Widths and stable isotopes of Larix tree rings in a glacier-fed river basin in the Swiss Alps respond to a changing climate

Tree-rings are a valuable proxy for reconstructing past environmental conditions such as climate at annual or intra-annual resolutions. Tree-ring dating combined with stable isotope analysis (e.g. δ ¹⁸O and δ ²H) has an enormous potential for better understanding climate dynamics and for tracing freshwater resource availability under a changing climate. In this study, we applied the standard statistical approaches of dendrochronology to understand climate-growth relationships, followed by analysis of the isotopic composition of tree rings for two different sites from upstream to downstream in the Turtmann river basin (2000 m a.m.s.l.) in south-western Switzerland. The climate-growth relationship shows that the growth of European Larch (Larix decidua) trees in this river basin is positively correlated with the October and November temperature of the previous year (r= 0.46, t=0.01). Some existing tree-ring chronologies (available at ITRDB) of same tree species from the nearest sites located at different elevational transects (e.g. 1500 m and 900 m a.m.s.l) show positive correlation between tree growth and current year June-July precipitation (r= 0.40, α=0.01). This switch of tree growth from temperature-limited at higher elevation to precipitation-limited at lower elevation can be due to numerous factors including the changes in elevational gradient, micro-climate, exposure to the sun due to aspect (north versus south facing) etc. A minor trend of switching from temperature-limited to precipitation-limited growth has been observed for more recent time periods between the trees at different sites in higher and lower elevation.. This finding is supported by the relationship between tree growth and δ ¹⁸O and δ ²H values for the last two decades. Therefore, it is important to identify a breakpoint where the signal changes from temperature-limited to precipitation-limited across the elevational gradient and how this threshold is shifting particularly in the Swiss Alps to fully understand climate-growth relationships and to trace freshwater resources under a changing climate.