Accelerating species loss in southern boreal understories as early warning sign of functional erosion

Biodiversity is changing rapidly worldwide. Although taxonomic and functional diversity are intrinsically linked, biodiversity change can decouple their trajectories, challenging biodiversity assessments that often focus on a taxonomic perspective. As a result, functional consequences of biodiversity change remain elusive. To better understand how taxonomic and functional diversity change are intertwined, we analytically partition functional diversity change. Specifically, using 38 years of boreal forest understorey data across Finland, we explicitly link species turnover to shifts in community functional composition. We show that, while stable in Mid and Northern regions, recent net loss of fast-growing species in Southern Finland has halted a decades-long increase in functional diversity. This slowdown emerges not from the loss of highly functionally unique species, but rather from the cumulative number of species lost, whose low abundances limit their contribution to functional diversity. Our approach uncovers mechanisms underlying the decoupling of taxonomic and functional diversity, illustrating how functional diversity can persist despite species loss, while also indicating a growing risk of functional erosion. Taken together, these findings underscore the importance and utility of integrating taxonomic and functional dimensions to better anticipate ecological consequences of biodiversity change.