Borealization of plant communities in the Arctic is driven by boreal-tundra species

Mariana García Criado , Isabel C Barrio, James D. M. Speed, Isla H Myers-Smith, Anne D. Bjorkman, Rien Aerts, Juha M. Alatalo, Katlyn R. Betway-May, Robert G. Björk, Mats P. Björkman, Daan Blok, Elisabeth J. Cooper, J. Hans C. Cornelissen, Sarah C. Elmendorf, William A. Gould, Ragnhild Gya, Greg H.R. Henry, Luise Hermanutz, Robert D. Hollister, Annika K. Jägerbrand, Ingibjörg Svala Jónsdóttir, Elina Kaarlejärvi, Olga Khitun, Simone I. Lang, Petr Macek, Jeremy L. May, Anders Michelsen, Signe Normand, Siri L. Olsen, Eric Post , Riikka Rinnan, Niels Martin Schmidt, Sofie Sjögersten, Anne Tolvanen, Joachim P. Töpper, Andrew Trant, Vigdis Vandvik, Tage Vowles

Following rapid climate change, tundra plant communities are experiencing extensive compositional shifts. A conservation concern is the potential encroachment of boreal species into the tundra (‘borealization’). Tundra borealization has been sporadically reported, but not systematically quantified. Here, we synthesized data from across 32 study areas, spanning 1,137 plots and 287 vascular plant species, resurveyed between 1981 and 2023. We i) quantified tundra borealization as the colonisation and increase in abundance of Boreal and Boreal-Tundra species, ii) assessed biogeographical, climatic and local borealization drivers, and iii) identified species contributing to borealization and their associated traits. Half of the plots experienced borealization, although borealization rates were not different to random expectation. Borealization was greater in Eurasia, closer to the treeline, at higher elevations, in warmer and wetter regions, where climate change was limited, and where initial boreal abundance was lower. Boreal coloniser species were generally short-statured, and more often shrubs and graminoids. Boreal species colonised around three times less frequently than Boreal-Tundra species. Hence, our findings indicate that tundra borealization is mainly driven by the spread of already established boreal-low Arctic tundra species. These plant community composition changes could have cascading impacts on land-atmosphere interactions, trophic dynamics and local and Indigenous livelihoods.