Figure 4. Proportional contribution to xylem water (mean ±SE) of the water in the fractured bedrock (bedrock; dark blue), deep (20-40 cm; medium blue), and shallow soil layers (0-20 cm; light blue) resulting from the mixing model, for P. nigra, P. sylvestris, Q. faginea , and Q. ilex in monospecific (dashed bars) and four-species mixtures (empty bars) for each sampling date. Asterisks denote significant differences between diversity levels for a given soil layers and date (*0.05≥P>0.01; **0.01≥P>0.001; ***P≥0.001).
We observed a significant negative relationship between water source partitioning (PW), i.e., the average difference between water-uptake depths, and the soil aridity index (P/PET) (P<0.001; Fig. 5b). When the soil moisture decreased in the summer (i.e., decreasing P/PET), PW consistently increased, indicating stronger water source partitioning. After the first rain replenished the soil in the fall of 2021, PWdecreased. In contrast, the intense 2022 drought delayed the recovery of PW in the fall (Fig. 5a).