Microclimatic benefits of urban shading trees: Synergies and trade-offs in Canberra, Australia

Urban forests, integral to Green Infrastructure (GI) and delivering Nature-based Solutions (NbS), play a pivotal role in mitigating the Urban Heat Island (UHI) effect and enhancing urban thermal comfort. The cooling effectiveness of urban trees is influenced by functional traits that determine their shading capacity and evapotranspiration rates. While trait-service relationships have been proposed to guide species selection for improved microclimate outcomes, their interplay with contextual factors, such as solar irradiance, remains underexplored. This study investigates the cooling potential of four morphologically distinctive urban tree species in Canberra, Australia, characterised by a distinctive inland Mediterranean climate. In contrast to many prior studies that rely on single heat metrics and summer midday snapshots, this study analyses seasonal and daytime variations by modelling interactions between seasons and surface materials, and between functional traits and solar irradiance. Linear mixed-effects models were employed to quantify the contribution and significance of these factors to reductions in Surface Temperature (ST) and Wet Bulb Globe Temperature (WBGT). Crown density emerged consistently as a significant trait that was positively correlated with reductions in both ST and WBGT, whereas other traits showed indicator-specific and context-dependent effects. Notably, traits that improve surface cooling might be counterproductive for thermal cooling, revealing potential trade-offs between UHI mitigation and thermal stress reduction. The findings underscore that trait-service relationships can vary with solar irradiance. This study highlights the need for strategic tree species selection, integrating appropriate functional traits within specific urban contexts, to optimise urban microclimatic benefits.