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

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