Strong resilience of stem CO2 fluxes from a mature temperate forest under elevated atmospheric CO2

There is substantial uncertainty regarding how different components of mature forests respond to rising atmospheric CO₂ concentrations under ongoing climate change. Tree stems, in particular, may act as increased carbon sinks due to enhanced growth under CO₂ fertilization, but they may also release more CO₂ as a consequence of higher metabolic rates and accelerated carbon cycling. Here, we investigated stem CO₂ fluxes in a mature oak (Quercus robur) stand exposed to elevated CO₂ since 2016 as part of a Free-Air CO₂ Enrichment experiment (BIFoR FACE, UK; +150 ppm above ambient concentrations). Stem CO₂ fluxes were measured over one year through monthly campaigns at 1.3 m height, and seasonally along the stem profile up to 4 m height. Stem CO₂ fluxes exhibited a pronounced seasonal pattern, with higher rates during the growing season, a decline in autumn, and consistently low fluxes during winter. However, neither the magnitude nor the seasonal dynamics of stem CO₂ fluxes were affected by elevated CO₂. Furthermore, partitioning total stem fluxes into maintenance respiration (associated with the metabolism of living stem tissues) and growth respiration (associated with the biosynthesis of new stem cells) revealed no significant response of either component to elevated CO₂. Stem CO₂ fluxes also showed no consistent vertical gradient along the stem, and this pattern was similarly unaffected by CO₂ enrichment. Overall, these findings indicate a strong functional resilience of stem CO₂ fluxes in mature trees to elevated atmospheric CO₂. This resilience may have important implications for predicting forest carbon balance responses to future climate conditions, particularly in mature temperate forests.