Water use strategies of Populus euphratica across age classes along groundwater depth gradients in the lower Tarim River.

How water source use and intrinsic water-use efficiency (iWUE) are linked across tree age classes under contrasting groundwater conditions remains unclear in desert riparian forests. We assessed water use strategies of young, middle-aged, and mature Populus euphratica in the lower Tarim River during ecological water conveyance (EWC) and non-EWC periods by combining hydrogen and oxygen stable isotopes of xylem and potential source waters with a Bayesian mixing model and leaf δ13C-derived iWUE. Across the five monitoring sections, mean groundwater depth was significantly lower during EWC than during non-EWC periods, averaging 4.7 and 4.9 m, respectively. At the Kunasite (DW) and Yingsu (F) sections, MixSIAR posterior mean contributions during EWC were 16.0-23.1% for groundwater, 14.4-17.6% for river water, and 6.6-19.0% for individual 40-cm soil layers. During non-EWC periods, posterior mean contributions from individual soil layers ranged from 10.9 to 20.3%, with relatively greater contributions from deeper layers. Associations between water source contributions and iWUE varied among age classes during EWC but were nonsignificant during non-EWC periods. Across sampling periods, mean iWUE was 58.29, 53.08, and 51.37 μmol mol-1 in young, middle-aged, and mature trees, respectively. The relationship between iWUE and groundwater depth also varied among sampling periods, with no consistent directional pattern. Based primarily on changes in the contributions of shallow and deep soil water along the groundwater depth gradient, we identified 4.5-6.0 m as a preliminary groundwater depth range associated with shifts in water source use by P. euphratica. These results show that water source use and iWUE in P. euphratica vary with hydrological conditions and tree age, providing empirical evidence for assessing its responses to EWC and groundwater variation in desert riparian forests.