Due to recent forest management, most of the natural occupation areas of beech (Fagus sylvatica L.) are now dominated by planted conifers. One of the aims of current forest policy is to revert this process, favouring the natural regeneration of beech and t he growth and quality improvement of adult trees (e.g. directive 92/43/CEE). However, current climatic models predict an overall raise in temperature throughout Europe (up to 1ºC by 2040), together with an increase in summer drought. Although beech might benefit from greater temperatures by expanding its vegetative period, the decrease in water availability would counteract the positive effect of temperature. Consequently, a forest policy promoting systematically beech regeneration, without considering future conditions, may be unsuccessful.This makes necessary to increase our efforts to characterise the eco-physiology of this species, particularly regarding water relations, in order to define the most adequate management criteria. In this context, the analysis of carbon and oxygen stable isotopes (d13C, d18O) in different plant fractions shown to be an useful ecophysiological tool, combining the ability to integrate physiological information in time with a relative simplicity, which allows to validate the results over a great number of sites.The aim of this proposal is to determine whether the ecophysiological traits of beech may be a disadvantage respect some of its competitors (conifers and oaks), in case of an overall increase in temperature and summer drought. This will be done by combining information from d13C and d18O with data derived from sap flow and meteorological measurements. Although the core of the project is the use of stable isotopes to determine water balance of trees, the proposal is strongly interdisciplinary, combining data from different disciplines: meteorology, hydrology, plant physiology and ecology. This project will help to adapt forest management policies to a changing environment.