Long‐term homogenization of Fennoscandian heathland and tundra vegetation is connected to the expansion of an allelopathic dwarf shrub

Boreal and tundra plant communities are expected to change in biodiversity due to increasing global change pressures such as climate warming. One long‐term scenario is increasing compositional similarity, i.e. biotic homogenization, which has been relatively little studied in high‐latitude plant communities. Here, we study how the composition and diversity of heathland and tundra plant communities have changed in northern Fennoscandia over several decades. In 2013–2023, we resurveyed 275 historic vegetation plots, originally surveyed in 1964–1975, with percentage covers for vascular plant, bryophyte and lichen species. We analyzed temporal changes in community composition and diversity across the study area and in different biogeographic zones, continentality‐humidity classes and habitat types. We found a strong homogenization trend across the study area, with plant communities becoming more similar in composition over the decades when all taxa were treated together. The observed homogenization was driven especially by the increased similarity of vascular plant and lichen communities and was largely independent of biogeographic zones or continentality‐humidity gradient. Homogenization was particularly associated with the drastic encroachment of the evergreen dwarf shrub Empetrum nigrum in habitat types originally dominated by other species, and with the decrease in lichen cover. In general, our findings suggest that Fennoscandian heathland and tundra vegetation is transforming towards a more homogeneous evergreen dwarf shrub‐dominated system, which may threaten ecosystem multifunctionality. Our results highlight the importance of exploring biodiversity among different metrics and growth forms to understand the overall changes in heathland and tundra biodiversity.