Frédéric Berger

Three-dimensional analysis of the anatomical growth response of European conifers to mechanical disturbance

Studies on tree reaction after wounding were so far based on artificial wounding or chemical treatment. For the first time, type, spread and intensity of anatomical responses were analyzed and quantified in naturally disturbed Larix decidua Mill., Picea abies (L.) Karst. and Abies alba Mill. trees. The consequences of rockfall impacts on increment growth were assessed at the height of the wounds, as well as above and below the injuries. A total of 16 trees were selected on rockfall slopes, and growth responses following 54 wounding events were analyzed on 820 cross-sections. Anatomical analysis focused on the occurrence of tangential rows of traumatic resin ducts (TRD) and on the formation of reaction wood. Following mechanical disturbance, TRD production was observed in 100% of L. decidua and P. abies wounds. The radial extension of TRD was largest at wound height, and they occurred more commonly above, rather than below, the wounds. For all species, an intra-annual radial shift of TRD was observed with increasing axial distance from wounds. Reaction wood was formed in 87.5% of A. alba following wounding, but such cases occurred only in 7.7% of L. decidua. The results demonstrate that anatomical growth responses following natural mechanical disturbance differ significantly from the reactions induced by artificial stimuli or by decapitation. While the types of reactions remain comparable between the species, their intensity, spread and persistence disagree considerably. We also illustrate that the external appearance of wounds does not reflect an internal response intensity. This study reveals that disturbance induced under natural conditions triggers more intense and more widespread anatomical responses than that induced under artificial stimuli, and that experimental laboratory tests considerably underestimate tree response.

Dendrogeomorphic reconstruction of past landslide reactivation with seasonal precision: the Bois Noir landslide, southeast French Alps

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 200xa0mm, pointing to the important role of summer rainstorms in the triggering of events at the Bois Noir landslide body.

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.

Support Vector Regression for the Estimation of Forest Stand Parameters Using Airborne Laser Scanning

Airborne laser scanning is nowadays widely used for the estimation of forest stand parameters. Prediction models have to deal with high-dimensional laser data sets as well as limited field calibration data. This problem is enhanced in mountainous areas where forest is highly heterogeneous and field data collection is costly. Artificial neural network models and support vector regression (SVR) have already demonstrated their ability to address such issues for species-specific plot volume prediction. In this letter, we compare the stand parameter prediction accuracies of support vector machines and ordinary least squares multiple-regression models for dominant height, basal area, mean diameter, and stem density. The sensitivity of these techniques to the input variables is investigated by testing data sets which include different numbers and types of laser metrics, and by reducing their dimension with principal component and independent component analyses. Whereas usual variables only reflect the vertical distribution, we also integrate the entropy of the horizontal distribution of the point cloud in the laser metrics. The results show that SVR prediction models are of similar accuracy with multiple-regression models but are more robust regarding the metrics included in the data sets. Preliminary dimension reduction of the data set by principal component analysis generally benefits more to SVR than to multiple regression. The optimal combination of laser metrics to be included in the data sets mainly depends on the forest parameter to be estimated.

Characterization of rockfalls from seismic signal: insights from laboratory experiments

The seismic signals generated by rockfalls can provide information on their dynamics and location. However, the lack of field observations makes it difficult to establish clear relationships between the characteristics of the signal and the source. In this study, scaling laws are derived from analytical impact models to relate the mass and the speed of an individual impactor to the radiated elastic energy and the frequency content of the emitted seismic signal. It appears that the radiated elastic energy and frequencies decrease when the impact is viscoelastic or elasto-plastic compared to the case of an elastic impact. The scaling laws are validated with laboratory experiments of impacts of beads and gravels on smooth thin plates and rough thick blocks. Regardless of the involved materials, the masses and speeds of the impactors are retrieved from seismic measurements within a nfactor of 3. A quantitative energy budget of the impacts is established. On smooth thin plates, the lost energy is either radiated in elastic waves or dissipated in viscoelasticity when the impactor is large or small with respect to the plate thickness, respectively. In contrast, on rough thick blocks, the nelastic energy radiation represents less than 5% of the lost energy. Most of the energy is lost in plastic deformation or rotation modes of the bead owing nto surface roughness. Finally, we estimate the elastic energy radiated during field scale rockfalls experiments. This energy is shown to be proportional to the boulder mass, in agreement with the theoretical scaling laws.

Managing coppice forests for rockfall protection: lessons from modeling

Abstract• ContextOne eighth of Europe’s forests are still managed as coppice. In some European countries, more than half of the forest exhibits coppice structures from the past or present coppice management. Many of these forests grow in the broadleaf zone of mountainous regions, often on steep slopes. Here, they play an important role in rockfall protection. However, it remains unclear how coppice forests should be structured for optimal rockfall protection or how the protection effect changes during the aging of the coppice.• AimA few studies have applied rock trajectory analyses, but so far, no process-based model has been used to quantify the protective effect of differently structured coppice forests. The present study compared 40 coppice patches from two chronosequences in South Tyrol, North Italy, regarding their protective effect against rocks of two sizes using the rockfall simulation model Rockyfor3D.• ResultsThe results indicate that coppice stands older than 30xa0years better protect against rockfall than medium-aged and young stands, although the old ones have lower stem densities. Surprisingly, a random stem distribution had a better protective effect than the clumped stem distribution typical for coppice stands.• ConclusionImplications for future management are discussed in detail, including the relevance of standards in coppice forests.

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.

Management of Austrian Black Pine on Marly Lands for Sustainable Protection Against Erosion (Southern Alps, France)

The old afforestations of Austrian black pine dating from more than a century ago on marly substrates in the French Southern Alps play a protective role against erosion and torrential floods. However, these forests have now reached maturity and it is necessary to consider their renewal within the framework of minimal management to ensure sustainable protection against erosion. We first present methods for determining and mapping priority areas for forestry actions on different spatial scales, after which we define various forestry application rules.

Predicting load path and tensile forces during cable yarding operations on steep terrain

Cable yarding systems constitute an adapted solution for steep-slope harvesting in mountain forests. However, it requires many specific skills for both forest managers and operators. The objectives of this research were to: (1) develop a CableHelp model for the set-up of cable yarding systems where inputs are operational field data and outputs are load path and tensile forces, and (2) to validate it with field experiments. The results show a high accuracy between the data predicted by the model and field measurements. Furthermore, this work stresses the importance of taking into account both the mainline effect and the friction between skyline and intermediate supports to properly calculate the skyline tension and load path. The CableHelp model shows great adaptability and ensures highly accurate predictions for any position on the line profile and for different configurations: single-span or multiple-span profiles, uphill or downhill yarding and for different kinds of carriage. A direct application of this research is to optimize the set-up of cable lines in order to reduce equipment wear, as well as operating cost, while respecting operator safety.