La restauration des écosystèmes calcicoles de la basse vallée de la Seine : l’importance des échelles spatio-temporelles pour définir un écosystème de référence

Résumé En Fr

Ecological restoration of the chalky slopes of the lower Seine river valley is considered in its historical and geographical context. Two issues are emphasised : — Chalk grassland, on which restoration usually focuses, is only one possible reference among others which can be recorded along the complex history of these landscapes. Several historical ecosystems could thus be considered as possible references. Biogeographical data suggest that endemic species settled on chalk scree are probably inherited from the Ice Age. Soil seed bank and historical analysis have revealed the importance of former cultivation and vineyard in the past centuries while paleo-ecological analysis, based on shells of snail studies, suggests that forested periods are probably important in the past land uses. — Considering the present situation, there is a myriad of ecological gradients which influence chalk grassland biodiversity, acting at different scales, in time and space. Our data suggest that biogeographical gradient, at the regional level, and successional gradient, at local level, play a major role to explain species occurrence or coexistence. Restoration purposes have to take into account both of these aspects in order to define the kind of ecosystem or landscape to be restored and the structure and functioning of communities to be managed. Two major alternatives can be considered : i) the choice of one historical reference in past ecosystems and the design of management in order to lead to an “optimal stage”, generally grasslands, or ii) the consideration of the whole set of past ecosystems through changing management in a changing and heterogeneous landscape mosaic. Criteria for such alternatives are to be found in biodiversity analysis, taking in account several taxonomic groups, diversity dimensions, scales, etc. We therefore need to adopt, for restoration programs, an integrative, interdisciplinary and hierarchical approach of biodiversity analysis in order to define such criteria.

La restauration des écosystèmes des coteaux crayeux de la vallée de la Seine est resituée dans son contexte historique et géographique. D’une part, les pelouses calcicoles, qui sont souvent les systèmes privilégiés dans cette restauration, ne sont que l’un des écosystèmes de référence possibles parmi d’autres qui caractérisent la trajectoire complexe de ces paysages. D’autre part, les gradients actuels permettant d’expliquer la biodiversité des coteaux crayeux sont multiples et agissent à des échelles spatio-temporelles distinctes. La démarche de restauration se doit donc d’aborder ces deux aspects non indépendants, en réfléchissant à la structure et au fonctionnement des écosystèmes ou des paysages visés. Elle doit pour cela adopter une démarche intégrative, interdisciplinaire et hiérarchique, lui permettant de prendre en compte les diverses dimensions du concept de biodiversité.