Abstract
French Guiana forest presents a high originality in terms of biodiversity, a good state of preservation, a strong contribution to the overall Amazonian carbon stocks and a very fast socio-economic dynamic. However, our knowledge about forest ecosystems diversity remains insufficient to ensure an optimal management of the various ecosystems and of the services they may provide across the territory. This thesis aims to answer this central issue by studying the diversity of forest ecosystems at the scale of the entire forested area of French Guiana. Specifically we targeted highlighting the factors that determine ecosystems variability and evaluating the consequences of this variability in terms of ecosystem services and management. Firstly, we bring out complex patterns of diversity at different scales including strong floristic variation at the regional scale, using a recent forest inventories campaign (2006-2013) that has been conducted according to a stratified sampling design which includes 3,132 0.2ha-plots covering the whole territory. Our results verify the hypothesis of a strong control of forest beta and alpha diversity by geomorphology, especially at the landscape scale and for dominant species (Simpson metric). This first step required several preliminary methodological developments including an assessment of the taxonomic uncertainty in forest inventories, and spatial models to accurately characterize the geomorphological context at various scales.Secondly, we aimed at interpreting the influence of geomorphological landscapes on the composition of forest vegetation by exploring the relationship between geomorphology and soils. Using more than 400 soil sampling locations coupled with the forest inventory plots, we demonstrated that nature and properties of soils are dependent on geomorphological landscapes and locally distributed along regional elevation gradients. A significant soil filtering effect was found underlying the geomorphological influence on forest composition and diversity. However, a major part of this influence proved to be independent of soil filtering and is interpreted as a combination of two mechanisms at two different time-scales: (i) the control of forest turn-over by the geomorphological dynamics during the Late Quaternary and (ii) the integration of biogeographic effects under the influence of tectonic evolution and ancient climate changesLast of all, we addressed the consequences of these biotic and abiotic variations, which combine into habitats diversity, on forest management and on the variability of ecosystem services provided by forests, particularly for carbon storage in biomass and soils. The amount of carbon stored is highly variable at local scale but proves to be significantly influenced by habitats diversity at the landscape level. This regulation service is related with the local species richness (pertaining to the category of support services) and the amount of usable wood (pertaining to thecategory of supply services). Complex relationships, including correlations and compromises, are evidenced between these different services. This illustrates the variability of the responses of ecosystem services to the habitats diversity.Finally our work allowed us to provide a new typology of natural habitats present in French Guiana forest and to propose suggestions in order to improve land uses and multifunctional forest management in French Guiana