Role of urban trees in mitigating pedestrian heat exposure across diverse climates

Cities now host more than half of the world population and account for nearly three-quarters of global energy use. Their compact geometry, extensive use of impermeable low-albedo materials, and limited vegetation amplify urban heat and intensify the urban heat-island effect. In parallel, climate change is increasing the frequency and severity of heat waves, compounding risks to human health. Urban vegetation, especially trees, is often advanced as a prominent nature-based solution. Through shading and evapotranspiration, trees can lower surface and air temperatures and reduce radiative loads, while offering co-benefits. Yet these cooling effects are not uniform and can be offset by potential drawbacks, including ventilation obstruction, higher humidity, and reduced nocturnal radiative cooling. The net impact on thermal comfort depends strongly on background climate, tree form, canopy density, and surrounding urban morphology. Current knowledge remains fragmented, with most studies limited to single sites or simplified models that cannot capture the mechanistic interactions of vegetation within complex urban contexts. This thesis addresses this gap by developing a nested modeling framework coupling mesoscale meteorology with microscale simulations, enabling explicit quantification of the complex interactions between tree physiological responses, radiative exchanges, and advection effects under varying climatic conditions. The open-source solver urbanMicroclimateFoam is employed to resolve urban phenomena that govern tree-atmosphere interactions at neighborhood scale. Three representative urban contexts are studied: Montreal, Canada (continental), Singapore (tropical hot-humid), and Oujda, Morocco (arid hot-dry). Within each, multiple vegetation configurations are examined, including street trees or parks. Thermal comfort is assessed using the Universal Thermal Climate Index (UTCI), from which generalized assessment tools are developed. A cooling efficiency (CE) metric quantifies reductions in heat exposure relative to baseline conditions, while clustering identifies recurring spatiotemporal patterns to support targeted planning interventions. Results reveal that tree cooling varies strongly with climate and generates both local benefits and non-local trade-offs. In continental climate, trees reduce thermal stress by up to 8 °C UTCI locally (CE of -50%) but obstruct ventilation corridors, causing non-local heating up to +3 °C UTCI (CE of +25%). In tropical climate, high background humidity limits the comfort benefits of transpiration, yet shading still provides local reductions up to 8 °C UTCI (−40% CE), but non-local heating reaches up to +6 °C UTCI (+35% CE). In arid climate, transpiration is particularly effective when water is sufficient, achieving only positive impact of −50% CE locally and −15% non-locally. However, drought trees lead to non-local heating up to +15% CE. Across all cases, tree arrangement and canopy density are essential to control local cooling and non-local heating effects. Additional simulations apply the three climatic conditions to a common idealized geometry, isolating how background climate alone governs tree cooling effect and spatial extent. The work concludes that trees remain a key adaptation strategy, but their effectiveness is context-specific and requires climate-sensitive design guidelines. Future perspectives include extending analyses to long-term climate scenarios, integrating complementary heat mitigation strategies, enriching guidance for city dwellers and planners, and assessing socio-economic and ecological co-benefits.

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