Background
In the United States, most of California's species-rich native grasslands growing on nutrient-rich soils have been overrun and replaced by species-poor Eurasian complexes of annual grasses.  However, native grasslands still maintain their presence and diversity in nutrient-poor and heavy-metal-rich serpentine soils that occur along the Coast Range and Sierra Nevada Mountains.  Recently, an annual forb - Centaurea solstitialis - has also been invading California's grasslands on nutrient-rich soils, but it has not yet established a significant presence in the highly-diverse native grasslands common to nutrient-poor serpentine soils.

What was done
The author grew Centaurea solstitialis in monocultures on nutrient-poor serpentine soils at ambient and twice-ambient atmospheric CO2 concentrations to determine if this normally-invasive species is responsive to elevated CO2 and can survive on this soil type without competition from other species.  In addition, the author constructed model microcosms of native California grasslands containing the invasive forb Centaurea solstitialis on nutrient-poor serpentine soils and subjected them to atmospheric CO2 concentrations of 350 and 700 ppm for one growing season to determine if elevated CO2 will favor the expansion of this non-native species into these species-rich grassland communities.

What was learned
In the monoculture experiments, elevated CO2 dramatically affected the growth of Centaurea solstitialis; plants grown at twice-ambient atmospheric CO2 concentrations achieved rates of photosynthesis that were130% greater than rates displayed by ambiently-grown plants.  In addition, elevated CO2 increased the aboveground biomass of the invader by 70%.  On the other hand, when grown in competition with native serpentine grassland species, Centaurea solstitialis did not exhibit much of a response.  Under ambient CO2 concentrations, for example, the invasive species comprised 4.3% of the total community biomass, while at the elevated CO2 concentration it comprised only 1.2% more, i.e., 5.5%.  At the same time, however, the extra CO2 increased total community biomass by 28%.

What it means
As the CO2 content of the air increases, it is unlikely that the invasive forb Centaurea solstitialis will significantly affect the productivity of native grasslands growing on California's nutrient-poor serpentine soils.  Thus, the high species richness that exists in these grasslands will likely be maintained in the future.  Furthermore, these grasslands should exhibit significant increases in total community productivity as the air's CO2 content continues to rise.