The quandary of sources and sinks of CO2 efflux in tree stems—new insights and future directions

Stem respiration (R_S) substantially contributes to the return of photo-assimilated carbon to the atmosphere and, thus, to the tree and ecosystem carbon balance. Stem CO₂ efflux (E_CO2) is often used as a proxy for R_S. However, this metric has often been challenged because of the uncertain origin of CO₂ emitted from the stem due to post-respiratory processes. In this Insight, we (i) describe processes affecting the quantification of R_S, (ii) review common methodological approaches to quantify and model R_S, and (iii) develop a research agenda to fill the most relevant knowledge gaps that we identified. Dissolution, transport and accumulation of respired CO₂ away from its production site, reassimilation of respired CO₂ via stem photosynthesis and the enzyme phosphoenolpyruvate carboxylase, axial CO₂ diffusion in the gas phase, shifts in the respiratory substrate and non-respiratory oxygen (O₂) consumption are the most relevant processes causing divergence between R_S and measured stem gas exchange (E_CO2 or O₂ influx, I_O2). Two common methodological approaches to estimate R_S, namely the CO₂ mass balance approach and the O₂ consumption technique, circumvent some of these processes but have yielded inconsistent results regarding the fate of respired CO₂. Stem respiration modelling has recently progressed at the organ and tree levels. However, its implementation in large-scale models, commonly operated from a source-driven perspective, is unlikely to reflect adequate mechanisms. Finally, we propose hypotheses and approaches to advance the knowledge of the stem carbon balance, the role of sap pH on R_S, the reassimilation of respired CO₂, R_S upscaling procedures, large-scale R_S modelling, and shifts in respiratory metabolism during environmental stress.