Cities as stepping stones for climate-driven tree species range shifts

As climate change accelerates, many tree species fail to track shifting climatic conditions. Assisted migration - the human-driven relocation of species beyond their natural range - has emerged as a controversial yet potentially useful strategy. Cities often host non-native tree species and thus already function as informal platforms of assisted migration. Using species distribution models, we assessed future climate suitability across Europe for 96 tree species introduced in urban areas. Results show that urban plantings extended species’ ranges by over 650 km northward, surpassing what would be possible through natural migration alone. In several cases, urban populations are already near or within future suitable habitats, reducing the required migration distance by up to 40-fold. These findings highlight the potential of cities to act as stepping stones for range expansion and testbeds for assisted migration, and show that, with ecological oversight, cities could support targeted introductions and conservation under climate change.