Drought and heat waves frequently co-occur and represent major stressors for forest ecosystems. However, the mechanistic links between plant hydraulic status and photosystem II (PSII) heat tolerance remain poorly understood. This study investigated how progressive drought affects stomatal conductance, intrinsic water-use efficiency (iWUE), and PSII thermotolerance (critical temperatures T_c, T_15(Fv/Fm)) in seedlings of four riparian tree species ( Alnus incana , Populus alba , Salix alba , and Salix fragilis ). In addition, xylem embolism resistance (P₅₀), turgor loss point (P_TLP) and stomatal morphological traits were quantified. All species exhibited strongly isohydric behaviour, with leaf water potential at stomatal closure (P_gs90) of approximately −1.5 MPa, followed by P₅₀ around −1.7 MPa and tight stomatal safety margins ranging from 0.07 to 0.46 MPa. We observed a unimodal relationship between iWUE and leaf water potential, suggesting optimal iWUE just prior to the P₅₀ threshold. Declining water potential led to a reduction in PSII heat tolerance in the two Salix species, but not in A. incana and P. alba . PSII critical temperatures were significantly lower after crossing the P₅₀ threshold compared to pre-P₅₀ measurements in the two S. species. T_c was reduced by approximately 2 °C and T_15(Fv/Fm) by 1–3 °C under severe drought stress. Notably, the P₅₀ threshold appears to represent a critical hydraulic tipping point beyond which the thermal stability of the photosynthetic apparatus declines significantly.