Elevated CO₂ Increases the Canopy Temperature of Mature Quercus robur (Pedunculate Oak)

The canopy thermal response of natural forests to elevated CO₂ (eCO₂) is an understudied biophysical feedback in the global climate system. We investigated the effects of eCO₂ (150 μmol mol⁻¹ above ambient) on canopy temperature (T_can) dynamics of mature (> 175 years) Quercus robur (oak) at the Birmingham Institute for Forest Research Free Air CO₂ Enrichment (BIFoR-FACE) facility in Staffordshire, England, during the growing seasons of 2021, 2022 and 2023. We employed long-term, high-frequency thermal infrared (TIR) imaging to measure T_can. Our results show that daily maximum oak T_can under eCO₂ was, on average, approximately 1.3°C higher than under ambient (aCO₂) conditions (21.5°C ± 4.4°C for aCO₂ vs. 22.8°C ± 5.2°C for eCO₂ oaks). Moreover, daily maximum T_can–air temperature (T_air) differences were significantly higher under eCO₂, resulting from more frequent extreme temperature excursions. These differences appear primarily to be driven by reduced stomatal conductance under eCO₂, which limits transpirational cooling and alters the surface energy balance. This effect was evident in the different relationship between T_can–T_air and vapour pressure deficit (VPD) for eCO₂ compared to aCO₂, showing a reduction in transpirational cooling under high VPD. Also, CO₂-induced leaf structural and anatomical modifications, such as increased leaf mass per area, may have enhanced solar radiation absorption, thereby enabling greater canopy warming under high radiation conditions. Thus, eCO₂ could likely cause a reduction in leaf transpiration in oaks, reducing its contribution to processes such as humidification of the lower atmosphere and precipitation in local and regional climates. Our findings highlight how high CO₂ conditions may intensify thermal stress in temperate forests, influencing water and carbon cycles and potentially impacting forest resilience. Furthermore, T_can will be essential for refining global Earth system models, which often use T_air as a proxy for T_can, despite the latter's direct influence on carbon and hydrological cycles.