Mitigating urban heat: Spatial reach of cooling effect in nine urban forests of Milan

Heat stress is the leading climate-related cause of premature deaths in Europe, with major heat waves expected to increase in intensity and duration. Urban areas are especially vulnerable to this threat due to their building materials and lack of permeable surfaces. Urban forests help mitigate heat by providing shade and cooling through evapotranspiration. However, there are still many uncertainties about the spatial extent of the mitigation effect provided by urban forests on Air Temperature (AirT), and on how it may be influenced by forest size, canopy cover (CC), structure (density, height and deciduous-to-evergreen ratio), and their interactions with other climate variables (e.g., humidity). In this study, we accessed data from 169 AirT sensors at 3 m height at different distances (0 to 300 m) from the centre of nine urban forests in Milan Metropolitan Area (mean forest size: 1.5 ha). The sensors measured AirT every half hour for 15 months. We observed a strong cooling effect up until 180 m from the forest centre, with a higher intensity for summer temperatures. The cooling effect in the hottest months on record (July 2023) for distances up to 180 m was on average -3.5 °C for mean temperature and -5.5 °C for maximum temperatures. A higher canopy cover significantly increased the intensity and extent of the cooling effect. Correlating the spatial reach of heat mitigation with urban forest size and structure could help future decision makers in planning nature-based solutions that mitigate heatwaves for the largest residential area possible.