New research published in Global Change Biology unveils a striking but overlooked consequence of elevated atmospheric carbon dioxide (CO2) levels: a measurable rise in the temperature within forest canopies. Conducted over three growing seasons at the University of Birmingham’s Institute for Forest Research Free Air CO2 Enrichment (BIFoR-FACE) facility in Staffordshire, UK, this unprecedented study employed advanced thermal imaging and multiple sensor arrays to scrutinize the thermal dynamics of mature pedunculate oak (Quercus robur) trees. The findings reveal that by 2050, under CO2 concentrations projected by climate models, leaf temperatures could increase by over one degree Celsius on average, reaching even higher spikes during extreme heat events—posing new threats to forest ecosystems globally.
Physiologically, the study centers on the intricacies of leaf thermoregulation, a critical function largely governed by transpiration. Transpiration—the evaporation of water vapor through micro-pores called stomata—allows leaves to dissipate heat, moderating temperature stress. Under elevated CO2 scenarios, plants often reduce stomatal opening to conserve water, effectively diminishing transpirational cooling. This physiological adjustment, while beneficial for water conservation, paradoxically causes the leaves to retain more heat, amplifying canopy temperatures. This research measured a mean canopy temperature increase from the current 21.5°C to approximately 22.8°C in environments simulating mid-21st-century CO2 levels.
What differentiates this investigation is the temporal and environmental scale at which temperature records were captured. Over the 22-month period, infrared thermal cameras mounted strategically within the canopy imaged leaf temperatures every ten minutes. During the severe UK heatwave of summer 2022, when ambient temperatures soared beyond 40°C, leaf temperatures within elevated CO2 plots peaked at nearly 40°C. These extreme temperature elevations carry significant ecological implications, potentially pushing trees closer to their physiological thermal limits and impairing critical processes such as photosynthesis and water transport.
The ripple effects of diminished transpiration extend beyond individual trees. Forests play a fundamental role in the global hydrological cycle by transpiring vast quantities of water back into the atmosphere. A sustained decrease in oscillatory water flux due to reduced stomatal conductance could therefore alter regional and global climate patterns. The research stresses how altered tree physiology under elevated CO2 not only raises canopy temperatures but could also disrupt water vapor fluxes, thereby affecting rainfall regimes and ecosystem stability on a planetary scale.
Importantly, the study highlights species-specific responses to this climatic stress. While oak trees demonstrated some degree of thermal resilience, likely because of their adaptive evolutionary history and robust physiological mechanisms, the researchers caution that other species may be far more vulnerable to the compound stressors of heat and altered CO2 concentrations. This variability in response underscores the complexity of projecting forest ecosystem futures and the need for species-specific data to guide conservation and reforestation efforts.
Lead author William Hagan Brown, a PhD researcher affiliated with the University of Plymouth and the Forestry Research Institute of Ghana, emphasized the study’s comparative aspect. Parallel projects in tropical forest ecosystems in Ghana are underway to gauge how canopy temperature dynamics and species-specific physiological traits interface in vastly different biomes. Such comparative research aims to inform adaptive management strategies that can sustain or restore forest resilience worldwide amid climate change.
The implications of elevated leaf temperatures span beyond growth and survival metrics. Elevated thermal stress can influence plant-pathogen interactions and pest infestations, with hotter conditions potentially facilitating the proliferation of harmful organisms. Moreover, temperature-induced reductions in leaf gas exchange can curtail carbon assimilation, thereby weakening forests’ capacity to act as carbon sinks, a crucial element in climate change mitigation strategies.
Methodologically, this study represents a landmark integration of cutting-edge measurement techniques within an open-air experimental setup. The BIFoR-FACE platform uniquely simulates elevated CO2 conditions in a naturalistic forest setting, avoiding the limitations of enclosed chamber experiments and thus yielding highly applicable ecological insights. The continuous, high-resolution thermal imaging enabled that correlations between CO2 enrichment, microclimate fluctuations, and leaf temperature could be robustly quantified.
The researchers advocate urgent action in the global context of environmental policy. Their findings caution against simplistic narratives that regard tree planting as an unequivocal solution to elevated CO2 and climate change. Without concurrently addressing emissions reductions, the altered physiological and thermal responses of forests could undermine their role as climate stabilizers. The study thereby integrates plant physiological responses into broader dialogue on climate mitigation and adaptation strategies.
As forests already face threats from deforestation, habitat fragmentation, and increasing climatic extremes, understanding the intersecting effects of elevated CO2 and thermal stress is imperative. This research encourages a recalibration of predictive forest models to include thermal feedback mechanisms within canopies. Doing so will enhance the precision of ecosystem service projections and help prioritize forestry interventions under dynamic future climate scenarios.
Dr. Sophie Fauset of the University of Plymouth, senior author of the study, underscored the urgency of this research: “Our forests, long considered bulwarks against climate change, are experiencing physiological stresses previously underappreciated. As leaf temperature rises independent of other factors, trees’ adaptive capacity will be tested like never before. This challenges assumptions that current forest populations can simply adjust to rapid environmental shifts.”
Overall, the study paints a nuanced picture of forest health in a high-CO2 future, highlighting complex physiological trade-offs and broader ecological ramifications. While oak trees serve as a resilient model, the variable impacts on other species demand targeted empirical research. Such knowledge is vital for forging resilient forest ecosystems capable of sustaining biodiversity, carbon sequestration, and hydrological functions amid accelerating climate change.
Subject of Research:
Article Title: Elevated CO2 increases the canopy temperature of mature Quercus robur (pedunculate oak)
News Publication Date: 5-Nov-2025
Web References: http://dx.doi.org/10.1111/gcb.70565
Image Credits: Peter Ganderton/University of Plymouth
Keywords: Climate change, Climatology, Climate data, Climate sensitivity, Climate change adaptation, Climate change effects, Environmental sciences, Ecology, Ecosystems, Biomes, Forests, Tropical forests, Forest diversity, Forest ecosystems, Temperature, Heat, Carbon dioxide, Atmospheric carbon dioxide, Hydrological cycle
