Elevated CO 2 does not affect stem CO 2 efflux nor stem respiration in a dry Eucalyptus woodland, but it shifts the vertical gradient in xylem [CO 2 ]

To quantify stem respiration (R S ) under elevated CO 2 (eCO 2 ), stem CO 2 efflux (E A ) and CO 2 flux through the xylem (F T ) should be accounted for, because part of respired CO 2 is transported upwards with the sap solution. However, previous studies have used E A as a proxy of R S , which could lead to equivocal conclusions. Here, to test the effect of eCO 2 on R S , both E A and F T were measured in a free‐air CO 2 enrichment experiment located in a mature Eucalyptus native forest. Drought stress substantially reduced E A and R S , which were unaffected by eCO 2 , likely as a consequence of its neutral effect on stem growth in this phosphorus‐limited site. However, xylem CO 2 concentration measured near the stem base was higher under eCO 2 , and decreased along the stem resulting in a negative contribution of F T to R S , whereas the contribution of F T to R S under ambient CO 2 was positive. Negative F T indicates net efflux of CO 2 respired below the monitored stem segment, likely coming from the roots. Our results highlight the role of nutrient availability on the dependency of R S on eCO 2 and suggest stimulated root respiration under eCO 2 that may shift vertical gradients in xylem [CO 2 ] confounding the interpretation of E A measurements.