Spatial and temporal biodiversity variations in a high mountain environment: the case of the proglacial margin of the Evettes, Natura 2000 area (Savoie, French Alps)

International audienceThe aim of this paper is to contribute to the understanding of the changes in plant cover after the recent glacier retreat, in an alpine environment. The selected study site in Savoie in the French Alps (2502-2509m asl), belonging to the European network Natura 2000, provides favourable conditions for the study due to the flat glacier foreland, where the glacier did not advance since the Little Ice Age. Data collected from 110 botanical plots were correlated with dated glacial and proglacial landforms. Species diversity has been analysed using the Shannon-Weaver index, the Grime classification, and the uncommon species described in the European list of Natura 2000 European Network. Classical and multivariate analyses have been made to determine the impact of the glacial retreat on the biodiversity variation. We show that the changes in species richness and vegetation cover were related to the distance from the glacier front. The biodiversity index was less than 2.5 near the glacier but above 4 at the furthest point. The highest plant diversity, however, was observed at an intermediate position, where competitive and pioneer plants were equally represented in the field. This location, concurrently, showed the highest heterogeneity in the activity of periglacial processes. It seems that the most heterogeneous and disturbed soils, due to inherited deposits and currently active periglacial processes, are responsible for an increase in biodiversity.L'objectif de cet article est de contribuer à la compréhension des changements en cours dans un environnement alpin, au front d'une marge proglaciaire, en relation avec un retrait glaciaire. Le secteur étudié correspond à la plaine proglaciaire du glacier des Evettes, situé en Savoie dans les Alpes françaises, vers 2500m d'altitude. Ce site appartient au réseau Natura 2000. Les conditions écologiques locales ont permis l'élaboration d'une vaste plaine proglaciaire en raison du retrait du glacier depuis la fin du Petit Âge Glaciaire (stades de retrait jalonnés depuis 1860). Les données collectées reposent sur l'échantillonnage floristique sur 110 placettes dont les résultats ont été analysés statistiquement avec plusieurs paramètres : la distance par rapport au front glaciaire actuel (et donc l'âge depuis la déglaciation), le contexte morphopédologique, la diversité spécifique déterminée à partir de l'indice Shannon-Weaver, la classification du type de stratégie dite de Grime et la valeur de la rareté de l'espèce lorsque celle-ci est signalée dans les listes du réseau européen de Natura 2000. Des analyses univariées et multivariées ont été appliquées afin de déterminer l'impact du retrait glaciaire sur la variation de la biodiversité. Si l'on aboutit très classiquement à la démonstration qu'il y a augmentation globale de la biodiversité en allant du front glaciaire actuel vers la marge la plus externe, notre étude prouve que la biodiversité la plus élevée concerne un point intermédiaire. Cette position correspond en réalité au secteur où les conditions locales périglaciaires permettent la cohabitation de plantes à la fois pionnières, rudérales et compétitrices. A terme, cette activité périglaciaire, ici responsable de l'augmentation de la biodiversité, est pourtant en régression sous l'impact du réchauffement

Les variations glaciaires en Haute Durance (Briançonnais, Hautes-Alpes) depuis la fin du XIXe siècle : mise au point d'après les documents d'archives et la lichénométrie

Les travaux reconstituant les variations glaciaires récentes se sont longtemps fondés sur des séries de mesures directes établies sur quelques glaciers sélectionnés. Nous proposons de compléter ces séries en étudiant les fluctuations, depuis la fin du XIXe siècle, du front d’une trentaine de glaciers, situés sur la retombée orientale du Massif des Ecrins. Des relevés géomorphologiques, la lichénométrie, et les documents anciens ont été utilisés conjointement, ce qui a permis d’identifier trois générations de moraines, délaissées respectivement entre 1915-1930, autour de 1950 et lors de la décennie 1970. Ces formations suggèrent l’existence de trois stades d’avancée et/ou de stationnement des glaces dans un contexte général de retrait glaciaire depuis le Petit Âge de Glace. Au cours du XXe siècle, la surface englacée a diminué dans ce secteur de 30% environ (35 à 25 km2). Cette évolution s’explique par un bilan de masse déficitaire de l’ordre de -0,2 à -0,4 mee.an-1, valeurs qui sont de même ordre de grandeur que celles établies ailleurs dans les Alpes Occidentales.Until recently, studies on glacial fluctuations were mostly relying on direct measurements of a few, selected glaciers. In order to complete these former datasets, we studied thirty glaciers located in the eastern part of the Massif des Ecrins. By using geomorphology surveys, lichenometry dating method, coupled with archives investigations, we identified three main stages of moraines deposition: between 1915-1930, around 1950, and in the 1970’s. These moraines suggest the occurrence of three stages of glacial advances and/or stationnary equilibrium in a general context of glacier recession subsequent to the Little Ice Age. The glaciated surface has decreased from 35 to 25 km2 during the 20th century, thus representing a mass-balance ranging from -0,2 to -0,4 mwe.yr-1, in agreement with what is known from other parts of western Alps

Climate reconstruction of the Little Ice Age maximum extent of the tropical Zongo Glacier using a distributed energy balance model

This study assessed the climate conditions that caused the tropical Zongo Glacier (16° S, Bolivia) to reach its Little Ice Age (LIA) maximum extent in the late 17th century. We carried out sensitivity analyses of the annual surface mass balance to different physically coherent climate scenarios constrained by information taken from paleoclimate proxies and sensitivity studies of past glacier advances. These scenarios were constrained by a 1.1 K cooling and a 20% increase in annual precipitation compared to the current climate. Seasonal precipitation changes were constructed using shuffled input data for the model: measurements of air temperature and relative humidity, precipitation, wind speed, incoming short and longwave radiation fluxes, and assessed using a distributed energy balance model. They were considered plausible if conditions close to equilibrium glacier-wide mass balance were obtained. Results suggest that on top of a 1.1 K cooling and ${\sim }$20% increase in annual precipitation, only two seasonal precipitation patterns allow LIA equilibrium: evenly distributed precipitation events across the year and an early wet season onset

The snow avalanches risk on Alpine roads network

Road accessibility is highly strategic for the maintenance of economic activities but also for the emergency services. In mountains, snow avalanches are a particularly strong threat because, in addition to the victims and direct damage, they cause a loss of accessibility more or less prolonged when the networks are already strongly altered by seasonal closures. Specifically, risk to traffic roads caused by snow avalanches has been very rarely assessed at a regional scale. To assess the physical, human and functional vulnerabilities of road networks in three Alpine departments (Alpes-de-Haute-Provence, Hautes-Alpes, Alpes-Maritimes), the first step of this research was to geo-locate and harmonize within a GIS all information sources about the consequences of avalanches on roads. This allowed identifying the road impacts of avalanches since 1937, to characterize the intensity and typology of damages and to evaluate the functional vulnerability of networks. The second step was to produce simple risk indexes of dysfunction and isolation at this regional scale. These indicators were modeled using the graph theory in a GIS framework, integrating avalanche activity indicators derived from the past activity with the road network. The obtained output maps should facilitate the decision support for crisis management and a comparative spatial analysis at the regional scale

Climate reconstruction of the Little Ice Age maximum extent of the tropical Zongo Glacier using a distributed energy balance model

This study assessed the climate conditions that caused the tropical Zongo Glacier (16° S, Bolivia) to reach its Little Ice Age (LIA) maximum extent in the late 17th century. We carried out sensitivity analyses of the annual surface mass balance to different physically coherent climate scenarios constrained by information taken from paleoclimate proxies and sensitivity studies of past glacier advances. These scenarios were constrained by a 1.1 K cooling and a 20% increase in annual precipitation compared to the current climate. Seasonal precipitation changes were constructed using shuffled input data for the model: measurements of air temperature and relative humidity, precipitation, wind speed, incoming short and longwave radiation fluxes, and assessed using a distributed energy balance model. They were considered plausible if conditions close to equilibrium glacier-wide mass balance were obtained. Results suggest that on top of a 1.1 K cooling and ${\sim }$20% increase in annual precipitation, only two seasonal precipitation patterns allow LIA equilibrium: evenly distributed precipitation events across the year and an early wet season onset

Be‐10 dating of ice‐marginal moraines in the Khumbu Valley, Nepal, Central Himalaya, reveals the response of monsoon‐influenced glaciers to Holocene climate change

The dynamic response of large mountain glaciers to climatic forcing operates over timescales of several centuries and therefore understanding how these glaciers change requires observations of their behavior through the Holocene. We used Be-10 exposure-age dating and geomorphological mapping to constrain the evolution of glaciers in the Khumbu Valley in the Everest region of Nepal. Khumbu and Lobuche Glaciers are surrounded by high-relief lateral and terminal moraines from which seven glacial stages were identified and dated to 7.4 ± 0.2, 5.0 ± 0.3, 3.9 ± 0.1, 2.8 ± 0.2, 1.3 ± 0.1, 0.9 ± 0.02, and 0.6 ± 0.16 ka. These stages correlate to each of the seven latest Holocene regional glacial stages identified across the monsoon-influenced Himalaya, demonstrating that a coherent record of high elevation terrestrial palaeoclimate change can be extracted from dynamic mountain landscapes. The time-constrained moraine complex represents a catchment-wide denudation rate of 0.8–1.4 mm a−1 over the last 8 kyr. The geometry of the ablation area of Khumbu Glacier changed around 4 ka from a broad, shallow ice tongue to become narrower and thicker as restricted by the topographic barrier of the terminal moraine complex

Moraine crest or slope: An analysis of the effects of boulder position on cosmogenic exposure age

Terrestrial cosmogenic nuclide dating of ice-marginal moraines can provide unique insights into Quaternary glacial history. However, pre- and post-depositional exposure histories of moraine boulders can introduce geologic uncertainty to numerical landform ages. To avoid geologic outliers, boulders are typically selected based on their depositional context and individual characteristics but while these criteria have good qualitative reasoning, many have not been tested quantitatively. Of these, boulder location is critical, as boulders located on moraine crests are prioritised, while those on moraine slopes are typically rejected. This study provides the first quantitative assessment of the relative utility of moraine crest and moraine slope sampling using new and published 10Be and 36Cl ages (n = 19) and Schmidt hammer sampling (SH; n = 635 moraine boulders, ∼19,050 SH R-values) in the northern and southern Pyrenees. These data show that for many of the studied moraines, the spatial distribution of “good” boulders is effectively random, with no consistent clustering on moraine crests, ice-proximal or -distal slopes. In turn, and in contrast to prior work, there is no clear penalty to either moraine crest or moraine slope sampling. Instead, we argue that landform stability exerts a greater influence on exposure age distributions than the characteristics of individual boulders. For the studied landforms, post-depositional stability is strongly influenced by sedimentology, with prolonged degradation of matrix-rich unconsolidated moraines while boulder-rich, matrix-poor moraines stabilised rapidly after deposition. While this pattern is unlikely to hold true in all settings, these data indicate that differences between landforms can be more significant than differences at the intra-landform scale. As ad hoc assessment of landform stability is extremely challenging based on geomorphological evidence alone, preliminary SH sampling, as utilised here, is a useful method to assess the temporal distribution of boulder exposure ages and to prioritise individual boulders for subsequent analysis

The origin and collapse of rock glaciers during the Bølling-Allerød interstadial: A new study case from the Cantabrian Mountains (Spain)

.During the Late Pleistocene, the main mountain ranges of the Iberian Peninsula were covered by small icefields and cirque and alpine glaciers. The deglaciation triggered paraglacial processes that generated landforms, mostly within the ice-free glacial cirques. In this research we analyse the deglaciation process in the Muxivén Cirque (42°15′N – 6°16′W), in the upper Sil River Basin, which includes some of the largest relict rock glaciers of the Cantabrian Mountains. We addressed this objective by means of accurate geomorphological reconstructions, sedimentological analysis, Schmidt-hammer surface weathering measurements and a dataset of 10 10Be Cosmic-Ray Exposure ages. Results reveal that after ~16 ka, glaciers retreated to the bottom of the cirques at the headwaters of the valley, leaving the walls free of ice and triggering rock avalanches onto the remnants of these glaciers. This paraglacial process supplied debris to a small glacier within Muxivén Cirque, which transformed in two rock glaciers. These debris isolated the ice inside the rock glaciers only for a very short period of time and ended up melting completely before the Younger Dryas. The lower sector of the largest one stabilized at 14.5 ± 1.5 ka, while the upper sector remained active until 13.5 ± 0.8 ka. Previous to the stabilization of the lower sector of the northern rock glacier, at its margin a high-energy debris avalanche occurred at ~14.0 ± 0.9 ka. These data agree with previous research, corroborating the paraglacial origin of most Iberian rock glaciers during the Bølling-Allerød interstadial.S

A community-based geological reconstruction of Antarctic Ice Sheet deglaciation since the Last Glacial Maximum

A robust understanding of Antarctic Ice Sheet deglacial history since the Last Glacial Maximum is important in order to constrain ice sheet and glacial-isostatic adjustment models, and to explore the forcing mechanisms responsible for ice sheet retreat. Such understanding can be derived from a broad range of geological and glaciological datasets and recent decades have seen an upsurge in such data gathering around the continent and Sub-Antarctic islands. Here, we report a new synthesis of those datasets, based on an accompanying series of reviews of the geological data, organised by sector. We present a series of timeslice maps for 20ka, 15ka, 10ka and 5ka, including grounding line position and ice sheet thickness changes, along with a clear assessment of levels of confidence. The reconstruction shows that the Antarctic Ice sheet did not everywhere reach the continental shelf edge at its maximum, that initial retreat was asynchronous, and that the spatial pattern of deglaciation was highly variable, particularly on the inner shelf. The deglacial reconstruction is consistent with a moderate overall excess ice volume and with a relatively small Antarctic contribution to meltwater pulse 1a. We discuss key areas of uncertainty both around the continent and by time interval, and we highlight potential priorit. © 2014 The Authors