This study is part of a research program examining the effects of elevated atmospheric carbon dioxide on a pasture. It was designed to examine (i) the interaction between disturbance and the atmospheric CO2concentration at the community level, and (ii) the response of a major weedChenopodium albumto CO2enrichment in a natural field situation. Although both the total number of species and Simpson's index increased upon disturbance, these traits did not respond to CO2fertilization. Counter to our expectation, we found no significant interaction between disturbance and CO2. The composition of the community that established in the open, disturbed spaces was a function of seed availability and as such independent of the atmospheric CO2concentration. Using height of the background vegetation to assess the impact of elevated CO2, we found some evidence for density dependence in the undisturbed quadrats but not in the disturbed ones. ForC.album, the disturbance regimes outweigh the CO2increase in importance. Neighboring plants have a strong influence onC.albumgrowth, this even though theC.albumphotosynthetic mechanisms are potentially responsive to elevated CO2. The present study highlights the complex feed-back interactions occurring when a community is exposed to elevated CO2concentration.Key words: elevated carbon dioxide, disturbance,Chenopodium album, open-top chambers.