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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]

Long-term records of rockfalls have proven to be scarce and typically incomplete, especially in increasingly urbanized areas where inventories are largely absent and the risk associated with rockfall events rises proportionally with urbanization. On forested slopes, tree-ring analyses may help to fill this gap, as they have been demonstrated to provide annually-resolved data on past rockfall activity over long periods. Yet, the reconstruction of rockfall chronologies has been hampered in the past by the paucity of studies that include broadleaved tree species, which are, in fact, quite common in various rockfall-prone environments. In this study, we test the sensitivity of two common, yet unstudied, broadleaved species — Quercus pubescens Willd. ( Qp ) and Acer opalus Mill. ( Ao ) — to record rockfall impacts. The approach is based on a systematic mapping of trees and the counting of visible scars on the stem surface of both species. Data are presented from a site in the Vercors massif (French Alps) where rocks are frequently detached from Valanginian limestone and marl cliffs. We compare recurrence interval maps obtained from both species and from two different sets of tree structures (i.e., single trees vs. coppice stands) based on Cohen's k coefficient and the mean absolute error. A total of 1230 scars were observed on the stem surface of 847 A. opalus and Q. pubescens trees. Both methods yield comparable results on the spatial distribution of relative rockfall activity with similar downslope decreasing recurrence intervals. Yet recurrence intervals vary significantly according to tree species and tree structure. The recurrence interval observed on the stem surface of Q. pubescens exceeds that of A. opalus by > 20 years in the lower part of the studied plot. Similarly, the recurrence interval map derived from A. opalus coppice stands, dominant at the stand scale, does not exhibit a clear spatial pattern. Differences between species may be explained by the bark thickness of Q. pubescens , which has been demonstrated to grow at twice the rate of A. opalus , thus constituting a mechanical barrier that is able to buffer low energy rockfalls and thus can avoid damage to the underlying tissues. The reasons for differences between tree structures are related to the clustered coppice-specific spatial stem distribution in clumps that could result on one hand in bigger gaps between clumps, which in turn decreases the probability of tree impacts for traveling blocks. On the other hand, data also indicate that several scars on the bark of coppice stands may stem from the same impact and thus may lead to an overestimation of rockfall activity. [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: The purpose of this study was to reconstruct spatio-temporal patterns of past landslide reactivation and the possible occurrence of future events in a forested area of the Barcelonnette basin (Southeastern French Alps). Analysis of past events on the Aiguettes landslide was based on growth-ring series from 223 heavily affected Mountain pine (Pinus uncinata Mill. ex Mirb.) trees growing on the landslide body. A total of 355 growth disturbances were identified in the samples indicating 14 reactivation phases of the landslide body since AD 1898. Accuracy of the spatio-temporal reconstruction is confirmed by historical records and aerial photographs. Logistic regressions using monthly rainfall data from the HISTALP database indicated that landslide reactivations occurred due to above-average precipitation anomalies in winter. They point to the important role of snow in the triggering of reactivations at the Aiguettes landslide body. In a subsequent step, spatially explicit probabilities of landslide reactivation were computed based on the extensive dendrogeomorphic dataset using a Poisson distribution model for an event to occur in 5, 20, 50, and 100yr. High-resolution maps indicate highest probabilities of reactivation in the lower part of the landslide body and increase from 0.28 for a 5-yr period to 0.99 for a 100-yr period. In the upper part of the landslide body, probabilities do not exceed 0.57 for a 100-yr period and somehow confirm the more stable character of this segment of the Aiguettes landslide. The approach presented in this paper is considered a valuable tool for land-use planners and emergency cells in charge of forecasting future events and in protecting people and their assets from the negative effects of landslides. [Copyright &y& Elsevier]

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: Extensive areas of the French Alps are underlain by Jurassic black marls. Wherever these “terres noires” crop out they become subject to intense erosion, causing major sedimentation in regional reservoirs and river systems. In the badlands near Draix (Alpes de Haute-Provence, France), measured sediment rates were obtained at the plot scale by surface elevation change-based methods and at the catchment scale by monitoring sedimentation in dams. In this study, we use a dendrogeomorphic approach based on anatomical changes in exposed roots of Pinus sylvestris L. to accurately quantify continuous denudation rates. A total of 123 cross sections (75 from buried and 48 from exposed roots of 23 trees) were sampled in the Moulin basin. The size and position of roots at the time of exposure was determined via anatomical variations in the annual growth rings of roots. In cross sections of buried roots, a sharp reduction of earlywood tracheid lumen area — a growth signature which has traditionally been used to determine the moment of root denudation — was observed as soon as erosion reduced soil cover to ≤3cm. As a consequence, estimates of eroded soil thickness had to be adjusted to take account of this bias. Bias-adjusted, averaged, medium-term erosion rates derived from exposed roots vary between 6 and 7mm y–1 at Moulin basin depending on the importance accorded to the uplift of roots after exposure. Values are significantly correlated to slope angle and match with erosion rates derived from monitored iron stakes (5.7mm y–1) or measurements of sediment yield in retention dams (4.7mm y–1) at the outlet of the Moulin basin. Besides demonstrating that the interpretation of anatomical signatures in tree roots to erosion have to be revised, this paper also shows that dendrogeomorphic analyses of roots are indeed a powerful tool for the quantification of minimal rates of soil erosion in environments where measurements of past activity are not available. [Copyright &y& Elsevier]