Elevation-dependent breakpoint in climate forcing of tree growth in the Turtmann River Basin, Switzerland

Tree rings are valuable proxies for reconstructing changes in climate at annual and intra-annual resolutions. In Alpine regions, changes in climate may well lead to switches between tree-growth being temperature-limited and precipitation-limited. Distinguishing between these is important as they may express themselves differently in tree rings. However, such switches cannot be separated from more local environmental influences such as elevation and aspect. In this study, we seek to understand how tree growth evolves as a function of elevation in the context of climate change. For the growth of European larch (Larix decidua Mill) in the Turtmann River Basin, a glacier-fed high-elevation river basin in the Swiss Alps, located in southwestern Switzerland, we find that the average temperature increase leads to a switch from temperature-limitation to precipitation-limitation as a function of elevation. The growth of Larix decidua Mill in this river basin is influenced by previous year’s September-October-November (SON) temperature and current year’s January- February-March (JFM) precipitation in the higher- and lower-elevations across an elevational gradient respectively. Based on the analysis from four sites, assuming a linear response, our study suggests that there is a transition zone (i.e., an elevational breakpoint) between 1800 and 900 m AMSL where the signal changes from precipitation-limited to temperature-limited. We also conclude that this elevation breakpoint appears to be rising with time due to increasingly warmer annual average temperatures in this Alpine setting, where this warming is noted to be more than twice compared to the global average temperature change.