Carbon dioxide transport in xylem causes errors in estimation of rates of respiration in stems and branches of trees

Complementary laboratory and field experiments showed that the internal transport of carbon dioxide (CO2) in the xylem of trees is an important pathway for carbon movement. Carbon dioxide released by respiration dissolves in sap and moves upward in the transpirational stream. The concentration of CO2 in xylem sap can be up to three orders of magnitude greater than that found in the atmosphere. In the present experiments, diffusion outward of a portion of xylem-transported CO2 caused a substantial overestimation of the apparent rate of stem and branch respiration. Rates of CO2 efflux were linearly related to sap CO2 concentration. Direct manipulations of xylem sap CO2 concentration produced rapid and reversible changes in CO2 efflux from stems and branches, in some cases quadrupling the rate of efflux. These results demonstrated that apparent rates of stem and branch respiration of trees are in large part a byproduct of the rate of CO2 diffusion from xylem sap.