New research shows that elevated atmospheric CO₂ levels projected for mid‑century can raise temperatures inside forest canopies, with implications for tree physiology, water cycling, and the ability of forests to buffer climate change.
The study, conducted at the University of Birmingham’s Institute for Forest Research Free Air CO₂ Enrichment (BIFoR-FACE) facility during the 2021–2023 growing seasons, with the support of researchers at the University of Plymouth University of Leeds, Northern Arizona University, and the Forestry Research Institute of Ghana, utilised thermal imaging and canopy sensors to continuously record leaf temperatures under both current and elevated CO₂ treatments.
On average, it found canopy leaf temperatures rose by about 1.3°C — from 21.5°C under ambient conditions to 22.8°C at the CO₂ levels modelled for 2050. The effect was more pronounced during extreme heat, with differences exceeding 2°C during the 2022 heatwave, and the highest recorded leaf temperature reached 40°C.
Dr Sophie Fauset, Associate Professor of Terrestrial Ecology at the University of Plymouth and the study’s senior author, explicitly stated, “We are seeing a direct rise in leaf temperatures due to increased CO₂, and this is separate from any other variables. Our reliance on trees to mitigate environmental change is at risk because the conditions are now negatively affecting our forests. Trees have adapted for centuries, but it is now uncertain whether they can keep pace with rapid environmental changes.”
The team links the warming to reductions in transpiration: when CO₂ levels rise, many trees partially close their stomata, losing less water, which in turn reduces evaporative cooling from leaves. The reduced transpiration not only elevates leaf temperatures but could also weaken trees’ capacity to return water to the atmosphere, with potential knock‑on effects for local and regional water cycles.
William Hagan Brown, the study’s lead author, said, “We are running a parallel project in Ghana to directly measure canopy temperatures across tree types and environments over different seasons. Although the focus here is on mature Quercus robur, and these oaks generally show resilience, it is possible that other species will experience even greater impacts.”
The experiment benefited from long-term, high-frequency observations: a thermal camera mounted on a tower within the canopy collected infrared images every 10 minutes for approximately 22 months, producing an exceptionally detailed record of leaf-scale thermal responses under varying weather and CO₂ regimes. The project also coincided with the UK’s record‑breaking temperatures in 2022, providing a real‑world stress test for the canopy under combined heat and CO₂ forcing.
Authors say the results underscore two policy-relevant points: first, reducing global CO₂ emissions remains critical because elevated CO₂ levels alone can alter forest microclimates and physiology; second, tree-planting strategies that assume uniform resilience across species may need revision if canopy warming differentially affects species and ecosystem services.
For further information: William Hagan Brown et al., Elevated CO2 Increases the Canopy Temperature of Mature Quercus robur (Pedunculate Oak), Global Change Biology (2025). DOI: 10.1111/gcb . 70565.
