Beyond summer cooling: Evaluating winter thermal performance of urban trees in subtropical coastal region

Urban trees are widely recognized as effective nature-based solutions for urban heat mitigation, yet current design strategies focus predominantly on optimizing summer cooling, leaving their potential for winter thermal regulation understudied. This oversight raises a question: Do tree configurations optimized for summer cooling remain thermally beneficial in winter? To address this issue, this study conducted a parametric study using ENVI-met to evaluate the seasonal thermal performance of urban trees in subtropical coastal city Hong Kong under extreme summer and winter conditions. Multi-scenario simulations were run, incorporating eight urban canyons characterized by discrete sky view factors (SVF=0.1–0.8) from typical morphological categories in previous research, two street orientations (N-S, E-W), and two tree species (evergreen Ficus microcarpa , deciduous Lagerstroemia speciose ). Results indicate that both evergreen and deciduous trees provided comparable cooling in summer, with mean differences of 0.21 °C in PET and 0.35 °C in UTCI. A consistent peak cooling of up to 8 °C in PET and 5 °C in UTCI was observed at SVF = 0.7 for both species. In winter, evergreen trees improved comfort by wind shielding in deep, low-SVF canyons (up to 1 °C in PET; 6 °C in UTCI), while deciduous trees enhanced comfort through solar access in shallow, high-SVF canyons (over 0.3 °C PET; 1 °C in UTCI). For subtropical coastal context, our findings suggest that tree selection and placement should be seasonally adaptive. This study provides a methodological framework and preliminary insights for climate-adaptive planting strategies in urban canyons from a year-round perspective.