Tropical leaf phenology as a thermoregulation trait: how changes in crown structure modulate thermal dynamics for two dominant Hawaiian trees.

Canopy and leaf temperatures often diverge from air temperature, influencing photosynthesis, water loss, and heat stress. While leaf traits are known to affect heat exchange, how changes in crown structure interact with environmental drivers to regulate canopy thermal dynamics is poorly understood, particularly in tropical forests. In a Hawaiian wet tropical montane forest, we used repeated UAV-based (Unmanned Aerial Vehicle) thermal and LiDAR (light detection and ranging) measurements of sunlit leaves within and across 397 tree crowns of Acacia koa and Metrosideros polymorpha to investigate (1) seasonal and species-specific variation in crown traits; (2) how crown traits and environmental drivers interact to determine canopy-air temperature differences (Tdiff), within-crown thermal variability (TcanSD), and maximum leaf temperatures (TcanMAX); and (3) whether crowns with greater seasonal structural change exhibit enhanced thermoregulation. All crown traits showed species-specific seasonal dynamics, with trait-trait and trait-environment interactions shaping thermal metrics differently across species. In M. polymorpha, crowns with greater seasonal structural change exhibited lower relative Tdiff, whereas A. koa did not show this buffering effect. These results indicate that canopy thermoregulation emerges from interacting crown traits dynamically reshaped across phenological and seasonal cycles. Seasonal crown dynamics provide partial thermal buffering in some species, highlighting species-specific pathways of thermal acclimation or vulnerability under climate change.