Boreal and subarctic peatlands contain 20-30 f the world's soil organic carbon, and if growing, they constitute sinks for atmospheric CO2. We hypothesized that even in the nutrient-poor bog environment, elevated CO2 would stimulate Sphagnum growth more than vascular plant growth, thereby improving Sphagnum's competitive strength and enhancing carbon (C) sequestration. Free-air carbon dioxide enrichment (FACE) experiments took place on predominantly ombrotrophic peatbog-lawns in Finland (FI), Sweden (SW), The Netherlands (NL), and Switzerland (CH). After 3 yr of treatment, increased CO2 concentration (560 ppm on volume basis) had no significant effect on Sphagnum or vascular plant biomass at either site. This research suggests that, just as with other nutrient-poor ecosystems, increased atmospheric CO2 concentrations will have a limited effect on bog ecosystems.