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.