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 biological species living in cities. Adaptation to the UHIE may be necessary for urban wild flora to persist in cities, but experimental evidence is scarce. Here, we report evidence of adaptive evolution in a perennial 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 vernalization treatments on flowering phenology. We found that urban plants accumulate more biomass at higher temperatures and require shorter vernalization periods, corresponding to milder winters, to induce flowering compared to rural plants. Differentiation was also observed between different intra-urban subhabitats, with park plants displaying a higher vernalization requirement than street plants. Our results show genetic differentiation between urban and rural dandelions in temperature-dependent growth and phenology, consistent with adaptive divergence in response to the UHIE. Adaptation to the UHIE may be a potential explanation for the persistence of dandelions in urban environments. The urban heat island effect (UHIE) is a globally consistent characteristic of cities where temperatures are several degrees higher than their rural surroundings due to the absorption and retention of heat by buildings, pavements, and other artificial surfaces. This has a considerable effect on urban inhabitants, including wild flora and fauna, which may require biological adaptation for survival. Very few experiments, however, have demonstrated how plants evolve in response to the UHIE. We fill this knowledge gap here by providing evidence in urban dandelions (Taraxacum officinale s.l.) of (1) superior growth at elevated temperatures and (2) a reduced requirement of cold periods to activate flowering, as compared to rural populations of the same species. Temperature conditions in temperate cities, due to the UHIE, reflect future global warming scenarios, providing an insight into the biological adaptation of wild flora and fauna that is required for future survival. Vegetation plays a key role in the mitigation of (urban) heat and other effects of climate change, making knowledge of plant adaptation to urbanization and global warming essential for the sustainable planning of urban environments.