Spatio-temporal reconstruction of snow avalanche activity using tree rings: Pierres Jean Jeanne avalanche talus, Massif de l'Oisans, France

Snow avalanches are a major threat in many parts of the Alps, where they periodically damage infrastructure, disrupt transportation corridors or even cause loss of life. Nonetheless, the spatial behavior of past avalanche activity and the analysis of areas affected during particular events remain often imprecise. It was therefore the purpose of this study to reconstruct spatio-temporal patterns of past avalanche activity on a forested avalanche talus in the French Alps (Pierres Jean Jeanne talus, Massif de l'Oisans, France). A total of 232 European larches (Larix decidua Mill.) with clear signs of snow wasting events was analyzed and growth disturbances (GD) related to avalanche activity was assessed, such as tangential rows of traumatic resin ducts, the onset of compression wood or abrupt growth suppression and release. In total, 901 GD were identified in the tree-ring samples, indicating that 20 high-magnitude avalanches occurred between AD 1919 and 1994. The mean return period of snow avalanches was similar to 4 years with a similar to 26% probability that an avalanche occurs in any particular year. Interpolated maps allowed for explicit spatial estimates of return periods throughout the talus, showing a rapid increase of return frequency from 2.5 to 50 years with increasing distance from the talus apex. The distribution of avalanche years seems to be quite homogeneous in time with a gap between 1951 and 1959 and since 1994. Snowfall from a nearby meteorological station (Saint-Christophe en Oisans; 10 km from the study site) indicated that the five most recent high-magnitude events on record occurred due to above-average snowfall anomalies in December and January associated with abnormally low air temperatures. Findings suggest that a strong snow metamorphism under high temperature gradients in January could explain the occurrence of high-magnitude snow avalanches. (C) 2010 Elsevier B.V. All rights reserved.