Elevated [CO2] does not ameliorate the negative effects of elevated temperature on drought-induced mortality in Eucalyptus radiata seedlings

It has been reported that elevated temperature accelerates the time-to-mortality in plants exposed to prolonged drought, while elevated [CO2] acts as a mitigating factor because it can reduce stomatal conductance and thereby reduce water loss. We examined the interactive effects of elevated [CO2] and temperature on the inter-dependent carbon and hydraulic characteristics associated with drought-induced mortality in Eucalyptus radiata seedlings grown in two [CO2] (400 and 640μL L-1) and two temperature (ambient and ambient +4°C) treatments. Seedlings were exposed to two controlled drying and rewatering cycles, and then water was withheld until plants died. The extent of xylem cavitation was assessed as loss of stem hydraulic conductivity. Elevated temperature triggered more rapid mortality than ambient temperature through hydraulic failure, and was associated with larger water use, increased drought sensitivities of gas exchange traits and earlier occurrence of xylem cavitation. Elevated [CO2] had a negligible effect on seedling response to drought, and did not ameliorate the negative effects of elevated temperature on drought. Our findings suggest that elevated temperature and consequent higher vapour pressure deficit, but not elevated [CO2], may be the primary contributors to drought-induced seedling mortality under future climates. To date, very few studies have investigated drought-induced mortality as a function of two key global change factors (i.e. rising [CO2] and temperature), particularly with the respect to their interaction. In this manuscript, we addressed the main and interactive impacts of elevated [CO2] and temperature on tree seedling response to drought and mechanisms associated with drought-induced mortality in Eucalyptus radiata seedlings. We found that elevated temperature treatments triggered more rapid mortality than ambient temperature treatments, and were associated with increased drought sensitivities of gas exchange traits and earlier substantial xylem cavitation. Elevated [CO2] had a negligible effect on drought responses, and did not ameliorate the negative effects of elevated temperature on drought stress.