On-going global changes have already affected ecosystems and threaten the biodiversity all over the world. In order to maintain the ecosystems services provided to humans and adapt conservation planning, the challenge is to improve our understanding of the mechanisms underlying the maintenance of biodiversity and to predict its response to global changes. In mountainous areas, where the environment is very heterogeneous, the modifications of both climate and land use are expected to strongly influence the landscapes and current biodiversity. This PhD thesis has for main objective to assess the vulnerability of species and habitat to environmental changes in the French Alps. It uses three different approaches and relies on the large databases accumulated by two institutions: the National Alpine Botanical Conservatory and the Ecrins National Park (PNE). The first part of the PhD confronts theoretical hypotheses for species coexistence to observations and describes the characteristics of the regional flora. The species ecological niche breadth has been estimated and related to other rarity facets and trade-off between plant functional strategies. A second analysis disentangles the drivers of the presence or the local abundance of 21 focal species and highlights the importance of the dispersion and the source-sink dynamics. The second part is based on the same conceptual background and aims to develop a dynamic model of the vegetation structure and diversity. The model has been validated for the vegetation of the PNE. The last part proposes an application of this dynamic model to provide multiple biodiversity scenarios in respect to change in both climate and land management. The simulations showed that the consequences of climate change might be visible only after a certain time-lag, demonstrating the interest of considering the spatial but also temporal vegetation dynamics. Furthermore, the analysis pointed out the importance of the interplay effects between climate and land use abandonment. Such a model should pave the way for the exploration of multiples scenarios and will be able to describe not only the potential future landscapes but also the transition states leading to it.
