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The effect of xylem sap flow in stems of mature Betula pendula Roth on radial CO"2 efflux was studied from April to October 2001. Temperature-controlled respiration cuvettes allowed measurements of CO"2 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 CO"2 efflux caused by sap flow was expressed as the difference between gross and apparent CO"2 release. Gross CO"2 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 CO"2 efflux reached 1.8-3.9@mmol CO"2m^-^2g^-^1 xylem sap transpired. Assuming tree-specific maximum transpiration rates of 30kg H"2Od^-^1 this is up to 40% of gross CO"2 release. In relation to photosynthetic CO"2 fixation the endogenous supply of dissolved CO"2 to the leaves acccounted for 0.5-3.7%. This study indicates a negative correlation between sap flow velocity and radial CO"2 efflux from B. pendula stems. Periods of unbalanced CO"2 partial pressures between aqueous and gaseous pathways during increase and decrease of sap flow seem to affect gaseous CO"2 release through lenticels. It is concluded that CO"2 efflux rates are not simply equivalent to respiration rates because of the interference of aqueous CO"2 transport by xylem sap flow in the wood-body of trees.
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