Shifting Diversities in the Anthropocene: Impact of Alien Species on Plant and Macrofungal Diversity in Native Forests

This study evaluates the ecological impact of Robinia pseudoacacia L. (black locust) invasion on native chestnut (Castanea sativa Mill.) groves on Mount Amiata (Central Italy), focusing on both plant and macrofungal community dynamics. Surveys were conducted over a three-year period (2022–2024) across 16 plots to assess shifts in taxonomic alpha diversity, species richness, and trophic guild structure. Our results demonstrate that while R. pseudoacacia stands exhibit a higher Shannon–Wiener index for plants, native chestnut groves host significantly greater species richness and higher taxonomic distinctiveness across both biological groups. A major shift in fungal functional structure was observed with chestnut-dominated plots characterized by a predominance of ectomycorrhizal species (58.3%), whereas invaded stands were heavily dominated by saprotrophic fungi (73.4%). Non-metric Multidimensional Scaling (NMDS) further confirmed a clear separation in community composition between the two forest types, indicating that R. pseudoacacia invasion leads to a homogenization of the forest biota and a potential decline in ecosystem health, as evidenced by the sharp reduction in mycorrhizal diversity. These findings highlight the importance of monitoring macrofungal communities as sensitive bioindicators of the ecological degradation caused by invasive woody species.