From Tree Quantity to the Climatic Quality of Shade: Empirical Evidence from Coimbra (Portugal)

Abstract

The climatic effectiveness of urban tree cover depends not only on the presence of trees but also on the interaction between canopy architecture, species phenology and the surrounding urban context. Against this background, this study presents a conceptual framework distinguishing "Hot Shade" and "Cool Shade" as analytical categories for interpreting the differential thermal performance of urban tree cover. The research is based on in situ air temperature measurements (2022–2024), using data loggers installed beneath the canopies of deciduous and evergreen species in green spaces and mineralised areas, accounting for seasonal variability and vertical thermal stratification (3 m and 6 m). The results show that the climatic effectiveness of tree cover stems from the interaction between canopy density, phenology, measurement height and the characteristics of surrounding surfaces. Deciduous species with dense canopies, such as Quercus robur, demonstrate greater cooling capacity during summer, although leaf loss modifies their thermal behaviour in winter. In heavily impervious urban environments, shade may not significantly reduce air temperature because of the accumulation and re-emission of heat by urban materials. The distinction between "Hot Shade" and "Cool Shade", operationalised through relative thermal differences (ΔT), provides an analytical framework for interpreting intra-urban thermal variability. The integration of empirical observations, vegetation structure and urban context strengthens the interpretation of microclimatic processes. These findings have direct implications for urban planning by highlighting the need to integrate bioclimatic criteria into species selection and urban green infrastructure planning, shifting the focus from tree quantity to the climatic quality of shade.