Mechanical resistance of different tree species to rockfall in the French Alps Alexia Stokes 1,5 , Franck Salin 1 , Adzo Dzifa Kokutse 1 , St´ ephane Berthier 1,2 , Henri Jeannin 1 , Shaun Mochan 2 , Luuk Dorren 3 , Nomessi Kokutse 1 , Murad Abd. Ghani 1 & Thierry Fourcaud 1,4 1 Laboratoire du Rh´ eologie du Bois de Bordeaux, (Mixed unit: CNRS/INRA/Universit´ e Bordeaux I), 69, rte d’Arcachon, 33612, Cestas cedex, France. 2 Forest Research, Northern Research Station, EH25 9SY, Midlothian, Scotland. 3 Cemagref Grenoble, 2 rue de la Papeterie, B.P. 76, 38402, St. Martin d’H` eres cedex, France. 4 AMAP, CIRAD, TA40/PS2, Boulevard de la Lironde, 34398, Montpellier Cedex 5, France. 5 Corresponding author ∗ Key words: protection forest, root anchorage, scar formation, stem breakage, tree stability, uprooting Abstract In order to determine the mechanical resistance of several forest tree species to rockfall, an inventory of the type of damage sustained in an active rockfall corridor was carried out in the French Alps. The diameter, spatial position and type of damage incurred were measured in 423 trees. Only 5% of trees had sustained damage above a height of 1.3 m and in damaged trees, 66% of broken or uprooted trees were conifers. Larger trees were more likely to be wounded or dead than smaller trees, although the size of the wounds was relatively smaller in larger trees. The species with the least proportion of damage through stem breakage, uprooting or wounding was European beech (Fagus sylvatica L.). Winching tests were carried out on two conifer species, Norway spruce (Picea abies L.) and Silver fir (Abies alba Mill.), as well as European beech, in order to verify the hypothesis that beech was highly resistant to rockfall and that conifers were more susceptible to uprooting or stem breakage. Nineteen trees were winched downhill and the force necessary to cause failure was measured. The energy ( E fail ) required to break or uproot a tree was then calculated. Most Silver fir trees failed in the stem and Norway spruce usually failed through uprooting. European beech was either uprooted or broke in the stem and was twice as resistant to failure as Silver fir, and three times more resistant than Norway spruce. E fail was strongly related to stem diameter in European beech only, and was significantly higher in this species compared to Norway spruce. Results suggest that European beech would be a better species to plant with regards to protection against rockfall. Nevertheless, all types of different abiotic stresses on any particular alpine site should be considered by the forest manager, as planting only broadleaf species may compromise the protecting capacity of the forest, e.g., in the case of snow avalanches. Introduction The use of protection forests against the impact of nat- ural hazards, e.g., rockfall and snow avalanches is be- coming more and more common in Europe (Brang, 2001; Dorren and Berger, 2006; Dorren et al., 2004; Hurand and Berger, 2002; Motta and Haudemand, 2000; Ott, 1996). With the increase in catastrophic events both in the European Alps (Interreg IIIb, 2001; Sauri et al., 2003) and in mountainous regions around the world (Tianchi et al., 2002), research into this phenomenon has accelerated. However, although it is ∗ E-mail: stokes@liama.ia.ac.cn understood that the structure of the forest plays a vital role in determining its effectiveness as a protective bar- rier (Jahn, 1988; Kr¨ auchi et al., 2000), little information exists concerning the mechanical resistance of differ- ent tree species to different types of natural hazards. One particular natural hazard which has been much neglected until recent years is that of rockfall. Not only is the movement of rocks and stones a hazard to both people and infrastructures, but rockfall safety nets are expensive and difficult to install and they deteriorate with time (Dorren, 2003). If further information on the structure of a protection forest, and the most mechan- ically resistant species to use against rockfall could be obtained, these data could be used as input to models of This article has been previously published in the following journal—Plant and Soil, Volume 278, 107–117. Stokes et al. (eds), Eco- and Ground Bio-Engineering: The Use of Vegetation to Improve Slope Stability, 155–164. C  2007. Springer. 155