MODELING 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. DeltaC-13 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 evapotranspirations as well as the soil water content on a daily basis. DeltaC-13 shows a significant decrease from -24-5 parts per thousand to -25 parts per thousand 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 deltaC-13 for the previous year, predicts as much as 70% of this variance. Air temperature or precipitation accounted for less variation. DeltaC-13 is strongly correlated with basal area increment, but appeared a more reliable indicator of water status at the stand level.