Elevated CO2 buffers negative impacts of heat stress on short-term growth adjustments of 180 year-old Quercus robur

Abstract

Rising atmospheric carbon dioxide (CO2) concentration are a primary driver of global warming and are expected to be associated with more frequent droughts, greater temperatures, and increased vapour pressure deficit (VPD). These factors are all key drivers of tree mortality. While elevated CO2 (eCO2) enhances photosynthesis (Anet) and intrinsic water use efficiency (iWUE), its capacity to mitigate heat stress in mature trees under field conditions remains poorly understood.

We investigated the effects of eCO2 on the physiological and growth responses of mature Quercus robur (~180 years old) at a forest Free Air Carbon Enrichment (FACE) experiment. We combined measurements of tree growth, canopy conductance and leaf gas exchange, together with leaf morphological traits, collected during a naturally occurring heat events (> 32°C), to assess whether eCO2 buffered the impacts of extreme heat events (>32°C). The leaf-level measurements also enabled us to determine whether the physiological enhancements previously observed during the early years of BIFoR FACE were maintained following prolonged exposure to elevated CO2.

After eight years of CO2 enrichment (+ 150 ppm above ambient), eCO2-grown trees showed increased iWUE (+ 33 %), driven by increased Anet (+ 26.1 %) and modest reductions in stomatal conductance (- 11.1 %), with no significant changes in stomatal anatomy. Elevated CO2 increased the sensitivity of canopy-level conductance to VPD during heat events, indicating stronger stomatal regulation under high atmospheric demand in eCO2 compared to aCO2-grown trees. Heat stress reduced tree growth in both conditions, but the reduction was less pronounced under eCO2, suggesting partial mitigation of heat stress effects.

These findings indicate that eCO2 can confer partial physiological buffering against heat stress in mature Q. robur, enhancing resilience without compromising structure. Shifts in water use highlight the importance of integrating CO2–climate interactions when predicting forest responses to future climate extremes.