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The exchange of water for carbon between the vegetation and the atmosphere constitutes the most important feedback mechanism underlying the regulation of water and carbon cycles by the terrestrial biosphere. Water use efficiency (WUE) is the trait that serves to describe this intimate link between the carbon and water cycles and is a crucial parameter incorporated in many land-surface-models to predict vegetation-climatic feedbacks under future climate change scenario. WUE can be estimated using multiple approaches and methodologies concerning different temporal spatial and temporal scales. Empirical comparison of WUE estimates reveal that important disagreements exist among methodologies; however, these comparisons could not reveal the underlying mechanisms driving these discrepancies. The aim of this PhD project is to develop a novel experimental approach to reconcile WUE estimates from different methodologies, temporal and spatial scales, with special emphasis on the use of carbon stable isotopes measured on different plant tissues. The model species will be European beech (Fagus sylvatica). The PhD candidate will combine modelling with experimental work under controlled conditions and in the field. The field work will be located in beech forests in the Iberian Peninsula with some intensive campaigns in forests in the Basque Country.