The influence of elevated temperature, elevated atmospheric CO2 concentration and water stress on net photosynthesis of loblolly pine (Pinus taeda L.) at northern, central and southern sites in its native range

We investigated the effect of elevated [CO2] (700 mu mol mol-1), elevated temperature (+2 degrees C above ambient) and decreased soil water availability on net photosynthesis (A(net)) 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 (A(net)), stomatal conductance (g(s)), leaf water potential and leaf fluorescence were made in June and September, 2008. We observed a significant increase in A(net) in response to elevated [CO2] regardless of site or temperature treatment in June and September. An increase in air temperature of over 2 degrees C had no significant effect on A(net) at any of the sites in June or September despite over a 6 degrees C difference in mean annual temperature between the sites. Decreased water availability significantly reduced A(net) in all treatments at each site in June. The effects of elevated [CO2] and temperature on g(s) 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 A(net), while predicted increases in air temperature will have little effect on A(net) across the native range of loblolly pine. Potential decreases in precipitation will likely cause a significant reduction in A(net), though this may be mitigated by increased [CO2].