The effect of an increase in air temperature and [CO2] on northern red oak biomass accumulation

Background/Question/Methods
Climate reports released by the IPCC have predicted a doubling of atmospheric [CO₂] by the year 2050, which is expected to increase mean annual air temperature.  An increase in atmospheric [CO₂] is expected to boost biomass productivity; however, an increase in air temperature may mitigate the positive effect of elevated [CO₂].  This may be especially pronounced at the southern margins of a species’ distribution where high temperatures may limit growth.  If an increase in air temperature is large enough biomass production may be suppressed or tree mortality may occur.  We investigated the effect of elevated [CO₂] (700 μmol mol^-1) and elevated temperature (+3 and +6°C) on well watered and well fertilized Quercus rubra seedlings grown in treatment chambers at Athens, GA.  We measured photosynthesis and leaf respiration, as well as light and dark acclimated fluorescence, three times during the growing season and determined seedling biomass at the end of the growing season.  

Results/Conclusions

At ambient air temperature biomass production in Q. rubra was stimulated by elevated [CO₂]; however, elevated air temperature mitigated the positive effect of elevated [CO₂].  Biomass of seedlings grown at elevated temperature (+3°C) and elevated [CO₂] was not significantly different than biomass of seedlings grown at ambient temperature and ambient [CO₂].  There was an additional reduction in biomass production of seedlings grown at +6°C.  While elevated [CO₂] boosted photosynthesis, elevated temperature reduced photosynthesis to levels similar to seedlings grown at ambient [CO₂].  Concurrently, there were no significant treatment effects on temperature response curves of foliar respiration, suggesting that respiration did not acclimate to growing temperatures.  Exposure to elevated temperature did not appear to increase foliar damage, as determined by fluorescence.  These results suggest that the reduction in biomass of seedlings grown at higher temperatures was due to both a reduction in carbon fixation and an increase in foliar respiration.  Thus, predicted future climate conditions may suppress biomass production of some hardwood species, depending on the extent of the temperature increase.  At a minimum, we suggest that biomass production of northern red oak near the southern portion of its distribution will not benefit from predicted climate changes.