The IPCC predicts a doubling of atmospheric [CO₂] by the year 2050 accompanied by a related increase in global air temperature. Elevated atmospheric [CO₂] has the potential to increase plant productivity, but thermal stress may diminish this effect, especially at the warm limit of a species range. We investigated the seedling growth response to ambient and elevated [CO₂] (380  vs. 700 µmol mol^-1) and air temperature (ambient vs. +2°C) in two hardwood tree species, Quercus rubra and Liriodendron tulipifera, grown in treatment chambers at Tifton, GA. The site is within the natural distribution of L. tulipifera, but south of the natural distribution of Q. rubra, suggesting the potential for differing responses to the treatments between the two species. We measured photosynthesis and leaf respiration three times and seedling height and diameter five times during the growing season. Final seedling biomass was determined at the end of the growing season.

Results/Conclusions

Final biomass of L. tulipifera was not significantly different among treatments, suggesting that it was insensitive to changes in both [CO₂] and temperature. In contrast, Q. rubra responded to both elevated [CO₂] and elevated temperature. Biomass of Q. rubra seedlings was significantly greater when grown in elevated [CO₂], but significantly smaller when grown in elevated temperature, compared to seedlings grown in ambient conditions. Seedlings grown in the combination of elevated temperature and elevated [CO₂] had similar biomass to those in ambient conditions, suggesting that elevated [CO₂] mitigated the reduction in growth of seedlings subjected to elevated temperature. Sequential measurements of seedling height and diameter showed no difference in growth patterns in L. tulipifera among treatments, but Q. rubra seedlings grown in elevated temperature showed a reduction in both height and diameter which began early in the growing season and became more pronounced as the season progressed. In both species, foliar respiration did not appear to acclimate to temperature and elevated [CO₂] had no consistent effect on photosynthesis. Our findings show that the predicted future increase in atmospheric [CO₂] accompanied by an increase in air temperature did not have an effect on biomass accumulation of either of these hardwood species, but the mechanisms for these responses differ. Grown within its native range, L. tulipifera, did not respond to either elevated [CO₂] or elevated temperature, while Q. rubra, growing outside its native range, had a negative response to increased temperature that was mitigated by the positive effects of elevated [CO₂].