Dégradation du bois du pin noir d'Autriche après coupe dans les Alpes su sud française

[Departement_IRSTEA]Territoires [Departement_IRSTEA]Eaux [TR1_IRSTEA]SEDYVIN [TR2_IRSTEA]RIVAGEInternational audienceForest management has different impacts on mountain slope. Forests play an important role of protection against natural hazards such as rockfalls, avalanches, soil erosion and floods. Timber harvesting, whether intended or not, can have dramatic geomorphic or ecological consequences. The result is some slash or dead wood on the slope surface. From slashings, protection structures against rockfalls or avalanches can be built. However, slashings decayed in time and the protection decreased. At the same time, on forested slopes prone to erosion or shallow landslide, wood decay and erosion or mass movement increase. This paper is focused on the wood decay process. The main objective is to determine decay rate of Austria black pine (Pinus nigra) under Mediterranean mountain climate, unknown today. A chronosequence has been established in the Saint-Genis forest in the Drôme valley, Southern French Prealps. A sum of 93 stumps and 134 coarse woody roots (> 2 cm in diameter) has been sampled within the study sites on many stumps cut between 1992 and 2011. Three methods of sampling have been employed to characterize decay rate indicators: (i) basic dry density obtained by a classical method, (ii) drilling resistance measured with a resistograph, and (iii) penetrating resistance obtained with a pilodyn. Data showed an exponential curve with a decay rate of 0.073 for stump, and 0.033 for roots. The time necessary for loosing 50% and 90% of the initial density for stumps and roots were 9 and 21 years, and 41 and 91 years, respectively. Logistic fits gave better statistical fits for both drilling and penetrating resistance data. Two practical applications of these results are presented. The first is to show the rockfall bioprotection structures life time efficiency through the example of slashings (high stumps) decay. The second application concerns the timing of the slope erosion response to planned deforestation, a practice used in the Drôme River basin to replenish incised river channels with sediment. Regarding the decay rate, the life time of the protection structures would be optimum between 5 to 10 years. The remobilization of sediment would be optimum in the first 5 years following the deforestation. Then, the sediment production tends to stabilize because of spontaneous regeneration. This research based on wood decay allow us to improve the knowledge on decomposition of Pinus nigra in Southern Alps, and confirm effects of logging on protection forest and sediment supply from small ravines over time