Temporal and spatial effects of elevated CO2 on greenhouse gas fluxes from tree stems in an upland temperate forest

Tree stems emit CO₂ and can exchange CH₄ with the atmosphere (either emitting or uptaking), with a significant contribution to the C budgets from local to regional scales. However, there is still a need to better understand the spatial and temporal variability of stem CO₂ and CH₄ fluxes to quantify the role of vegetation on the C cycle and how these fluxes will behave under future environmental conditions such as atmospheric elevated CO₂. An increment of atmospheric CO₂ concentrations might result in higher photosynthetic rates, which would spin the C cycle in the trees, potentially increasing stem CO₂ emissions due to higher stem respiration and higher soil-derived CO₂ contribution. Higher photosynthetic rates might also stimulate fine roots exudation, which could stimulate methanotrophic or methanogenic communities. Additionally, elevated CO₂ would increase water use efficiency at the leaf level, reducing the amount of water transpired, and potentially increasing soil moisture, which would favour conditions for CH₄ production. In this study, we present one year of monthly measurements of stem CO₂ and CH₄ fluxes from mature oaks (Quercus robur) growing under elevated CO₂ (~150 ppm above atmospheric concentrations) and ambient conditions, in a second-generation FACE experiment (Free Air CO₂ Enrichment; BIFoR-FACE UK). Trees growing under ambient conditions emitted 76% more CO₂ than those under elevated atmospheric CO₂, which was not what we hypothesized. Despite stem CH₄ fluxes have been reported in multiple upland ecosystems for lots of tree species, our preliminary results did not show clear evidence of CH₄ stem fluxes (emissions or uptake) for the oaks at our study site. Similar measurements in other FACE experiments are needed to determine if our results on the effect of elevated CO₂ on stem CO₂ and CH₄ fluxes could be extrapolated to other ecosystems and species.