• Energy Research

AbstractIt 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 640 μL 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.

Elevated [CO2] and temperature may alter the drought responses of tree seedling growth, photosynthesis, respiration and total non-structural carbohydrate (TNC) status depending on drought intensity and duration. Few studies have addressed these important climatic interactions or their consequences. We grew Eucalyptus globulus Labill. seedlings in two [CO2] concentrations (400 and 640 μl l(-1)) and two temperatures (28/17 and 32/21 °C) (day/night) in a sun-lit glasshouse, and grew them in well-watered conditions or exposed them to two drought treatments having undergone different previous water conditions (i.e., rewatered drought and sustained drought). Progressive drought in both drought treatments led to similar limitations in growth, photosynthesis and respiration, but reductions in TNC concentration were not observed. Elevated [CO2] ameliorated the impact of the drought during the moderate drought phase (i.e., Day 63 to Day 79) by increasing photosynthesis and enhancing leaf and whole-plant TNC content. In contrast, elevated temperature exacerbated the impact of the drought during the moderate drought phase by reducing photosynthesis, increasing leaf respiration and decreasing whole-plant TNC content. Extreme drought (i.e., Day 79 to Day 103) eliminated [CO2] and temperature effects on plant growth, photosynthesis and respiration. The combined effects of elevated [CO2] and elevated temperature on moderate drought stressed seedlings were reduced with progressive drought, with no sustained effects on growth despite greater whole-plant TNC content.

AbstractEucalypts are likely to play a critical role in the response of Australian forests to rising atmospheric CO2 concentration ([CO2]) and temperature. Although eucalypts are frequently phosphorus (P) limited in native soils, few studies have examined the main and interactive effects of P availability, [CO2] and temperature on eucalypt morphology, physiology and anatomy. To address this issue, we grew seedlings of Eucalyptus tereticornis Smith across its P-responsive range (6–500 mg kg−1) for 120 days under two [CO2] (ambient: 400 μmol mol−1 (Ca) and elevated: 640 μmol mol−1 (Ce)) and two temperature (ambient: 24/16 °C (Ta) and elevated: 28/20 °C (Te) day/night) treatments in a sunlit glasshouse. Seedlings were well-watered and supplied with otherwise non-limiting macro- and micro-nutrients. Increasing soil P supply increased growth responses to Ce and Te. At the highest P supplies, Ce increased total dry mass, leaf number and total leaf area by ~50%, and Te increased leaf number by ~40%. By contrast, Ce and Te had limited effects on seedling growth at the lowest P supply. Soil P supply did not consistently modify photosynthetic responses to Ce or Te. Overall, effects of Ce and Te on growth, physiological and anatomical responses of E. tereticornis seedlings were generally neutral or negative at low soil P supply, suggesting that native tree responses to future climates may be relatively small in native low-P soils in Australian forests.