From roots to canopy: Unraveling the influence of species diversity on tree water relations under warmer and drier climates

The worsening of drought events with rising air temperature alters tree water relations causing severe hydraulic impairments and widespread forest mortality. Mixing tree species with contrasting hydraulic traits could reduce forest vulnerability to extreme events through positive species interactions, such as facilitation and niche partitioning. However, it remains unclear if and how species diversity modulates tree water relations as very few studies investigated such relationships mechanistically and in the field.In this thesis, I aimed to evaluate how tree species diversity affects tree water relations in response to a drying and warming climate, from the roots up to the canopy. The first objective was to identify the physiological mechanisms driving species interaction effects on the leaf- and stem-level water relations in European trees exposed to chronically drier and warmer conditions. In a multi-year climatic manipulation using open-top chambers, I assessed how heat and drought affect the leaf hydraulic traits and time to hydraulic failure of juvenile F. sylvatica and Q. pubescens trees in monospecific and mixtures. Then, I investigated how tree species diversity alters the seasonal water dynamics in both belowground and aboveground compartments in natural dry forests subjected to seasonal variation in precipitation and temperature. In these mature forests, I studied the seasonal dynamics of in-situ hydraulic traits at the leaf, stem, and belowground compartments in four co-existing Pinus and Quercus species over two years in stands with increasing tree species diversity (from monospecific to four-species mixtures).My work highlighted mainly adverse impacts of species diversity in mixed compared to monospecific stands for almost all tree species. This trend was observed both in experimental settings and in natural forests with adult trees. The work in open-top chambers showed that differences in canopy size and transpiration rates (driven mainly by contrasting stomatal regulation strategies between species) drove the observed leaf water dynamics. More specifically, higher water use rates and larger crowns in Q. pubescens exacerbated drought and heat impacts on F. sylvatica in mixtures. Similarly, I mainly observed adverse impacts of species diversity in mixed forests compared to monospecific stands for all tree species, including higher hydraulic impairments, especially for the two pines. However, I still observed important soil water source partitioning in more diverse stands, particularly as conditions became drier during the summer, suggesting that reduced competition for water in more diverse ecosystems is insufficient to buffer the adverse impacts of severe droughts.To conclude, my work highlighted that diversity effects in forests are not systematically beneficial and highly depend on the species composition, especially the specific set of trait and degree of acclimation of all interacting species to drier and warmer conditions. The drought vulnerability and competitiveness of tree species can vary in response to species interactions and are mainly driven by the species-specific canopy size, the stomatal regulation strategy, the maximum rooting depth, and local environmental conditions (i.e., heat and drought intensity) found in each forest stand. Hence, to combine forest multifunctionality and drought tolerance, my work provide key information to improve the selection of species combinations adapted to future climat.