La dégradation du bois dans les processus biogéomorphologique sur versant forestier

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVINInternational audienceIn mountain, forest can play a protective role against natural hazard such as rockfalls, avalanches, shallow landslides, debris flow and erosion. Complementary with the classical researches on the dissipate effect of standing trees, the protective capacity of wood slash are increasingly taken into account in the European protection forest. Slash are the results of logging, and are composed by stumps and logs. Slash can be a real barrier, if they are strategically positioned in the corridors subject to rockfalls or avalanches. Thus, after logging, slash can maintain or increase the roughness on the ground, and slow down or stop the gravitational phenomena. However, wood slash is a living material and it is subjected to decomposition process. After a cut and due to the wood decay, mass movement (e.g. erosion or shallow landslide) can be then reactivated. Loggings have different consequences on the biogeomorphological processes over time. The initial objective of our study is to analyze the decomposition and mechanical resistance loss of slash for the protection against rockfalls or avalanches over time. A chronosequence study has been established for three main species in European Alps: Pinus nigra, Fagus sylvatica and Picea abies. Several methods were used to assess wood density loss as an indicator: classical method for dry density measurement, resistograph, pilodyn, and spectrometry. All the results are presented via the calculation of different indexes of density decay. These results have been compared to crash test realized in laboratory to simulate a rock impact. To do a parallel with other topics as erosion or shallow landslide, we will also present the assessment of sediment production after clear-cutting in the black marls of the southern French Alps