Integrating forest inventory and LiDAR observations to uncover the role of plant traits on cooling and humidifying effects in urban area

Outdoor heat stress possesses significant health risks, contributing to thousands of premature deaths each summer. Urban greening has been widely recognized as a potential solution to mitigate this heat threat. However, the optimal way to maximize cooling effects from urban forest—specifically through the influences of plant leaf traits and canopy structure on local climate (temperature and humidity)—remains underexplored. To address this issue, we combined traditional forest inventory methods with advanced LiDAR observations to assess 3883 individual trees and 77 plant species in the urban forests of Shanghai during the summer of 2021. For all trees, we analysed six leaf traits: nitrogen content, phosphorus content, potassium content, leaf area, specific leaf area and leaf dry matter content. Additionally, three canopy structural characteristics—mean foliage height, foliage height diversity, and canopy coverage—were investigated. Near-surface air temperature and relative humidity within and outside the forests were measured repeatedly using a state-of-the-art mobile monitoring system during four time intervals: 07:00–10:00, 11:00–14:00, 16:00–19:00 and 21:00–24:00. Our findings revealed that leaf stoichiometric traits significantly contribute to the variation in cooling effects, with their relative importance being twice as high as that of canopy structure during morning and afternoon periods. Specifically, nitrogen (N) and phosphorus (P) in leaves positively influenced cooling and humidification, whereas potassium (K) had a negative impact. Synthesis and applications. To enhance the summer cooling potential of urban forests, we recommend incorporating tree species with high leaf N, leaf P and low leaf K in urban park designs, rather than solely expanding canopy coverage. This study highlights the importance of considering plant traits in future urban green design, planning and management for combating heat effectively.