using the lower Tarim River, where environmental flows have been implemented continuously since 2000, as a case study, we combined field surveys, multi-source remote-sensing imagery, and groundwater data to establish a spatially continuous 10 m vegetation-association baseline for 2025, distinguishing 13 representative riparian vegetation associations. Among six classifiers evaluated using spatial block cross-validation, XGBoost achieved the highest performance, with an overall accuracy of 80 %. Using this association-level baseline, we developed an integrated hydrogeographic framework to quantify vegetation differentiation across three interrelated dimensions representing longitudinal environmental-flow conveyance, lateral distance from the river, and water-table depth. The vegetation associations exhibited pronounced differentiation across these dimensions. Most Populus euphratica- and Tamarix-related associations had their main distribution centres at intermediate water-table depths of approximately 4–6 m, whereas the Phragmites australis wetland association was concentrated under shallower groundwater conditions. With increasing water-table depth, vegetation composition progressively shifted towards drought-tolerant shrub and desert-herb associations. Association-specific realized groundwater niches further revealed empirical compositional transition zones around water-table depths of approximately 4 and 6 m, representing shifts in community organization rather than universal physiological thresholds or groundwater optima. Longitudinal zonation and lateral differentiation further showed that association patterns reflected the interconnected influences of downstream water delivery, river proximity, and groundwater accessibility. Together, these results characterize the present-day ecohydrological organization of riparian vegetation under a hydrological regime shaped by a quarter-century of environmental-flow releases. By advancing environmental-flow assessment from aggregate vegetation responses to spatially explicit association-level ecohydrological organization, this study provides a finer ecological framework for identifying hydrologically differentiated vegetation units and supporting spatially differentiated environmental-water management and riparian restoration in regulated dryland rivers.