What was done
Over the first three years (1998-2000) of this Oak Ridge National Laboratory FACE study of a closed-canopy sweetgum (Liquidamber styraciflua) stand, where 25-m-diameter circular plots of trees were exposed to either ambient air having an average mid-day CO2 concentration of 364 ppm or CO2-enriched air having a corresponding concentration of 553 ppm, the authors measured the net photosynthetic rates of upper-canopy leaves on a regular basis throughout all three growing seasons.

What was learned
The authors report that "net photosynthetic rates of upper-canopy leaves were 44% higher in trees grown in elevated CO2 compared with ambient CO2 over the 3-yr period," and that "there were no significant CO2 treatment effects on photosynthetic or biochemical capacity of L. styraciflua leaves."

What it means
The results of this study clearly demonstrate that photosynthetic acclimation or down regulation did not occur, under conditions once thought to promote a tapering off of the positive effects of the aerial fertilization effect of atmospheric CO2 enrichment.  As a result, Sholtis et al. conclude that "the enhancement of photosynthesis in L. styraciflua in elevated CO2 is likely to continue for many years, thereby providing more carbon for growth in this closed-canopy deciduous forest."