Le couvert forestier, marqueur spatio temporel de l'activité d'un glissement de terrain

Abstract

Trees produce reaction wood when they are exposed to geomorphological and hydrolocical processes like snow avalanches, floods or landslides that lead to changes of the tree position, of the growth conditions or to direct mechanical damage. This wood has a different cell structure that is clearly visible when extracted from a tree. In this way trees record, from a certain threshold of intensity, duration or recurrence, geomorphological and hydrological processes that lead to disturbances in their environment. Consequently the identification of zones of reaction wood in tree stems can be used to identify previous disturbance and to reconstruct the triggering events in time, space and intensity. In this study we reconstructed the activity of four landslides in the Southern French Alps by using the forest cover on the landslide body as a silent witness of the past. We assessed the growth disturbances related to landslide activity of 75 damaged Pinus uncinata trees. In total we identified 139 growth disturbances that indicate 6 different periods of landslide activity between 1947 and 2004 although only one event was clearly identified by historical records. Besides, this study shows that dendrogeomorphical methods are innovative techniques offering perspectives for a better understanding of natural hazards.