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 ]

Abstract

To quantify stem respiration (R_S ) under elevated CO₂ (eCO₂ ), 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 eCO₂ 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 eCO₂ , 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 eCO₂ , 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 eCO₂ and suggest stimulated root respiration under eCO₂ that may shift vertical gradients in xylem [CO₂ ] confounding the interpretation of E_A measurements.

Keywords: CO2 enrichment; EucFACE; Eucalyptus tereticornis; growth respiration; nutrient-limited growth; woody tissue respiration; xylem CO2 transport.