Elevated CO2 does not affect stem CO2 efflux nor stem respiration in a dry Eucalyptus woodland, but it shifts the vertical gradient in xylem [CO2]

To quantify stem respiration (R-S) under elevated CO2 (eCO(2)), stem CO2 efflux (E-A) and CO2 flux through the xylem (F-T) should be accounted for, because part of respired CO2 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 CO2 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 CO2 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 CO2 was positive. Negative F-T indicates net efflux of CO2 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 [CO2] confounding the interpretation of E-A measurements