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).