Photosynthesis, growth and density for the dominant species in a CO2-enriched grassland

Although increased atmospheric CO2 frequently increases short-term photosynthetic rates, longer-term photosynthetic responses are more variable. Plant size, reproduction and ecosystem carbon gain are determined, in part, by such photosynthetic responses. Here we examine photosynthetic regulation for the dominant species in a grassland exposed to elevated CO2 and examine whether the observed photosynthetic responses contribute to changes in growth, reproduction and plant density in the same grassland. Avena barbata in the field showed little evidence of photosynthetic downregulation with elevated CO2 at the end of the growing season (differences between treatments <10%). Glasshouse studies also showed little evidence for downregulation of photosynthesis measured at various light and intercellular CO2. concentrations. Although specific leaf mass (leaf mass per unit leaf area) for Avena increased 20% in the field with elevated CO2, leaf nitrogen concentrations decreased 25%, resulting in an 11% reduction in leaf N on a leaf-area basis. For the relatively wet 1993 growing season, Avena barbata increased its size and reproduction approximately 30% in elevated CO2, with a 21% decrease in population density. For the relatively dry 1994 season Avena density was almost doubled in elevated CO2, but increases in individual size and reproduction with CO2 were small (6-18%). The primary effect of CO2 in the drier year appears to have been greater Avena survival, rather than increased individual size.