What was done
Open-top and -bottom chambers of 1.2-m² soil surface area were established in a species-rich grassland located near Basal, Switzerland, and fumigated continuously with atmospheric CO2 concentrations of 350 and 600 ppm, except during the winter months, for four growing seasons to study the effects of elevated CO2 on plant community structure.  The study site is dominated by the perennial grass Bromus erectus and contains many other graminoid, non-legume forb, and legumes species.  It is mown twice a year, to simulate grazing, in order to maintain species diversity.

What was learned
No significant effects of elevated CO2 on community biomass production were evident until the end of the second year of treatment, whereupon community biomass in CO2-enriched plots was found to be 20% greater than that observed in ambiently-grown plots.  This positive effect of atmospheric CO2 enrichment on biomass production slowly increased to 21 and 29% after the third and fourth years of CO2 fumigation, respectively.

As expected, elevated CO2 had different effects on plant functional groups that changed over the years.  Non-leguminous forbs, for example, had the smallest CO2-induced growth response of any functional group after two years of treatment (16%), but exhibited the largest growth response after three years of atmospheric CO2 enrichment (40%).  For comparison, graminoids exhibited less variation over the same period, displaying CO2-induced growth responses of 24 and 23% respectively, while legumes increased from 28 to 34%.  The proportionally greater increases observed in non-leguminous and leguminous forbs, relative to the small decrease observed in graminoid biomass, led to a marginally significant increase in biodiversity, as measured by using Simpson's index, which is sensitive to changes in the relative contributions of the most abundant species in a community.

What it means
As the atmospheric CO2 concentration continues to rise, it is likely that nutrient-stressed calcareous grasslands of Switzerland will exhibit significant increases in biomass that, in the words of the authors, "could reasonably be sustained over the long term."  Such increases in biomass, which are driven by increases in photosynthetic productivity, will likely maintain, or even increase, species richness, as was the case in this study, in these endangered grassland communities.  In addition, carbon sequestration in these grasslands should help to reduce the rate of rise in atmospheric CO2 concentration.