EFFECTS OF ELEVATED CO2, WATER-STRESS, AND NITROGEN LEVEL ON COMPETITIVE INTERACTIONS OF SIMULATED LOBLOLLY-PINE AND SWEETGUM STANDS

Loblolly pine (Pinus taeda L.) and sweetgum (Liquidambar styraciflua L .) were grown in mixed stands and in monocultures at 2.54 X 2.54 cm sp acing in controlled-environment chambers. Treatments consisted of pres ent (ambient) and projected future (ambient + 400 ppm) carbon dioxide (CO2) concentrations, drought-stressed, and well-watered conditions, a nd low (20 kg N/ha) and high (474 kg N/ha) nitrogen application rates. After two accelerated growing cycles, total biomass of both species w as significantly greater under elevated CO2. No significant interactio ns between CO2 concentration and water availability, nitrogen availabi lity, or stand type were observed. Competitive interactions between lo blolly pine and sweetgum were strongly influenced by water availabilit y, but not CO2 concentration. Assessment of species response to CO2 wa s dependent upon growth in monoculture or mixture. Under low water ava ilability, data from monocultures suggested that sweetgum had a strong er growth response to elevated CO2 concentrations than loblolly pine. In contrast, results from mixed-species stands showed that the competi tive status of loblolly pine and sweetgum did not change under the hig h CO2 concentration. These results underscore the value of growing co- occurring species in mixed stands under varying levels of multiple res ources for the determination of relative performance under future envi ronments.