Agricultural intensification and climate change threaten plant diversity in pastoral landscapes

1. Combined effects of agricultural intensification and climate change on plant communities remain little studied. To preserve plant diversity and ecosystem functioning, it is vital to understand in which contexts climate change may lead to adaptive trade-offs in plants or exacerbate the selection pressures resulting from agricultural intensification. 2. Using trait-based and bioindication approaches, we analyse changes in the diversity and composition of plant communities in 546 resurveyed plots across Northern Ireland, over 15 years characterized by agricultural intensification (increase in intensive pig, poultry, and dairy production) and climate change (+1.12°C between 2009 and 2023). 3. We report a significant decline in grassland plant diversity. Species richness declined in 69% of grassland study sites, with an average of −2.27 species per 4m² plot. Grassland plant communities also showed functional convergence over time, becoming dominated by faster-growing and taller species, less dependent on pollinators, with higher affinities for nutrients and temperature (more thermophilous). These changes were even apparent in seminatural grasslands with low cover of commonly sown species, suggesting nitrogen and phosphorus leaching or atmospheric deposition across the country. 4. The shift towards more thermophilous plant communities was consistent across habitats (grassland, heath-wetland, woodland) and indicator metrics (community mean based on presence-absence or abundance), suggesting short-term climate-induced changes in plant communities across the country. 5. Synthesis: Climate warming is likely to exacerbate the pervasive impacts of soil eutrophication on plant communities, by increasing the productivity of the most competitive species, as well as potentially affecting plant-pollinator interactions. Our results call for an urgent change in agricultural practices, as the subsequent loss of plant diversity could have major negative consequences for biodiversity conservation, ecosystem service provision, and agricultural production.