• Rising atmospheric CO2 concentrations are likely to have direct effects on terrestrial ecosystems. Here we describe effects of elevated concentrations of CO2 on an understory plant community in terms of production and community composition. • In 2001 and 2002 total and species-specific above-ground net primary produc- tivity (ANPP) were estimated by harvesting above-ground biomass within an under- story community receiving ambient [CO2] and elevated [CO2] at Oak Ridge National Laboratory’s free-air carbon dioxide enrichment (FACE) facility. • Duringawetyearcommunitycompositiondifferedbetweenplotsreceivingambient [CO2] and elevated [CO2] but total ANPP did not differ. By contrast during a drier year community composition did not differ but total ANPP was greater in elevated than ambient [CO2] plots. These patterns were driven by the response of two codo- minant species Lonicera japonica and Microstegium vimineum both considered invasive species in the south-eastern United States. The ANPP of L. japonica was consistently greater under elevated [CO2] whereas the response of M. vimineum to CO2 enrichment differed between years and mediated total community response. • These data suggest that community and species responses to a future CO2- enriched atmosphere may be mediated by other environmental factors and will depend on individual species responses.