Lethal drought stress is modified by elevated temperature but not elevated [CO2] in eucalypt tree seedlings

Background/Question/Methods

Extreme drought associated with rising atmospheric [CO₂] and temperature may have significant impacts on tree seedling survival. However, very few studies have investigated tree seedling sensitivity to drought as a function of these two key global change factors. Therefore, we designed an experiment to test the following questions: (1) What the proximate mechanisms generating tree seedling mortality during drought? (2) Will elevated temperature exacerbate drought stress and accelerate mortality, potentially through higher water loss or greater carbon utilization? (3) Will elevated [CO₂] ameliorate drought stress and delay mortality by increasing carbohydrate storage and reducing water loss? (4) Will there be interactive effects of elevated [CO₂]  and temperature on seedling sensitivity to drought? We addressed these questions by assessing carbon (i.e. leaf gas exchange, growth and non-structural carbohydrate) and hydraulic (i.e. leaf water potential, vulnerability to xylem cavitation and whole-plant water use) characteristics in Eucalyptus radiata seedlings exposed to two [CO₂] (400 ml l^-1 and 640 ml l^-1) and two temperature (ambient and ambient + 4 °C) treatments in a sun-lit glasshouse.

Results/Conclusions

Mortality of E.radiata seedlings was primarily driven by hydraulic failure (e.g. impacts on leaf water potential, xylem cavitation and whole-plant water use), rather than carbon depletion (e.g. non-structural carbohydrates) during long-term progressive drought. Elevated temperature accelerated declines in both carbon exchange (i.e. photosynthesis, stomotal conductance and night respiration) and hydraulic (i.e. predawn and midday leaf water potential) traits as drought progressed. Additionally, elevated temperature induced greater xylem cavitation and triggered earlier mortality (30 ~ 50 days) than ambient temperature. Elevated [CO₂] increased photosynthesis and growth in seedlings exposed to drought, but it did not reduce whole plant water use or improve hydraulic status; hence, it did not delay mortality. Interestingly, elevated [CO₂] and temperature together induced the greatest xylem cavitation and plants died about 20 days earlier than in ambient [CO₂] and elevated temperature conditions. Overall, our findings indicate that elevated temperature rather than elevated [CO₂] may be more important in regulating drought-induced mortality of E.radiata. Furthermore, the interactive effects of elevated [CO₂] and temperature may generate the greatest susceptibility of tree seedlings to xylem cavitation and ultimately, mortality.