Tree canopy structure and phenology as drivers of microclimate regulation in Mediterranean urban ecosystems.

Urban trees constitute a fundamental biotic component of urban ecosystems, mediating local climate through shading, evapotranspiration and modification of radiative and aerodynamic processes. While their cooling role is widely acknowledged, the ecological mechanisms governing vertical and seasonal microclimatic regulation within urban tree canopies remain insufficiently quantified, particularly in Mediterranean cities. This study investigates how tree functional traits, canopy structure and phenology interact with urban ecosystem typologies to regulate near-surface air temperature. Continuous air temperature measurements were conducted at two vertical levels (3 m and 6 m above ground) beneath four representative urban tree species-Tilia europaea, Platanus × hispanica, Quercus robur (deciduous broadleaf) and Cupressus sempervirens (evergreen conifer)-across contrasting urban ecosystems in Coimbra, Portugal, including open parks, a street canyon and a densely built square. Results demonstrate that dense deciduous canopies function as effective ecological regulators during summer, reducing pedestrian-level air temperature by approximately 1-5 °C and generating consistent daytime vertical thermal stratification, with warmer air retained in the upper canopy and cooler air maintained below. This stratification vanished during winter leaf-off conditions, highlighting the dominant role of phenology in seasonal ecosystem functioning. Urban ecosystem typology strongly mediated canopy performance, with open park systems maximising cooling benefits and confined street canyons constraining ecosystem functioning. These findings provide empirical evidence that microclimate regulation in cities emerges from the coupled effects of tree functional traits, vertical canopy structure and urban ecosystem configuration, reinforcing the role of urban trees as key components of climate-regulating ecosystem services in Mediterranean environments. The results carry direct implications for urban heat island mitigation, public health adaptation strategies and the evidence-based design of nature-based solutions in climate-vulnerable cities.