Vertical regulation of thermal stress by canopy structure in urban forests: The role of species composition

Urban forest canopies serve as effective thermal buffers, yet their potential to mediate human heat stress through structural complexity remains underexplored. This study quantified canopy height, heterogeneity, and diversity characteristics, and examined their relationships with mean radiant temperature (T_mrt) across tree species with distinct cooling capacities. It further evaluated the overall cooling effects of forest community structure using a Generalized Additive Model (GAM) combined with Non-Metric Multidimensional Scaling (NMDS), and identified key structural thresholds influencing thermal regulation under varying levels of species compositional diversity via a segmented GAM. We found that: (1) Tree species with varying cooling performance differed by 1.43 °C in cooling intensity (mean T_mrt) and 0.013 in cooling stability (CV of T_mrt), with T_mrt in Strong–Stable performers exhibiting significant negative correlations with canopy height and vertical heterogeneity (r = –0.35 to –0.69, p < 0.001); (2) Overall T_mrt variation across communities was primarily driven by canopy height, with the NMDS axis explaining 30.9 % of the variation (p < 0.001). Heterogeneity characteristics, as indicated by mean outer canopy height, top rugosity, and canopy relief ratio, accounted for 27.9 %, while diversity characteristics contributed less (11.6 %); (3) With increasing species compositional variability, the structural response of T_mrt shifted from being dominated by a single factor to a coordinated regulation by multiple structural characteristics. In high-variability communities, mean height, top rugosity, canopy relief ratio, and vegetation area index jointly influenced T_mrt, with the model explaining 53.8 % of its variation. This study sheds light on the dynamic role of tree canopy structures in regulating thermal stress, offering insights for microclimate-responsive urban landscape design.