Urban warming favours C₄ plants in temperate European cities

Elucidating the mechanisms responsible for the structure of urban communities is a key aim of urban ecology, but one that is often confounded by the multitude of environmental changes that are caused by urbanization. We applied trait-based techniques to identify the specific environmental drivers that shape urban plant assemblages and predict how these drivers will further impact biotas with increasing urbanization and global environmental change. Urbanized areas across temperate Europe have significantly higher abundance and richness of plants using the C4 photosynthetic pathway, relative to the total number and species richness of all plant records, than non-urban areas. Urban warming, mediated by the contrasting physiological responses of C3 and C4 plants to warming, is the main driver of observed patterns of plant assemblage structure. This mirrors broadscale and historical distribution patterns of C3 and C4 plants. The increased relative abundance of C4 plants in cities demonstrated here may be indicative of more geographically widespread assemblage changes to be expected in temperate environments under continuing global climate change. Synthesis. Applying a combined trait-based, ecoinformatic and remote-sensing approach provides new insight into the landscape-level consequences of urbanization. Specifically, we show that localized urban warming in cities across temperate Europe favours C4 plant species, which respond positively to increased temperatures. Urban plant assemblages are shaped by environmental warming and exhibit significant increases in C4 plant relative abundance compared to non-urban assemblages.