Abstract
Elevated atmospheric CO 2 concentration (e C 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 (Δ 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 e C a in a water‐limited ecosystem, we tested whether 2 years of e C 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 e C a reduced stand water‐use irrespective of L , which was unaffected by e C a in this timeframe. We hypothesized that e C 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 Δ S , R or D g . We computed E t , Δ S , R and D g from measurements of sapflow velocity, L , soil water content (θ), understory micrometeorology, throughfall and stemflow. We found that e C a did not affect E tree , E floor , Δ S 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 e C a did not significantly affect any of these relationships. Our results suggest that in the short‐term, e C a does not significantly affect ecosystem water‐use at this site. We conclude that water‐savings under e C 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.