Christophe Corona

Dendroecological Dating of Geomorphic Disturbance in Trees

Abstract The initial employment of tree rings in geomorphic studies was simply as a dating tool and only rarely were other environmental information and records of damage contained within the tree exploited. However, these annually resolved tree-ring records also preserve valuable archives of past geomorphic processes on timescales of decades to centuries. As many of these processes are significant natural hazards, understanding their distribution, timing and controls provides crucial information that can assist in the prediction, mitigation and defense against these hazards and their effects on society. This contribution aims at presenting a proposal on the types of growth disturbances to be included in future work focusing on geomorphic disturbance, the intensity of reactions, and on the minimum requirements needed for growth disturbances to be considered in event histories. We present possibilities and limitations of dendrogeomorphic applications in geomorphic research and propose a range of techniques and approaches that may become standard practice in the analysis and understanding of earth-surface processes and related natural hazards in the future.

Seven centuries of avalanche activity at Echalp (Queyras massif, southern French Alps) as inferred from tree rings

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.

Climate change increases frequency of shallow spring landslides in the French Alps

In this contribution, past process activity is reconstructed on seven landslide bodies of the Riou Bourdoux catchment (southeastern French Alps). Based on an unusually dense data set of 3036 tree-ring series extracted from 759 conifers, we provide evidence for 61 landslide reactivations since A.D. 1898. Based on logistic regressions and threshold analyses of monthly rainfall data and temperature anomalies, we determine that the combination of snow-rich winters and positive temperature anomalies in spring (enhanced snowmelt) seems to have driven landslide reactivations in the past. Since the early 1990s, however, landslide reactivations clearly have been on the rise and thereby exhibit excessive and unprecedented rates of activity (12.5 events per 10 yr) at the scale of the Riou Bourdoux catchment. From the data, evidence exists for a shift from snowmelt-induced landslides (controlled by winter precipitation) to reactivations controlled by spring temperatures. Therefore, this contribution also adds evidence to the hypothesis that climate change (and related warmer springs) could further enhance landslide activity in the course of the 21 st century.

Rolling stones and tree rings A state of research on dendrogeomorphic reconstructions of rockfall

This progress report focuses on the contribution of tree-ring series to rockfall research and on recent development and challenges in the field. Dendrogeomorphic techniques have been used extensively since the early 2000s and several approaches have been developed to extract rockfall signals from tree-ring records of conifer trees. The reconstruction of rockfall chronologies has been hampered in the past by sample sizes that decrease as one goes back in time, as well as by a paucity of studies that include broadleaved tree species, which are in fact quite common in rockfall-prone environments. In this report, we propose a new approach considering impact probability and quantification of uncertainty in the reconstruction of rockfall time series as well as a quantitative estimate of presumably missed events. In addition, we outline new approaches and future perspectives for the inclusion of woody vegetation in hazard assessment procedures, and end with future thematic perspectives.

Defining an Adequate Sample of Earlywood Vessels for Retrospective Injury Detection in Diffuse-Porous Species

Vessels of broad-leaved trees have been analyzed to study how trees deal with various environmental factors. Cambial injury, in particular, has been reported to induce the formation of narrower conduits. Yet, little or no effort has been devoted to the elaboration of vessel sampling strategies for retrospective injury detection based on vessel lumen size reduction. To fill this methodological gap, four wounded individuals each of grey alder (Alnus incana (L.) Moench) and downy birch (Betula pubescens Ehrh.) were harvested in an avalanche path. Earlywood vessel lumina were measured and compared for each tree between the injury ring built during the growing season following wounding and the control ring laid down the previous year. Measurements were performed along a 10 mm wide radial strip, located directly next to the injury. Specifically, this study aimed at (i) investigating the intra-annual duration and local extension of vessel narrowing close to the wound margin and (ii) identifying an adequate sample of earlywood vessels (number and intra-ring location of cells) attesting to cambial injury. Based on the results of this study, we recommend analyzing at least 30 vessels in each ring. Within the 10 mm wide segment of the injury ring, wound-induced reduction in vessel lumen size did not fade with increasing radial and tangential distances, but we nevertheless advise favoring early earlywood vessels located closest to the injury. These findings, derived from two species widespread across subarctic, mountainous, and temperate regions, will assist retrospective injury detection in Alnus, Betula, and other diffuse-porous species as well as future related research on hydraulic implications after wounding.

Exploring the impact of regional climate and local hydrology on Pinus sylvestris L. growth variability - A comparison between pine populations growing on peat soils and mineral soils in Lithuania

AimsTo compare growth variability of Scots pine (Pinus sylvestris L.) on different soil types, and to assess the potential of peat-soil pines for climatological and hydrological studies.MethodsWe used extensive dendrochronological analyses to investigate temporal and spatial responses of pines growing on peat soils and mineral soils in three regions of Lithuania.ResultsSignificant correlations were observed between tree populations growing on similar soil types in different geographical regions, whereas synchronicity was absent between neighbouring stands growing on different soil types. At mineral soils, tree growth was significantly correlated with winter and early summer temperatures, whereas a more complex response was detected in peat-soil trees, presumably reflecting a multi-annual synthesis of moisture variability and changing hydrology. Synchronous long-term peat soil tree-growth variations observed over large parts of the Baltics point to a possible regional hydrological forcing. Our results may therefore improve hydrological reconstructions using living and subfossil peat-soil trees, and could be of prime importance given the major influence peatland water-table fluctuations have on a range of environmental processes.ConclusionResults reveal that peat-soil pines are unsuitable for high-frequency climate reconstruction, but demonstrate their potential for the reconstruction of multi-annual to decadal hydrological fluctuations. Mineral-soil pines, by contrast, should be used for temperature reconstructions.

A New Tree-Ring-Based, Semi-Quantitative Approach for the Determination of Snow Avalanche Events: use of Classification Trees for Validation

Abstract On forested paths, dendrogeomorphology has been demonstrated to represent a powerful tool to reconstruct past activity of avalanches, an indispensable step in avalanche hazard assessment. Several quantitative and qualitative approaches have been shown to yield reasonable event chronologies but the question of the completeness of tree-ring records remains debatable. Here, we present an alternative semi-quantitative approach for the determination of past snow avalanche events. The approach relies on the assessment of the number and position of disturbed trees within avalanche paths as well as on the intensity of reactions in trees. In order to demonstrate that no bias was induced by the dendrogeomorphic expert, we carry out a statistical evaluation (Classification and Regression Trees, or CART) of the approach. Results point to the consistency and replicability of the procedure and to the fact that the approach is not restricted to the identification of high-magnitude avalanches. Evaluation of the semi-quantitative approach is illustrated on a well-documented path in Chamonix, French Alps. For the period 1905–2010, comparison between the avalanche years recorded in a substantial database (Enquête Permanente sur les Avalanches, or EPA) and those defined with dendrogeomorphic techniques shows that the avalanche record reconstructed from tree-ring series contains 38% of the observed events.

Can we infer avalanche–climate relations using tree-ring data? Case studies in the French Alps

Abstract Dendrogeomorphology is a powerful tool to determine past avalanche activity, but whether or not the obtained annually resolved chronologies are sufficiently detailed to infer avalanche–climate relationships (in terms of temporal resolution) remains an open question. In this work, avalanche activity is reconstructed in five paths of the French Alps and crossed with a set of snow and weather variables covering the period 1959–2009 on a monthly and annual (winter) basis. The variables which best explain avalanche activity are highlighted with an original variable selection procedure implemented within a logistic regression framework. The same approach is used for historical chronologies available for the same paths, as well as for the composite tree-ring/historical chronologies. Results suggest that dendrogeomorphic time series allow capturing the relations between snow or climate and avalanche occurrences to a certain extent. Weak links exist with annually resolved snow and weather variables and the different avalanche chronologies. On the contrary, clear statistical relations exist between these and monthly resolved snow and weather variables. In detail, tree rings seem to preferentially record avalanches triggered during cold winter storms with heavy precipitation. Conversely, historical avalanche data seem to contain a majority of events that were released later in the season and during episodes of strong positive temperature anomalies.

Probability Maps of Landslide Reactivation Derived from Tree-Ring Records

Probability maps of landslide reactivation are presented for the Aiguettes landslide located in the southern French Alps based on results obtained with dendrogeomorphic analysis. Spatio-temporal activity was derived from tree-ring series of 223 disturbed trees. 355 growth disturbances were identified in the samples indicating 14 reactivation phases of the landslide body between 1898 and 2011. Probabilities of landslide reactivation were computed and illustrated using a Poisson distribution model. For example, in the toe, the probability increases from 0.28 for a 5-year period to 0.99 for a 100-year period. This method differs from conventional approaches, which have demonstrated to have limitations in the prediction of spatiotemporal reactivation of landslides. Based on extensive data and therefore allowing determination of quantitative probability maps of reactivation, this approach is considered a valuable tool for land managers in charge of protecting and forecasting people as well as for those responsible for land-use planning and management.

Debris-flow risk analysis in a managed torrent based on a stochastic life-cycle performance.

Two key factors can affect the functional ability of protection structures in mountains torrents, namely (i) infrastructure maintenance of existing infrastructures (as a majority of existing works is in the second half of their life cycle), and (ii) changes in debris-flow activity as a result of ongoing and expected future climatic changes. Here, we explore the applicability of a stochastic life-cycle performance to assess debris-flow risk in the heavily managed Wartschenbach torrent (Lienz region, Austria) and to quantify associated, expected economic losses. We do so by considering maintenance costs to restore infrastructure in the aftermath of debris-flow events as well as by assessing the probability of check dam failure (e.g., as a result of overload). Our analysis comprises two different management strategies as well as three scenarios defining future changes in debris-flow activity resulting from climatic changes. At the study site, an average debris-flow frequency of 21 events per decade was observed for the period 1950-2000; activity at the site is projected to change by +38% to -33%, according to the climate scenario used. Comparison of the different management alternatives suggests that the current mitigation strategy will allow to reduce expected damage to infrastructure and population almost fully (89%). However, to guarantee a comparable level of safety, maintenance costs is expected to increase by 57-63%, with an increase of maintenance costs by ca. 50% for each intervention. Our analysis therefore also highlights the importance of taking maintenance costs into account for risk assessments realized in managed torrent systems, as they result both from progressive and event-related deteriorations. We conclude that the stochastic life-cycle performance adopted in this study represents indeed an integrated approach to assess the long-term effects and costs of prevention structures in managed torrents.