Each year in mountains, natural hazards generate extensive damage and even sometimes casualties. For rockfalls, forests can play an important role of passive protection by increasing the energy dissipation of falling rocks. Up to now, many studies have shown the efficiency of forests standing against rockfalls, but these studies are only considering living trees. In Austria and Switzerland, managers are using stumps, logs and other timber products to create natural dams against rockfalls since twenty years ago. Today, French foresters have adopted these techniques, but there is still a lack of knowledge on the efficiency and the lifetime of such protective structures. The main objectives of this research are to quantify the mechanical resistance loss according to the wood decay kinematics. The materials studied include logs, stumps and roots. Preliminary works have been done in order to select experimental plots in the Alpine Space. The species studied are: silver fir, norway spruce, european beech and pinus nigra. The wood decay rates are quantified by using a chronosequence approach. Different methods are used to quantify the wood density loss. A specific protocol, based on both non-destructive and destructive tests, has been built for analyzing the loss of mechanical resistance of these woody elements. The final objective is to integrate, in rockfall trajectory simulation models in 3D, the actions of this biological engineering protective works taking into account their efficiency evolution within their lifetime.