Mechanisms causing drought mortality are currently unresolved, and the role of carbon starvation and hydraulic failure as the cause of mortality is currently under significant debate. We tested whether hydraulic failure and/or carbon starvation were causes of mortality in Eucalyptus sideroxylon plants subject to well watered conditions and extreme drought for 60 days. Theory suggests that reduced stomatal conductance and increased water use efficiency under elevated CO2 will lead to higher soil water availability and ‘water savings’. We tested whether elevated CO2 led to water savings, mitigated drought stress and delayed mortality due to drought, and, conversely, whether elevated temperatures increased drought stress and reduced time to mortality. Water relations were measured on Eucalypt seedlings grown across a CO2 gradient of 280, 400 and 640 ppm CO2 concentrations, and ambient and elevated (ambient +4 degrees C) temperatures. Stomatal conductance, transpiration, leaf and soil water potential, leaf area and xylem vulnerability curves for each treatment were compared to determine the effects of CO2 and temperature on hydraulic architecture.
Carbohydrate, leaf and soil water potential data and xylem vulnerability curves suggest that mortality in the Eucalypt saplings was caused by hydraulic failure rather than carbon starvation. Interestingly, elevated CO2 did not lead to increased water availability, rather, it caused trees to grow larger, and use more water, which led to faster mortality from drought. As expected, high temperatures increased water stress and reduced time to mortality. Results from this experiment suggest that Eucalypt saplings may increase their growth rates rather than experience water savings under elevated CO2. These increased growth rates, combined with higher temperatures, suggest that under future climates, Eucalypts may be at higher risk of drought mortality. Furthermore, we provide evidence that hydraulic failure, not carbon starvation, caused mortality in a Eucalypt subject to an extreme 60 day drought.