We investigated the effect of elevated [CO2] (700 μmol mol−1), elevated temperature (+2 °C above ambient) and decreased soil water availability on net photosynthesis (Anet) and water relations of one-year old potted loblolly pine (Pinus taeda L.) seedlings grown in treatment chambers with high fertility at three sites along a north-south transect covering a large portion of the species native range. At each location (Blairsville, Athens and Tifton, GA) we constructed four treatment chambers and randomly assigned each chamber one of four treatments: ambient [CO2] and ambient temperature, elevated [CO2] and ambient temperature, ambient [CO2] and elevated temperature, or elevated [CO2] and elevated temperature. Within each chamber half of the seedlings were well watered and half received much less water (1/4 that of the well watered).
Measurements of net photosynthesis (Anet), stomatal conductance (gs), leaf water potential and leaf fluorescence were made in June and September, 2008. We observed a significant increase in Anet in response to elevated [CO2] regardless of site or temperature treatment in June and September. An increase in air temperature of over 2 °C had no significant effect on Anet at any of the sites in June or September despite over a 6 °C difference in mean annual temperature between the sites. Decreased water availability significantly reduced Anet in all treatments at each site in June. The effects of elevated [CO2] and temperature on gs followed a similar trend. The temperature, [CO2] and water treatments did not significantly affect leaf water potential or chlorophyll fluorescence. Our findings suggest that predicted increases in [CO2] will significantly increase Anet, while predicted increases in air temperature will have little effect on Anet across the native range of loblolly pine. Potential decreases in precipitation will likely cause a significant reduction in Anet, though this may be mitigated by increased [CO2].