Impact of heat waves, drought stress and elevated CO2 on northern red oak seedlings

Background/Question/Methods

The frequency and intensity of heat waves are predicted to increase in the future. We investigated whether heat waves of different severities would have the same impact as a constant increase in temperature with the same heat sum, and whether elevated [CO₂] would mitigate, and soil drought would exacerbate, plant response to severe heat. We measured gas exchange and biomass yield of Quercus rubra L. seedlings grown in pots in climate-controlled chambers at a field site near Athens GA.  Treatments consisted of ambient or elevated [CO₂] (380 or 700 μmol CO₂ mol^-1) and four temperature treatments: ambient (+0°C), ambient +3°C, moderate heat wave (+6°C), or severe heat wave (+12°C).  Heat wave treatments were applied as a week-long heat wave every other week of +6°C, or every fourth week of +12°C throughout the growing season. This schedule yielded the same heat sum and average temperature for all three elevated temperature treatments for each four week period, and for the entire growing season.  Half of the seedlings in each treatment combination were well-watered and half were grown under low soil moisture.

Results/Conclusions

In ambient [CO₂], well-watered plants had significantly less biomass when exposed to either the +6^oC or +12^oC heat waves compared to a constant +3^oC increase, or ambient, temperatures.  The trend was the same for plants in the low water treatment, but was not statistically significant.  In all temperature x [CO₂] treatment combinations the low-water treatment substantially reduced biomass compared to the well-watered treatment.  Under well-watered conditions biomass accumulation in elevated [CO₂] was generally higher than under ambient [CO₂], and significant differences among temperature treatments were absent.  Biomass accumulation under elevated [CO₂] was lower in the low-water treatment compared to the well-watered treatment, and also higher than in those treatments under ambient [CO₂].  Heat waves significantly reduced afternoon net photosynthesis (A_net), but not morning A_net. Elevated [CO₂] increased A_net but did not completely mitigate effects of heat and drought stress on instantaneous performance. Low soil moisture significantly reduced A_net outside of the heat wave treatments, and diminished the heat wave-induced A_net decrease in both [CO₂] treatments.  We conclude that a) heat waves produced more stress than the same amount of heat applied uniformly and b) elevated [CO₂] mitigated many of the negative effects of the heat waves.