Water source partitioning
To estimate the plot-level vertical water source distribution in the mixed forest plots, we calculated the belowground water source partitioning at the community level (PW, unitless) from the sum of the differences in either the natural abundance of δ2H or δ 18O in plant water between all interacting species in every four-species mixtures for each sampling date. As different water uptake depths between trees correspond to greater differences in the xylem water stable isotope within a tree cluster, higher PW indicates that the trees are taking up water from more distant water sources from each other. Therefore, to calculate PW based on δ2H, we used the following equation developed by Grossiord et al. (2018):
PW = |δ2HPN - δ2HPS| + |δ2HPN - δ2HQF| + |δ2HPN - δ2HQI| +
2HPS - δ2HQF| + |δ2HPS - δ2HQI| + |δ2HQF - δ2HQI| (Equ. 1)
where δ2HPN, δ2HPS, δ2HQF, and δ2HQI correspond to δ2H of P. nigra, P. sylvestris, Q. faginea, andQ. ilex , respectively. As the PW resulted from δ18O and δ2H weighted similarly, only the PW based on δ2H was further used in this paper (Fig. S4).