In the context of ecology studies on biodiversity and ecosystem functioning relationships, mixed-species forests have been highly studied over the past decade. Forests with a high diversity of treespecies are usually found to be more productive than monospecific ones (Toïgo et al. 2015). However,the aboveground and belowground mechanisms leading to these patterns are still not well understood(Forrester 2014). Furthermore, in the context of climate change, it is interesting to analyze whetherthese mechanisms can help mediate the effects of harsher climatic conditions such as drought(Grossiord et al. 2014). The objective of this study was to characterize the influence of tree species interactions on functionaltraits related to carbon and water acquisition of trees under various environmental conditions in mixedforests, in order to discuss the mechanisms leading to diversity effects. We measured tree-level water use efficiency using carbon isotope composition of the wood in treerings, leaf functional traits related to carbon and water acquisition and use, light availability for thedominant trees, and soil water acquisition. A triplet approach (3 different forest stands) was used tocompare these functional traits in pure and two-species stands. Measurements were conducted alonga latitudinal gradient in natural forests (5 sites in the South-East of France) in beech-silver fir or beech-pubescent oak forests. All data are available right now and statistical analyses are currently being processed. We expecttrees in mixed-stands to have enhanced access to water resources and thus display differentfunctional traits. Moreover, we hypothesize that aboveground interactions between species result inhigher light interception in mixed-stands. Finally, we expect that these effects should be stronger in thesites that are the most exposed to water stress.< Réduire