The importance of carbon dioxide in the xylem of trees was examined. A technique was developed to continuously measure the internal CO2 concentration ([CO2]) in xylem of trees. Experiments using this technique showed a diurnal pattern in [CO2] that was negatively correlated with xylem sap flow, suggesting that CO2 was transported in xylem and that previous estimates of stem respiration based on measurements of CO2 efflux were erroneous due to the confounding influence of storage and transport of CO2 in the xylem. Based on this idea, a mass balance approach that accounted for internal as well as external flux of respired CO2 was developed to more accurately estimate stem respiration. This approach revealed that different fluxes of CO2 dominated estimates of respiration at different times of day and in different environmental conditions and that stem efflux was not a good predictor of stem respiration. To further examine the effects of sap velocity and temperature on internal and external fluxes of respiratory CO2, an experiment was conducted in the lab with detached branch segments that removed the confounding influence of previously-stored CO2 and controlled sap velocity and temperature. When sap velocity was high, sap [CO2] decreased but more respired CO2 was transported and less fluxed to the atmosphere. Respiratory flux rates were similar when calculated on a branch volume basis, but were positively correlated with diameter when calculated on a surface area basis, suggesting that the cellular source of respired CO2 was predominantly xylem parenchyma rather than cambium and phloem. Temperature had a typical exponential effect on branch respiration, but at higher temperature, a larger proportion of respired CO2 fluxed to the atmosphere. These measurements showed unequivocally that not all respired CO2 fluxed to the atmosphere; some remained in the stem and was transported in the sap stream. To quantitatively determine the fate of xylem-transported CO2, an experiment was designed to simulate the transport of CO2 from stems into branches by allowing detached sycamore branches to transpire water enriched with the stable isotope 13C. Subsequent analysis of the branch tissues showed that about two-thirds of the transported 13CO2 fluxed to the atmosphere and one-third was assimilated by photosynthetic cells in woody tissue, and to a lesser extent, in leaves. The results of all of these experiments suggested that there were many factors that influenced the fate of respired CO2. A conceptual diagram is presented that illustrates some of these factors and their effects on woody tissue respiration and the fate of respired CO2.
