<p>Floodplains are known to be hotspots of biodiversity, but are threatened due to water regulation and land-use practices by humans. The Gülpe case study on the Havel River in Germany offers a valuable opportunity to analyze the change in plant diversity and habitat function due to a transition from intensive agricultural management to an extensive management aligned with Natura2000 goals over the past 40 years. This study compiled a vegetation survey of 407 plots based on a vegetation survey conducted in 2020, combined with three previous surveys spanning several decades in the same floodplain. For each plot, plant diversity (number of species, Shannon Diversity Index) and further indicators of rareness (Red List species), of representativeness (river corridor plants), and of hemeroby (neophytes) were calculated for the total dataset and a stratified grassland dataset. Non-metric multidimensional scaling was performed to obtain deeper insights into temporal changes of species composition and selected species. Hydrological data and information on agricultural practices as well as Ellenberg Indicator Values were compiled to gain information on habitat function. The results show that the Gülpe floodplain exhibits high plant diversity, though species composition has changed significantly in recent decades. The number of river corridor plants and Red List species remain at a high level. The indicator species Selinum dubium (former Cnidium dubium) of the Natura2000 Habitat Type Cnidion dubii appeared at the study site in 1995 and has since established widespread populations, indicating that the land-use extensification has supported the reestablishment of important habitat functions in Gülpe.</p>