Georges Rovéra

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.

Impacts of more frequent droughts on a relict low-altitude Pinus uncinata stand in the French Alps

Cold microclimatic conditions provide exceptional microhabitats to Pinus uncinata stands occurring at abnormally low altitudes in seven relict stands of the northern French Alps. Here, P. uncinata is located at the lower bounds of its ecological limits and therefore expected to provide a sensitive indicator of climate change processes. We used dendrochronological analysis to study the growth patterns of closely spaced chronologies across an elevational transect and compare a relict low-altitude to a P. uncinata stand located at the alpine treeline. Two detrending procedures are used to reveal high- and low-frequencies embedded in annually resolved ring-width series. Growth response of P. uncinata to instrumental temperature and precipitation data is investigated by means of moving response function analyses. Results show an increase in the sensitivity of tree-ring widths to drought during previous summer in both stands. At the treeline stand, an increasing response to fall temperature is observed whereby fall temperature and radial tree growth increased in two synchronous steps around ~1930 and from ~1980-present. At the low-altitude stand, P. uncinata appears more drought sensitive and exhibits a sharp growth decline since the mid-1980s, coinciding with increasing summer temperatures. Growth divergence between the two stands can be observed since the mid-1980s. We argue that the positive growth trend at the high-altitude stand is due to increasing fall temperatures which would favor the formation of metabolic reserves in conjunction with atmospheric CO2 enrichment that in turn would facilitate improved water use efficiency. At the relict low-altitude stand, in contrast, it seems that improved water use efficiency cannot compensate for the increase in summer temperatures.

Quantification of cliff retreat in coastal Quaternary sediments using anatomical changes in exposed tree roots: Quantification of cliff retreat using exposed tree roots

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 cmyr 1 (average: 2.1 cmyr ). Highest rates are observed at Pointe de la Tufière (2.6–3.9 cmyr ), 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 cmyr ). 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.