Water source contribution and partitioning
As the soil dried out in the summer, all species, regardless of the diversity (i.e., monospecific and four-species mixtures), shifted their water uptake depth from shallow (i.e., 0-20 cm) in spring to deeper (i.e., >20 cm) layers in the summer (Fig. 4). In 2021, this trend was followed by a recovery to shallower layers in the fall. In contrast, the water uptake depth did not return to shallower sources in the fall of 2022 (Fig. 4 & Table S3), probably due to the reduced precipitation in fall 2022 (Fig. 1). The uptake of the water from the bedrock was consistently low for all species throughout the growing season (19±0.5%), except in the summer of 2021, where it increased by up to 23% compared to the springtime for the oaks in monospecific plots (Fig. 4).
However, we found no significant effect of species diversity on the water uptake depth for all species except for Q. faginea . In 2021, Q. faginea trees took up more water from the shallower layers in the spring and the deep layers in the summer in the four-species mixtures compared to monospecific plots (Fig. 4 & Table S3). We further observed a clear distinction in the seasonal variability between functional types (i.e., oaks and pines). Under wetter conditions (i.e., spring and fall), oaks took up a similar proportion of water from the shallowest layers as pines (44±11% and 39±6%, respectively). In the summer, oaks more sharply shifted towards deeper sources than pines (Fig. 4). This pattern resulted in less water extracted from the shallowest layers in summer than in spring, independently of the year (reduction of about 28% and 17% for oaks and pines, respectively).