What was done
The authors grew two adequately watered and fertilized C3 grasses - native Wallaby grass, i.e., Austrodanthonia eriantha (Lindl.), and non-native Vulpia myuros (L.) C.C. Gmel - in either monoculture or competition in pots within controlled environment chambers maintained at either 350 or 700 ppm CO2 at day/night temperatures of either 20/10°C or 23/13°C for a period of 50 days, while comparing their relative rates of aboveground growth (as calculated from rates of total aboveground "biovolume" production) at 10-day intervals.

What was learned
Hely and Roxburgh report that "above-ground growth of all plants was enhanced by increased CO2 and temperature alone, however the combined temperature and CO2 treatment showed a sub-additive effect, where growth was less than expected based on the responses to each factor independently."  In addition, and more to the point of their study, they found that "competition was clearly evident ... with Vulpia significantly reducing the above-ground growth and hence final plant volume of Austrodanthonia," and in this regard they further note that although temperature had no effect upon the competitive interaction, "there was some suggestion that the suppression of the Austrodanthonia by the Vulpia was less severe under elevated CO2."

What it means
Contrary to what is often claimed by climate alarmists, the experiment of Hely and Roxburgh revealed that increases in the air's CO2 concentration need not enhance the competitiveness of invading plant species.  In fact, it provided a suggestion of a CO2-induced weakening of their invasive tendencies.