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Le présent article se propose de faire le point sur les connaissances actuelles quant au rôle de la végétation dans la protection contre l'érosion hydrique de surface. Il expose, dans un premier temps, les mécanismes d'action de la végétation vis-à-vis des processus d'érosion et de sédimentation. Dans un deuxième temps, il présente des résultats de recherches sur l'efficacité des différentes formations végétales et l'importance du taux de couverture végétale pour la lutte contre l'érosion de ravines. Pour citer cet article : F.

The purpose of this study was to reconstruct spatiotemporal patterns of avalanche events in a forested avalanche path of the Queyras massif (Echalp avalanche path, southeast French Alps). Analysis of past events was based on tree-ring series from 163 heavily affected multicentennial larch trees ( Larix decidua Mill.) growing near or next to the avalanche path. A total of 514 growth disturbances, such as tangential rows of traumatic resin ducts, the onset of compression wood as well as abrupt growth suppression or release, were identified in the samples indicating 38 destructive snow avalanches between 1338 and 2010. The mean return period of snow avalanches was 22 years with a 4% probability that an avalanche occurs in a particular year. On a temporal plan, three maxima in snow avalanche frequency were reconstructed at the beginning of the 16th and 19th centuries and around 1850, correlating with below-average winter temperatures and glacier advances. Analysis of the spatial distribution of disturbed trees contributed to the determination of four preferential patterns of avalanche events. The comparison of dendrogeomorphic data with historical records demonstrate that at least 18 events – six of which were undocumented – reached the hamlet of Echalp during the last seven centuries, but no significant temporal trend was detected concerning the frequency of these extreme events.

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Black marls form very extensive outcrops in the Alps and constitute some of the most eroded terrains, thus causing major problems of sedimentation in artificial storage systems (e.g. reservoirs) and river systems. In the experimental catchments near Draix (France), soil erosion rates have been measured in the past at the plot scale through a detailed monitoring of surface elevation changes and at the catchment scale through continuous monitoring of sediment yield in traps at basin outlets. More recently, erosion rates have been determined by means of dendrogeomorphic techniques in three monitored catchments of the Draix basin. A total of 48 exposed roots of Scots pine have been sampled and anatomical variations in annual growth rings resulting from denudation analysed. At the plot scale, average medium-term soil erosion rates derived from exposed roots vary between 1·8 and 13·8 mm yr −1 (average: 5·9 mm yr −1 ) and values are significantly correlated with slope angle. The dendrogeomorphic record of point-scale soil erosion rates matches very well with soil erosion rates measured in the Draix basins. Based on the point-scale measurements and dendrogeomorphic results obtained at the point scale, a linear regression model involving slope angle was derived and coupled to high-resolution slope maps obtained from a LiDAR-generated digital elevation model so as to generate high-resolution soil erosion maps. The resulting regression model is statistically significant and average soil erosion rates obtained from the areal erosion map (5·8, 5·2 and 6·2 mm yr −1 for the Roubine, Moulin and Laval catchments, respectively) prove to be well in concert with average annual erosion rates measured in traps at the outlet of these catchments since 1985 (6·3, 4·1 and 6·4 mm yr −1 ). This contribution demonstrates that dendrogeomorphic analyses of roots clearly have significant potential and that they are a powerful tool for the quantification and mapping of soil erosion rates in areas where measurements of past erosion is lacking.

Résumé Dans les Alpes du Sud, à 800 m d’altitude, les conditions climatiques hivernales génèrent des processus d’érosion périglaciaires dans les bassins-versants marneux à bad-lands. Sur le site de Draix (bassins-versants expérimentaux du CEMAGREF-RTM, Préalpes de Digne), des enregistrements thermiques et des mesures de l’érosion obtenues au cours de l’hiver 2000‑2001 ont permis de préciser les caractéristiques suivantes de la morphogenèse périglaciaire : 114 cycles de gel-dégel dans l’air, opposition morphoclimatique entre les versants adret et ubac des ravins, gélireptation et gélifluxion en surface du régolite marneux et présence d’un gélisol saisonnier jusqu’à 30 cm de profondeur en versant ubac. La gélireptation et la gélifluxion régularisent les versants, comblent les fonds de ravins et provoquent une ablation du régolite de 1 à 3 mm par hiver. Sur les versants ubacs, la gélifraction provoque une ablation des corniches de 0,3 à 1 mm par hiver. Dans les premiers décimètres du substrat, la glace participe à la disjonction des strates et à la genèse du régolite. La gélifraction expérimentale sur blocs confirme la sensibilité au gel de la marne et l’importance des plans de fissuration dans la fragmentation. Malgré une durée d’action limitée aux trois mois les plus froids, les processus périglaciaires participent au bilan érosif annuel des marnes, fragilisent le substrat et facilitent l’action du ruissellement.

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Sea cliffs represent 80% of the world's coasts and can be found virtually in all types of morphogenetic environments. Most studies on rocky environments focused on the impacts of modern sea level rise on cliff stability derived from sequential surveys, direct measurements or erosional features in anthropogenic structures. In this study, we explore the potential of dendrogeomorphic techniques to quantify multidecadal changes in coastal environments on Porquerolles Island (France). We sampled a total of 56 cross‐sections from 16 Pinus halepensis Mill. roots growing on former alluvial deposits and on sandy‐gravelly cliffs to quantify mean annual cliff retreat rates as well as changes in cliff geometry. Anatomical changes in roots have been used successfully in the past to quantify continuous denudation rates on slopes, channel incision and gullying processes but the approach has not been used so far in a coastal cliff context. At Porquerolles Island, reconstructed rates of cliff retreat cover 30–40 years and show average erosion rates between 0.6 and 3.9 cm yr−1 (average: 2.1 cm yr−1). Highest rates are observed at Pointe de la Tufière (2.6–3.9 cm yr−1), a small rock promontory that is more exposed to wave and storm surges than the remainder of the study area. By contrast, lower erosion rates are recorded at cliffs protected by the La Courtade pocket beach (0.6–1.9 cm yr−1). This contribution demonstrates that dendrogeomorphic analyses of roots clearly have a significant potential and that they are a powerful tool for the quantification of multidecadal rates of cliff retreat in areas where measurements of past erosion are lacking. More specifically, the approach also has clear advantages over the shorter time series obtained with repeat monitoring (e.g. terrestrial laser scanning, sensors, erosion pins) or over longer, but more coarsely resolved records obtained from aerial photographs or radio‐nuclides. © 2018 John Wiley & Sons, Ltd.