• Urban tree canopy cover is a promising solution for mitigating heat island. • Data-driven guidance on tree selection and planting locations is still limited. • Four research priorities are proposed, requiring a collaborative research effort. • Cross-climate morphological and physiological characteristics are desired. • Integration with atmospheric boundary layer models is suggested.

• Tree growth and cooling provisioning affected by impervious surfaces, primarily due to low soil moisture. • Tree species showed differing responses to imperviousness. • Increased imperviousness is linked to higher PET under trees. • Leaf Area Index is the most influencing tree trait for human thermal comfort.

Urban areas face high particulate matter (PM10) levels, increasing the risk of respiratory and cardiovascular diseases. Green spaces can significantly reduce PM10 concentration, as shown at various scales, from boroughs to whole cities. However, long-term monitoring is needed to understand the specific mechanisms and cumulative impact of green spaces on air quality to changing pollution levels. We investigated the influence of neighbourhood green space percentage, climatic variables, and population density on PM10 deposition during the vegetation period across eight cities in contrasting climate zones over 20 years (2000-2020). We used a correlation matrix, generalized additive model, one-way ANOVA, and Tukey HSD test to analyze the impact of these factors on PM10 deposition rates, assess the role of green space percentage in reducing it, and identify significant differences in PM10 parameters at different proximities to emission sources. Cities with higher population density in warmer, drier climates had higher PM levels, since land surface temperature and wind pressure positively correlated with PM10 deposition, while relative humidity showed a negative correlation. The study found significantly higher PM10 concentrations in industrial areas (36.25 μg/m³) than in roadside areas (25.73 μg/m³) and parks (20.17 μg/m³) (p < 0.01). This highlights the need for targeted interventions in different zones. The study found a complex relationship between green space percentage and PM10 deposition rate onto plant surfaces. Our model suggests that at least 27% of green spaces as land cover can significantly reduce the particulate matter flux, although the minimum threshold can vary depending on the specific urban contexts. The study focused on the proportionate cover of green spaces; still, further investigation including quantitative aspects of urban surface forms, and traffic emissions can comprehend the climatic context and determine the optimal extent of green space required for strategic planning toward future urban sustainability initiatives.

Urban forests can mitigate urban heat effectively during hot summer days. Their cooling effects are largely determined by the plant canopy structure, which can be classified into three features: canopy density and horizontal and vertical canopy structures. This study aimed to identify the contribution of these canopy structural features to cooling effect of urban forests and how their contribution varies during the day. Near-surface air temperature and relative humidity were measured repeatedly between 07:00 - 10:00, 11:00 - 14:00, 16:00 - 19:00 and 21:00 - 24:00 during 16 summer days of 2021 using a mobile monitoring system at five urban parks and representative reference areas in Shanghai. To measure canopy structural features around each measurement point with a scale of 5 m, 10 m and 20 m radius buffers, each park was scanned by an unmanned aerial vehicle (UAV) laser scanner and an UAV multiple-spectrum image system. We found vertical canopy structure had great importance in explaining cooling effects and even exceeded the magnitude of air temperature reductions caused by leaf area index and canopy coverage during noon and nighttime. With increasing foliage height diversity (FHD), the cooling effect first decreased and then increased, when the turning point of FHD was approximately 0.5. Generally, sites with a high canopy coverage and an unevenly distributed canopy (FHD < 0.3) in high layers are recommended to provide better cooling effects in summer. Along with canopy density and horizontal structure, vertical canopy structure must be considered to optimize the cooling potential of urban forests in future planning.

Green infrastructure (GI) has emerged as a feasible strategy for promoting adaptive capacities of cities to climate change by alleviating urban heat island (UHI) and thus heat stress for humans. However, GI can also intensify the winter cold stress. To understand the extent of UHI within a city as well as the link between outdoor thermal stress both diurnally and seasonally, we carried out an empirical study in Würzburg, Germany from 2018 to 2020. At sub-urban sites, relative humidity and wind speed (WS) was considerably higher and air temperature (AT) lower compared to the inner city sites. Mean AT of inner city sites were higher by 1.3 °C during summer and 5 °C during winter compared to sub-urban sites. The magnitude followed the spatial land use patterns, in particular the amount of buildings. Consequently, out of 97 hot days (AT > 30 °C) in 3 years, 9 days above the extreme threshold of wet bulb globe temperature of 35 °C were recorded at a centre location compared to none at a sub-urban site. Extreme heat stress could be halved with 30–40% cover of greenspaces including grass lawns, green roofs, and green walls with little compromise in increasing winter cold stress.