Reconstruction des températures estivales dans les alpes françaises au moyen de données dendrochronologiques (749-2008 AD)

Abstract

To date, in the Alps, only three reconstructions extend back prior to 1000 AD. This lack is related to the scarcity of wood material in the western and southern parts of the Alps, especially during the early part of the last millennium. On the basis of a dense tree-ring width network (34 unpublished multi centennial larch chronologies), this paper attempts, for the first time, a reconstruction of the summer temperatures over the French Alps. This network combine 548 tree-ring width series from living trees, dead trees and construction wood. All sites sampled are located near tree-line in high-altitude forest stands. The Adaptative Regional Growth Curve standardization method was applied to preserve interannual to multicentennial variations in this high-elevation proxy data-set. The proxies are calibrated using the June to August mean temperatures from the last revised version of the HISTALP database. The method combines an analogue technique, which is able to extend the too short tree-ring series, an artificial neural network technique for an optimal non-linear calibration including a bootstrap technique for calculating error assessment on the reconstruction. About 50% of the temperature variance is reconstructed. The proxy record evidences a prolonged Medieval Warm Period persisting until 1500 with warm periods that resemble twentieth century conditions but also cold phases before 1000 synchronous to the Swiss glacier advances. The Little Ice Age is rather mild until 1660 if compared to other alpine reconstructions. Then, summers are 0. 7°C cooler than the 1961—1990 mean until 1920.