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The medium-high mountain ranges of Western Europe are undergoing rapid socio-environmental changes. The aim of this study is to show that their landscape can be strongly shaped by the actions of avalanche activity, humans and climate. The study area is the Rothenbachkopf-Rainkopf complex, a site representative of avalanche-prone areas of the Vosges Mountains (France). A geo-historical approach documents regular avalanche activity over more than 200 years on 13 paths. A diachronic analysis of historical maps and photographs demonstrates substantial afforestation (from 60 to 80% of the total surface since 1832). LIDAR data and field surveys highlight the existence of a longitudinal and transversal structure of trees. Hence, the avalanche, human and climate activity footprints are retained by the landscape, which rapidly adapts to these changing drivers. Specifically, the pattern of tree species and heights results from a near equilibrium with regular avalanche activity sometimes disturbed by major avalanches that induce quasi-cyclic changes in the landscape mosaic. The afforestation trend is attributable to profound changes in silvo-pastoral practices, supplemented by the impact of climate change. The wider relevance of the results in relation to the local context is discussed, as well as outlooks that can refine our understanding of this complex system. [ABSTRACT FROM AUTHOR]

The purpose of dendrogeomorphic analyses is to amplify the signal related to the geomorphic process under investigation, and to minimize the noise induced by other signals in the tree-ring series. Yet, to date, no study accounts specifically for interferences induced by climate conditions or exogenous disturbances and which can, potentially, affect the quality of tree-ring based process reconstructions. In this paper, we develop a specific procedure allowing evaluation of the quality of reconstructions in five avalanche paths at Oberwald (Swiss Alps). The study is based on possible interferences between snow avalanches, climatic conditions and ecological signals in the tree-ring series. Analysis of past events was based on tree-ring series from 564 heavily affected, multi-centennial European larch trees ( Larix decidua Mill.) growing near or next to the avalanche paths. A total of 2389 growth disturbances, such as scars, tangential rows of traumatic resin ducts, compression wood as well as abrupt growth suppressions or releases, were identified in the samples, indicating 43 destructive snow avalanches since AD 1780. At the same time, 31 potential events, which were detected with the conventional Shroder index value, were rejected from the final reconstruction due to potentially strong interferences between the different signals. This high rejection rate underlines the necessity to systematically–and carefully–discriminate ecological and climatic noise from avalanche-related disturbances. This discrimination is even more so crucial as a significant proportion of dendrogeomorphic studies in the Alps are based on L. decidua trees which are cyclically affected by larch budmoth outbreaks. [ABSTRACT FROM AUTHOR]

Abstract: Trees affected by mass movements record the evidence of geomorphic disturbance in their growth-ring series, and thereby provide a precise geochronological tool for the reconstruction of past process activity. At the tree scale, identification of past mass movements was typically based on the presence of growth anomalies and focused on the presence of scars, tilted or buried trunks, as well as on apex decapitation. In terms of sampling strategy, however, clear guidelines have been largely missing. Most previous work was based either on the sampling of visibly disturbed trees irrespective of their position at the study site or on the systematic sampling of trees evenly distributed along transects. Based on a dense dataset of 323 trees growing on an active landslide body, this study aims at defining guidelines for future tree-ring sampling of landslides. Using random extractions of trees and iterative mapping, we investigate subsets of the full tree-ring sample to define optimal sampling strategy, sample depth and trees for the development of frequency maps of landslide reactivations. We demonstrate that (i) the sampling of 50–100 trees can be sufficient to obtain satisfactory results on landslide frequency without including noise to the dendrogeomorphic record; (ii) increasing growth disturbance thresholds should be adjusted to sample size and are preferable to fixed values; (iii) an even distribution of sampled trees is crucial to increase the reliability of frequency maps, even more so if the reconstruction is based on small sample sizes; and that (iv) the selection of the most frequently disturbed trees is key to reduce uncertainties in the frequency maps. The optimization of sample sizes and the adjustment of sampling strategy will not only facilitate fieldwork and render analyses and interpretation more reliable, but will also ultimately allow reconstruction of time series of past mass movements with reasonable temporal efforts and excellent cost-benefit ratios. [Copyright &y& Elsevier]

Abstract: On forested talus slopes which have been build up by rockfall, a strong interaction exists between the trees and the falling rocks. While the presence and density of vegetation have a profound influence on rockfall activity, the occurrence of the latter will also exert control on the presence, vitality, species composition, and age distribution of forest stands. This paper exploits the interactions between biotic (tree growth) and abiotic (rockfall) processes in a mountain forest to gather and obtain reliable input data on rockfall for the 3D process based simulation model RockyFor3D. We demonstrate that differences between the simulated and observed numbers of tree impacts can be minimized through (i) a careful definition of active source areas and (ii) a weighted distribution of block sizes as observed in the field. As a result of this field-based, optimized configuration, highly significant values can be obtained with RockyFor3D for the number of impacts per tree, so that results of the model runs can be converted with a high degree of certainty into real frequencies. The combination of the field-based dendrogeomorphic with the modeling approaches is seen as a significant advance for hazard mapping as it allows a reliable and highly-resolved spatial characterization of rockfall frequencies and a realistic representation of (past) rockfall dynamics at the slope scale. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

The purpose of this study was to reconstruct spatiotemporal patterns of past landslide reactivation in a forested area of the Barcelonnette Basin (Bois Noir landslide, Southern French Alps). Analysis of past events was based on tree ring series from 79 heavily affected Mountain pine ( Pinus uncinata Mill. ex Mirb) trees growing near or next to the landslide body. Dendrogeomorphic analysis focused on the presence of compression wood and growth reductions, with the first reaction being used for a dating of past reactivations with seasonal precision. A total of 151 growth disturbances were identified in the samples representing eight different stages of reactivation of the landslide body between 1874 and 2008. The spatiotemporal accuracy of the reconstruction is confirmed by historical records from neighboring sites and by aerial photographs. The onset of compression wood formation allows identifying five stages of landslide reactivation during the dormant season or the very beginning of the growing season of trees, i.e., between early October and late May, and three stages toward the end of the growth period. Monthly rainfall data from the HISTALP database demonstrate that the rainfall during four out of the eight reactivations are characterized by summer rainfall totals (July-August) exceeding 200 mm, pointing to the important role of summer rainstorms in the triggering of events at the Bois Noir landslide body. [ABSTRACT FROM AUTHOR]

Abstract: In snow-rich areas, snow avalanches endanger settlements and cause heavy damage to infrastructure or transportation routes. In wooded avalanche paths, dendrogeomorphology has been used extensively to reconstruct snow avalanche histories or to complement existing archival records. Several authors noted (i) that avalanche chronologies reconstructed from tree rings would depend on the number of trees sampled, and on (ii) the minimum number of tree-ring responses; and (iii) that they would always represent minimum frequencies. These restrictions gave rise to the question of how much of the real avalanche activity can be captured in tree-ring records. We therefore performed a dendrogeomorphic analysis based on 175 Larix decidua Mill. and 34 Picea abies (L.) Karst. trees from an extensively and accurately documented (1905–2010) avalanche path located in the Arve valley (French Alps) to obtain optimal thresholds for sample size and index values (i.e. percentage of responses in relation to the number of trees alive for a given year). Results clearly demonstrate that a sample size of ~100 trees is needed to obtain the best match between reconstruction (tree rings) and documentation (archives) while minimizing the inclusion of noise in the dendrogeomorphic record. Validation of the reconstruction (1771–2010) with historical archives shows that 13 undocumented events could be added to the archival record and that 43% of all documented events were deciphered with dendrogeomorphic techniques. The reconstruction of the spatial extent and reach of past snow avalanches matches with historical archives as far as the longitudinal extent of the largest avalanches is concerned. Yet, tree-ring records tend to underestimate runout elevations for a majority of minor events. Large discrepancies are also reported between the lateral limits derived with dendrogeomorphic techniques and the data reported in historical reports and hazard maps, with tree-ring data suggesting larger lateral spread of avalanche snow. [Copyright &y& Elsevier]

Abstract: Probability maps of landslide reactivation are presented for the Pra Bellon landslide located in the southern French Alps based on results obtained with dendrogeomorphic analysis. Spatiotemporal patterns of past landslide activity was derived from tree-ring series of 403 disturbed mountain pine trees growing in the landslide body. In total, 704 growth disturbances were identified in the samples indicating 22 reactivation phases of the landslide body between 1910 and 2011. The mean return period was 4.5years. Given the spatiotemporal completeness of the reconstruction, probabilities of landslide reactivation were computed and illustrated using a Poisson distribution model and for 5, 20, 50, and 100years. Probability of landslide reactivation is highest in the central part of the landslide body and increases from 0.13 for a 5-year period to 0.94 for a 100-year period. Conversely, probabilities of reactivation are lower at its margins. The proposed method differs from conventional approaches based on statistical analyses or physical modeling that have demonstrated to have limitations in the prediction of spatiotemporal reactivation of landslides. Our approach is, in contrast, based on extensive data on past landslides and therefore allowed determination of quantitative probability maps of reactivation derived directly from the frequency of past events. This approach is considered a valuable tool for land managers in charge of protecting and forecasting people and their assets from the negative effects of landslides as well as for those responsible for land use planning and management. It demonstrates the reliability of dendrogeomorphic mapping that should be used systematically in forested shallow landslides. [Copyright &y& Elsevier]

Abstract: Trees affected by mass movements record the evidence of geomorphic disturbance in the growth-ring series, and thereby provide a precise geochronological tool for the reconstruction of past activity of mass movement. The identification of past activity of processes was typically based on the presence of growth anomalies in affected trees and focused on the presence of scars, tilted or buried trunks, as well as on apex decapitation. For the analyses and interpretation of disturbances in tree-ring records, in contrast, clear guidelines have not been established, with largely differing or no thresholds used to distinguish signal from noise. At the same time, processes with a large spatial footprint (e.g., snow avalanches, landslides, or floods) will likely leave growth anomalies in a large number of trees, whereas a falling rock would only cause scars in one or a few trees along its trajectory. Based on the above considerations, we examine issues relating to the interpretation and dendrogeomorphic dating of mass movements. Particular attention is drawn to sampling in terms of sample distribution across a study site, the actual selection of trees as well as to sample size (i.e., number of trees sampled). Based on case studies from snow avalanche, debris flow, and landslide sites, we demonstrate that thresholds can indeed improve dating quality and, at the same time, minimize noise in time series. We also conclude that different thresholds need to be used for different processes and different periods of the reconstruction, especially for the early stages of the reconstruction when the number of potentially responding trees will be much smaller. This paper seeks to set standards for dendrogeomorphic fieldwork, analysis, and interpretation for different processes of mass movements. [Copyright &y& Elsevier]

Expected runout distances and related return periods are the most important parameters needed for zoning in terrain prone to snow avalanching. Hazard mapping procedures usually allocate areas of land to zones with a different degree of danger based on return periods estimated for given snow volumes in the starting zone or with statistical/dynamical models. On forested avalanche paths, dendrogeomorphology has a great potential to add critical input data to these calculations in terms of recurrence intervals or return periods. However, quite paradoxically, recurrence interval maps of snow avalanches have only rarely been retrieved from tree-ring analysis and mostly represent the inverse of the mean frequency of avalanches that could be retrieved locally rather than the return period. The purpose of this study therefore was to propose a consistent approach for tree-ring based recurrence interval mapping of snow avalanche events. On the basis of 71 snow avalanches retrieved from 2570 GD growth disturbances identified in 307 larch trees from three avalanche paths located in the vicinity of Täsch (Canton of Valais, Swiss Alps), we first followed the classical approach used in dendrogeomorphology and derived recurrence interval maps through interpolation from recurrence intervals observed at the level of individual trees. We then applied an expert delineation of the spatial extent of past events based on the location of disturbed trees. Our results show that the second step improved representation of expected patterns of recurrence intervals that typically increase as one moves down the centerline of the avalanche path. Despite remaining limitations and uncertainties precluding from direct use of our maps for hazard mapping purpose, these results suggest that dendrogeomorphic time series of snow avalanches can yield valuable information for the assessment of recurrence intervals of avalanches on forested paths for which only very limited or no historical data exists, and that this data can be obtained independently from meteorological data or numerical modeling. [ABSTRACT FROM AUTHOR]