Background
The Duke Forest FACE experiment, which began full operation in August of 1996, supplies an extra 200 ppm of CO2 to three of six 30-m-diameter circular plots within a community of loblolly pine (1733 stems per ha), sweetgum (620 stems per ha) and yellow poplar (68 stems per ha) trees, which also contains lesser amounts of several additional hardwood species.  In a previous study of this ecosystem's sweetgum trees, Herrick and Thomas (1999) found that the extra CO2 increased the net photosynthetic rates of its sun and shade leaves by 92 and 54%, respectively, in June, while in August it enhanced them by 166 and 68%, respectively.  Likewise, Herrick and Thomas (2001) observed mean photosynthetic enhancements of 63 and 48% in sun and shade leaves during the middle portion of the third full growing season, indicative of little to no down-regulation of photosynthesis over the first three years of the experiment.

What was done
In the current late-season study (late September to early November), Herrick and Thomas continue to assess the potential down-regulation aspect of the sweetgum trees' response to elevated atmospheric CO2, as well as the potential for extra CO2 to alter various aspects of leaf phenology.

What was learned
The authors report they "found no evidence that atmospheric CO2 enrichment affected leaf phenology of overstory sweetgum trees growing in the Duke Forest FACE experiment."  Specifically, they note that "elevated CO2 did not affect emergence date, abscission date or leaf longevity of sun or shade leaves."  However, there were still large increases in the net photosynthetic rates of the leaves of the CO2-enriched trees: 51 to 96% in sun leaves and 23 to 51% in shade leaves.

The two scientists additionally report that they observed no effect of elevated CO2 on stomatal conductance in either the sun or shade leaves of the sweetgum trees during the latter part of the growing season, in contrast to what had been observed in some earlier experiments.  But they note that in a review of 48 different tree studies, Curtis and Wang (1998) found that the average stomatal conductance response to elevated CO2 was highly variable with a non-significant reduction of only 11%.

What it means
Herrick and Thomas conclude that atmospheric CO2 enrichment stimulates the seasonally integrated carbon gain of sweetgum trees by increasing rates of leaf photosynthesis, but not by changing leaf phenology.  They also demonstrate that the CO2-induced stimulation of photosynthesis has now persisted in these specific trees for several years, with no indication it is in process of significantly declining.

References
Curtis, P.S. and Wang, X.  1998.  A meta-analysis of elevated CO2 effects on woody plant mass, form, and physiology.  Oecologia 113: 299-313.

Herrick, J.D. and Thomas, R.B.  1999.  Effects of CO2 enrichment on the photosynthetic light response of sun and shade leaves of canopy sweetgum trees (Liquidambar styraciflua) in a forest ecosystem.  Tree Physiology 19: 779-786.

Herrick, J.D. and Thomas, R.B.  2001.  No photosynthetic down-regulation in sweetgum trees (Liquidambar styraciflua L.) after three years of CO2 enrichment at the Duke Forest FACE experiment.  Plant, Cell and Environment 24: 53-64.