Cross-continental, integrated phenological observations reveals that the effect of urbanization on plant phenology depends on regional temperature

Plant phenology is shifting in response to urbanization, but generalities regarding the direction and magnitude of phenological response to urbanization have not yet emerged. Here, we used a database with >22 million in situ plant phenological observations to study the joint influence of varying human population density and of regional temperature on median flowering and leaf-out dates across a wide plant phylogenetic spectrum. Separately, increasing population density and warmer regional temperature both advanced plant flowering and leaf-out. However, high population density advanced plant phenology in cold areas but this effect disappeared or even reversed in warm areas.