1 INTRODUCTION

Mediterranean ecosystems are biodiversity hotspots and prime targets for conservation efforts (Myers et al., 2000). These iconic ecosystems count as a global change epicentre that is expected to experience stronger temperature rises compared with the global average (e.g. Giorgi & Lionello, 2008; Hoegh-Guldberg et al., 2018; Mariotti et al., 2015; Polade et al., 2017). More frequent and intense droughts with global warming will alter plant carbon and water exchange within these ecosystems, including leading to severe hydraulic impairments (e.g. Fontes et al., 2018; Klein et al., 2022) and amplified tree mortality (e.g. Anderegg et al., 2016; Breshears et al., 2005; Hartmann et al., 2022; McDowell et al., 2018). A strategy often advocated to mitigate adverse drought effects is to promote and restore tree species diversity via management efforts (e.g. reforestation and selective thinning; Vadell et al., 2022) (Anderegg et al., 2018; Liu et al., 2022; Steckel et al., 2020). Yet, the underlying mechanisms driving tree diversity effects on water dynamics are poorly understood. Therefore, it remains unclear whether more diverse forests tolerate better extreme events (Grossiord, 2020).

The effect of drought on above-ground water use has been well studied, allowing us to gain a good understanding of the leaf- to tree-level processes leading to drought-induced tree decline. When soil moisture is reduced, the leaf predawn water potential (Ψ_pd) decreases. During the day, if trees continue to transpire, midday leaf water potential (Ψ_md) drops, which increases the difference between the predawn and midday water potentials (Δ_Ψ, an index for stomatal regulation). Eventually, trees will close their stomata, reducing their net carbon uptake through photosynthesis (A_net) and their stomatal conductance (g_s) (e.g. Brodribb & Holbrook, 2003). As the drought progresses, negative tensions in the vascular system will eventually surpass critical thresholds (Choat et al., 2018; Cochard, 2006; Morcillo et al., 2022; Tyree & Sperry, 1989), leading to hydraulic failure and tree desiccation. However, compared with above-ground processes, we have limited knowledge of the below-ground ones and their consequences for tree carbon and water relations, particularly the temporal and spatial dynamics of tree soil water uptake (Phillips et al., 2016). Mediterranean regions are often characterized by the duality of shallow soils where water quickly evaporates and a fractured deep bedrock that can store water for extended periods (Peñuelas & Filella, 2003; Rose et al., 2003). Consequently, Mediterranean plants tend to develop larger below-ground biomass than above-ground ones, with roots reaching depths up to seven times the canopy projection (Moreno et al., 2005). Because of the prominence of dual root systems (i.e. shallow and vertical deep sinker roots; Devi et al., 2016) in dry regions, trees often transition their primary water source from superficial layers in the spring to water stored in the bedrock cracks in the summer (e.g. Barbeta et al., 2015; David et al., 2013; Eliades et al., 2018; Grossiord et al., 2017). Indeed, water from shallow soil layers is easier to extract due to its higher porosity, lower soil matric potential, and higher water storage than deeper layers (Dawson et al., 2020; Klos et al., 2018; Or et al., 2002). Accessing water from deep horizons and the bedrock could allow maintenance of vital plant functions during extreme droughts (Hanson et al., 2007; Rempe & Dietrich, 2018). Yet, studies investigating the dynamics of tree water sources tend to focus on single species (e.g. Brinkmann et al., 2019), so the impacts of species interactions on water uptake are largely unknown. Moreover, because of technical challenges associated with below-ground measurements, our knowledge of tree water uptake and its impact on tree carbon and water use is limited in natural ecosystems (but see Andrews et al., 2012; Ding et al., 2021; Grossiord et al., 2017; Kukowski et al., 2013). In this context, isotope profiling offers a non-destructive method, relating the stable isotopic signature of the plant water to that of the soil at different depths (Ehleringer & Dawson, 1992).

In forests, the co-existence of functionally contrasting species with distinct architectures (e.g. mixtures of broadleaf and conifer species) can lead to complementary aboveground structural traits, resulting in denser canopies (i.e. enhanced canopy packing; e.g. Jucker et al., 2015) and stronger shading (Duarte et al., 2021; Ligot et al., 2016). Additionally, denser canopies improve the forest microclimate and buffer temperature extremes, especially in dry regions (e.g. De Frenne et al., 2021). Moreover, trees can differ in their aboveground water use strategy by ranking along a gradient from isohydric to anisohydric (Martínez-Vilalta et al., 2014; Tardieu & Simonneau, 1998), with some species tracking soil moisture reductions by dropping their leaf water potential (i.e. high Δ_Ψ; anisohydric) while others maintain a relatively constant water potential by closing their stomata (i.e. low Δ_Ψ; isohydric). Differences in species-specific stomatal sensitivity affect the competition intensity and timing as water resources are differently used throughout the year. On the contrary, species with similar water use strategies could severely compete during drought, increasing water stress in monospecific stands (e.g. Grossiord, Gessler, et al., 2014; Grossiord, Granier, et al., 2014). Nevertheless, below-ground complementarity mechanisms are undoubtedly the ones that could play the most considerable role in Mediterranean systems. Indeed, interacting species may extend their roots at different depths to partition water sources and reduce tree–tree competition (Grossiord et al., 2018; Hooper, 1998; Rodríguez-Robles et al., 2020; Silvertown, 2004), inducing a slower reduction in water availability during drought and delaying the onset of hydraulic dysfunctions (Hajek et al., 2022). For instance, rather anisohydric oak species (Roman et al., 2015) are characterized by a deep dimorphic root system (i.e. deep taproot and secondary roots poorly developed horizontally) reaching up to 5.2 m depth (Moreno et al., 2005). In contrast, isohydric pines (Klein et al., 2011) tend to have more extended shallow root systems (Čermák et al., 2008; Moreno et al., 2005). Hence, when these two rooting habits coexist in mixed forests, they could, to some degree, exhibit water source partitioning. Additionally, processes of facilitation such as hydraulic redistribution whereby deep-rooted species passively transfer water from deep, moist soils to dry superficial ones can provide additional moisture to shallow-rooted species (e.g. Lubczynski, 2009; Rodríguez-Robles et al., 2020; Schwendenmann et al., 2015). However, during extreme events, soil moisture reductions may be too severe for these mechanisms to overcome the water stress experienced by trees (e.g. Grossiord et al., 2018; Haberstroh & Werner, 2022). Species interactions can shift from positive to negative due to enhanced competition (i.e. below-ground water niche overlapping) depending on environmental conditions (Ratcliffe et al., 2017), with most benefits observed at intermediate stress levels (Rodríguez-Robles et al., 2020). Still, the tree's functional characteristics and environmental conditions giving rise to beneficial or detrimental diversity effects remain unclear, mainly because the temporal below-ground mechanisms have rarely been addressed.

The objective of this study was to investigate how tree species diversity modulates the seasonal dynamics of above- and below-ground water use and carbon fixation in four co-existing Mediterranean tree species with contrasting water use strategies and rooting habit: two shallow-rooted isohydric conifers, that is Pinus nigra and Pinus sylvestris, and two deep-rooted anisohydric broadleaves, that is Quercus faginea and Quercus ilex (Čermák et al., 2008; Moreno et al., 2005). We monitored the seasonal dynamics in above-ground (Ψ_pd, Ψ_md, Δ_Ψ, A_net, g_s) and below-ground (water uptake depth and water source partitioning determined by water stable isotope profiling) water dynamics over 2 years in 30 mature forest plots with increasing tree diversity (from monospecific to four-species mixtures). Because of complementarity and facilitation between functional groups, we expected a lesser decrease in Ψ_pd, Ψ_md, Δ_Ψ, A_net, and g_s during the summer drought and a more rapid recovery in the fall in mixed conifer-broadleaf stands. Below-ground, these responses should be driven by moisture partitioning between the two functional groups. Above-ground, enhanced canopy packing and shading could mitigate the heat stress experienced by trees in diverse forests during the hot summer months.