Modelling carbon isotope fractionation in tree rings based on effective evapotranspiration and soil water status

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. δ 13 C 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. δ 13 C shows a significant decrease from –24·5‰ to –25‰ over this period. Internal CO 2 concentration changes from 200 to 220 ppm in relation with the rise of atmospheric CO 2 . 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 δ 13 C for the previous year, predicts as much as 70% of this variance. Air temperature or precipitation accounted for less variation. δ 13 C is strongly correlated with basal area increment, but appeared a more reliable indicator of water status at the stand level.