Effects of xylem sap flow on carbon dioxide efflux from stems of birch (Betula pendula Roth)

The effect of xylem sap flow in stems of mature Betula pendula Roth on radial CO2 efflux was studied from April to October 2001. Temperature-controlled respiration cuvettes allowed measurements of CO2 efflux without interference from temperature gradients between stem surface and sapwood. Variations of sap flow in different stem sectors, and in a given sector at different heights were analysed. Daytime reduction of CO2 efflux caused by sap flow was expressed as the difference between gross and apparent CO2 release. Gross CO2 release was calculated from Arrhenius-equations derived from night-time data records of the same day, which were free from interference by sap flow. In mid-July, daytime reductions of CO2 efflux reached 1.8-3.9 μmol CO2 m−2 g−1 xylem sap transpired. Assuming tree-specific maximum transpiration rates of 30 kg H2O d−1 this is up to 40% of gross CO2 release. In relation to photosynthetic CO2 fixation the endogenous supply of dissolved CO2 to the leaves acccounted for 0.5-3.7%. This study indicates a negative correlation between sap flow velocity and radial CO2 efflux from B. pendula stems. Periods of unbalanced CO2 partial pressures between aqueous and gaseous pathways during increase and decrease of sap flow seem to affect gaseous CO2 release through lenticels. It is concluded that CO2 efflux rates are not simply equivalent to respiration rates because of the interference of aqueous CO2 transport by xylem sap flow in the wood-body of trees.