Leaf-level gas exchange and water potential
In both years, as drought stress increased during the summer, predawn
leaf water potential (Ψpd), net photosynthesis
(Anet), and stomatal conductance (gs)
significantly decreased for all species (Fig. 2, Table S2). As
precipitation increased soil water content in the early fall,
Ψpd, Anet, and gsrecovered to similar values as in spring for all species in 2021. In
contrast, Ψpd for all species and Anetand gs for pines did not recover following the extreme
summer drought in 2022 (Fig. 2). As Ψpd decreased, the
difference between predawn and midday water potentials
(ΔΨ) significantly decreased for all species
(P<0.001), with oaks reaching lower Ψpd while
maintaining higher ΔΨ than pines (Fig. 3).
Throughout all seasons, we found lower Ψpd,
Anet, and gs in the four-species
mixtures compared to the monospecific plots for P. sylvestris(Fig. 2 & Table S2). Similarly, in the two-species mixtures (i.e.,
monofunctional and multifunctional), significant reductions in
Ψpd, Anet, and gscompared to the monospecific plots were observed in the fall of 2021 and
throughout 2022 for P. sylvestris . A similar pattern was found
for P. nigra but with less significant diversity effects (Fig. 2
& Table S2). Indeed, for P. nigra , while Ψpdvalues were lower in the diverse plots compared to the monospecific ones
in the summer and fall of 2022, Anet and
gs rates were only reduced in the four-species and
monofunctional mixtures in the summer and fall of 2021. For oaks, the
effect of species diversity was less consistent, with lower recovery of
Ψpd, Anet, and gs in the
four-species mixtures compared to the monospecific plots following the
summer of 2021. Further, in the fall of 2022, Ψpd was
higher in multifunctional and four-species mixtures for Q. ilexand Q. faginea, respectively, compared to the monospecific plots.
Similarly, Anet and gs were higher for
the multifunctional mixtures than the monospecific plots for both oak
species (Fig. 2).