Figure 6. Relationships between the predawn leaf water potential (Ψpredawn), the light-saturated stomatal conductance (gs), the difference in predawn and midday leaf water potential (Δ Ψpredawn-Ψmidday), and the xylem water oxygen isotopic composition (δ18O) for P. nigra , P. sylvestris , Q. faginea and Q. ilex in monospecific (empty circles), and four-species mixtures (plain circles). Colors represent the aridity index of all the measurement months with a gradient from the driest (light blue) to the wettest (dark blue) in 2021 and 2022. Lines represent significant linear regressions with dotted and solid lines standing for monospecific and four-species plots.
We found variable effects of PW on the leaf hydraulic traits. For all species, we observed a decrease of Ψpdand gs with increasing PW(P<0.001), indicating reduced gas exchange with increased water source partitioning (mainly during the summer; Fig. 5). In addition, ΔΨ decreased with increasing PW for pines but not oaks, highlighting the differential stomatal regulation strategies in the two functional groups (Fig.7).