Model evaluation for reconstructing the oxygen isotopic composition in precipitation from tree ring cellulose over the last century

The oxygen isotope composition of tree rings is controlled by many factors, including temperature, amount of precipitation, and changes in relative humidity. In this study we present a modified leaf-water model (from Dongmann et al. [Radiation and Environmental Biophysics 11 (1974) 41]) that can be used to calculate the isotopic composition of the source-water (and thus of precipitation) that a tree used during the growing season. The calibration of the model was accomplished by comparing a previously measured oxygen isotope tree ring chronology from Central Switzerland with the Swiss Network for Isotopes in the Hydrologic Cycle station at Bern from 1971 to 1995 and integrating temperature, relative humidity data, and ring-width. In particular, our efforts focused on understanding the significance of the dampening factor f, which we relate to changes in humidity, and its variability over both the calibration period (1971 to 1995) and the study period (1913 to 1995). Our results indicate that f (ranging between 0.27 to 0.49) is variable, based on correlation with relative humidity, average daily temperature and ring width index. Using this model, we have constructed a record of the oxygen isotope composition of precipitation during the growing season (May through September) for the last century in central western Europe. This approach can be potentially used in other locations where isotopic and meteorological data are available to extend records of the isotopic composition of precipitation back in time beyond observational records.