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.