Beyond tree cover: Vertical canopy structure is associated with the cooling impact of pocket parks in Mediterranean cities

As cities continue to densify, opportunities to introduce green areas decrease. Small urban green spaces, such as pocket parks, often represent the only remaining non-built-up areas within compact urban fabrics. Through shading, evapotranspiration, and the enhancement of air circulation, these spaces can provide microclimatic benefits; yet the cooling capacity of pocket parks and the role of their internal forest structure remain insufficiently explored. This study quantifies the association between forest structure, configuration, and species composition of urban pocket parks and morning cooling performance during a heat wave in the city of Barcelona, Spain. Moving beyond the conventional emphasis on tree canopy cover alone, the role of vertical structure of 627 urban trees was investigated. Five cooling indices—Park Cooling Distance (PCD), Park Cooling Area (PCA), Park Cooling Efficiency (PCE), Park Cooling Intensity (PCI), and Park Cooling Gradient (PCG)—were used to evaluate 18 pocket parks in densely built-up areas of the city. Statistical analyses assessed relationships between internal park characteristics and cooling performance, identifying the structural configurations with the strongest association. Results showed an average PCD of 55.8 m, PCA of 1.44 ha, PCE of 7.9, PCI of 1.2 °C, and PCG of 0.02 °C∙m⁻¹ in the studied pocket parks. Tree height, tree density, percentage of tree cover, and vertical structural variability were significantly correlated with PCE, PCI, and PCG, whereas no significant relationships were observed with PCD. The pocket parks exhibiting the strongest cooling performance association shared distinct structural characteristics, i.e., dense (>150 trees∙ha -1 ), predominantly monospecific trees concentrated in the upper canopy layer (>8.6 m) and were characterized by high canopy cover (>86%). These findings reveal that internal tree structure correlates to pocket park cooling performance. Design and management-oriented actions, focused on forest structure, may significantly enhance urban cooling and thermal comfort, contributing to climate adaptation strategies in compact urban environments. • High-performing pocket parks combine dense plantings, wide crowns, and continuous canopy • PPs are associated with a measurable differences in morning LST patterns • Smaller green spaces show higher cooling efficiency per unit area than larger parks • Tree height, density, and canopy cover are consistently related with cooling indices • High canopy cover in upper tree layers and low crown ratios are related to cooling • Design benchmarks suggest ≥150 trees/ha and canopy heights above 8.6 m