Aside from the aesthetic benefits, trees and other vegetation help mitigate the effects of the urban heat island (UHI) by increasing the latent heat flux through evapotranspiration, resulting in cool leaves and hence lower air temperatures. In addition, they cast shadows on buildings and other artificial surfaces that otherwise would heat up more and store more energy, thus reducing the total energy surplus of an urban environment. Nevertheless, what kind of vegetation is most efficient and should be used for mitigation strategies in urban areas?
Around 85% of the world’s vegetation are so-called C3 plants, which can be found in moderate climate zones mainly. Due to urban warming in cities across Europe, C4 plants that are able to perform the C4 photosynthetic pathway, are favored and respond positively to increased temperatures. C3 plants require less energy and nutrients to perform photosynthesis, but under dry and hot conditions their stomata close and carbon fixation terminates to protect the plant. Contrary, due to a different chemical process in the CO2 fixation, C4 plants can still perform photosynthesis under these stressful conditions. Thus, studies have shown that in urban regions, the amount of C4 species is increasing significantly. For this reason, mitigation strategies should not only involve the general utilization of vegetation but also consider a specific selection of suitable plants. To determine the beneficial effects of different plant species and to decide about most efficiently locations, microclimate analysis tools can be used.