Differential responses to elevated atmospheric CO2 concentration exhibited by different plant functional types may alter competition for above- and below ground resources in a higher CO2 world. Because C allocation to roots is often favored over C allocation to shoots in plants grown with CO2 enrichment, below ground function of forest ecosystems may change significantly. We established an outdoor facility to examine the effects of elevated CO2 on root dynamics in artificially constructed communities of five early successional forest species: (1) a C3 evergreen conifer (longleaf pine, Pinus palustris Mill.); (2) a C4 monocotyledonous bunch grass (wiregrass, Aristida stricta Michx.); (3) a C3 broadleaf tree (sand post oak, Quercus margaretta); (4) a C3 perennial herbaceous legume (rattlebox, Crotalaria rotundifolia Walt. ex Gemel); and (5) an herbaceous C3 dicotyledonous perennial (butterfly weed, Asclepias tuberosa L.). These species are common associates in early successional longleaf pine savannahs throughout the southeastern USA and represent species that differ in life-form, growth habit, physiology, and symbiotic relationships. A combination of minirhizotrons and soil coring was used to examine temporal and spatial rooting dynamics from October 1998 to October 1999. CO2-enriched plots exhibited 35% higher standing root crop length, 37% greater root length production per day, and 47% greater root length mortality per day. These variables, however, were enhanced by CO2 enrichment only at the 10-30 cm depth. Relative root turnover (flux/standing crop) was unchanged by elevated CO2. Sixteen months after planting, root biomass of pine was 62% higher in elevated compared to ambient CO2 plots. Conversely, the combined biomass of rattlebox, wiregrass, and butterfly weed was 28% greater in ambient compared to high CO2 plots. There was no difference in root biomass of oaks after 16 months of exposure to elevated CO2. Using root and shoot biomass as a metric, longleaf pine realized the greatest and most consistent benefit from exposure to elevated CO2. This finding suggests that the ability of longleaf pine to compete with sand post oak, a common deciduous tree competitor, and wiregrass, the understory herbaceous species, in regenerating ecosystems may be significantly enhanced by rising atmospheric CO2 concentrations.
