Tougher leaves under global urbanization

Urban environments expose resident organisms to intense, multifaceted selective pressures. Studies employing intraspecific responses of functional traits have proven particularly valuable for understanding vegetation ecological strategy under urbanization. Here, we compiled a dataset of ten leaf functional traits of 1051 observations of 322 species from 104 cities to assess the impacts of urbanization. Urban-related moderators (climatic region, air quality and build up area), phylogenetic group, leaf habits and study design were considered. Results showed that leaf thickness (LT) and leaf dry matter content (LDMC) significantly increased, while the photosynthetic rate under saturating light (A _sat), specific leaf area (SLA), chlorophyll content (Chl), and leaf area (LA) significantly decreased with urbanization. This pattern suggests that urbanization has driven plants to reorient their resource investment-return strategies toward a more conservative ecological strategy. Greater responses to urbanization were observed for angiosperm and deciduous species compared to gymnosperm and evergreen species. We also found leaf trait responses to urbanization are not uniform but depend on the interplay between anthropogenic (e.g., population density, built-up area) and natural (e.g., aridity, MAT) gradients. Regardless of whether this variation arises from genetically based local adaptation or phenotypic plasticity, the results of this study provide key insights into how vegetation responds to urbanization.