Abstract Plants have shown responses to elevated CO 2 in many experiments under controlled conditions. Yet, predicting responses under field conditions is still difficult and the number of long-term field studies on elevated CO 2 is limited. Here the results from 4 years’ physiology and production studies in the field are presented. In a species-rich semi-natural grassland in central Sweden open-top chambers were used to study the effects of elevated carbon dioxide concentration (twice the ambient level) on plant production, physiology and species composition. The first three growing seasons showed a 30–60% increase in above-ground biomass at harvest under elevated CO 2. During the fourth year there was no difference in above-ground biomass between the treatments. For all years, leaf-level photosynthesis for measured species was 30–60% higher and stomatal conductance 20–40% lower at elevated CO 2 than at ambient. Nitrogen concentration in stems and leaves was 5–20% lower at elevated CO 2. Specific leaf area (SLA) did not show any response to elevated CO 2. The variation in the effect of CO 2 on above-ground production was attributed to variation in water stress, with low water stress (high precipitation) giving the least effect. It is concluded that even in this relatively low-production system CO 2 effects can persist for at least several years and even increase.