Response of biogeomorphological processes on mountain slopes : two pratical applications of harvesting un Southern French Prealps
Résumé
Rockfall risk is one of the most present risks in mountain regions. A forest management intervention can increase the protection role played by mountain forests (Bigot et al., 2009). Traditionally forest managers are using dead wood, in particular slash wood, against rockfall in mountain forest. Dead wood is an important structural and functional component of forest ecosystem (Harmon, 1986). The using materials are high stumps and logs. The "dead woods" used are mainly the timbers produced by thinning or clear-cutting harvesting. These structures have been principally developed in Austria at the end of last century. Today, use of slash as protection is present across all the alpine space. However, up to now there is no scientific knowledge on the efficiency of protection in time. Wood is a living material and it is subject to decomposition of these physical and chemical components. For dead wood, annual temperature is a main driver of decomposition. Initial wood density and diameter are also important drivers. Decomposition of dead wood is generally expressed as a decay constant k which is made up of annual mass or density losses due to respiration, leaching, and fragmentation(Garrett et al., 2007). Climatic factors and forest stands may vary greatly in mountain. Thus, the decomposition of wood is a complex process to study. A chronosequence study was established to evaluate this efficiency of protection in space and time. The main objective is to develop a first approach to study the loss of mechanical properties of slashes according to their decay rates. The experimental areas have been selected based on knowledge of harvesting years. For sampling, slashes were selected with different time since death (0, 5, 10, 15, etc., N-1years). The study areas are conducted in France, Switzerland and Austria. The mains species presented in the Alpine Arc are being sampled: Spruce, European beech, pine for Northern Alps and black pine for Southern Alps. Changes in the mass of logs and density of roots and stumps are used to estimate the decay rate constants. Multiple approaches are used to determine the decomposition of wood as an index. (i) Wood density is defined as the oven dry weight determined on disc sample or disc subsamples of the large sample, where samples are highly decomposed. (ii) Wood chemical properties loss (lignin and cellulose analysis) is determined by NIRS (Near Infrared Spectroscopy) after grinding of drying samples. (iii) Relative density is measured by drilling resistance and use of resistograph on the fields. (iiii) Penetration depth is determined directly by two penetrometers (pilodyn and nail gun)on the field. Later, these decay indexes will be validated and correlate by mechanic and destructive test within field works and laboratory. (i) Winching test to quantify root anchorage of stump. (ii) Real size experiments of rockfall to quantify the energy due for stumps and logs failures at experimental area Vaujany, France. (iii) Impact bending test with Charpy method to measure the resistance of wood material. The different approaches will be presented with the best ones retained for foresters' practical uses and fieldworks. For the study areas, the predominant factor of decomposition of wood will be identified. Decay rates will be correlated with density and dbh (diameter at breast high) of each species. NIRS will conduct to investigate relationships between decomposition and chemical composition. A first correlation will be established between decay rate index and mechanical resistance of slash wood. Finally, we will present the results on the lifetime per species of these protective structures. This paper proposed to introduce practical methods for field works dedicated to the assessment of rockfall protection structures made with slash woods. This research program is part of the European Interreg IV Alpine Space project "MANFRED".