Effects of elevated CO2, water stress, and nitrogen level on competitive interactions of simulated loblolly pine and sweetgum stands

Abstract

Loblolly pine (Pinustaeda L.) and sweetgum (Liquidambarstyraciflua L.) were grown in mixed stands and in monocultures at 2.54 × 2.54 cm spacing in controlled-environment chambers. Treatments consisted of present (ambient) and projected future (ambient + 400 ppm) carbon dioxide (CO2) concentrations, drought-stressed, and well-watered conditions, and low (20 kg N/ha) and high (474 kg N/ha) nitrogen application rates. After two accelerated growing cycles, total biomass of both species was significantly greater under elevated CO2. No significant interactions between CO2 concentration and water availability, nitrogen availability, or stand type were observed. Competitive interactions between loblolly pine and sweetgum were strongly influenced by water availability, but not CO2 concentration. Assessment of species response to CO2 was dependent upon growth in monoculture or mixture. Under low water availability, data from monocultures suggested that sweetgum had a stronger growth response to elevated CO2 concentrations than loblolly pine. In contrast, results from mixed-species stands showed that the competitive status of loblolly pine and sweetgum did not change under the high CO2 concentration. These results underscore the value of growing co-occurring species in mixed stands under varying levels of multiple resources for the determination of relative performance under future environments.