Larix decidua δ18O tree-ring cellulose mainly reflects the isotopic signature of winter snow in a high-altitude glacial valley of the European Alps

We analyzed the chronologies of cellulose stable isotopes (δ13C and δ18O) and tree-ring widths from European larch (Larix decidua) in a high-altitude site (2190 m a.s.l.) at the bottom of a glacial valley in the Italian Alps, and investigated their dependence on monthly meteorological variables and δ18O precipitation values. The δ18O of tree-ring cellulose appears to be strongly driven by the δ18O of winter snowfall (November to March), which suggests that larch trees mostly use the snow-melt water of the previous winter during the growing season. This water, which also comes from the slope streams and from the underground flow of nearby steep slopes, infiltrates the soil in the valley bottom. The tree-ring cellulose δ18O values were also found to be influenced by the August precipitation δ18O and mean temperature. The associated regression model shows that the δ18O chronology from the tree rings explains up to 34% of the variance in the winter precipitation δ18O record, demonstrating the potential for reconstructing the δ18O isotopic composition of past winter precipitation in the study region. Unlike most other tree-ring studies that focus on growing season signals, in our study the summer signal was small and the winter signal dominant due to the special conditions of the glacial valley. Site topography, geomorphology and soil characteristics in particular influence the stable isotope signal in tree-ring cellulose.