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

1. 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. 2. To address
this issue, we combined traditional forest inventory methods with advanced
LiDAR observations to assess 3,883 individual trees and 77 plant species
in the urban forests of Shanghai during the summer of 2021. For all trees,
we analyzed 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. 3. 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. 4. 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.