Beaucoup de parasites à cycle complexe ont développé la capacité d’altérer le phénotype de leurs hôtes. Il est reconnu que ces modifications d’apparence ou de comportement sont responsables d’une augmentation de la probabilité de prédation de l’hôte intermédiaire par l’hôte définitif. Ce phénomène de manipulation parasitaire peut avoir de nombreuses conséquences à l’échelle de l’écosystème, modifiant les interactions entre les populations d’hôtes et bouleversant leur rôle écologique. Cependant, les parasites manipulateurs sont peu étudiés sur le plan écologique, et l’effet de l’environnement sur l’interaction entre ces parasites et leurs hôtes, notamment en termes de manipulation, est encore largement inconnu. Au cours de cette thèse, j’ai étudié l’effet de l’environnement sur l’interaction entre les gammares, des crustacés amphipodes de grande importance écologique dans les rivières, et leurs parasites acanthocéphales. Mes travaux ont montré que les conditions de ressources alimentaires et de température subies par les hôtes durant le développement des parasites influençaient plusieurs paramètres de l’infection mais n’affectaient pas la manipulation en termes d’utilisation de refuges. Toutefois, tandis que la géotaxie ne dépendait pas non plus de la température, l’impact des parasites sur la phototaxie des gammares était plus fort à haute température. De plus, mes travaux ont montré que l’impact conjoint de la température et des parasites pouvait modifier le rôle détritivore des gammares. Dans un contexte de changements globaux, ces travaux permettent de mieux comprendre l’importance de l’impact des parasites sur leurs hôtes et en prévoir les conséquences écologiques. / Many parasites with complex life cycle have developed the ability to alter the phenotype of their hosts. It is recognized that such changes in appearance and behavior are responsible for an increase in the probability of predation of their intermediate hosts by their definitive hosts. This phenomenon of parasite manipulation can have numerous consequences at the scale of the ecosystem, modifying the interactions between host populations and altering their ecological role. However, manipulative parasites received little attention from an ecological point of view. Thus, the effect of the environment on the interaction between these parasites and their hosts, in particular in terms of manipulation, is largely unknown. In this thesis, I studied the effect of the environment on the interaction between gammarids – ecologically important crustacean amphipods in rivers – and their acanthocephalan parasites. My experiments showed that the conditions of food resources and temperature experienced by gammarids during the development of their parasites influenced several infection parameters, but did not affect behavioral manipulation in terms of use of refuges. Nevertheless, while the geotaxis was not either effected by temperature, the impact of parasites on gammarids phototaxis was stronger at high temperature. Moreover, my studies showed that the cumulative effect of temperature and parasitism could alter the shredder role of gammarids. In a context of global changes, this work provides a better understanding of the importance of the impact of parasites on their hosts, allowing to make previsions on their subsequent ecological consequences.

The demise of reef-building corals potentially lies on the horizon, given ongoing climate change amid other anthropogenic environmental stressors. If corals cannot acclimatize or adapt to changing conditions, dramatic declines in the extent and health of the living reefs are expected within the next half century. The primary and proximal global threat to corals is climate change. Reef-building corals are dependent upon a nutritional symbiosis with photosynthetic dinoflagellates belonging to the group Symbiodinium. The symbiosis between the cnidarian host and algal partner is a stress-sensitive relationship; temperatures just 1°C above normal thermal maxima can result in the breakdown of the symbiosis, resulting in coral bleaching (the loss of Symbiodinium and/or associated photopigments) and ultimately, colony death. As ocean temperatures continue to rise, corals will either acclimatize or adapt to changing conditions, or will perish. By experimentally preconditioning the coral Acropora millepora via sublethal heat treatment, the coral acquired thermal tolerance, resisting bleaching during subsequent hyperthermal stress. The complex nature of the coral holobiont translates to multiple possible explanations for acclimatization: acquired thermal tolerance could potentially originate from the host itself, the Symbiodinium, or from the bacterial community associated with the coral. By examining the type of in hospite Symbiodinium and the bacterial community prior acclimation and after thermal challenge, it is shown that short-term acclimatization is not due to a distinct change in the dinoflagellate or prokaryote community. Though the microbial partnerships remain without considerable flux in preconditioned corals, the host transcriptome is dynamic. One dominant pattern was the apparent tuning of gene expression observed between preconditioned and non-preconditioned treatments, showing a modulated transcriptomic response to stress. Additionally several genes were upregulated in association with thermal tolerance, including antiapoptotic genes, lectins, and oxidative stress response genes. Upstream of two of these thermal tolerance genes, inhibitor of NFκB and mannose-binding lectin, DNA polymorphisms were identified which vary significantly between the northern and southern Great Barrier Reef. The impact of these mutations in putative promoter regions remains to be seen, but variation across thermally-disparate geography serves to generate hypotheses regarding the role of regulatory element evolution in a coral adaptation context.

Comprendre l'impact des changements globaux sur la dynamique des populations animales représente un enjeu sociétal majeur pour favoriser le maintien de la biodiversité. Le milieu de montagne permet de travailler sur de petites communautés d'herbivores à forte valeur patrimoniale, cynégétique ou emblématique. Il s'agit en effet d'un milieu en pleine mutation depuis plusieurs décennies à cause de la déprise agricole et du développement récent de l'écotourisme et des activités humaines récréatives. Ces changements ont conduit à l'aménagement et à la modification de nombreux habitats. Par ailleurs, le milieu montagnard est fortement saisonnier et semble particulièrement sensible au changement global actuel. L'effet des changements climatiques récents reste pourtant encore mal évalué. L'augmentation de la température moyenne des 30 dernières années est susceptible de modifier la dynamique des ressources disponibles pour les herbivores, mais aussi la chronologie d'accès à ces ressources, notamment à travers la diminution du couvert neigeux en hiver et sa fonte accélérée au printemps. L'objectif de cette thèse est donc d'évaluer l'effet des variables climatiques locales et globales sur les traits d'histoire de vie des herbivores de montagne, principalement à travers l'exemple de la marmotte alpine (Marmota marmota), un mammifère social et hibernant. Ce travail a permis de mieux appréhender les mécanismes d'action du climat sur la démographie des espèces de montagne, mais aussi de mettre en évidence le déclin continu d'une population de marmottes Alpines, dans les Alpes françaises, depuis les années 1990. Il pourrait ainsi servir de base pour établir les règles de gestion de cette espèce emblématique, et maintenir une forte biodiversité dans les écosystèmes de montagne / Natural systems responses to global change are of major concern for human societies to maintain high species diversity. Mountains, and particularly alpine climate, offer the opportunity to work with small mammalian herbivore communities, with a strong patrimonial, hunting or emblematic value. Since the last decades, mountains are indeed facing major changes, due to the loss of agricultural lands and the recent development of ecotourism and human recreational activities. Those changes have modified natural habitats and their management planning a lot. Additionally, alpine climate is strongly seasonal and seem particularly sensitive to the actual global change. However, the recent impact of climate change on species and natural habitats is still under-evaluated. The increase in the average global temperature of the last 30 years could modify resources dynamics for a wide variety of species. The availability of resources, but also the chronology of access to these resources, for example through the decrease of snow cover in winter and its accelerated thaw in spring, is of crucial importance for herbivore populations. The aim of this thesis is therefore to evaluate the potential role of local and global climatic factors on life history traits of mountain herbivores, mostly through the example of the alpine marmot (Marmota marmota), a social and hibernating mammal. This work allowed us to better understand the mechanisms underlying the effect of climate change on mountain mammal population dynamics and demography; but also to highlight the continuous decline of an alpine marmot population in the French Alps since the 90s. Conservation strategies and practices could thus arise from this work, in order to maintain biodiversity in mountainous ecosystems

Water resources in the semi-arid to arid areas of Central Asia are often limited by low precipitation, and hence vulnerable to impacts of global change, i.e. socio-economic development and climate change. Both, socio-economic development and climate change are very likely causing significant changes as water resources are affected by two main effects: Firstly, growing population and industrial activities in the region raise the pressure on water resources due to increasing water abstractions. Secondly, air temperature in the region has been rising in the past far above global average and it is expected to increase further, which will lead to changes in runoff generation and therefore water availability. Increasing temperature as well as increasing water abstractions will affect water quantity and consequently water quality as a result of higher pollution intake or reduction in dilution capacity. Thus, it is of crucial importance to analyse and assess the state of current and future water resources to implement sustainable water management as the above mentioned effects very likely causing significant changes of water resources. Within the last years, the number of scientific research studies using large-scale models to simulate water availability and water use has increased substantially. Several new datasets from earth observations and new or improved models have been published (Werth et al. 2009; Werth and Güntner 2010; van Beek et al. 2011). Nevertheless, those studies focussed on water quantity and did not take into account impacts on water quality induced by global change although changes in water quality affecting aquatic ecosystems and species. Furthermore, spatially explicit large-scale modelling studies have not been carried out for Mongolia and Central Asia to get a comprehensive overview and assessment. To address this research gap, the large-scale water resource modelling framework WaterGAP 3 was applied to Central Asia with a focus on Mongolia to simulate impacts on current and future water resources. WaterGAP 3 consists of hydrology, water use and water quality sub-models in order to simulate current and future water quantity and quality.

Freoner är ett stort problem för människors hälsa och andra levande organismer på grund av freons nedbrytande effekt på ozonlagret. Forskare upptäckte problemen med freon och varningar från forskningssamhället uppmärksammades av huvudproducenter av freoner samt av allmänheten då påverkan på ozonlagret ökar risken för bland annat hudcancer. Montrealprotokollet blev starten på de internationella konventioner som senare implementerades. Dock finns det ett flertal växthusgaser idag som också är ozonnedbrytande som inte ingår under någon konvention. Den globala uppvärmningen är en effekt av ökad mängd växthusgaser i atmosfären och detta kräver ett internationellt samarbete för att kunna lösas. I Montrealprotokollet ingår ODS (Ozonnedbrytande substanser) där freon innefattas. Syftet för denna studie var att belysa vad som gjorde freonförbudet lyckat, och om det är möjligt att tillämpa dessa nyckelaspekter på en av dagens växthusgaser som liknar freon. NO2 valdes för att begränsa urvalet.  Denna litteraturstudie använde sig av databaser för att finna relevanta källor genom nyckelord, därefter valdes källor utefter relevansen till frågeställningen samt att de skulle vara opartiska. Resultatet av denna studie pekar på svårigheterna med att implementera det arbetssätt som ledde till freonförbudet, på gasen NO2, som hade liknande egenskaper som freon. Detta på grund av att dagens växthusgaser är svåra att reglera utsläppet på. Växthusgaser finns globalt och det finns inte en tydlig utsläppskälla som går att reglera samt hitta ett substitut för. Korrelationen till den mänskliga hälsan och miljön, är också en nyckelaspekt till varför förbudet mot freoner var lyckat. / Freons became a large problem for society when scientists proved the ozone depletion effect, which proved to have a negative effect on human health and other living organisms. The ozone layer which was known for its protective qualities against UV-radiation lead to a new field of study, ODS (ozone depleting substances). The warnings from the scientist society and the public were eminent. The research pointed out, among other things, the increasing risk of skin cancer. This forced fast new research and several international conventions to save the ozone layer. But there are several greenhouse gases, which also have the degrading effect on the ozone layer, that are in use today. This require an international cooperation for the environment to be able to solve. The aim of this report was to illustrate why the freon ban was a success and why it would be hard to implement these key concepts on a similar present greenhouse gas. In this literature study, several databases were used to find relevant sources for the main questions and unbiased for the topic. The key aspects in this study is presented in the result section. The result pointed at the difficulties with implementing the same way of work as at the freon ban to the greenhouse gas ban. Different aspects play an important role in this, but one of the greater difficulties for implementing this is to pinpoint it to a specific source for gas emissions and finding a sustainable solution for it. The correlation between human health and environmental issues was a key factor in the freon ban.

Global change holds complex consequences for Earth's ecosystems, with global warming simultaneously affecting multiple aspects including individual physiology, population dynamics and ecosystem processes. In a recent study on stream decomposition under global warming, Réveillon, et al. (2022) combined individual-level laboratory assessments of metabolic rates and leaf-litter ingestion with experimentally parameterized consumer-resource models, designed to reveal how stream-detritivore populations respond to combined impacts of warming and declining body size. Their findings of reduced energetic efficiency, weakened detritivore populations and reduced decomposition in warmed streams expand our understanding of how global change mechanistically links changes from the individual to the ecosystem level.

<p>The purpose of this thesis is to analyse interactions between freshwater flows, terrestrial ecosystems and human well-being. Freshwater management and policy has mainly focused on the liquid water part (surface and ground water run off) of the hydrological cycle including aquatic ecosystems. Although of great significance, this thesis shows that such a focus will not be sufficient for coping with freshwater related social-ecological vulnerability. The thesis illustrates that the terrestrial component of the hydrological cycle, reflected in vapour flows (or evapotranspiration), serves multiple functions in the human life-support system. A broader understanding of the interactions between terrestrial systems and freshwater flows is particularly important in light of present widespread land cover change in terrestrial ecosystems. </p><p>The water vapour flows from continental ecosystems were quantified at a global scale in Paper I of the thesis. It was estimated that in order to sustain the majority of global terrestrial ecosystem services on which humanity depends, an annual water vapour flow of 63 000 km3/yr is needed, including 6800 km3/yr for crop production. In comparison, the annual human withdrawal of liquid water amounts to roughly 4000 km3/yr. A potential conflict between freshwater for future food production and for terrestrial ecosystem services was identified. </p><p>Human redistribution of water vapour flows as a consequence of long-term land cover change was addressed at both continental (Australia) (Paper II) and global scales (Paper III). It was estimated that the annual vapour flow had decreased by 10% in Australia during the last 200 years. This is due to a decrease in woody vegetation for agricultural production. The reduction in vapour flows has caused severe problems with salinity of soils and rivers. The human-induced alteration of vapour flows was estimated at more than 15 times the volume of human-induced change in liquid water (Paper II). </p>

La compréhension des interactions qui associent les plantes et les microorganismes du sol est une étape incontournable pour une gestion durable de nos écosystèmes notamment en agriculture. Parmi les services écosystémiques résultant de leurs interactions, on peut citer la productivité végétale répondant, en partie, aux besoins alimentaires de la population mondiale et la régulation des cycles biogéochimiques. Les services écosystémiques, qui émergent de telles interactions, reposent sur des liens trophiques pouvant être représentés par un compromis entre coûts et bénéfices pour les différents partenaires de l’interaction. Les plantes, organismes autotrophes ou producteurs primaires, sont des organismes clefs qui font entrer le carbone dans l’écosystème, via la photosynthèse. Une partie de ce carbone est libérée sous forme de molécules plus ou moins complexes, au niveau de leurs racines, par le processus de rhizodéposition. Ces composés servent de molécules signal et de nutriments pour les microorganismes du sol, essentiellement hétérotrophes, c’est l’effet rhizosphère. Ce processus est donc coûteux pour la plante mais bénéfique aux microorganismes. Les microorganismes contribuent, en retour, à la nutrition et la santé des plantes ce qui est coûteux mais leur assure une source bénéfique de nutriments. Ces échanges trophiques reposent néanmoins sur un équilibre dépendant des conditions biotiques et abiotiques qui affectent chaque partenaire. La biodiversité microbienne, de par la multitude d’interactions au sein des communautés microbiennes, est un facteur biotique important. Parmi les facteurs abiotiques, le contexte environnemental actuel, soumis aux changements globaux, est propice à une déstabilisation de ces interactions. L’objectif de ce travail est donc de comprendre comment vont varier les coûts et bénéfices, pour chaque partenaire, suite à des modifications de l’environnement affectant l’un ou l’autre. L’intérêt étant de savoir si les bénéfices pour les plantes et les microorganismes, qui permettent les services écosystémiques, seront affectés. Pour répondre à cet objectif, un cadre d’interaction plantes-microbes simplifié a été choisi et une déstabilisation, au niveau de la plante, a été effectuée au moyen d’une augmentation en CO2 atmosphérique. L’interaction entre Medicago truncatula et Pseudomonas fluorescens a ainsi été étudiée. Les interactions ont ensuite été complexifiées en utilisant une communauté microbienne dans son ensemble et, cette fois, la modification a été appliquée au compartiment microbien soumis à une dilution de sa diversité. L’effet du gradient de diversité microbienne obtenu a été mesuré sur la croissance et la reproduction de trois espèces végétales modèles (Medicago truncatula, Brachypodium distachyon et Arabidopsis thaliana). Enfin, l’analyse s’est focalisée sur la communauté microbienne en identifiant la part active, c'est-à-dire les microorganismes qui utilisent les composés libérés par la plante. Ces microorganismes, qui interagissent réellement avec la plante, ont été détectés grâce à une analyse ADN SIP utilisant l’isotope 13C. Les principaux résultats observés, que la modification affecte l’un ou l’autre des partenaires, sont une déstabilisation des coûts et bénéfices. La première étude montre une variation temporaire des interactions en faveur de la plante en condition de CO2 augmenté. Dans le cas d’une dilution de la diversité microbienne, les coûts pour la plante sont conditionnés par la dépendance naturelle des plantes vis-à-vis des microorganismes symbiotiques qui interagissent avec le reste de la communauté. Cela est confirmé par la dernière expérimentation qui met en évidence les interactions microbes-microbes qui conditionnent la structure de la communauté microbienne interagissant avec la plante. [...] / Understanding the interactions that bind plants and soil microorganisms is an essential step for the sustainable management of ecosystems, especially in agriculture. The ecosystem services resulting from such interactions include plant productivity which responds, in part, to the food requirements of the world's population and the regulation of biogeochemical cycles. These ecosystem services depend on trophic links between the two partners in the interaction and can be represented by a tradeoff between the costs and benefits for each partner. Plants, being autotrophic organisms or primary producers, are key organisms which introduce carbon into the ecosystem, through photosynthesis. Part of this carbon is released as more or less complex molecules at the roots level, thanks to the rhizodeposition process. These compounds act as signal molecules and nutrients for soil microorganisms, which are mainly heterotrophic, in the so-called rhizosphere effect. This process is costly for the plant but beneficial to the microorganisms. In return, microorganisms contribute to plant nutrition and health, which is costly but provides them with a beneficial source of nutrients. These trophic exchanges, however, are based on a balance which depends on the biotic and abiotic conditions that affect each partner. Microbial biodiversity, through the multitude of interactions occurring within microbial communities, is a significant biotic factor. Among the abiotic factors, the current environmental context, subject to global change, is tending to destabilize these interactions. The objective of this work was to understand how environmental changes affect the costs and benefits for each partner by applying changes to one or the other, the aim being to determine whether these changes would affect the benefits for plants and microorganisms that provide ecosystem services. To achieve this objective, a simplified framework for plants-microbes interaction was first chosen. Destabilization at the plant level was carried out by increasing the atmospheric CO2 and studying the interaction between Medicago truncatula and Pseudomonas fluorescens. The interactions were then made more complex by using a whole microbial community but this time the change was applied to the microbial compartment by subjecting it to diversity dilution. The effect of the resulting microbial diversity gradient was measured on the growth and reproduction of three model plant species (Medicago truncatula, Brachypodium distachyon and Arabidopsis thaliana). Finally, the microbial community was subjected to a DNA SIP analysis, with the isotope 13C, to identify the active portion, i.e., those microorganisms which really interacted with the plant and used compounds released by it. The main result, when the change affected one or other partner, was a destabilization of the costs and benefits. The first study showed a transient variation in the interactions in favour of the plant under increased CO2 conditions. In the case of a dilution of microbial diversity, the costs for the plant are conditioned by the natural dependency of plants on symbiotic microorganisms that interact with the rest of the community. This was confirmed by the last experiment that highlighted the between-microbes interactions which determined the composition of the microbial community that interacted with the plant. This work has helped to clarify the functioning of relationships between plants and soil microbes and the factors that contribute to their maintenance which is essential to the functioning of ecosystems. These studies also provide ways for predicting the impacts of global change on ecosystems. The conservation or restoration of ecosystem services is essential for human well-being

Projektionen der Effekte von Umweltveränderungen auf sozio-ökologische Systeme sind ein fester Bestandteil der Nachhaltigkeitsforschung. Solche Projektionen beruhen auf Modellen und Modellketten. In jedem Modellierungsschritt werden modelspezifische Unsicherheiten bezüglich Parameterwerten, Eingabedaten und Modelstruktur akkumuliert und führen zu einer Kaskade der Unsicherheiten. Ziel dieser Dissertation ist es, die Kaskade der Unsicherheiten der Wirkungen von Umweltveränderungen am Beispiel der Waldökosystemmodellierung zu behandeln. Dies führt zu zwei übergreifenden Forschungsfragen: 1. Wie beeinflussen unterschiedliche Typen von Unsicherheiten die Projektionen der Wirkungen sich verändernder Umweltbedingungen auf Waldökosysteme? 2. Gibt es einen übergeordneten Rahmen für nachhaltiges Ressourcenmanagement in sozio-ökologischen Systemen, in den Unsicherheiten eingebettet werden können? Diese Dissertation zeigt, dass die Produktivität von Wäldern unter Bedingungen des Klimawandels in kühleren und feuchteren Regionen zunehmen und in wärmeren und trockeneren abnehmen kann. Diese Ergebnisse sind qualitativ konsistent über eine Vielzahl von Modellstrukturen, Klimaszenarien und Modelparameter, die jedoch quantitativ zu nennenswerten Unsicherheiten in Projektionen führen. Diese Arbeit zeigt, dass es Methoden gibt, um bestimmte Unsicherheiten einzuschätzen, aber auch, dass viele Klimawirkungsstudien die Wirkung von Veränderungen im Mittelwert von Klimavariablen betrachten und nicht die von Extremwerten. Außerdem zeigt diese Arbeit, dass adaptive, sektorenübergreifende Strategien für ein nachhaltiges Ressourcenmanagement existieren, die mit Unsicherheiten von Klimawirkungen umgehen können und nachhaltige, regionale Entwicklungen fördern. Die Kaskade der Unsicherheiten ist eine zentrale Herausforderung für nachhaltiges Ressourcenmanagement. Eine systematischere Behandlung von Unsicherheiten ermöglicht robuste Projektionen der Wirkungen sich verändernder Umweltbedingungen. / Projecting the effects of environmental change on social-ecological systems is a crucial component of sustainability science. Such projections rely on models and modeling chains. At each modeling step, model-specific uncertainties about parameter values, input data or structure accumulate and lead to a cascade of uncertainty. The aim of this thesis is to explore the cascade of uncertainties in responses to environmental change in a structured way at the example of forest ecosystem modeling. This leads to two overarching research questions: 1. How do different types of uncertainties affect projections of the effects of environmental change on forest ecosystems? 2. What is the general framework of sustainable natural resource management in coupled social-ecological systems in which uncertainties need to be integrated? This thesis shows that forest productivity under climate change may increase in cool and wet regions and decrease in already warm and dry regions. These findings are robust despite large differences in model structure, climate change scenarios and model parameters that induce considerable uncertainty into future projections. It also stresses that there are methods available to assess uncertainties but also that many climate change impact studies have focused on testing the response of plants to changes in mean climate rather than climatic extremes. Finally, this thesis shows that adaptive, cross-sectoral natural resource management strategies exist that accommodate uncertain impacts of environmental and societal change and foster sustainable regional development. I conclude that the cascade of uncertainty challenges sustainable natural resource management and that a more systematic treatment of uncertainties is strongly needed to generate robust projections of the impacts of environmental change. The findings of this thesis provide a general framework in which both modelers and decision-makers can integrate model results and assess their robustness.

Les écosystèmes prairiaux alpestres sont supposés être très sensibles au changement climatique. Cependant leur long historique fait de variation du climat et de multiples siècles d'utilisation des terres peut avoir sélectionné des mécanismes de résilience écologique à la variabilité climatique et aux stress climatiques extrêmes. Nous avons utilisé un dispositif expérimental de grande envergure pour explorer les mécanismes de réponses des prairies subalpines à une combinaison d’extrêmes météorologiques hivernaux (fonte des neiges précoce) et estivaux (sécheresse) en fonction de la composition fonctionnelle des communautés végétales et des pratiques de gestion des terres. La composition fonctionnelle des prairies a été manipulée à l’aide d’assemblages de trois espèces de poacées en différentes abondances relatives, représentant in fine un gradient d’utilisation des ressources allant de la conservation à l'exploitation. De manière générale, la composition fonctionnelle a été le principal déterminant de tous les paramètres observés pour la performance individuelle des plantes, les réponses intraspécifiques des plantes, la décomposition de la litière et les processus de recyclage de l'azote. Le fonctionnement des écosystèmes prairiaux dominés par des plantes conservatrices a été remarquablement résistant aux traitements climatiques extrêmes, tandis que les écosystèmes prairiaux dominés par des plantes plus exploitatrices ont été plus résilients. Les pratiques de gestion ont modulé ces réponses et plus particulièrement dans le cas des communautés exploitatrices. Les allocations souterraines aux réserves glucidiques et aux pools d'azote microbiens ont été identifiées comme deux mécanismes clés sous-tendant les réponses des communautés résilientes. Néanmoins, des répercussions à plus long terme du changement climatique pourraient être observées, causées par un épuisement successif des réserves végétales et une diminution des retours azotés vers le sol via les processus de décomposition. / Alpine grassland ecosystems are presumed to be highly sensitive to climate change, yet their long history of climate variability, and multiple centuries of land use may have selected for mechanisms of ecological resilience to climate variability and climate extremes. We used a large experimental design to explore patterns and mechanisms for responses of subalpine grasslands to combined winter (snow removal) and summer (drought) weather extremes depending on plant functional composition and management. Plant functional composition was manipulated by establishing grass mixtures with three species representing a conservation to exploitation gradient planted at varying relative abundances. Overall, functional composition was the primary determinant of all observed parameters for plant individual performance, intraspecific plant trait responses, litter decomposition and nitrogen recycling processes. The functioning of grassland ecosystems dominated by conservative plants was remarkably resistant to extreme weather treatments, while grassland ecosystems dominated by more exploitative plants were more resilient. Management altered these responses mostly in the case of exploitative communities. Belowground allocation to carbohydrate reserves and to microbial nitrogen pools were identified as two key mechanisms underpinning these resilient responses. Longer-term impacts of climate change may however unfold through the exhaustion of plant reserves and decreasing nitrogen returns to soils via decomposition process.