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

Elevated atmospheric CO ₂ 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 CO ₂ 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.