Stem radial CO2 conductance affects stem respiratory CO2 fluxes in ash and birch trees

The CO2 released from respiring cells in woody tissues of trees can contribute to one of three fluxes: efflux to the atmosphere (E-A), internal xylem sap transport flux (F-T), and storage flux (S). Adding those fluxes together provides an estimate of actual stem respiration (R-S).We know that the relative proportion of CO2 in those fluxes varies greatly among tree species, but we do not yet have a clear understanding of the causes for this variation. One possible explanation is that species differ in stem radial CO2 conductance (g(c)). A high g(c) would favor the E-A pathway and a low g(c) would favor the F-T pathway. However, g(c) has only been measured once in situ and only in a single tree species. We measured g(c) using two methods in stems of Fraxinus mandshurica Rupr. (ash) and Betula platyphylla Suk. (birch) trees in situ, along with R-S, E-A, F-T and S. Stem radial CO2 conductance was substantially greater in ash trees than in birch trees. Corresponding to that finding, in ash trees over 24h, E-A constituted the entire flux of respired CO2, and F-T was negative, indicating that additional CO2, probably transported from the root system via the xylem, was also diffusing into the atmosphere. In ash trees, F-T was negative over the entire 24h, and this study represents the first time that has been reported. The addition of xylem-transported CO2 to E-A caused E-A to be 9% higher than the actual R-S over the diel measurement period. Birch trees, which had lower g(c), also had a more commonly seen pattern, with E-A accounting for about 80% of the CO2 released from local cell respiration and F-T accounting for the remainder. The inorganic carbon concentration in xylem sap was also lower in ash trees than in birch trees: 2.7 versus 5.3mmol L-1, respectively. Our results indicate that stem CO2 conductance could be a very useful measurement to help explain differences among species in the proportion of respired CO(2)that remains in the xylem or diffuses into the atmosphere.