Drought responses of two gymnosperm species with contrasting stomatal regulation strategies under elevated [CO2] and temperature

Future climate regimes characterized by rising [CO₂], rising temperatures and associated droughts may differentially affect tree growth and physiology. However, the interactive effects of these three factors are complex because elevated [CO₂] and elevated temperature may generate differential physiological responses during drought. To date, the interactive effects of elevated [CO₂] and elevated temperature on drought-induced tree mortality remain poorly understood in gymnosperm species that differ in stomatal regulation strategies. Water relations and carbon dynamics were examined in two species with contrasting stomatal regulation strategies: Pinus radiata D. Don (relatively isohydric gymnosperm; regulating stomata to maintain leaf water potential above critical thresholds) and Callitris rhomboidea R. Br (relatively anisohydric gymnosperm; allowing leaf water potential to decline as the soil dries), to assess response to drought as a function of [CO₂] and temperature. Both species were grown in two [CO₂] (C_a (ambient, 400 μl l⁻¹) and C_e (elevated, 640 μl l⁻¹)) and two temperature (T_a (ambient) and T_e (ambient +4 °C)) treatments in a sun-lit glasshouse under well-watered conditions. Drought plants were then exposed to a progressive drought until mortality. Prior to mortality, extensive xylem cavitation occurred in both species, but significant depletion of non-structural carbohydrates was not observed in either species. T_e resulted in faster mortality in P. radiata, but it did not modify the time-to-mortality in C. rhomboidea. C_e did not delay the time-to-mortality in either species under drought or T_e treatments. In summary, elevated temperature (+4 °C) had greater influence than elevated [CO₂] (+240 μl l⁻¹) on drought responses of the two studied gymnosperm species, while stomatal regulation strategies did not generally affect the relative contributions of hydraulic failure and carbohydrate depletion to mortality under severe drought.