Relationship between stem CO 2 efflux, stem sap velocity and xylem CO 2 concentration in young loblolly pine trees

We measured diel patterns of stem surface CO 2 efflux ( E s , µ mol m −2  s −1 ), sap velocity ( v s , mm s −1 ) and xylem CO 2 concetration ([CO 2 ]) ( X s , %) in 8‐year‐old loblolly pine trees during the spring to determine how v s and X s influence E s . All trees showed a strong diel hysteresis between E s and stem temperature, where at a given temperature, E s was lower during the day than at night. Diel variations in temperature‐independent E s were correlated with v s ( R 2  = 0.54), such that at maximum v s , E s was reduced between 18 and 40%. However, this correlation may not represent a cause‐and‐effect relationship. In a subset of trees, v s was artificially reduced by progressively removing the tree canopy. Reducing v s to near zero had no effect on E s and did not change the diel hysteretic response to temperature. Diel X s tended to decrease with v s and increase with E s , however, in defoliated trees, large increases in X s , when v s  ≈ 0, had no effect on E s . We conclude that at this time of the year, E s is driven primarily by respiration of cambium and phloem tissues and that sap flow and xylem transport of CO 2 had no direct influence on E s .