Background
The authors write that "it is generally believed that characteristics that contribute to the invasiveness of a plant, namely broad environmental tolerance, high relative growth rate and high fecundity, are the very traits that would be favored in a warmer, high-CO2 world," and they note that "previous research has demonstrated substantial impacts of elevated CO2 on selected invasive species, mostly indicating that elevated CO2 does increase weed invasion success, particularly when the invasive species [are] C3 plants."

What was done
Among other things, Williams et al. investigated this hypothesis at the Tasmanian Free-Air CO2 Enrichment (TasFACE) facility, which is located in a native lowland grassland in the southern midlands region of Tasmania, Australia, where they studied the impacts of an imposed 170-ppm increase in atmospheric CO2 concentration and a 2°C rise in air temperature over the period stretching from the spring of 2003 to the summer of 2006, during which time they documented annual seed production, seedling emergence, seedling survival and adult survival of four abundant perennial species, including the two most dominant invading weeds: Hypochaeris radicata L. and Leontodon taraxacoides (Vill.) Merat, which are members of the Asteraceae family.

What was learned
The six researchers determined there were no significant CO2-induced differences in the population growth rates of either weed species; but they found that the population growth rates of both of them "were substantially reduced by warming."

What it means
Williams et al. concluded from their findings that "global warming may be a more important determinant of the success of invasive species than CO2 concentration," and they say their results suggest that both of the invading weed species they studied in Tasmania "are likely to be excluded [our italics] from the grassland community by increasing temperatures."