What was done
Field-sown meadow fescue (Festuca pratensis cv. Kalevi) was grown in open-top chambers and glasshouses receiving atmospheric CO2 concentrations of 350 and 700 ppm in combination with ambient and elevated (ambient plus 3°C) air temperatures for four consecutive years to determine the effects of these parameters on aboveground biomass production in this important forage crop.

What was learned
Elevated CO2 significantly increased aboveground biomass by an average of 18% in each of the experimental years.  However, the effect was only present when plants were concomitantly exposed to elevated air temperatures.  At ambient air temperatures, elevated CO2 significantly increased total aboveground biomass in only one of the four experimental years.  Nonetheless, elevated CO2 did not reduce biomass production at ambient temperatures; it simply failed to stimulate it.

What it means
As the CO2 content of the air increases, meadow fescue plants will likely exhibit significant increases in aboveground biomass production, but only if ambient air temperatures also increase.  If ambient air temperatures do not increase, then plants will likely continue to display their current patterns of growth and biomass production.