Stable isotopes in biological systems provide a valuable tool to infer environmental information from the present and the past. From isotope fractionation models, it is known that the ¹³C/¹²C ratio of plant material is dependent on the stomatal limitation of photosynthesis and on the intercellular CO₂-concentration, c_i. As these physiological parameters are dependent on micro-climatic conditions, the isotope ratios are also influenced by climate. We present data from cellulose of Fagus sylvatica covering the period from 1935 to 1990 from several sites in Switzerland and demonstrate that increased ¹³C/¹²C ratios are correlated with warm and dry conditions during the growing season. Further, the calculated c_i-values for F. sylvatica increase by about 20 ppm during the last 50 years which is significantly less than the increase of the atmospheric CO₂-concentration, c_a, during the same period. Accordingly, there is a strongly increasing trend for c_a-c_i whereas c_i/c_a is decreasing. The ¹⁸O/¹⁶O ratio in plant material is determined (i) by the oxygen isotope variations in precipitation which in turn are dependent on temperature, and (ii) by the ¹⁸O enrichment in leaf water due to transpiration which is influenced mainly by the relative humidity of the atmosphere. From our tree-ring data, we conclude that ¹⁸O/¹⁶O variations in precipitation are preserved in tree-ring cellulose, whereas the signal for leaf-water enrichment is strongly dampened.