Afforestation with River red gum (Eucalyptus camaldulensis) drives biotic homogenization in coastal sand dunes

• Demonstrated that Eucalyptus afforestation drives biotic and functional homogenization in coastal-sand grassland ecosystems. • Used a multi-taxa approach including plants, arthropods, and lizards. • Showed loss of psammophile and endemic species, rise in generalists and aliens. • Linked structural vegetation changes under Eucalyptus to cascading trophic shifts • Raised concerns about global afforestation policies in naturally open habitats. Afforestation in naturally open ecosystems is increasingly promoted as a climate mitigation strategy. However, its ecological consequences, particularly in terms of biodiversity, community composition and biotic homogenization - remain underexplored. This process, contributing to convergence in species assemblages in different regions, landscapes, and ecosystems, is widely documented in human-modified ecosystems but rarely studied in the context of afforestation. Here, we assessed the long-term ecological impacts of Eucalyptus camaldulensis afforestation in a coastal dune grassland in Israel using a multi-taxa approach over three years. We sampled vascular plants, ground-dwelling arthropods, and lizards in afforested and adjacent unplanted plots. We found strong evidence of biotic and functional homogenization, with significantly lower richness and abundance across all taxa, and altered community composition characterized by the loss of psammophilic and endemic species and a rise in generalist and alien taxa. We observed a marked decline in annual and perennial herbaceous plant richness and cover under Eucalyptus. These shifts in vegetation structure reflect a transition from open, herbaceous-dominated habitats to shaded, woody vegetation, with cascading impacts on microclimate and ground-level biodiversity. Eucalyptus litter, rich in allelopathic substances, further reduces habitat suitability for specialist taxa. Structural habitat modification altered the prey community, replacing typical psammophile arthropod taxa with litter-associated detritivores, that in turn caused trophic shift from specialist lizards toward arthropod mesopredators like spiders and centipedes. These findings challenge the assumption that afforestation in open ecosystems offers net ecological benefits. We urge policymakers to distinguish afforestation from ecological restoration and avoid tree planting in naturally open habitats.