Résumé
Nitrogen is an essential nutrient for life on Earth, but in excess, it can lead to environmental issues (e.g., N saturation, loss of biodiversity, acidification of lakes, etc.). Understanding the nitrogen budget (i.e., inputs and outputs) is essential to evaluate the prospective decay of the ecosystem services (e.g., freshwater quality, erosion control, loss of high patrimonial-value plant species, etc.) that subalpine headwater catchments provide, especially as these ecosystems experience high atmospheric nitrogen deposition. Here, we use a multi-isotopic tracer (Δ17O, δ15N and δ18O) of nitrate in aerosols, snow, and streams to assess the fate of atmospherically deposited nitrate in the subalpine watershed of the Lautaret Pass (French Alps). We show that atmospheric N deposition contributes significantly to stream nitrate pool year-round, either by direct inputs (up to 35%) or by in situ nitrification of atmospheric ammonium (up to 35%). Snowmelt in particular leads to high exports of atmospheric nitrate, most likely fast enough to impede assimilation by surrounding ecosystems. Yet, in a context of climate change, with shorter snow seasons, and increasing nitrogen emissions, our results hint at possibly stronger ecological consequences of nitrogen atmospheric deposition in the close future.
Auteurs, date et publication :
Auteurs Ilann Bourgeois , Joël Savarino , Nicolas Caillon , Hélène Angot , Albane Barbero , Franck Delbart , Didier Voisin , Jean-Christophe Clément
Publication : Environmental Science & Technology
Date : 2018
Volume : 52
Issue : 10
Pages : 5561-5570
Catégorie(s)
#CNRS #Lautaret #UGARésumé
The leaf economics spectrum (LES) is based on a suite of leaf traits related to plant functioning and ranges from resource-conservative to resource-acquisitive strategies. However, the relationships with root traits, and the associated belowground plant functioning such as N uptake, including nitrate (NO3−) and ammonium (NH4+), is still poorly known. Additionally, environmental variations occurring both in time and in space could uncouple LES from root traits. We explored, in subalpine grasslands, the relationships between leaf and root morphological traits for three dominant perennial grass species, and to what extent they contribute to the whole-plant economics spectrum. We also investigated the link between this spectrum and NO3− and NH4+ uptake rates, as well as the variations of uptake across four grasslands differing by the land-use history at peak biomass and in autumn. Although poorly correlated with leaf traits, root traits contributed to an economic spectrum at the whole plant level. Higher NH4+ and NO3− uptake abilities were associated with the resource-acquisitive strategy. Nonetheless, NH4+ and NO3−-uptake within species varied between land-uses and with sampling time, suggesting that LES and plant traits are good, but still incomplete, descriptors of plant functioning. Although the NH4+:NO3− uptake ratio was different between plant species in our study, they all showed a preference for NH4+, and particularly the most conservative species. Soil environmental variations between grasslands and sampling times may also drive to some extent the NH4+ and NO3− uptake ability of species. Our results support the current efforts to build a more general framework including above- and below-ground processes when studying plant community functioning.
Auteurs, date et publication :
Auteurs Nicolas Legay , Fabrice Grassein , Cindy Arnoldi , Raphael Segura , Philippe Laîné , Sandra Lavorel , Jean-Christophe Clément
Publication : Oikos
Date : 2025
Volume : 129
Issue : 6
Pages : 830-841
Catégorie(s)
#CNRS #Lautaret #UGARésumé
Uncertainties of snowpack models and of their meteorological forcings limit their use by avalanche hazard forecasters, or for glaciological and hydrological studies. The spatialized simulations currently available for avalanche hazard forecasting are only assimilating sparse meteorological observations. As suggested by recent studies, their forecasting skills could be significantly improved by assimilating satellite data such as snow reflectances from satellites in the visible and the near-infrared spectra. Indeed, these data can help constrain the microstructural properties of surface snow and light absorbing impurities content, which in turn affect the surface energy and mass budgets. This paper investigates the prerequisites of satellite data assimilation into a detailed snowpack model. An ensemble version of Météo-France operational snowpack forecasting system (named S2M) was built for this study. This operational system runs on topographic classes instead of grid points, so-called ‘semi-distributed’ approach. Each class corresponds to one of the 23 mountain massifs of the French Alps (about 1000 km2 each), an altitudinal range (by step of 300 m) and aspect (by step of 45°). We assess the feasability of satellite data assimilation in such a semi-distributed geometry. Ensemble simulations are compared with satellite observations from MODIS and Sentinel-2, and with in-situ reflectance observations. The study focuses on the 2013–2014 and 2016–2017 winters in the Grandes-Rousses massif. Substantial Pearson R2 correlations (0.75–0.90) of MODIS observations with simulations are found over the domain. This suggests that assimilating it could have an impact on the spatialized snowpack forecasting system. However, observations contain significant biases (0.1–0.2 in reflectance) which prevent their direct assimilation. MODIS spectral band ratios seem to be much less biased. This may open the way to an operational assimilation of MODIS reflectances into the Météo-France snowpack modelling system.
Auteurs, date et publication :
Auteurs Bertrand Cluzet , Jesus Revuelto , Matthieu Lafaysse , François Tuzet , Emmanuel Cosme , Ghislain Picard , Laurent Arnaud , Marie Dumont
Publication : Cold Regions Science and Technology
Date : 2025
Volume : 170
Pages : 102918
Catégorie(s)
#CNRS #Lautaret #UGARésumé
Sensitivity of grassland biomass production to climate is critical to impacts on multiple ecological processes and ecosystem services. Understanding its climate determinants is essential for climate change adaptation. This requires long-term monitoring, using robust methods that are appropriated by stakeholders. We tested the sensitivity of easily measured sward height to interannual climate variation in mountain grasslands. Using twelve consecutive years of measurements across 67 grassland plots representative of six land-use types associated with different landscape positions, we show that peak green biomass increased with mean summer months (June and July) maximum temperature. Different land-use types responded to specific combinations of climate parameters, but all except higher-elevation summer pastures were sensitive to summer months temperatures. We did not detect any effects of drought, with summer precipitation instead decreasing peak biomass of some grasslands due to cooling and cloudiness, also suggesting that soil water recharge from snowmelt was enough to sustain the first growth cycle. Summer pasture peak biomass decreased with number of frosts during the onset of growth in May. These result support the robustness and sensitivity of sward height as an indicator for climate response of peak fodder biomass. Differential responses across land-use types suggest some resource complementarity which can support tactical adaptation for farmers. During the three recent hottest summers (2015, 2017 and 2018) production was well below predicted values from actual temperatures, suggesting a potential regime shift when the vegetative growth period is shortened by temperature-driven acceleration in phenology and/or heat stress combined with high light intensity causing physiological damage. The baseline regime and the anomalies in hottest years need confirmation for longer time series and across a greater geographic extent. Further effects of drought and of an extended growing season are also likely for post-harvest or grazing regrowth.
Auteurs, date et publication :
Auteurs Karl Grigulis , Sandra Lavorel
Publication : Ecological Indicators
Date : 2025
Volume : 116
Pages : 106519
Catégorie(s)
#CNRS #Lautaret #UGARésumé
Most models simulating snow albedo assume a flat and smooth surface, neglecting surface roughness. However, the presence of macroscopic roughness leads to a systematic decrease in albedo due to two effects: (1) photons are trapped in concavities (multiple reflection effect) and (2) when the sun is low, the roughness sides facing the sun experience an overall decrease in the local incidence angle relative to a smooth surface, promoting higher absorption, whilst the other sides have weak contributions because of the increased incidence angle or because they are shadowed (called the effective-angle effect here). This paper aims to quantify the impact of surface roughness on albedo and to assess the respective role of these two effects, with (1) observations over varying amounts of surface roughness and (2) simulations using the new rough surface ray-tracing (RSRT) model, based on a Monte Carlo method for photon transport calculation. The observations include spectral albedo (400-1050 nm) over manually created roughness surfaces with multiple geometrical characteristics. Measurements highlight that even a low fraction of surface roughness features (7% of the surface) causes an albedo decrease of 0.02 at 1000 nm when the solar zenith angle (theta(s)) is larger than 50 degrees. For higher fractions (13 %, 27% and 63 %), and when the roughness orientation is perpendicular to the sun, the decrease is of 0.03-0.04 at 700 nm and of 0.06-0.10 at 1000 nm. The impact is 20% lower when roughness orientation is parallel to the sun. The observations are subsequently compared to RSRT simulations. Accounting for surface roughness improves the model observation agreement by a factor of 2 at 700 and 1000 nm (errors of 0.03 and 0.04, respectively) compared to simulations considering a flat smooth surface. The model is used to explore the albedo sensitivity to surface roughness with varying snow properties and illumination conditions. Both multiple reflections and the effective-angle effect have a greater impact with low specific surface area (SSA; < 10m(2) kg(-1)). The effective-angle effect also increases rapidly with theta(s) at large theta(s). This latter effect is larger when the overall slope of the surface is facing away from the sun and has a roughness orientation perpendicular to the sun. For a snowpack where artificial surface roughness features were created, we showed that a broadband albedo decrease of 0.05 may cause an increase in the net shortwave radiation of 80% (from 15 to 27Wm(-2)). This paper highlights the necessity of considering surface roughness in the estimation of the surface energy budget and opens the way for considering natural rough surfaces in snow modelling.
Auteurs, date et publication :
Auteurs Fanny Larue , Ghislain Picard , Laurent Arnaud , Ines Ollivier , Clement Delcourt , Maxim Lamare , Francois Tuzet , Jesus Revuelto , Marie Dumont
Publication : CRYOSPHERE
Date : 2020
Volume : 14
Issue : 5
Pages : 1651-1672
Catégorie(s)
#ANR-Citation #CNRS #Lautaret #UGARésumé
Asexual reproduction has often been regarded as an evolutionary dead end, but asexual lineages (most notably those that are apomictic) are present in several angiosperm families and often comprise a large number of taxa, both widespread and endemic. Investigating correlations between genetic, environmental and taxonomic factors and the incidence of apomixis has represented a challenge for many years, with previous analyses frequently omitting one or more of these variables. Here, flow cytometric seed screening, cytological data and ecological variables have been integrated in a phylogenetic framework to create a comprehensive dataset for 229 of Asteraceae from the European Alps. Data were analysed using phylogenetically informed generalized linear mixed models (pMCMCglmm) where elevation, ploidy and phenology were assessed for their potential correlation with asexual reproduction and apomixis type. Although apomixis is not dominant among the species studied, our results confirm that an odd ploidy (e.g. 3x) and, to a lesser extent, an even polyploid level (i.e. 4x) significantly increase its probability, most probably due to chromosome misalignments during meiosis. The distribution of apomictic species does not correlate with elevation, and there is a weak correlation between early flowering initiation and aposporous apomixis. Although current and future changes in climate may severely impact the survival of the flora of the European Alps, asexual reproduction and polyploidization may prove to be, at least temporarily, lifelines for the survival of a species under the novel climatic conditions. Therefore, uncovering how apomicts and polyploids evolve and persist will be essential for understanding the ecology of the European Alps and hence informing future conservation strategies.
Auteurs, date et publication :
Auteurs Luca Pegoraro , Ellen C Baker , David Aeschimann , Manica Balant , Rolland Douzet , Teresa Garnatje , Maïté S Guignard , Ilia J Leitch , Andrew R Leitch , Luis Palazzesi , Jean-Paul Theurillat , Oriane Hidalgo , Jaume Pellicer
Publication : Botanical Journal of the Linnean Society
Date : 2020
Volume : 194
Issue : 4
Pages : 410-422
Catégorie(s)
#CNRS #Lautaret #UGARésumé
Aim Plant–soil interactions can be major driving forces of community responses to environmental changes in terrestrial ecosystems. These interactions can leave signals in aboveground plant functional traits and belowground microbial activities and these signals can manifest in observed covariations. However, we know little about how these plant–soil linkages vary in response to environmental conditions at biogeographic scales for which experiments are impossible. Here, we investigate patterns of direct and indirect linkages between plant functional traits, soil microbial activities and environmental conditions in mountain grasslands along elevational gradients. Location The French Alps. Taxon Vascular plants and soil microbiota. Methods We analysed observational grassland data sampled along 14 elevational gradients across the entire French Alps (between 1500 and 2800 m of elevation). Using Graphical Lasso, we inferred a partial correlation network to tease apart direct and indirect plant–soil linkages without defining the direction of interactions a priori. Results We found tight spatial associations of plant traits with microbial activities, climate driving the former and soil properties the latter. In these plant–soil linkages, the dominance of specific plant traits was more important than their diversity. We then showed that in sites with conservative plant traits and reduced organic matter quality, soil microbes invested strongly in nutrient acquisition. Main conclusions By investigating plant–soil linkages along elevational gradients in the French Alps, we showed that plant functional traits and belowground microbial activity are tightly linked and how they depend on environmental conditions. Overall, we demonstrated how soil functioning can be integrated in studies of ecosystem shifts under environmental change at large spatial scales.
Auteurs, date et publication :
Auteurs Sarah-Sophie Weil , Camille Martinez-Almoyna , Gabin Piton , Julien Renaud , Louise Boulangeat , Arnaud Foulquier , Amélie Saillard , Philippe Choler , Jérôme Poulenard , Orchamp Consortium , Tamara Münkemüller , Wilfried Thuiller
Publication : Journal of Biogeography
Date : 2025
Volume : 48
Issue : 11
Pages : 2755-2770
Catégorie(s)
#CNRS #Lautaret #UGARésumé
Data assimilation of snow observations significantly improves the accuracy of snow cover simulations. However, remotely-sensed snowpack observations made in areas of complex topography are typically subject to large error and biases, creating a challenge for data assimilation. To improve the reliability of ensemble snowpack simulations, this study investigated the appropriate conditions for assimilating MODIS-like synthetic surface reflectances. We used a simulation system that included the Particle Filter data assimilation technique. More than 270 ensemble simulations involving assimilation of synthetic observations were conducted in a twin experiment procedure for three snow seasons. These tests were aimed at establishing the spectral combination of MODIS-like reflectances that convey the more information in the assimilation system, rendering the most reliable snowpack simulation, and determining the maximum observation errors that the assimilation system could tolerate. The assimilation of the first seven MODIS-like bands, covering visible and near-infrared wavelengths, provided the best scores compared with any other band combination, and thus are highly recommended for use when possible. The simulation system tolerated a maximum deviation from ground truth of 5% without loss of performance. However, the assimilation of the first seven bands of true MODIS surface of reflectance fails on improving simulation results in rouged mountain areas.
Auteurs, date et publication :
Auteurs J. Revuelto , B. Cluzet , N. Duran , M. Fructus , M. Lafaysse , E. Cosme , M. Dumont
Publication : Journal of Hydrology
Date : 2025
Volume : 603
Pages : 126966
Catégorie(s)
#CNRS #Lautaret #UGARésumé
Rice poses a major source of the toxic contaminant cadmium (Cd) for humans. Here, we elucidated the role of Cd storage forms (i.e., the chemical Cd speciation) on the dynamics of Cd within rice. In a pot trial, we grew rice on a Cd-contaminated soil in upland conditions and sampled roots and shoots parts at flowering and maturity. Cd concentrations, isotope ratios, Cd speciation (X-ray absorption spectroscopy), and micronutrient concentrations were analyzed. During grain filling, Cd and preferentially light Cd isotopes were strongly retained in roots where the Cd storage form did not change (Cd bound to thiols, Cd-S = 100%). In the same period, no net change of Cd mass occurred in roots and shoots, and the shoots became enriched in heavy isotopes (Delta Cd-114/110(maturity-flowering) = 0.14 +/- 0.04 parts per thousand). These results are consistent with a sequestration of Cd in root vacuoles that includes strong binding of Cd to thiol containing ligands that favor light isotopes, with a small fraction of Cd strongly enriched in heavy isotopes being transferred to shoots during grain filling. The Cd speciation in the shoots changed from predominantly Cd-S (72%) to Cd bound to O ligands (Cd-O, 80%) during grain filling. Cd-O may represent Cd binding to organic acids in vacuoles and/or binding to cell walls in the apoplast. Despite this change of ligands, which was attributed to plant senescence, Cd was largely immobile in the shoots since only 0.77% of Cd in the shoots were transferred into the grains. Thus, both storage forms (Cd-S and Cd-O) contributed to the retention of Cd in the straw. Cd was mainly bound to S in nodes I and grains (Cd-S > 84%), and these organs were strongly enriched in heavy isotopes compared to straw (Delta Cd-114/110(grains/nodes-)straw = 0.66-0.72 parts per thousand) and flag leaves (Delta Cd-114/110(grains/nodes-flag leaves) = 0.49-0.52 parts per thousand). Hence, xylem to phloem transfer in the node favors heavy isotopes, and the Cd-S form may persist during the transfer of Cd from node to grain. This study highlights the importance of Cd storage forms during its journey to grain and potentially into the food chain.
Auteurs, date et publication :
Auteurs Matthias Wiggenhauser , Anne-Marie Aucour , Philippe Telouk , Hester Blommaert , Geraldine Sarret
Publication : FRONTIERS IN PLANT SCIENCE
Date : 2021
Volume : 12
Catégorie(s)
#ANR-Citation #CNRS #Lautaret #UGARésumé
This contribution focuses on scientific mediation implemented within scientific tourism initiatives in mountain areas. Three case studies in the French Alps support this research: the Jardin Alpin du Lautaret of the University of Grenoble Alpes, the CREA Mont-Blanc, a private research center organizing participatory science and student travel abroad programs and the scientific hut to hut trekking routes in the Écrins National Park, organized jointly with a scientific research project (Labex ITEM, Reflab). Scientific tourism and mediation are first defined. Next, a quantitative and qualitative approach (with interviews and participant observations) allows us to understand the impact and role of scientific mediation within tourism. Results show a high level of interest of the issue within involved publics and a close connection between experience and knowledge. It appears that it is also a tool for public awareness of the challenges of our modern societies.
Auteurs, date et publication :
Auteurs Yannick Vialette , Pascal Mao , Fabien Bourlon
Publication : Journal of Alpine Research | Revue de géographie alpine
Date : 2021
Issue : 109-2