Water source contribution
From the natural abundance of δ2H and
δ18O in plant xylem and soil water, we used a Bayesian
stable isotope mixing model to quantify the contribution of potential
tree water sources in each species in the monospecific and four-species
mixtures for each sampling date. As the rainwater isotope ratio differs
throughout the season (i.e., isotopically more depleted rainwater in
winter compared to summer) and the water evaporation decreases with soil
depth, each water source has a significantly different stable isotopic
composition (Fig. S3). These distinct soil isotopic profiles allow us to
determine the contribution of each water source to the tree xylem water
under the assumption that there is no isotopic fractionation during
water uptake by the roots (Dawson & Ehleringer, 1991). Therefore, the
natural isotopic abundance of xylem sap should reflect the water sources
used by the plant. We used the package simmr in R (Parnell,
2019), where the isotopic composition (δ 18O and δ2H) for each potential source (i.e., 0-10 cm, 10-20
cm, 20-30 cm, 30-40 cm, rainwater) and each target tree were assigned
into the model. We set the TEF (trophic enrichment factor) and the
concentration dependence to 0 due to the absence of isotopic
fractionation by the roots. We ran the model where 3600 iterations out
of 10000 runs were produced over 4 Markov chain Monte Carlo (MCMC) for
the isotopic values from each plant with the isotopic values from the
soil water source of the corresponding plot in each date (Sun et al.,
2022). To increase the clarity of presentation, the contributions from
the water sources were grouped a posteriori into three layers:
shallow (i.e., 0–20 cm), deep (i.e., 20–40 cm), and water stored in
the fractured bedrock (i.e., rainwater). Indeed, the winter
precipitation that penetrates deep soil layers and bedrock cracks could
be a substantial water source for trees in Mediterranean forests during
summer droughts (Eliades et al., 2018). As the bedrock water is not
subjected to evaporation (Ehleringer & Dawson, 1992), we used the
precipitation collected at our site as a proxy, similar to Grossiord et
al., (2017). The cumulative rainwater collected during the winter of
2021 (i.e., January-April) and 2022 (i.e., October-April) was considered
bedrock water source for the May campaigns of 2021 and 2022,
respectively. During the remaining growing season, the stable isotope
composition of the precipitation until the sampling date was added to
the winter precipitation (e.g., winter, May, and June for the campaigns
in July).