Dr. Pierre Mariotte
Plant-Soil Interactions & Global Change Ecology

Dr. Pierre Mariotte

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Research

Plant-soil interactions and feedbacks

    Above- and below-ground interactions play a fundamental role in plant community structure and dynamics and in ecosystem functioning. My research aim at understanding how these interactions modify or improve the functioning of grassland ecosystems and in return provide ecosystem services. During my PhD thesis, I focused my research on the role of soil microbial communities, mainly bacteria and mycorrhiza, in plant community composition and hierarchy. In addition, I am interested in determining the role of plant-soil feedbacks in community assembly (competition/facilitation) and in ecosystem functions such as productivity, decomposition, nutrient cycling and N:P stoichiometry, but also in the mechanisms of species invasion, which was the focus of my postdoctoral research in California.


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Climate change

       Climate is one of the major drivers of plant species distribution around the world and climate changes are expected to shift species distributions or modify their abundance and dominance patterns and to perturb ecosystem functioning. While many experiments have been carried out to determine the effects of climate change, the results are contradictory, insufficient, or limited in view of ecosystem multifunctionality. In my previous work in Swiss mountain and Australian grasslands, I highlighted the importance of plant functional diversity (e.g. subordinate species) and productivity (biomass dependent hypothesis) in the resistance of plant and soil community against during drought. Moreover, I am also interested in exploring the relative importance of the different components of climate change (e.g. warming, flooding, drought etc.) and in showing how these factors interact together (interactions) and with local environmental changes (e.g. diversity, land-use, invasion). 


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Functional diversity - Subordinate insurance hypothesis

      Despite their low relative abundance, subordinate plant species may have larger impacts on ecosystem functioning than expected, but their role in plant communities remained poorly understood. While dominant species are supposed to be the most important species-group in plant communities, I demonstrated, during my PhD research, that subordinate species may also play a role in ecosystem functioning. Using removal experiments in two field sites of semi-natural mountain grasslands (Swiss Jura), I demonstrated that the loss of subordinate species can have significant consequences for soil microbial communities (especially fungi) and ecosystem functions (biomass production, soil respiration, litter decomposition), suggesting that subordinate species are important drivers of ecosystem properties. 


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Community structure and land-use change

      Changes in agricultural practices of mountain grasslands are expected to modify plant community structure and shift dominance patterns. By analysing vegetation survey of semi-natural grasslands, I highlighted a humpbacked diversity-productivity relationship, which is due to an increase of subordinate species number at intermediate productivity. Abandonment of pasturing led to a loss of species, which was mainly due to a higher reduction in the number of subordinate species, as a consequence of the increasing proportion of dominant species. When plant biodiversity maintenance is the aim, my results have direct implications for the way grasslands should be managed. Indeed, while intensification and abandonment have been accelerated since few decades, these findings confirm the importance of maintaining intermediate level of pasturing to preserve biodiversity.