Search

Summer mountain pastures (also called alpages) are a central element for many agro-pastoral livestock systems in the alpine region, by providing the feedstock for herds during the summer transhumance. However, vegetation phenology and productivity in mountain pastures are increasingly affected by climate hazards exacerbated by climate change, such as early snow removal, late frost events, or droughts. Difficulties can then arise to match animal demand with forage resource on alpages and, in the long term, threaten the sustainable management of these highly multifunctional socio-ecological systems. To help agro-pastoral actors adapt, an essential step is to quantify the risk of impacts on the forage resource, due to an increased occurrence or intensity of climate hazards.Exposure to climate hazards on alpages is defined locally by topographic aspects in combination with the broader influence of the regional climate. Our work therefore aimed at providing a tailored assessment of potential climate risk for the forage resource at the individual scale of each alpage in the French Alps. To this end, we developed agro-climatic indicators based on atmospheric and snow cover data accounting for geographic and topographic conditions, and applied them to a database providing unique spatially explicit information at the alpage level.For the first time, we introduce a description of agro-climatic conditions and provide a classification of agro-climatic profiles of alpages in the French Alps, ranging from low to high potential risk for the forage resource, mainly following a North-South gradient combined with altitude. We also bring insights on the evolutions of the climate risk with climate change and discuss management implications for agro-pastoral livestock systems using alpages. We finally present a web-based visualization tool that aim at communicating agro-climatic profiles and their evolution to practitioners and at assisting decision makers in understanding climate-related risks on the alpages of the French Alps.

International audience; Summer mountain pastures are complex coupled ecological and human systems. They provide vital forage for livestock during summer, and their traditional use is decisive for the maintenance of biodiversity, ecosystem services, and open landscapes, which benefit local populations and tourists. With climate change, the increased intensity and frequency of climatic hazards threaten the sustainable management of these systems. To foster climate adaptation in such complex systems, we developed a tool to assess their climate change-related vulnerability. The tool consists of a 3-step vulnerability analysis: first, of the inherent exposure of mountain pastures to climatic hazards based on their physical features; second, of vegetation sensitivity to climatic hazards and changes in practices; and third, of adaptive capacities that lie in the options for managing mountain pastures together with the farms using them. This work was carried out within the research and development network Sentinel Mountain Pastures, which addresses climate change adaptation issues on mountain pastures across the French Alps. We used a transdisciplinary approach that included participatory work with experts and interviews with stakeholders. We believe this diagnostic tool has high potential for practical application to support adaptation on summer mountain pastures, by allowing a shared integrative understanding of the complexity of mountain pasture systems by stakeholders. We hope this will provide new information for policymaking that enhances the resilience of summer mountain pasture systems.

Herbivores alter plant biodiversity (species richness) in many of the world’s ecosystems, but the magnitude and the direction of herbivore effects on biodiversity vary widely within and among ecosystems. One current theory predicts that herbivores enhance plant biodiversity at high productivity but have the opposite effect at low productivity. Yet, empirical support for the importance of site productivity as a mediator of these herbivore impacts is equivocal. Here, we synthesize data from 252 large-herbivore exclusion studies, spanning a 20-fold range in site productivity, to test an alternative hypothesis—that herbivore-induced changes in the competitive environment determine the response of plant biodiversity to herbivory irrespective of productivity. Under this hypothesis, when herbivores reduce the abundance (biomass, cover) of dominant species (for example, because the dominant plant is palatable), additional resources become available to support new species, thereby increasing biodiversity. By contrast, if herbivores promote high dominance by increasing the abundance of herbivory-resistant, unpalatable species, then resource availability for other species decreases reducing biodiversity. We show that herbivore-induced change in dominance, independent of site productivity or precipitation (a proxy for productivity), is the best predictor of herbivore effects on biodiversity in grassland and savannah sites. Given that most herbaceous ecosystems are dominated by one or a few species, altering the competitive environment via herbivores or by other means may be an effective strategy for conserving biodiversity in grasslands and savannahs globally. Fil: Koerner, Sally E.. University of North Carolina at Greensboro. Department of Biology; Estados Unidos Fil: Smith, Melinda D.. State University of Colorado - Fort Collins; Estados Unidos Fil: Burkepile, Deron E.. University of California; Estados Unidos Fil: Hanan, Niall P.. New Mexico State University; Estados Unidos Fil: Avolio, Meghan L.. University Johns Hopkins; Estados Unidos Fil: Collins, Scott L.. University of New Mexico; Estados Unidos Fil: Knapp, Alan K.. State University of Colorado - Fort Collins; Estados Unidos Fil: Lemoine, Nathan P.. State University of Colorado - Fort Collins; Estados Unidos Fil: Forrestel, Elisabeth J.. University of California at Davis; Estados Unidos Fil: Eby, Stephanie. Northeastern University; Estados Unidos Fil: Thompson, Dave I.. University of the Witwatersrand; Sudáfrica. Kruger National Park. South African Environmental Observation Network. Ndlovu Node. Scientific Services; Sudáfrica Fil: Aguado-Santacruz, Gerardo A.. Tecnológico Nacional de México; México Fil: Anderson, John P.. New Mexico State University; Estados Unidos Fil: Anderson, T. Michael. University Wake Forest; Estados Unidos Fil: Angassa, Ayana. Botswana University of Agriculture and Natural Resources; Sudáfrica. Hawassa University; Etiopía Fil: Bagchi, Sumanta. Indian Institute of Science; India Fil: Bakker, Elisabeth S.. Netherlands Institute of Ecology; Países Bajos Fil: Bastin, Gary. Alice Springs; Australia Fil: Baur, Lauren E.. University of New Mexico; Estados Unidos Fil: Beard, Karen H.. State University of Utah; Estados Unidos Fil: Beever, Erik A.. United States Geological Survey; Estados Unidos. State University of Montana; Estados Unidos Fil: Bohlen, Patrick J.. University of Central Florida; Estados Unidos Fil: Boughton, Elizabeth H.. MacArthur Agroecology Research Center; Estados Unidos Fil: Canestro, Don. University of California; Estados Unidos Fil: Cesa, Ariela. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Buenos Aires; Argentina Fil: Chaneton, Enrique Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Cheng, Jimin. Northwest A&F University; China Fil: D?Antonio, Carla M.. University of California; Estados Unidos Fil: Deleglise, Claire. Université Grenoble Alpes; Francia Fil: Dembélé, Fadiala. Institut de Formation et de Recherche Appliquee; Sudáfrica Fil: Dorrough, Josh. Ecosystem Mangement Science; Austria Fil: Eldridge, David J.. University of New South Wales; Australia Fil: Fernandez Going, Barbara. University of Texas at Austin; Estados Unidos Fil: Fernández Lugo, Silvia. Universidad de La Laguna; España Fil: Fraser, Lauchlan H.. Thompson Rivers University; Canadá Fil: Freedman, Bill. Dalhousie University Halifax; Canadá Fil: García Salgado, Gonzalo. Dalhousie University Halifax; Canadá Fil: Goheen, Jacob R.. University of Wyoming; Estados Unidos Fil: Guo, Liang. Northwest A&F University; China Fil: Husheer, Sean. New Zealand Forest Surveys; Nueva Zelanda Fil: Karembé, Moussa. Université des Sciences; Malí Fil: Knops, Johannes M. H.. Universidad de Nebraska - Lincoln; Estados Unidos Fil: Kraaij, Tineke. Nelson Mandela University; Sudáfrica Fil: Kulmatiski, Andrew. State University of Utah; Estados Unidos Fil: Kytöviita, Minna Maarit. Universidad de Jyvaskyla; Finlandia Fil: Lezama, Felipe. Universidad de la República; Uruguay Fil: Loucougaray, Gregory. Université Grenoble Alpes; Francia Fil: Loydi, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida. Universidad Nacional del Sur. Centro de Recursos Naturales Renovables de la Zona Semiárida; Argentina Fil: Milchunas, Dan G.. State University of Colorado - Fort Collins; Estados Unidos Fil: Milton, Suzanne J.. South African Environmental Observation Network; Sudáfrica Fil: Morgan, John W.. La Trobe University; Australia Fil: Moxham, Claire. Arthur Rylah Institute for Environmental Research; Australia Fil: Nehring, Kyle C.. State University of Utah; Estados Unidos Fil: Olff, Han. University of Groningen; Países Bajos Fil: Palmer, Todd M.. University of Florida; Estados Unidos Fil: Rebollo, Salvador. Universidad de Alcalá; España Fil: Riginos, Corinna. The Nature Conservancy; Estados Unidos Fil: Risch, Anita C.. Swiss Federal Institute for Forest, Snow and Landscape Research; Suiza Fil: Rueda, Marta. Consejo Superior de Investigaciones Científicas. Estación Biológica de Doñana; España Fil: Sankaran, Mahesh. National Centre For Biological Sciences; India. University of Leeds; Reino Unido Fil: Sasaki, Takehiro. Yokohama National University; Japón Fil: Schoenecker, Kathryn A.. Fort Collins Science Center and Colorado State University; Estados Unidos Fil: Schultz, Nick L.. Federation University; Australia Fil: Schütz, Martin. Swiss Federal Institute for Forest, Snow and Landscape Research; Suiza Fil: Schwabe, Angelika. Universitat Technische Darmstadt; Alemania Fil: Siebert, Frances. North West University; Sudáfrica Fil: Smit, Christian. University of Groningen; Países Bajos Fil: Stahlheber, Karen A.. University of Wisconsin Green Bay; Estados Unidos Fil: Storm, Christian. Universitat Technische Darmstadt; Alemania Fil: Strong, Dustin J.. United States Department of Agriculture. Agriculture Research Service; Estados Unidos Fil: Su, Jishuai. Northwest A&F University; China Fil: Tiruvaimozhi, Yadugiri V.. National Centre for Biological Sciences; India Fil: Tyler, Claudia. University of California; Estados Unidos Fil: Val, James. Office of Environment and Heritage; Australia Fil: Vandegehuchte, Martijn L.. University of Ghent; Bélgica. Swiss Federal Institute for Forest, Snow and Landscape Research; Suiza Fil: Veblen, Kari E.. State University of Utah; Estados Unidos Fil: Vermeire, Lance T.. United States Department of Agriculture. Agriculture Research Service; Estados Unidos Fil: Ward, David. Kent State University; Estados Unidos Fil: Wu, Jianshuang. Lhasa National Ecological Research Station; China Fil: Young, Truman P.. University of California at Davis; Estados Unidos. Mpala Research Centre; Kenia Fil: Yu, Qiang. Chinese Academy Of Agricultural Sciences; China. Institute of Agricultural Resources and Regional Planning; China Fil: Zelikova, Tamara Jane. University Of Wyoming; . University of Wyoming; Estados Unidos

Extreme drought events can dramatically impact grassland ecosystems, causing potential loss of forage production for livestock in temperate grasslands. However, knowledge of drought effects on forage production, nutritive value and plant community stability in the real context of farming management is scarce. For this purpose, the effect of a simulated summer drought was studied under two realistic management types on a semi-natural grassland in the Swiss Jura mountains. The first management ("grazing") consisted in six consecutive utilizations by animals over the growing season, representing a rotational grazing system as regionally conducted. The second management ("mowing") consisted of three harvests, corresponding to the usual mowing frequency of hay meadows. In both managements, drought caused minor short-term modifications of species composition and almost no persistent effects were observed. Besides, important short-term changes were observed in community weighted mean of several key functional traits, reflecting a strong decline in community growth during the drought followed by a partial recovery two months later. Forage yields, and to a lesser extent its nutritive value, thus declined during the drought period. Both were still affected in the following months, but had recovered in spring of the following year. Forage loss was twofold higher in the grazing management, but recovery as well as nutritive value was slightly improved in this management. The results suggest that rotational grazing can amplify negative drought impacts, compared to traditional mowing, and highlight the need to adapt such management in dry years. They also demonstrate that, even under a fairly intensive management, resilience of such mountain grasslands after one extreme drought event can be expected to be high, with no persistent changes in species and trait compositions. (C) 2015 Elsevier B.V. All rights reserved.

Question: Does long-term grazing exclusion affect spatial patterns of canopy height, plant species and traits in subalpine grassland communities? Are spatial patterns of species and traits similarly affected by grazing exclusion? Are changes in spatial patterns of species associated with changes in species abundances? Location: Subalpine grasslands, Vercors and Oisans Mountains, Alps (France). Methods: Spatial sampling of vegetation and measurements of plant traits were carried out within nine pairs of grazed and ungrazed 10 m × 10-m plots in three species-rich communities with different productivities. We estimated within-plot spatial patterns of canopy height, species and aggregated trait values by measuring the extent (or patch size) and intensity of spatial dependence with Moran's I. Abundance-weighted averages for species patch size and intensity of spatial dependence were calculated across all species per plot and across species per life form. Such measures derived from analysis of spatial dependence were considered spatial traits. Results: Response of spatial patterns to grazing exclusion was only detected in patch size, whereas intensity of spatial dependence was not affected. Changes in spatial patterns were community-dependent because spatial traits based on patch size of canopy height and species increased following grazing exclusion only in the less productive community. Unexpectedly, changes in spatial patterns of species did not support changes in spatial patterns of trait values. Abundances and patch sizes of several life forms were significantly affected by grazing exclusion. However, at the scale investigated, changes in abundance of life forms did not correspond to changes in their spatial patterns and vice versa. Conclusion: In species-rich communities, grazing alters spatial spread of species (i.e. patch size) rather than intra-specific aggregation (i.e. intensity of spatial dependence). Results revealed possible mechanisms of species spatial reorganisation that are independent of abundance variation. Therefore, it is important to consider changes in spatial patterns in addition to changes in mean values of vegetation features when assessing impacts of grazing management.

In the context of climate change, grasslands will need to be populated with forage species that tolerate drought conditions. Planting forage mixtures is a possible solution, but questions remain regarding the specific varieties to use. Recently, different experiments have shown that both intra- and interspecific diversity could have a positive effect on grassland production under climate change and, more specifically, during droughts. However, to choose which species and genotypes to include, it is necessary to define the rules of assembly. These rules could be based on the choice of certain functional traits. For instance, research suggests that differences in growth-related traits among plants within the community, namely temporal differences in growth periods, have a positive effect. Defining the traits of greatest interest and their relative importance in grassland communities is a major consideration when selecting the plant varieties to include in forage mixtures., Le changement climatique s'accompagne d'un accroissement de la fréquence des sécheresses qui menace la pérennité et la production des prairies. Il est donc nécessaire d'adapter les couverts prairiaux à des conditions hydriques limitantes. Les mélanges offrent des perspectives intéressantes mais posent des questions quant aux critères de sélection des variétésDifférentes expérimentations récentes montrent la plus-value de la diversité spécifique et intraspécifique sur la production des prairies face au changement climatique et en particulier face aux épisodes de sécheresse. Mais le choix des espèces et des génotypes composant cette diversité nécessite de définir correctement les règles d'assemblage. Ces règles pourraient être basées sur des choix de traits (caractères) des espèces et des génotypes. La diversification de traits liés à des stratégies de croissance décalées dans le temps semble avoir un effet positif, contrairement aux traits racinaires. Ainsi, le choix de traits d'intérêt et de la valeur de ces traits dans les couverts prairiaux est à considérer dans les processus de sélection variétale.

International audience; The purpose of the present investigation was to examine the ability of the simple mechanistic model published by Jouven et al. (2006a) to simulate the effects of drought on the seasonal patterns of growth in managed grasslands. Data collected in Switzerland were used for reference. Different representations of the effects of water stress on transpiration and growth were tested, but otherwise all simulations were run using the standard model configuration. Results indicated that the model was able to correctly simulate the seasonality of growth across sites and years, as well as variations in soil water availability. Overall, the effects of drought on growth were realistically reproduced by the model. This was specifically the case with respect to the exceptional conditions characterizing the summer of 2003. On the other hand, there was a tendency for overestimating the impacts during conditions of moderate water stress. Replacing the original formulation of the water stress response with alternate schemes was not sufficient to bypass the problem. This suggests that it could be worth examining ways to represent drought avoidance or enhanced drought tolerance in the model. The failure to simulate a delayed start of growth in the spring of 2011 further indicate the necessity to address in a mechanistic way spring regrowth. Concerning future model developments, we argue that the inclusion of a root/soil compartment should be addressed with high priority to be able to account for the interactions between water and nutrient cycling. (C) 2015 Elsevier B.V. All rights reserved.

Bénéficiant de précipitations régulières et abondantes, la plupart des régions de polyculture élevage en Suisse sont favorables aux cultures herbagères. Les prairies semées occupent le deuxième rang des terres assolées et l’alimentation des herbivores est constituée de plus de 75 % d’herbe. Avec les changements climatiques, on s’attend à une augmentation des événements de sécheresse, en particulier dans l’ouest du pays. Les recherches en cours visent à adapter la composition et les modes d’exploitation des mélanges à cette contrainte. Ce travail s’appuie sur la connaissance des mécanismes de résistance au stress hydrique existant au sein des communautés végétales. Les prairies et les pâturages permanents du Jura, conditionnés par des sols karstiques et une mauvaise distribution des précipitations en été, offrent un terrain privilégié pour identifier les espèces et les écotypes susceptibles de convenir pour des conditions de sécheresse. Après des phases de sélection et d’assemblage d’espèces, les mélanges les plus performants sont distribués par les maisons de semences et bénéficient d’un label de qualité octroyé par l’Association pour le développement de la culture fourragère. Au sein d’un réseau de parcelles chez des agriculteurs, Agroscope réalise des mesures visant à adapter les modes d’exploitation et à optimiser la production des prairies semées, en particulier des fourrages stockés au printemps et de l’herbe pâturée en été., 168 Adaptation des prairies semées au changement climatique With regular and abundant rainfall, most mixed farming regions in Switzerland are favorable to grassland. Sown meadows occupy the second largest arable land surface and herbivorous diet consists on more than 75 % grass. With climate change, we expect an increase in drought events, especially in the western part of the country. Ongoing research aims to adapt the composition and management of grass-legumes mixtures to this constraint. This work is based on knowledge of the mechanisms of resistance to water stress within the plant communities. Permanent meadows and pastures of the Jura, conditioned by karst soils and poor distribution of rainfall in summer, offer a privileged field to identify species and ecotypes that may be suitable for dry conditions. After breeding and assembling species, the most effective mixtures are distributed by seed firms and enjoy a quality label granted by the Swiss grassland federation. Within a network of plots among farmers, Agroscope makes measurements to adapt the management and optimize the production of leys, especially stored forage in spring and grazed grass in summer.

International audience; The fine-scale spatial structure of plant communities is a key component for understanding the dynamics in vegetation following changes in land management but needs to be assessed at an appropriate scale. We studied the response of plant diversity and spatial variability of species and trait composition to grazing vs. non-grazing (>20 years) using different grain sizes of sampling in three subalpine plant communities. Species composition, diversity, and the aggregated values of 4 leaf traits were assessed at 7 grain sizes (within quadrats ranging from 25 cm2 to 1 m2) in 9 grazed and ungrazed paired plots. Evenness and species richness showed a strongly community-dependent response as they increased following grazing exclusion in the less productive community and decreased in the two more productive ones. Although species richness was influenced by grazing exclusion at grains larger than 625 cm2, evenness was affected at the finest grain investigated (25 cm2). In contrast, spatial variability of species composition was similarly affected across the three plant communities and increased following grazing exclusion from the 100 cm2-grain size. Grazing led to a sharper decline of spatial variability with increasing quadrat size revealing that the grain of spatial heterogeneity was finer in grazed plots relative to that in ungrazed ones. The response of spatial variability of aggregated trait values was less consistent, possibly because of functional redundancy between species. However, grazing exclusion increased the spatial variability of the aggregated values of leaf dry matter content and leaf carbon and nitrogen content. We suggest that spatial variability of species composition could be useful for detecting within-community changes in response to grazing management as it was detectable using a very fine scale of sampling and responded similarly to grazing treatment in every community. Such spatial variability in species composition can induce spatial organization of plant traits that may be important for community functioning.

A fertilisation experiment was started in the French Alps on an acid grassland at 2000 m in 1989 where lime as calcium carbonate (“liming”) and Thomas Slag enriched by potassium chloride (“fertilisation”) were added in a random block design until 1992. Since then, no further amendments were applied. Fifteen years after the last application, we revisited the experiment and observed that soil pH was still significantly higher on limed plots, while nitrogen (N) concentrations were lower. On fertilised plots, soil carbon (C) and N concentrations were lower compared to unfertilised plots. However, litter quality (C and N concentrations, near infrared spectroscopy [NIRS] data) was similar for both treatments. Vegetation composition, but not species richness, nor Shannon–Wiener or evenness differed between limed and unlimed plots, and fertilised and unfertilised plots. Liming explained about 18% and fertilisation about 6% of the variability of the vegetation composition. These changes in the vegetation composition are probably due to lower abundances of former dominant grass species and to an increase in generalist grasses. However, these changes did not influence the total above-ground productivity, which was similar for all treatments. Tissue N and C concentrations and NIRS data indicated a changed chemical composition of the biomass which persisted during time. We conclude that the 3 years of fertilisation and liming did substantially influence the vegetation composition at our site and lead to an increase in the agricultural value of the grassland. These changes are long-lasting as they changed key features of the functioning in the soil of grasslands ecosystems. From an ecological point of view, specialised vegetation was replaced by generalist species leading to a trivialisation of the vegetation.

Peut-on évaluer la diversité floristique d’une prairie à partir d’observations simples sur le terrain ? L’élaboration d’une démarche de diagnostic adaptée à des non-spécialistes de la flore nécessite de clarifier les différentes composantes de la biodiversité d’une parcelle. La démarche de diagnostic de la diversité floristique proposée ici s’applique à l’échelle du faciès de végétation et distingue deux composantes : le fonds prairial et les éléments paysagers. Le diagnostic du fonds prairial, qui fait référence à une typologie régionale de la biodiversité, est basé sur des comptages simplifiés d’espèces, de couleurs et de formes de fleurs, sans reconnaissance botanique précise. Le diagnostic de la diversité paysagère utilise le nombre et les différents types d’éléments paysagers présents sur le faciès. Les deux diagnostics différencient chacun cinq niveaux de diversité et se déclinent pour quatre catégories de prairies auvergnates définies par leur usage et leur milieu physique. A l’issue des diagnostics, des pistes d’interprétation sont suggérées à l’utilisateur en considérant que la diversité floristique d’une prairie est fonction du milieu et des pratiques agricoles mais aussi de la diversité paysagère., An easy method for the evaluation of a pasture's floristic diversity was developed in Auvergne, making it possible for a non-botanist to assess its bio-diversity. The aim was to make people working in the sphere of agriculture aware of the floristic diversity of ordinary grasslands. It relies on qualitative and quantitative observations made on pasture plots where different vegetation facies are distinguished. In our view, the floristic diversity of a pasture depends on the swards composition, but is also enhanced by the diversity of the landscape; we therefore assessed both these factors in our approach. The selection of indicators of the bio-diversity was based on the study of 126 vegetation facies, evenly distributed over four classes of pastures differing in their type of management (grazing or mowing) and in their environment (upland, lowland with a rich or a poor soil). We made for each facies both a botanical inventory and a simplified description of the vegetation, the fauna and the landscape features. To identify indicators, we looked for linear correlations between bio-diversity, as calculated from the inventories, and the data from the simplified descriptions. Nine indicators were found for the assessment of the herbaceous vegetation's diversity : total and average number of species with a different physiognomy, colours, forms of flowers... and one indicator was based on the diversity in butterflies. These indicators can be obtained on the field by a simplified counting, inside 10 circles of 1 m diameter for the flora, and along a transect for the fauna. They are more or less the same for each class, the main difference being in the identification thresholds, which are adapted to the Auvergne grasslands. The indicators of a given class were gathered in a single grid, in which five levels of bio-diversity were diagnosed. In addition to the assessment of each facies, we also propose a measure of the differences among the facies of the same plot. This point concerns only the number of species of different physiognomy. Separate grids were also designed for the assessment of the diversity of landscapes, as we did not find any correlation between the latter and the diversity of the herbaceous vegetation. Two indicators were used : the number and the types of landscape elements. The grids have also indication thresholds for each class and for the five levels of bio-diversity. Finally, various ways of interpreting the results are proposed to the users of the method, based on the hypothesis that the diversity depends not only on management and environment, but also on the landscapes. The way landscape diversity can increase and maintain the floristic diversity of grasslands is also emphasized.

Drought events can strongly affect ecosystem functioning by modifying relationship between plants, microbes and soil chemistry, with consequent impacts on nutrient cycling. However, the potential impacts of a soil moisture reduction on the nitrogen (N) and phosphorus (P) cycling in grasslands remain poorly understood, especially in regard to. forage production., To fill this knowledge gap, a drought experiment was carried out using rainout shelters in two permanent grasslands, characterized by similar vegetation communities but contrasted soil nutrient limitations. Drought treatments were applied during two months, either when plant growth was highest (Early-season drought) or after the peak of biomass production (Late-season drought). Dry matter production, forage N status (NNI) and P content as well as N and P contents in microbial biomass and soil were determined., Both early and late-season drought significantly reduced soil moisture during the vegetation growth period. Forage yield was also reduced by drought, but only when it occurred late in the season. Using a structural equation model, we showed that soil moisture reduction had a direct effect on forage N status, suggesting that water shortage induced lower transpiration and water fluxes. Soil moisture reduction also affected forage P by reducing the availability of soil P. However, other mechanisms played a larger role and were site-specific. At the more fertile site, reduction in soil moisture directly impaired forage P, suggesting that water stress mainly resulted in lower diffusion rates to roots, while at the less fertile site, an indirect reduction of forage P through a pathway implying microbes (decrease in microbial P) was detected., Our results suggest that the two grasslands suffered mainly from water shortage per se, but also from drought induced nutrient deficiency (mainly P), which amplified yield losses and further decreased forage quality. Overall, our findings emphasize the need for further research on the plant-soil-microbe system functioning, in order to secure a sustainable and resilient forage production in the context of climate change.

Spatial heterogeneity is now widely recognized as a key component for diversity and functioning of grazed plant communities but is still often neglected for the analysis of the impact of different pastoral uses. Subalpine grassland communities are species-rich habitats of high conservative, economic and cultural values. In large regions of the Alps, these grasslands have experienced a decrease in pastoral use and even abandonment, in the last decades. The investigation of the spatial response of these communities to these changes in pastoral use can be a key issue to better predict ecological and agronomical consequences of such changes. The main objective of this study is to identify the spatial organization of species and traits of subalpine grassland communities in contrasted situations of pastoral use: traditional grazing and abandonment (~20 years), to understand grazing-induced mechanisms influencing this spatial organisation and to explore the potential consequences of the spatial organization of species and traits on the pastoral value of these grasslands. This work reveals that grazing cessation leads to strong modifications of the spatial organization of both species and aerial functional traits. The spatial variability of these components are systematically increased at different spatial scales following grazing abandonment which revealed a coarser grain of spatial heterogeneity in relation to a lower species and trait local coexistence and to a stronger functional convergence than under the fine-scale diversifying action of traditional grazing. Besides, despite we also observed a response in terms of spatial patterns, this one is not systematic which indicates the absence of strongly structuring action of grazing that especially acts on variability. An in situ experiment indeed showed that spatial pattern creation in response to selective defoliation is limited in these relatively low productive grassland communities. In addition, we also evidenced that pastoral values (forage quality) exhibited poor spatial heterogeneity in these grasslands in relation to the fine grained spatial variability of species and plant traits in grazed areas. These results suggest the expression of a low selective grazing promoted by sheep herds grazing and low productivity of subalpine communities and the predominance of the abiotic filter in the determination of spatial assemblages of species and traits. Pronounced spatial patterns in response to grazing may be expected in more productive environments where competitive interactions between species are more intense and species regrowth promoted. Therefore, the consideration of the spatial variability in addition to the mean response of community components appeared to be essential in the analysis of subalpine grassland community dynamics in response to land use changes. One of the strong perspective following this work is to analyse the association between spatial responses of vegetation components and those of belowground components (root traits, microbial diversity...).; L'hétérogénéité spatiale est aujourd'hui reconnue comme un facteur primordial pour la diversité et le fonctionnement des écosystèmes prairiaux pâturés, mais reste souvent négligée dans l'analyse des impacts de différents modes d'utilisation pastorale. Les communautés prairiales subalpines sont des milieux hautement diversifiés, au coeur d'enjeux écologiques, socio-économiques et culturels. Elles sont soumises actuellement à des changements d'usage, notamment l'extensification, qui peut parfois aller jusqu'à l'abandon. Comprendre la réponse spatiale de ces communautés à ces changements d'utilisation des terres peut constituer unenjeu important pour mieux prédire les conséquences en termes de valeurs écologiques et agronomiques.L'objectif général de ce travail est d'identifier les mécanismes de réponse de l'organisation spatiale des composantes spécifiques et fonctionnelles de trois communautés prairiales subalpines en réponse à des situations contrastées d'usage : pâturage traditionnel et abandon sur le moyen terme (20 ans). Les conséquences potentielles de modifications d'organisation spatiale du couvert sur les valeurs d'usage de ces milieux sont ensuite explorées. Ce travail révèle que l'arrêt du pâturage conduit à de profondes modifications de l'organisation spatiale à la fois des espèces et de traits fonctionnels aériens. La variabilité spatiale de ces composantes est augmentée dès des échelles très fines (100 cm²) en situation d'abandon et ce jusque des échelles plus larges ce qui révèle un grain plus grossier de l'hétérogénéité spatiale des communautés. Ceci est relié à une action positive du pâturage sur la coexistence des espèces et des valeurs de traits dès les échelles fines, limitant la convergence fonctionnelle observée dans ces communautés et ainsi favorisant un grain très fin d'hétérogénéité. D'autre part, nous observons que les patrons spatiaux des espèces peuvent être renforcés en situation d'abandon, suggérant une plus grande extension spatiale des espèces, mais cette réponse est faible, ce qui indique l'absence d'une action structurante forte du pâturage qui agit donc surtout sur la variabilité. Une expérimentation in situ a permis de montrer que seuls des patrons spatiaux de défoliation assez marqués et entrainant la création de contrastes abiotiques forts entre zones perturbées et non perturbées pouvaient entrainer une modification des patrons spatiaux des espèces. La formation de patrons spatiaux marqués semble donc limitée dans ces prairies relativement peu productives. Ces résultats suggèrent ainsi l'expression d'un pâturage peu sélectif exprimé par les troupeaux ovins, et la prépondérance du filtre abiotique dans les assemblages, sur ces milieux faiblement productifs. La formation de patchs très contrastés et très structurés spatialement est plus probable dans des milieux très productifs où la compétition est plus intense et dans lesquels le pâturage pourrait cette fois conduire à plus d'hétérogénéité. Les différents modèles d'organisation spatiale que nous avons observés sont susceptibles d'influencer l'expression dans l'espace des composantes de la valeur d'usage fourragère de ces prairies. Notamment, en situation pâturée, les assemblages plus diversifiés dès des échelles fines permettent de capter très rapidement la valeur d'usage fourragère moyenne de la prairie. La prise en compte de la variabilité autour de la réponse moyenne des communautés se révèle donc un élément essentiel pour analyser l'évolution des prairies subalpines en réponse au changement d'usages pastoraux. Une des perspectives fortes suite à ce travail est l'analyse de la synergie entre la réponse de l'hétérogénéité spatiale du couvert et celle du compartiment souterrain (traits racinaires, mycorhization, diversité microbienne).