Elevated CO2 did not affect the hydrological balance of a mature native Eucalyptus woodland

Elevated atmospheric CO₂ concentration (eC_a) might reduce forest water‐use, due to decreased transpiration, following partial stomatal closure, thus enhancing water‐use efficiency and productivity at low water availability. If evapotranspiration (E_t) is reduced, it may subsequently increase soil water storage (ΔS) or surface runoff (R) and drainage (D_g), although these could be offset or even reversed by changes in vegetation structure, mainly increased leaf area index (L). To understand the effect of eC_a in a water‐limited ecosystem, we tested whether 2 years of eC_a (~40% increase) affected the hydrological partitioning in a mature water‐limited Eucalyptus woodland exposed to Free‐Air CO₂ Enrichment (FACE). This timeframe allowed us to evaluate whether physiological effects of eC_a reduced stand water‐use irrespective of L, which was unaffected by eC_a in this timeframe. We hypothesized that eC_a would reduce tree‐canopy transpiration (E_tree), but excess water from reduced E_tree would be lost via increased soil evaporation and understory transpiration (E_floor) with no increase in ΔS, R or D_g. We computed E_t, ΔS, R and D_g from measurements of sapflow velocity, L, soil water content (θ), understory micrometeorology, throughfall and stemflow. We found that eC_a did not affect E_tree, E_floor, ΔS or θ at any depth (to 4.5 m) over the experimental period. We closed the water balance for dry seasons with no differences in the partitioning to R and D_g between C_a levels. Soil temperature and θ were the main drivers of E_floor while vapour pressure deficit‐controlled E_tree, though eC_a did not significantly affect any of these relationships. Our results suggest that in the short‐term, eC_a does not significantly affect ecosystem water‐use at this site. We conclude that water‐savings under eC_a mediated by either direct effects on plant transpiration or by indirect effects via changes in L or soil moisture availability are unlikely in water‐limited mature eucalypt woodlands.