Elevated CO 2 Increases the Canopy Temperature of Mature Quercus robur (Pedunculate Oak)

Abstract

The canopy thermal response of natural forests to elevated CO 2 (eCO 2 ) is an understudied biophysical feedback in the global climate system. We investigated the effects of eCO 2 (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 CO 2 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 eCO 2 was, on average, approximately 1.3°C higher than under ambient (aCO 2 ) conditions (21.5°C ± 4.4°C for aCO 2 vs. 22.8°C ± 5.2°C for eCO 2 oaks). Moreover, daily maximum T can –air temperature ( T air ) differences were significantly higher under eCO 2 , resulting from more frequent extreme temperature excursions. These differences appear primarily to be driven by reduced stomatal conductance under eCO 2 , 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 eCO 2 compared to aCO 2 , showing a reduction in transpirational cooling under high VPD. Also, CO 2 ‐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, eCO 2 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 CO 2 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.

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