Differences between tree stem CO₂ efflux and O₂ influx rates cannot be explained by internal CO₂ transport or storage in large beech trees

Tree stem respiration (R-S) is a substantial component of the forest carbon balance. The mass balance approach uses stem CO2 efflux and internal xylem fluxes to sum up R-S, while the oxygen-based method assumes O-2 influx as a proxy of R-S. So far, both approaches have yielded inconsistent results regarding the fate of respired CO2 in tree stems, a major challenge for quantifying forest carbon dynamics. We collected a data set of CO2 efflux, O-2 influx, xylem CO2 concentration, sap flow, sap pH, stem temperature, nonstructural carbohydrates concentration and potential phosphoenolpyruvate carboxylase (PEPC) capacity on mature beech trees to identify the sources of differences between approaches. The ratio of CO2 efflux to O-2 influx was consistently below unity (0.7) along a 3-m vertical gradient, but internal fluxes did not bridge the gap between influx and efflux, nor did we find evidence for changes in respiratory substrate use. PEPC capacity was comparable with that previously reported in green current-year twigs. Although we could not reconcile differences between approaches, results shed light on the uncertain fate of CO2 respired by parenchyma cells across the sapwood. Unexpected high values of PEPC capacity highlight its potential relevance as a mechanism of local CO2 removal, which merits further research.