Some like it hot: adaptation to the urban heat island in common dandelion

The Urban Heat Island Effect (UHIE) is a globally consistent pressure on species living in cities. Rapid adaptation to the UHIE may be necessary for urban wild flora to persist in cities, but experimental evidence is lacking. Here, we report the first evidence of genetic differentiation in a plant species in response to the UHIE. We collected seeds from common dandelion ( Taraxacum officinale ) individuals along an urban-rural gradient in the city of Amsterdam (The Netherlands). In common-environment greenhouse experiments, we assessed the effect of elevated temperatures on plant growth and the effect of vernalisation treatments on flowering phenology. We found that urban plants accumulate more biomass at higher temperatures and require shorter vernalisation to induce flowering compared to rural plants. Differentiation was also observed between different intra-urban subhabitats, with park plants displaying a higher vernalisation requirement than street plants. Our results show strong differentiation between urban and rural dandelions in temperature-dependent growth and phenology, consistent with adaptive divergence in response to the UHIE. Rapid adaptation to the UHIE may be a potential explanation for the widespread success of dandelions in urban environments. Summary statement The urban heat island effect (UHIE) is the most prominent and globally consistent characteristic of environmental change due to urbanisation, severely impacting human populations in cities as well as the cohabiting wildlife. Despite the profoundly mitigating effect of vegetation on urban heat, evidence for plant adaptation to the UHIE has been lacking. Here we provide the first experimental evidence to date, demonstrating adaptation in urban dandelions in response to elevated temperatures, similar to the UHIE. We furthermore show an urban-rural differentiation in flowering response to shorter vernalisation times (cold winter period to activate the onset of flowering in early spring). Given the predominantly asexual apomictic mode of reproduction in dandelions, this evolution is likely the result of environmental filtering on a diverse population of clonal genotypes. We conclude that plant adaptation to the UHIE exists and recommend future studies to contrast our findings with those in outcrossing sexual plant systems. Studies of urban heat adaptation can bring impactful contributions to building climate change-resilient environments and plants should be an integral part of this research. ### Competing Interest Statement The authors have declared no competing interest.