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 (Tcan). Additionally, understanding variation in Tcan and regulation across species within ecosystems is crucial for understanding how climate change affects plants. Thus, this research aimed to measure Tcan in temperate and tropical forests under contrasting environmental conditions to advance understanding of Tcan. 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 Tcan consistently exceeded air temperature (Tair) by as much as 12 °C. Oaks grown under elevated CO2 (eCO2) exhibited Tcan 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 (Tcan – Tair), potentially altering forest energy exchange and physiological functioning. Complementary analyses of a tropical moist semi-deciduous forest showed pronounced interspecific differences in Tcan regulation. Daily maximum Tcan frequently exceeded 40 °C, with Tcan – Tair 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 (gs) maintained cooler canopies, whereas species with lower gs 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.

Awarding Institution(s)

University of Plymouth

Supervisor

Sophie Fauset, Ralph Fyfe, Emanuel Gloor, Rob MacKenzie

Keywords

Free-Air-CO2-Enrichment, Stomatal Conductance, leaf temperature, canopy temperature, Temperate Forests, Tropical forests, leaf traits, Climate change, Carbon dioxide, heat stress, Photosystem II

Document Type

Thesis

Publication Date

2026

Embargo Period

2026-06-30

Deposit Date

January 2026

Creative Commons License

Creative Commons Attribution-NonCommercial 4.0 International License
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License

This document is currently not available here.

This item is under embargo until 30 June 2026

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