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

Abstract

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 24 h, 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 24 h, 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 5.3 mmol 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 CO₂that remains in the xylem or diffuses into the atmosphere.

Keywords

Stem CO₂ conductance / Stem respiration / Stem CO₂ efflux / Transport flux / Stem temperature / Sap flow / Sap flux density

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