Response of an understory plant community to elevated [CO ] depends on differential responses of dominant invasive species and is mediated by soil water availability.

•  Rising atmospheric CO concentrations are likely to have direct effects on terrestrial ecosystems. Here, we describe effects of elevated concentrations of CO on an understory plant community in terms of production and community composition. •  In 2001 and 2002 total and species-specific above-ground net primary productivity (ANPP) were estimated by harvesting above-ground biomass within an understory community receiving ambient [CO ] and elevated [CO ] at Oak Ridge National Laboratory's free-air carbon dioxide enrichment (FACE) facility. •  During a wet year, community composition differed between plots receiving ambient [CO ] and elevated [CO ], 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 [CO ] plots. These patterns were driven by the response of two codominant 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 [CO ], whereas the response of M. vimineum to CO enrichment differed between years and mediated total community response. •  These data suggest that community and species responses to a future, CO -enriched atmosphere may be mediated by other environmental factors and will depend on individual species responses.