Response of canopy temperature and leaf heat tolerance to a heatwave: a study on Quercus robur under ambient (aCO2) and elevated (eCO2) conditions

Experimental and modelling studies indicate that elevated CO₂ (eCO₂) can alter leaf thermal dynamics through reduced stomatal conductance and leaf structural trait modifications. These changes weaken evaporative cooling and shift the leaf energy balance toward higher leaf temperatures. However, empirical evidence from mature natural forest ecosystems remains limited. Thermal infrared (TIR) imaging provides a robust approach for continuous, non-contact monitoring of surface temperature in natural ecosystems. Here, we used TIR imagery to quantify canopy temperature in mature Quercus robur at the Birmingham Institute of Forest Research Free-Air CO₂ Enrichment (BIFoR-FACE) facility in Staffordshire, central England, during the summers of 2021 to 2023, which included a heatwave in 2022. Elevated CO₂ induced structural and physiological shifts in oak leaves, including higher leaf mass per area and lower stomatal conductance, with implications for leaf energy balance and canopy heat dissipation. Across summers, canopies in eCO₂ plots were on approximately 1 °C warmer than those in ambient CO₂ (aCO₂) plots, with the largest differences occurring during high-temperature periods and an increased frequency of exceedance during heatwaves. We additionally assessed photosystem II heat tolerance before and during the 2022 heatwave using chlorophyll fluorescence (maximum quantum yield of photosystem II F_v/F_m). Following the July 2022 heatwave, leaves showed evidence of increased heat tolerance overall, but heat tolerance was reduced in eCO₂ compared with aCO₂. Together, these findings indicate that eCO₂ can elevate canopy temperatures in mature temperate forest canopies and may also alter physiological heat tolerance responses during extreme heat events.