Relationship between leaf hydraulic traits and xylem water stable isotope
With increasingly dry conditions in the summer, the xylem δ18O and δ2H decreased for oaks and increased for pines in the four-species mixtures, leading to contrasting relationships between xylem δ18O and the leaf hydraulic traits (Figs. 6 & S4; Table S2). With decreasing xylem δ18O, Ψpd, ΔΨ, and gs significantly increased for pines, suggesting that deeper water uptake resulted in lower water stress and higher gas exchange. For oaks, decreasing xylem δ18O led to lower Ψpd, ΔΨ, and gs(P<0.001; Fig. 6), indicating lower gas exchange despite deeper water sources. These relationships were only significant in the four-species mixtures and not in the monospecific plots (Fig. 6).