1. 4. Conclusions Extreme precipitation led to increased water stress and smaller plants. The detrimental impact of extreme water was larger than the ameliorative impact of elevated CO 2 on plant water stress (leaf water potential). Elevated CO 2 ameliorated the negative impact of extreme water on stomatal conductance in both species, and leaf area in Eucalypts. Acacia grew larger, had lower stomatal conductance during the day and night, and was more water stressed than Eucalyptus. Based on these results, savanna Eucalyptus tetrodonta may experience less water stress and perform better than the rainforest Acacia auriculiformis in future climates. Future climates are projected to include more intense, or ‘extreme precipitation’. Extreme precipitation is defined as larger, less frequent rain events, with droughts followed by floods. Few studies have quantified the response of deep rooted tree species to extreme precipitation. There has been debate over whether savanna boundaries and woody thickening/encroachment results from altered water availability or elevated CO 2.. Extreme precipitation increases water stress and reduces tree growth in elevated CO 2. Melanie Zeppel 1, Belinda Medlyn 1, Caroline Lehmann 1, JD Lewis 2, Macquarie University 1, Sydney Australia, Fordham University 2, NY Melanie.Zeppel@mq.edu.au This project tested the hypotheses that: 1.extreme precipitation would lead to increased water stress and decreased growth; 2.elevated CO 2 would ameliorate water stress and increase growth; 3.root depth would be deeper in extreme water and elevated CO 2 ; and 4.the savanna species, Eucalyptus tetrodonta, would be less water stressed with higher growth than the rainforest species Acacia auriculiformis. 3. Results: Extreme precipitation increased water stress and reduced leaf area. Elevated CO 2 had minimal impact on water stress and leaf area. However, the impact of extreme water was greater than the impact of elevated CO 2. Further elevated CO 2 ameliorated the reductions in stomatal conductance and leaf area imposed by extreme precipitation. Root depth of Eucalyptus tended to be deeper in extreme precipitation (p=0.078). Acacia had significantly less root biomass in extreme precipitation and elevated CO 2. Acknowledgements: Zeppel was funded by a Macquarie University Research Fellowship and an ARC Discovery Early Career Researcher DECRA. Lehmann was funded by a Macquarie University Research Fellowship Many thanks to Chris Blackman, Sean Gleason, Martin DeKauwe and MQ interns for lab assistance. Table 1. The influence of CO 2, water, species and two- and three- way interactions on mid-day leaf water potential (Ψ, MPa), stomatal conductance during the day (g s,d, mmol m -2 s -1 ) and night (g s,n, mmol m -2 s -1 ), leaf area (cm 2 ), root biomass (g), and root depth (% of shallow roots, < 15 cm). P values are reported and significant values are bold. Source of variationLeaf water potential Ψ (MPa) g s,d (mmol m -2 s -1 ) g s,n (mmol m -2 s -1 ) Leaf area (cm 2 ) Root biomass (g) Root depth (% shallow) CO 2 0.024n.s. Water0.0090.037n.s.<0.0010.012n.s. Species<0.001 0.009 CO 2 x Watern.s.0.001n.s. CO 2 x sppn.s. 0.006n.s. Water x sppn.s. 0.0740.0020.004n.s. CO 2 x Water x sppn.s. Fig 4. The impact of extreme water and elevated CO 2 on Leaf area. Fig 5. The impact of extreme water and elevated CO 2 on leaf water potential. Fig 1. Soil water content in extreme and control water and ambient and elevated CO 2. 1. Background 2. Aims Fig 3. The impact of extreme water and elevated CO 2 on root biomass and depth. Fig 3. The impact of extreme water and elevated CO 2 on day- time and night-time stomatal conductance.