Mouvements de masse et dynamique d’un géosystème alpestre : étude dendrogéomorphologique de deux sites de la vallée de Boulc (Diois, France)

Landslides represent major hazards in southern Alps where the plastic character of superficial deposits and regional climate create triggering conditions. They also provide an important sediment source for streambed dynamics. This article aims to date mass movements that affected two unstable sites within a watershed (Boulc valley). The reactions of 41 pines ( Pinus sylvestris L.) to the micro-movements on the Ravel-et-Ferriers site show that the January 1994 landslide is the result of a precarious stability pertaining to local factors since the end of the 1980's. The triggering conditions for landslides seem to result from long-term slope evolution (at least eight unstable periods occurred since the beginning of the century). The study of the mortality of 35 pines and 47 firs ( Abies alba Mill.) partially buried under sediments allowed to reconstruct the chronology of a dammed-lake development following the debris-flows of Bonneval-en-Diois. These events seem to be linked with some exceptional pluviometric conditions that dated recent movements. Results given by the dendrochronological analysis lead to set up the typological property of the two landslides, and to know the episods of water- course-sediment recharge, downstream to the sites. The Bonneval-en-Diois fossilization processes besides show analogies with the South Alps conditions during Holocene when part of thick fillings of valleys excavated in marlaceous formations should be the consequence of localized mass movements.