The anchorage of 40-year-old Sitka spruce (Picea sitchensis (Bong.) Carr.) trees grown in a plantation on a steep (ca. 30°) slope was compared with that of trees grown on an adjacent horizontal area. There was similar gleyed mineral soil on the sloping and horizontal areas. Trees were mechanically overturned using a winch, and anchorage was quantified by measuring load, stem angle, and tree dimensions. Trees on the slope were overturned upslope, downslope, or across-slope. Critical turning moments were calculated around the tree base and the actual hinge point. Critical wind speeds required to uproot or snap trees in this stand were modelled to compare the vulnerability of trees to upslope and downslope winds. No overall difference in anchorage was found between trees grown on the horizontal and sloping parts of the site. However, for trees on the slope, those pulled upslope showed significantly more resistance to overturning for a given stem mass than those pulled downslope. Critical turning moments calculated at the hinge point were smaller than those calculated at the stem base, but differences were small and had no effect on the comparison between treatments. Critical wind speeds for uprooting were estimated to be 28 m·s -1 for an upslope wind and 24 m·s -1 for a downslope wind on this site. The implications of these results are discussed in relation to windthrow-risk modelling and forest soil conservation.Résumé : L'ancrage de tiges d'épinette de Sitka (Picea sitchensis (Bong.) Carr.) dans une plantation âgée de 40 ans établie sur pente abrupte (30°) a été comparé à celui d'arbres croissant sur une surface horizontale adjacente. Dans les deux cas, le sol était semblable, soit un sol minéral gleyifié. Les arbres ont été déracinés mécaniquement à l'aide d'un treuil et l'ancrage a été quantifié en mesurant la charge, l'angle de la tige et les dimensions de l'arbre. Les arbres situés en pente ont été déracinés vers le haut de la pente, vers le bas et perpendiculairement à la pente. Les moments de flexion critiques ont été calculés par rapport à la base de l'arbre ainsi que par rapport à la charnière de déracinement. Les vitesses de vent critiques nécessaires au déracinement ou au bris de la tige ont été modélisées pour comparer la vulnérabilité des arbres à des vents soufflant vers le haut ou vers le bas de la pente. Globalement, aucune différence d'ancrage n'a été constatée entre les arbres du site qu'ils croissent sur une surface horizontale ou en pente. Toutefois, dans le cas des arbres situés en pente, ceux qui ont été treuillés vers le haut de la pente étaient significativement plus résistants au déracinement que ceux qui ont été treuillés vers le bas pour une même masse de tige. Les moments de flexion critiques calculés à la charnière étaient inférieurs à ceux calculés à la base de la tige, mais les différences étaient faibles et n'avaient pas d'effet sur la comparaison entre les traitements. Sur le site étudié, les vitesses de vent critiques pour le déracinement ont été estimées à 28 m·s -1 pour un vent s...

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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.

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