Environmental influences on carbon isotope fractionation in tree rings require further elucidation in order to use this parameter as a biological marker of climatic variations. δ13C values in tree-ring cellulose of beech (Fagus sylvatica L.) were analysed for the period from 1950 to 1990. A bioclimatic model of water balance was used to give the actual evapotranspiration as well as the soil water content on a daily basis. δ13C shows a significant decrease from –24·5‰ to –25‰ over this period. Internal CO2 concentration changes from 200 to 220 ppm in relation with the rise of atmospheric CO2. Beside a slight non-significant inter-individual variation, a large year-to-year variation exists. The relative extractable soil water of July, combined with the value of δ13C for the previous year, predicts as much as 70% of this variance. Air temperature or precipitation accounted for less variation. δ13C is strongly correlated with basal area increment, but appeared a more reliable indicator of water status at the stand level.