Figure 1. Mean monthly air temperature (± minimal and maximal temperature), vapor pressure deficit (VPD), and total monthly precipitation in 2021 and 2022. Dashed vertical lines indicate the measurement campaigns.
The area is characterized by the natural dominance of four tree species, i.e., two coniferous species (Pinus nigra subsp. salzmannii (Dunal) Franco and Pinus sylvestris L. ) and two broadleaved ones (Quercus faginea Lam. and Quercus ilex L. ), which were selected for this study. We selected plots with increasing tree species richness, including monospecific (where the target species represents more than 90% of the total basal area), monofunctional (i.e., two-species mixtures of either coniferous or broadleaved species), multifunctional (i.e., two-species mixtures of coniferous and broadleaved species) and mixtures of the four species. Each richness level was replicated three times for all species with all possible species combinations, except for the two-species mixture betweenP. sylvestris and Q. ilex , resulting in 30 plots (Table S1). In mixed plots, the target species had similar abundances with a lower limit of 60% of maximum evenness in the basal area (Baeten et al., 2013). The understory vegetation (representing less than 10% of the total basal area) was mainly composed of shrub species (Arctostaphylos uvaursi, Buxus sempervirens, and Genista scorpius ) and juveniles of the dominant tree species.
We randomly selected five dominant or co-dominant trees per species in each plot, leading to 265 trees. To assess the seasonal dynamics of aboveground and belowground tree water and carbon use, we conductedin-situ measurements (detailed below) on each tree in 2021 and 2022 at the beginning (i.e., in May when soil moisture was high), middle (i.e., in July, corresponding to the driest and hottest period), and the end of the growing season (i.e., in September, representing the recovery transition from dry to wet soils).