Climate Change Impacts on Forest Canopy Temperatures: from mechanisms to implications

Abstract

Forests are a vital interface between the biosphere and the atmosphere. As global temperatures rise due to increasing anthropogenic greenhouse gas emissions, it is important to understand the response of forest tree canopy leaf temperature (T_can). Additionally, understanding variation in T_can and regulation across species within ecosystems is crucial for understanding how climate change affects plants. Thus, this research aimed to measure T_can in temperate and tropical forests under contrasting environmental conditions to advance understanding of T_can. At the Birmingham Institute of Forest Research Free-Air CO₂ Enrichment (BIFoR-FACE) site, continuous thermal infrared (TIR) imaging of mature oak canopies during the 2021 – 2023 growing seasons showed that daytime T_can consistently exceeded air temperature (T_air) by as much as 12 °C. Oaks grown under elevated CO2 (eCO2) exhibited T_can approximately 1 °C higher than those under ambient CO₂ (aCO₂), driven largely by leaf structural and stomatal conductance modifications from CO₂ enrichment. Although eCO₂ did not directly alter leaf photosynthetic heat tolerance (PHT), a significant increase in tolerance was observed following a July 2022 heatwave, suggesting short-term acclimation of oaks to extreme heat. These findings indicate that elevated CO₂ amplifies canopy–air temperature divergence (T_can – T_air), potentially altering forest energy exchange and physiological functioning. Complementary analyses of a tropical moist semi-deciduous forest showed pronounced interspecific differences in T_can regulation. Daily maximum T_can frequently exceeded 40 °C, with T_can – T_air differences up to 14 °C. These exceedances were strongly modulated by solar radiation, relative humidity, wind speed, and species-specific leaf traits. Species with higher stomatal conductance (g_s) maintained cooler canopies, whereas species with lower g_s experienced greater heating. Evergreen pioneers exhibited the warmest canopies, high PHTs and narrow thermal safety margins (TSMs), while deciduous and shade-tolerant species had cooler crowns, low PHTs and broader TSMs, highlighting a trade-off between tolerance-based and avoidance-based thermoregulation strategies among coexisting species. Overall, these results show that forest canopies often operate at temperatures above ambient air, that eCO₂ can further intensify canopy warming, and that species differences in stomatal regulation and leaf traits underpin contrasting thermal strategies and heat risk. Thus, explicitly accounting for canopy temperature and trait-mediated thermoregulation is essential for predicting forest responses to ongoing climate change.

Date of Award	2026
Original language	English
Awarding Institution	University of Plymouth
Supervisor	Sophie Fauset (Director of Studies (First Supervisor)), Ralph Fyfe (Other Supervisor), Emanuel Gloor (Other Supervisor) & Rob MacKenzie (Other Supervisor)