Background
In the introduction to their study, the authors write that "there have been marked changes in plant phenology over the past century," and they indicate that these changes "have been interpreted as a consequence of the increase in temperature that has been observed over this time." In addition, they speculate that "the concentration of atmospheric CO2 may also directly affect time of flowering, even in the absence of temperature change."

What was done
In exploring this dual possibility, Johnston and Reekie examined the effects of elevated atmospheric CO2 concentration by itself (ambient and ambient + 330 ppm) as well as the combined effect of elevated CO2 and elevated air temperature (ambient + 1.5°C) on the flowering phenology of 22 species of plants in the family Asteraceae, which were grown under natural seasonally-varying temperature and daylength in separate compartments of a glasshouse in Wolfville, Nova Scotia, Canada.

What was learned
The two researchers report that, "on average, elevated CO2 by itself advanced flowering by four days," while "increasing temperature as well as CO2 advanced flowering by an additional three days." They also found that "CO2 was more likely to hasten phenology in long- than in short-day species," and that "early- and late-flowering species did not differ in response to elevated CO2, but the combined effect of elevated CO2 and temperature hastened flowering more in early- than late-flowering species."

What it means
In light of their several findings, Johnston and Reekie concluded that, with respect to time of flowering in Asteraceae species, "the direct effect of CO2 on phenology may be as important as its indirect effect through climate change."