What was done
In August 1996, circular FACE plots (30-m diameter) receiving 360 and 560 ppm CO2 were established in a now 17-year-old loblolly pine plantation in North Carolina, USA.  Although the plantation is dominated by loblolly pine, several deciduous species have naturally established themselves beneath the primary coniferous canopy and occasionally reach the canopy itself.  This paper describes the short- and long-term respiratory responses of loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) to elevated CO2 exposure.

What was learned
The modest 200-ppm increase in atmospheric CO2 concentration resulted in no significant short-term suppression of dark respiration rates in needles of loblolly pine.  It did, however, reduce rates of dark respiration in sweetgum leaves by an average of 10%.  The long-term exposure to elevated CO2 also did not appear to alter maintenance respiration, which is the amount of CO2 respired to maintain existing tissue, in either of the tested species.  However, growth respiration, which is the amount of CO2 respired when constructing new tissues, was reduced by 21% in loblolly pine and 39% in sweetgum leaves at the top of the canopy.

What it means
As the air's CO2 content continues to rise, it is likely that these two forest species will exhibit increased rates of photosynthesis that will provide them with more of the raw materials required for constructing new and greater amounts of biomass.  At the same time, respiratory construction costs during the synthesis of new tissues likely will be reduced, thus allowing greater amounts of carbon to be retained in the trees during this process.  Ultimately, more carbon should be sequestered by both species, helping to reduce the rate of rise of the atmosphere's CO2 concentration.