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 (Tcan) 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 Tcan. Our results show that daily maximum oak Tcan 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 Tcan–air temperature (Tair) 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 Tcan–Tair 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, Tcan will be essential for refining global Earth system models, which often use Tair as a proxy for Tcan, despite the latter's direct influence on carbon and hydrological cycles.