Observed and modelled stem respiration CO2 and O2 fluxes in mature beech trees
- 1Max-Planck Institute for Biogeochemistry, Biogeochemical Processes, Jena, Germany (jhelm@bgc-jena.mpg.de)
- 2Department of Natural Resources and Systems, Universidad Politécnica de Madrid (UPM), Madrid, Spain
- 3Laboratory of Plant Ecology, Department of Plants and Crops, Faculty of Bioscience Engineering, Ghent University, Ghent, Belgium
- 4Faculty of Forest Sciences and Forest Ecology, University of Göttingen, 37077 Göttingen, Germany
We lack a detailed understanding of tree carbon flux dynamics to quantify stem respiration correctly. Stem CO2 radial diffusivity and vertical CO2 transport with the xylem sap produce uncertainties in respiration estimates based on stem CO2 efflux measurements. Two independent approaches have been applied for comparison to assess such uncertainties: (1) the mass balance approach accounts for CO2 efflux from the stem to the atmosphere, CO2 transport through the xylem and CO2 stored within the stem to estimate total respiration rates, and (2) measurements of stem O2 consumption as a more robust proxy for stem respiration than stem CO2 efflux, because O2 is less soluble than CO2 in the sap solution and hence less affected by vertical transport.
In this study, we compare these two approaches and study CO2 and O2 flux dynamics along the stem height to capture CO2 transport. During summer 2019, we measured vertical CO2 and radial CO2 and O2 fluxes, sap flow, stem temperature and xylem sap pH from twigs. Stem CO2 and O2 fluxes were calculated along 4-m stem segments on mature beech trees in a managed forest in Germany.
We found that xylem [CO2] and CO2 and O2 fluxes did not vary with stem height. Interestingly, stem CO2 efflux was a poor predictor of stem respiration in the monitored mature trees. O2 influx was always higher than CO2 efflux (on average CO2-to-O2 ratios was 0.72), resulting in an underestimation of stem respiration when using CO2 measurements only, which is standard practice.
Preliminary results of the implementation of this dataset into a biophysical stem respiration model (TReSpire) suggest that CO2 respired in the xylem of mature trees encounter a relatively long diffusive pathway to reach the bark tissues, so that a significant fraction of CO2 dissolves in the sap and is transported upwards before being detected by traditional approaches. Our study provides insights into stem carbon flux dynamics in large trees, and thus helps to improve estimation of ecosystem carbon cycling.
How to cite: Helm, J., Salomón, R., Muhr, J., Steppe, K., Hilman, B., and Hartmann, H.: Observed and modelled stem respiration CO2 and O2 fluxes in mature beech trees, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4213, https://doi.org/10.5194/egusphere-egu22-4213, 2022.
