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 (S) 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 2years 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 S, R or D-g. We computed E-t, S, R and D-g from measurements of sapflow velocity, L, soil water content (), understory micrometeorology, throughfall and stemflow. We found that eC(a) did not affect E-tree, E-floor, S or at any depth (to 4.5m) 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 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.