Sixteen-year stimulation of sedge density by elevated CO2 reaches 100% and is salt-stress enhanced.

Increased atmospheric CO₂ concentration produces a short-term stimulation of plant growth across terrestrial ecosystems. However, the long-term response remains uncertain and is thought to depend on environmental constraints. In the longest experiment on natural ecosystem response to elevated CO₂, we observed for the wetland sedge Scirpus olneyi that the shoot-density stimulation by elevated CO₂ increased from 16% during the first four years to more than 100% in the 16th year. While most of the inter-annual variability of the stimulation was explained by this steady increase with time (r2 = 0.89), the residuals of this variability were positively correlated with the salinity of the marsh (r2 = 0.44). These results demonstrate that the inter-annual variability of plant response to elevated CO₂ is coupled to environmental stress and indicate that experiments based on more than a decade of measurements are necessary to ascertain such effects.