Understanding wet avalanche intensity and the role of past environmental changes on wet avalanche occurrence is a main concern especially in the context of a warming climate and accelerated environmental mutations. Avalanches are closely related to fast cryosphere changes and may cause major threats to human society. Here, we used the sedimentary archive of the Alpine Lake Lauvitel (Lac du Lauvitel; western French Alps) to establish the first long-term avalanche record in this Alpine region. For this purpose, we used a novel CT-scan methodology that allows the precise identification of coarse material – from sand to pebble – transported to the lake and embedded within the finer continuous sedimentation. We identified a total of 166 deposits over the last 3300 yr cal. BP. In parallel, a detailed pollen analysis gave an independent record of environmental changes. Based on modern observation, lake monitoring, seismic investigations and sedimentological evidences, coarse material deposits were attributed to wet avalanche events. Our results highlight the effect of vegetation cover on the avalanche hazard while a period of strong frequency increase occurred after 780 yr cal. BP. In Lake Lauvitel, this period corresponds to a major forest clearance induced by the rise of human land use. Climate forcing on the avalanche hazard was investigated before and after the vegetation shift. On a multicentennial scale, wet avalanches preferably occur during periods of larger glacier extent, in which higher winter precipitation probably generates a sufficiently thick snow cover. On a sub-centennial scale, avalanches are more frequent during periods of relative warming, resulting in a destabilization of the same snow cover in spring season. Our results highlight as well the role of forest cover in mitigating wet snow avalanches' occurrence. In the context of predicted warmer temperatures, this study raises the question of whether a wet avalanche hazard increase may be expected in the near future especially at higher altitudes.

We present a multidisciplinary approach to document the vegetation and landscape of Upper Guisane Valley around the Lautaret Pass area in the French Southwestern Alps since 7200 calyr BP. We combined pollen analysis in a peat bog (at 2044 m) with a leaf imprint study in two nearby travertine systems (at 2100 and 1950 m). During the last 7200 years, there was an open landscape with scarce trees such asconifers including Larix decidua Mill., Pinus spp., and Abies alba Mill. 2100 m was the highest altitude where fossils of A. alba Mill. have been reported in the Guisane Valley. These results, according to the pollen record of anthropogenic indicators and an increasing biodiversity of grasslands, suggest a human presence in the pass area since ca 6500 years with a pre-Roman deforestation. This human impact has become maximum from 1100 calyr BP to present, resulting from cereal crop agriculture and deforestation. The comparative study of the different records confirmed the role played by the exposition in the forest dynamics of the alpine valleys during the Holocene.

Soils have a substantial role in the environment because they provide several ecosystem services such as food supply or carbon storage. Agricultural practices can modify soil properties and soil evolution processes, hence threatening these services. These modifications are poorly studied, and the resilience/adaptation times of soils to disruptions are unknown. Here, we study the evolution of pedogenetic processes and soil evolution phases (progressive or regressive) in response to human-induced erosion from a 4000-year lake sediment sequence (Lake La Thuile, French Alps). Erosion in this small lake catchment in the montane area is quantified from the terrigenous sediments that were trapped in the lake and compared to the soil formation rate. To access this quantification, soil processes evolution are deciphered from soil and sediment geochemistry comparison. Over the last 4000 years, first impacts on soils are recorded at approximately 1600 yr cal. BP, with the erosion of surface horizons exceeding 10 t·km− 2·yr− 1. Increasingly deep horizons were eroded with erosion accentuation during the Higher Middle Ages (1400–850 yr cal. BP), reaching 1000 t·km− 2·yr− 1, and leading to the remobilization of carbonated and poorly weathered material, hence rejuvenating soil development. Erosion exceeded the soil formation rate and constituted a regression in the development of soils. The tolerable erosion limit is thus defined for erosion from 25 to 30 t·km− 2·yr− 1. Beyond this limit, the sustainability of the agroecosystem is limited and ecosystem services decrease. Afterwards, pedogenesis evolved again from progressive (700–300 yr cal. BP) to regressive (300 yr cal. BP-today) phases. Erosion was less important during the last 700 years than during the Middle Ages but with the same weathering stages, indicating that soils were deeply affected during the Middle-Age and have yet not recovered. Our results highlight the importance of the human factor in the pedogenesis over last millennia and suggest that the studied agro-ecosystem entered the Anthropocene 1400 years ago.

Le lac de La Thuile dans les préalpes du nord (874 m) fournit une séquence sédimentaire de 18 m de long. L’analyse multi-proxies haute résolution des 6,2 premiers mètres de sédiment incluant des données sédimentaires, palynologiques et géochimiques, associées à une chronologie bien établie, documente l’évolution holocène de l’érosion dans le bassin versant et les modifications du paysage. Le bassin versant est suffisamment déconnecté des grandes plaines pour capturer un signal sédimentaire spécifique à ce milieu. De 12 à 4,5 ka cal. BP, la végétation se développe avec l’apparition de feuillus alors que l’érosion diminue à la suite de la transition Tardiglaciaire-Holocène. La forêt devient plus dense, stabilise les pentes et empêche l’érosion des sols qui se développent en s’acidifiant. Une première phase érosive est initiée vers 4,5 ka cal. BP. Le paysage change avec le développement de la hêtraie-sapinière. Les taxons anthropiques font leur apparition autour de 3 000 cal. BP. Deux autres phases érosives sont à mettre en relation avec l’ouverture du milieu par les activités humaines : à partir de 2,5 ka cal. BP, à la fin de l’âge du Bronze et pendant la période Romaine, et après 1,6 ka cal. BP avec le Moyen-âge. Elles engendrent un rajeunissement et une recarbonatation des sols. L’érosion plus faible produite durant le Petit âge Glaciaire suggère que les activités anthropiques dominent les processus liés à l’érosion et masquent complètement l’impact du climat sur l’érosion à cette altitude.

Here we present the results from a bourgeoning tool, the analyses of lake sediment DNA applied to the reconstruction of agriculture and landscape histories (plant and mammal DNA) in the French Alps. As the tool is pioneer, the first part of the manuscript is focused on the study of the conditions to archive the DNA in lake sediments and on the reliability of the data. Results suggest that lakes submitted to high detrital inputs are more favourable to the record of DNA coming from the catchment. Moreover, we show that DNA and pollen analyses are complementary. Combining these two approaches, we can distinguish the local from the more regional landscape evolution. The reconstruction of local landscapes and the access to the composition of livestock farming are the main advantages of the lake sediment DNA tool. With this innovative tool, we can reconstruct a detailed picture of the alpine agriculture dynamic in space and time, as well as study their effects on the environment.

Lake La Thuile, in the Northern French Prealps (874 m a.s.l.), provides an 18-m long sedimentary sequence spanning the entire Lateglacial/Holocene period. The high-resolution multi-proxy (sedimentological, palynological, and geochemical) analysis of the uppermost 6.2 m reveals the Holocene dynamics of erosion in the catchment in response to landscape modifications. The mountain belt is at relevant altitude to study past human activities, and the watershed is sufficiently disconnected from large valleys to capture a local sedimentary signal. From 12,000 to 10,000 cal. BP (10–8 kyr cal. BC), the onset of hardwood species triggered a drop in erosion following the Lateglacial/Holocene transition. From 10,000 to 4500 cal. BP (8–2.5 kyr cal. BC), the forest became denser and favored slope stabilization, while erosion processes were very weak. A first erosive phase was initiated at ca. 4500 cal. BP without evidence of human presence in the catchment. Then, the forest declined at approximately 3000 cal. BP, suggesting the first human influence on the landscape. Two other erosive phases are related to anthropic activities: approximately 2500 cal. BP (550 cal. BC) during the Roman period and after 1600 cal. BP (350 cal. AD) with a substantial accentuation in the Middle Ages. In contrast, the lower erosion produced during the ‘Little Ice Age’, when climate deteriorations are generally considered to result in an increased erosion signal in this region, suggests that anthropic activities dominated the erosive processes and completely masked the natural effects of climate on erosion in the late Holocene.

The reconstruction of human-driven, Earth-shaping dynamics is important for understanding past human/environment interactions and for helping human societies that currently face global changes. However, it is often challenging to distinguish the effects of the climate from human activities on environmental changes. Here we evaluate an approach based on DNA metabarcoding used on lake sediments to provide the first high-resolution reconstruction of plant cover and livestock farming history since the Neolithic Period. By comparing these data with a previous reconstruction of erosive event frequency, we show that the most intense erosion period was caused by deforestation and overgrazing by sheep and cowherds during the Late Iron Age and Roman Period. Tracking plants and domestic mammals using lake sediment DNA (lake sedDNA) is a new, promising method for tracing past human practices, and it provides a new outlook of the effects of anthropogenic factors on landscape-scale changes.

This article reports on the progress in designing a new methodology to recreate paleo-landscapes using pollen analysis. It points out the need of restating incontrovertible methodological constraints. In a mountain environment, the potential decline in diversity of the pollen profile due to agitation of air masses mandates the localization of testing in homogeneous, ecologically restricted areas that best represent the spectrum of local vegetation. This is how the concept of key-massif emerged. This concept refers to small surface areas, less than one hectare, with different layers of vegetation and exposures. Synthesis on the regional scale is done using iterative approximation of results. Due to its capacity to reveal differences in changes of landscapes at the scale of human activities this methodology is adapted to the multidisciplinary programs to study human–environment relationships. During the Holocene the subalpine level of the French Alps was mostly wooded. The diversity of the tree cover was largely associated with the topography. Human activities caused a general decrease in the limit of altitudinal distribution of woody species and a decline of its diversity in open landscapes.

In central Western Europe, several studies have shown that colder Holocene periods, such as the Little Ice Age, also correspond to wet periods. However, in mountain areas which are highly sensitive to erosion processes and where precipitation events can be localized, past evolution of hydrological activity might be more complicated. To assess these past hydrological changes, a paleolimnological approach was applied on a 13.4-m-long sediment core taken in alpine Lake Anterne (2063m asl) and representing the last 3.5ka. Lake sedimentation is mainly composed of flood deposits triggered by precipitation events. Sedimentological and geochemical analyses show that floods were more frequent during cold periods while high-intensity flood events occurred preferentially during warmer periods. In mild temperature conditions, both flood patterns are present. This underlines the complex relationship between flood hazards and climatic change in mountain areas. During the warmer and/or dryer times of the end of Iron Age and the Roman Period, both the frequency and intensity of floods increased. This is interpreted as an effect of human-induced clearing for grazing activities and reveals that anthropogenic interferences must be taken into account when reconstructing climatic signals from natural archives.

A high-resolution sedimentological and geochemical study was performed on a 20 m long core from the alpine Lake Anterne (2063 m a.s.l., NW French Alps) spanning the last 10 ka. Sedimentation is mainly of minerogenic origin. The organic matter quantity (TOC%) as well as its quality (hydrogen (HI) and oxygen (OI) indices) both indicate the progressive onset and subsequent stabilization of vegetation cover in the catchment from 9950 to 5550 cal. BP. During this phase, the pedogenic process of carbonate dissolution is marked by a decrease in the calcium content in the sediment record. Between 7850 and 5550 cal. BP, very low manganese concentrations suggest anoxic conditions in the bottom-water of Lake Anterne. These are caused by a relatively high organic matter (terrestrial and lacustrine) content, a low flood frequency and longer summer stratification triggered by warmer conditions. From 5550 cal. BP, a decrease in TOC, stabilization of HI and higher sedimentation rates together reflect increased erosion rates of leptosols and developed soils, probably due to a colder and wetter climate. Then, three periods of important soil destabilization are marked by an increased frequency and thickness of flood deposits during the Bronze Age and by increases in topsoil erosion relative to leptosols (HI increases) during the late Iron Age/Roman period and the Medieval periods. These periods are also characterized by higher sedimentation rates. According to palynological data, human impact (deforestation and/or pasturing activity) probably triggered these periods of increased soil erosion.

A pollen study at Survilly (2235m asl, 06 degrees 49' 12 '' E, 45 degrees 59' 24 '' N), a small peatbog located on the Anterne mountain (Upper-Arve Valley, French north-western Alps) highlights the local role of human activities in Holocene vegetation dynamics of the currently treeless subalpine belt and the consecutive resumption of erosion. As early as 8890 cal. years BP (+/- 122), Pinus cembra grew close to the site. Grasslands without shrubs were established at around 4624 +/- 86 cal. years BP. Due to human activities, spruces extended little after 3600 cal. BP. The intense grazing that resulted in the current alpine meadows goes back to 1436 cal. years BP (+/- 81). After 4624 cal. BP three clay layers show that from this period, the erosion became as active as during the first steps of the colonization of the vegetation prior to 10,050 cal. BP. During peat growth only a millimetre of clay at the end of the 9400-9050 cal. BP climatic event was recorded. (C) 2010 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

Ce travail rappelle dans un premier volet les contraintes méthodologiques incontournables pour la reconstitution des paléo-paysages à partir de l'analyse pollinique et dans un second volet une synthèse des acquis dans les Alpes françaises. En milieu de montagne, l'uniformisation potentielle de l'image pollinique par le brassage des masses d'air impose la localisation des études dans des aires restreintes écologiquement homogènes et le choix de sites enregistrant au mieux l'image de la végétation locale. C'est ainsi qu'a émergé la notion de massif clé, constitué de sites de faible dimension dans les différents étages de végétation et à différentes expositions. La synthèse des acquis s'effectue par intégration des résultats de proche en proche. Une revue des données des cinquante dernières années est passée au crible de cette méthodologie qui met en évidence une uniformisation artificielle de l'image des paléo-paysages et un biais dans les datations. Pendant l'Holocène, l'étage subalpin était plus diversifié et majoritairement boisé avec des espèces dont la limite supérieure était plus élevée qu'actuellement. La mise en évidence de diachronismes des évolutions des paysages liés au relief contrasté constitue l'apport fondamental de cette méthodologie.

Pollen analyses were undertaken on a small peat bog (Ecuelles 06 degrees 49' 41''E, 45 degrees 58' 49''N, 1855 m asl), located on the Anterne mountain (Upper-Arve Valley, French north-western Alps). The study highlights the role of green alder (Alnus alnobetula [Ehrh] K. Koch) in Holocene vegetation dynamics of the nowadays treeless subalpine belt. At this place, the onset of human perturbation caused a retreat of fir and arolla-pine stands and an expansion of green alder, which consequently dominated the landscape from 3700 up to 1965 cal. BP. After 1965 cal. BP, the clearings led to the present grasslands with few ligneous species (spruce, larch) on inaccessible cliffs or green alder on the edges of torrents or in avalanche corridors. Picea percentages have increased after 3900 cal. BP, but, due to human activities, spruce has never constituted large stands in the study area. The present general expansion of green alder is due to the decreasing human impact and it constitutes the first step of re-afforestation that should lead to mixed stands of spruce and arolla-pine. The study gives a new evidence of the past diversity of the vegetation cover and do not support the idea that green alder colonization at the subalpine belt constitutes a long-term risk for the vegetation diversity.