Global functional shifts in trees driven by alien naturalization and native extinction

Human activities are driving simultaneous native extinctions and alien naturalizations, reshaping global tree diversity with major implications for ecosystem structure and function. Here we analysed functional traits and environmental niches of 31,001 tree species worldwide, comparing naturalized, threatened and non-threatened species to assess current patterns and project future shifts under intensified extinction and naturalization. Future tree-rich ecosystems are projected to become increasingly dominated by fast-growing, high-resource-use species with acquisitive traits, while slow-growing, conservative species face greater extinction risk. Although group means along the main functional axes do not differ significantly, naturalized species occupy broader functional and environmental spaces and thrive in colder and more variable climates, whereas threatened species are more specialized to warm, stable and nutrient-rich environments, with non-threatened species intermediate. Projected naturalizations expand local functional diversity, but their acquisitive strategies could reduce long-term ecosystem stability, while extinctions cause pronounced contractions of functional and environmental trait space, especially in climatically variable regions. Overall, our findings reveal an accelerating global shift towards faster-growing tree communities, with likely consequences for carbon storage and biodiversity, underscoring the need to safeguard slow-growing species and limit the dominance of acquisitive trees.