Introduction
Mediterranean ecosystems are biodiversity hotspots and prime targets for
conservation efforts (Myers et al., 2000). These iconic ecosystems count
as a global change epicenter that is expected to experience stronger
temperature rises compared to 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 if more diverse forests tolerate better extreme events
(Grossiord, 2020).
The effect of drought on aboveground 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 (Ψpd)
decreases. During the day, if trees continue to transpire, midday leaf
water potential (Ψmd) drops, which increases the
difference between the predawn and midday water potentials
(ΔΨ, an index for stomatal regulation). Eventually,
trees will close their stomata, reducing their net carbon uptake through
photosynthesis (Anet) and their stomatal conductance
(gs) (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 to aboveground processes, we have
limited knowledge of the belowground 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
belowground biomass than aboveground 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 belowground 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 ΔΨ; anisohydric) while others
maintain a relatively constant water potential by closing their stomata
(i.e., low ΔΨ; 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 (e.g.,
Grossiord et al., 2014). Nevertheless, belowground 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., belowground
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 belowground mechanisms have rarely been addressed.
The objective of this study is to investigate how tree species diversity
modulates the seasonal dynamics of above- and belowground 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, i.e., Pinus nigra and Pinus
sylvestris , and two deep-rooted anisohydric broadleaves, i.e.,Quercus faginea and Quercus ilex (Čermák et al., 2008;
Moreno et al., 2005). We monitored the seasonal dynamics in aboveground
(Ψpd, Ψmd, ΔΨ,
Anet, gs) and belowground (water uptake
depth and water source partitioning determined by water stable isotope
profiling) water dynamics over two 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 Ψpd,
Ψmd, ΔΨ, Anet, and
gs during the summer drought and a more rapid recovery
in the fall in mixed conifer-broadleaf stands. These responses should be
driven by belowground moisture partitioning between the two functional
groups.