Three decades later: A resurvey of vegetation biodiversity in Italian coastal dunes

Mediterranean coastal dunes have undergone substantial transformations over the last 70 years due to increasing anthropogenic pressure and environmental change. However, most studies on dune vegetation dynamics have been conducted at local scales, limiting our understanding of long-term plant diversity trends across broader regions. Here, we present the first national-scale assessment of long-term vegetation changes in Italian coastal dunes, based on ReSurveyDunes, a collaborative resurvey initiative. We analysed 519 vegetation plots originally surveyed on average 30 years ago and resampled in 2023-2024 along the entire Italian coastline. We quantified temporal changes in species richness and community composition, with a focus on ecological guilds, and analysed habitat transitions over time across three key dune habitats: upper beach, shifting dunes, and dune grasslands. Species richness increased across all habitats. However, this trend masked a marked decline of habitat-specialist psammophilous species, particularly in early-successional habitats. Upper beach and shifting dunes showed strong reductions in occurrence and cover of diagnostic species, accompanied by increases in ruderal taxa and species typical of more stabilised or inland habitats. These patterns reflect a redistribution of species along the coastal zonation gradient. Accordingly, nearly one-third of plots changed EUNIS habitat type or disappeared, indicating the coexistence of inland-directed succession and stabilisation with localised degradation and habitat loss, especially in foredune habitats. Our results show that apparent increases in species richness can conceal profound compositional and habitat-level changes. This highlights the importance of long-term, large-scale resurveys and of complementing richness-based metrics with compositional and habitat-level indicators when evaluating vegetation changes in dynamic coastal dune ecosystems.