Plant community borealization in the Arctic is driven by boreal-tundra boundary species

Following rapid climate change across the Arctic, tundra plant communities are experiencing extensive compositional shifts. One of the most prevalent changes is the encroachment of boreal species into the tundra (‘borealization’). Borealization has been reported at individual sites, but has not been systematically quantified across the tundra biome. Here, we use a dataset of 1,137 plots at 113 subsites across 32 study areas resurveyed at least once between 1981 and 2023 and encompassing 287 vascular plant species. We i) quantified the borealization of tundra ecosystems as the colonisation and the increase in abundance of boreal specialist and boreal-tundra boundary species, ii) assessed biogeographical, climatic and local drivers of borealization, and iii) identified species contributing most to borealization and their associated traits. Around half of the plots experienced borealization, especially at sites closer to the treeline, at higher elevations (mountains), in warmer and wetter regions, and at sites that had undergone the lowest magnitude of climate change. Boreal species were more likely to expand in Eurasia, and at sites with lower initial abundances of boreal species. Boreal species that colonised more plots were generally short, and more likely to be shrubs and graminoids than forbs. Boreal specialist species colonised three times less frequently than boreal-tundra boundary species, yet abundance changes were similar across groups. These findings indicate that borealization is mainly driven by the spread of already established species in the tundra, and suggest that future changes to Arctic ecosystems might not involve rapid, widespread replacement of Arctic species by boreal species. These observed and future plant community composition changes could affect land-atmosphere interactions, trophic dynamics and local and Indigenous livelihoods.