Differences between tree stem CO 2 efflux and O 2 influx rates cannot be explained by internal CO 2 transport or storage in large beech trees

Juliane Helm (1, 2) , Roberto L Salomón (3, 4) , Boaz Hilman (1) , Jan Muhr (1, 5) , Alexander Knohl (5) , Kathy Steppe (4) , Yves Gibon (6) , Cédric Cassan (6) , Henrik Hartmann (1)

1 MPI-BGC - Max Planck Institute for Biogeochemistry
2 Unibas - Université de Bâle = University of Basel = Basel Universität
3 UPM - Universidad Politécnica de Madrid
4 UGENT - Universiteit Gent = Ghent University
5 Georg-August-University = Georg-August-Universität Göttingen
6 BFP - Biologie du fruit et pathologie

Tree stem respiration (R S) is a substantial component of the forest carbon balance. The mass balance approach uses stem CO 2 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 CO 2 in tree stems, a major challenge for quantifying forest carbon dynamics. We collected a data set of CO 2 efflux, O 2 influx, xylem CO 2 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 CO 2 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 CO 2 respired by parenchyma cells across the sapwood. Unexpected high values of PEPC

Mots clés: CO2/O2 ratio; carbon dioxide transport; mature trees; oxygen consumption; temperate forest; vertical stem gradient.

Domaines: Sciences du Vivant [q-bio]; Biologie végétale

Juliane Helm, Roberto L Salomón, Boaz Hilman, Jan Muhr, Alexander Knohl, et al.. Differences between tree stem CO 2 efflux and O 2 influx rates cannot be explained by internal CO 2 transport or storage in large beech trees. Plant, Cell and Environment, 2023, Online first, ⟨10.1111/pce.14614⟩. ⟨hal-04121662⟩