Above-ground plant production under elevated carbon dioxide in a Swedish semi-natural grassland

Plants have shown responses to elevated CO2 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 CO2 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 CO2. 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 CO2 than at ambient. Nitrogen concentration in stems and leaves was 5-20% lower at elevated CO2. Specific leaf area (SLA) did not show any response to elevated CO2. The variation in the effect of CO2 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 CO2 effects can persist for at least several years and even increase.