Identification of an elevational breakpoint where climatic signal changes for the growth of Larix decidua tree rings in a glacier-fed river basin in the Swiss Alps

Tree-rings are a valuable proxy for reconstructing past environmental conditions such as climate at annual or intra-annual resolutions. Tree-ring dating has an enormous potential for better understanding climate dynamics under a changing climate. In Alpine regions, changes in climate may well lead to switches between temperature-limitation and precipitation-limitation. However, such changes cannot be separated from local environmental influences such as altitude and aspect. In this study, we applied the standard statistical approaches of dendrochronology to understand climate-growth relationships as a function of elevational gradients to understand how altitude conditions the impacts of climate change impacts on tree growth. For the growth of European Larch (Larix decidua) trees in the Turtmann river basin (2000 m a.m.s.l.), a glacier-fed river basin in the Swiss Alps, located in south-western Switzerland, we find that climate warming is leading to some switching from temperature limitation to precipitation limitation and vice-versa according to altitude. The climate-growth relationship further reveals that the growth of Larix decidua in this river basin is positively correlated with the October and November temperature of the previous year (r= 0.46, α=0.01). Comparing these changes with other tree-ring chronologies from the international tree-ring data bank (ITRDB) for the same species at much lower elevation transects  (e.g. 1500 m and 900 m a.m.s.l) show that the tree growth switches from temperature limitation to precipitation limitation. The growth of Larix decidua for these lower elevation trees correlates positively with the current year June-July precipitation (r= 0.40, α=0.01). A number of factors including differences in micro-climate and the effects of aspect (i.e. north versus south facing) across the elevational gradient are most likely to be responsible for these differences. Therefore, in the context of Swiss Alps where the temperature is rising at more than twice the global average, there is likely a breakpoint where the signal changes from temperature-limitation to precipitation-limitation across the elevational gradient and that climate change is causing this breakpoint to rise with altitude through time.

Keywords: Tree-rings, Larix decidua, Climate change, Turtmann river, Swiss Alps