Discussion
Understanding the environmental factors influencing drought vulnerability is crucial for effective forest management and conservation strategies. Our study indicates that while increasing tree diversity can promote water source partitioning and improve plant water availability to some extent, this mechanism alone is insufficient to overcome the adverse impacts of intense Mediterranean summer droughts. Indeed, we showed that the leaf hydraulic traits followed, as expected, the well-known seasonal variability in Mediterranean forests (Fotelli et al., 2019; Gulías et al., 2009). This trend is defined by increasing water stress (i.e., as shown by reduced Ψpd, Anet, and gs) when soil moisture decreases in the summer. Gas exchange then recovered in all species after the first rain replenished the soil in the fall of 2021 (Table S1 & Fig. 2). Interestingly, we observed lower or no recovery of gas exchange and hydraulic status in the fall of 2022 due to late precipitation events that kept soil moisture low at the end of the growing season (Fig.1). However, tree species diversity modulated the magnitude of this seasonal variation with contrasting impacts on oaks and pines. Decreasing Ψpd, Anet, and gs in pines in mixed plots compared to the monospecific ones indicates higher water stress during the summer. In contrast, oaks showed either no effect or a positive diversity effect (i.e., lower reduction in Ψpd, Anet, gs in mixtures) (Fig. 2). Previous work conducted in the same area also found that tree diversity enhanced or reduced tree transpiration of Q. faginea and P.sylvestris , respectively (Grossiord et al., 2015). However, our work highlights the predominance of functional diversity effects rather than species richness per se , as similar seasonal variations in carbon and water dynamics were observed in monospecific and monofunctional mixtures for both functional groups (Díaz & Cabido, 2001). Hence, despite the general consensus and our initial hypothesis that diversity enhances drought resistance and resilience (e.g., Anderegg et al., 2018; Liu et al., 2022), we found that increasing functional diversity could benefit some species but also exacerbate the water stress for others. Aboveground, this response could be driven by the water–saving strategy of the more isohydric pines that maintain high water potential (i.e., low ΔΨ) by closing their stomata earlier than the more anisohydric oaks (Fig. 3). As previously observed for Q. ilex and P. sylvestris in montane Mediterranean forests (Aguadé et al., 2015), contrasting stomatal regulations can result in faster exhaustion of soil moisture by oaks at the expense of pines.
Despite the essentially adverse impacts of drought on the aboveground carbon and water dynamics, all species shifted their water uptake depth from shallow to deep soil layers (>20cm) during the summer before going back to more superficial layers (0-20 cm) in the fall (Fig. 4). This finding supports the widely observed vertical plasticity of water sources in Mediterranean forests (e.g., Barbeta et al., 2015; David et al., 2013; Eliades et al., 2018; Grossiord et al., 2017). We further observed a constant contribution of water from the fractured bedrock (around 19%) (Fig. 4), supporting numerous studies that highlight the importance of such reservoirs in Mediterranean regions (e.g., Barbeta et al., 2015; Eliades et al., 2018; Hanson et al., 2007). Contrary to our expectations, species diversity did not modify the water sources of the studied species in the four-species mixtures (Table S3). Nevertheless, contrasting water uptake depths in pines and oaks discriminated tree water sources along a vertical niche axis during drier periods (Rodríguez-Robles et al., 2020). Hence, increasing water source partitioning with reduced soil moisture was observed (Fig. 5), as previously found in various forests during drought (e.g., Bello et al., 2019; Grossiord et al., 2018; Meiner et al., 2012; Rodríguez-Robles et al., 2020; Schwendenmann et al., 2015). As expected, shallow-rooted pines took preferentially water from the more superficial soil layers throughout the year (Čermák et al., 2008). In contrast, oaks’ deep and dual root systems facilitated access to significantly deeper water sources in the summer (Moreno et al., 2005). Whether the observed belowground niche partitioning could result from root growth competition remains unknown (del Castillo et al., 2016). Still, niche overlapping occurred when water was abundant, while niche partitioning ensued under dry conditions (e.g., Barbeta et al., 2015; Guo et al., 2018; Rodríguez-Robles et al., 2020; Rodríguez‐Robles et al., 2015).
Belowground moisture partitioning (PW; Figs. 4 & 5) had a limited beneficial impact on the aboveground carbon and water use dynamics. On the one hand, the shift to deeper sources in mixtures during the summer led to more negative xylem δ18O (i.e., reduced P/PET leading to deeper water sources) that still resulted in lower Ψpd, gs, and ΔΨ (Fig. 6) for oak. Thus, even if oaks maintained their water flux in mixed plots (i.e., no relationship between ΔΨ and PW; Fig. 7), access to deeper water reservoirs was insufficient to fully overcome the water stress induced by drought. This finding contradicts previous work inQuercus suber L. trees in central Portugal, suggesting that shifts in water sources could maintain high transpiration rates during summer droughts (David et al., 2007). The discrepancy between these findings could stem from the lower groundwater contributions observed at our plots (19% vs . 30%) associated with shallower soils, high stoniness, and the lesser importance of hydraulic redistribution, representing up to 37% of transpired water in this work. Although we cannot exclude that hydraulic redistribution occurred in our study, the very low gas exchange observed during the summer (Fig. 2) suggests that it played a limited role. On the other hand, for pines, the continuous reliance on the shallowest soil layers (as shown by the more positive xylem δ18O; Fig. S3) lowered Ψpd,gs, and ΔΨ during drought (Fig. 6). Hence, although water source partitioning increased as the soil dried out (mainly because of oak’s deeper water uptake), it still resulted in decreased Ψpd and gs in all species (Fig. 7). These findings contrast with previous studies highlighting the importance of belowground complementarity and water redistribution (e.g., David et al., 2007; Rodríguez-Robles et al., 2020). Overall, our findings feature the complex and multifaceted nature of forest responses to climate change by emphasizing that diversity-driven shifts in water sources can overcome water stress only up to a certain drought intensity (Figs. 2 & 6). Moreover, as species diversity effect on drought resistance depends on the environmental conditions (Grossiord et al., 2014; Liu et al., 2022), the conclusions of this study could change in different forest ecosystems. Further research in multiple ecosystems is warranted to elucidate if other diversity-driven mechanisms (e.g., hydraulic lift, canopy packing, microclimate feedback) could play a pivotal role in these ecosystems and identify strategies to enhance forest tolerance to climate change.