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Long-Term Studies (Woody Plants: Sweetgum) -- Summary
In the spring of 1988, a single-species sweetgum (Liquidambar styraciflua L.) plantation was established on a nutrient-rich soil at the Oak Ridge National Environmental Research Park in Roane County, Tennessee, USA, within which plantation a group of scientists constructed five circular FACE plots of 25 meters' diameter several years later.  Initial atmospheric CO2 enrichment to a daytime average concentration of 533 ppm (as opposed to the ambient concentration of 394 ppm) was begun in two of the plots in April of 1998, well after the 1996 date at which the young forest's canopy had achieved closure and the trees had entered into the stable linear growth phase in which yearly growth increments are approximately the same each year (as opposed to the exponential growth phase that precedes this more mature state of growth).

Monthly bole circumference measurements at a height of 1.3 meters above the ground were begun in April of 1997 (a full year before the start of differential CO2 treatments) on every tree within 10 meters of the centers of the ambient and CO2-enriched plots.  Several of these trees were subsequently sacrificed to determine their aboveground biomass; and a relationship was developed between this parameter and the tree basal area derived from the bole circumference measurements.  Based on these data, Norby et al. (2001) determined "there was no pretreatment bias to confound subsequent effects of CO2 on growth," and over the next two years of differential CO2 exposure, they thus determined that the increase in atmospheric CO2 concentration employed in their study increased the biomass production of the trees by an average of 24% over the first two years of the experiment, indicating, in their words, "that large trees have the capacity to respond to elevated CO2 just as much as younger trees that are in exponential growth," which was something that until that time had been highly conjectural.

After three years of exposure to different CO2 concentrations (reported at this subsequent time to be 360 and 550 ppm in the ambient and CO2-enriched plots, respectively), Gunderson et al. (2002) found that the 53% increase in the air's CO2 concentration they were imposing on the trees was boosting rates of net photosynthesis by 46% in both upper- and mid-canopy foliage.  In addition, they reported they had observed no decline in photosynthetic enhancement over the preceding three years of their study.

Simultaneously, Norby et al. (2002) reported that the elevated CO2 increased ecosystem net primary productivity by 21% in all of the three preceding years, stating yet again that "this experiment has provided the first evidence that CO2 enrichment can increase productivity in a closed-canopy deciduous forest."  And after yet an additional year of measurements, Norby et al. (2003) determined that net primary productivity had actually been enhanced by an average of 22% over years 2-5 of the study, reaffirming their conclusions of the year before.

A second long-term FACE study that is still ongoing and includes sweetgum trees has been running concurrently at Duke Forest in the Piedmont region of North Carolina, USA, where the soils are low in available nitrogen and phosphorus.  There, in August of 1996, three 30-m-diameter CO2 delivery rings began to enrich the air around the then-13-year-old trees they encircled -- including loblolly pine (1733 stems per ha), sweetgum (620 stems per ha) and yellow poplar (68 stems per ha) -- to 190 ppm above the atmosphere's ambient CO2 concentration, while three other FACE rings were used as ambient-air control plots.

In the initial stages of this study, Herrick and Thomas (1999) found that the elevated CO2 significantly increased photosynthetic rates in both sun and shade leaves, with the greatest CO2-induced photosynthetic stimulation occurring in August, when the mean maximum air temperature was 4C higher and monthly rainfall was 66% lower than it was in June.  In June, for example, the extra CO2 increased photosynthetic rates of sun and shade leaves by 92 and 54%, respectively, while in August, corresponding increases were 166 and 68%.

Two years later, Herrick and Thomas (2001) observed mean photosynthetic enhancements of 63 and 48% in sun and shade leaves during the middle portion of the study's third full growing season, indicative of little to no down-regulation of photosynthesis over the first three years of the experiment.  And after two more years, Herrick and Thomas (2003) found 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.

In conclusion, as these two ongoing long-term FACE studies of mature sweetgum trees move into their sixth year and beyond, they continue to display significant increases in net primary production that give no evidence of declining as the years progress.

References
Gunderson, C.A., Sholtis, J.D., Wullschleger, S.D., Tissue, D.T., Hanson, P.J. and Norby, R.J.  2002.  Environmental and stomatal control of photosynthetic enhancement in the canopy of a sweetgum (Liquidambar styraciflua L.) plantation during 3 years of CO2 enrichment.  Plant, Cell and Environment 25: 379-393.

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.

Herrick, J.D. and Thomas, R.B.  2003.  Leaf senescence and late-season net photosynthesis of sun and shade leaves of overstory sweetgum (Liquidambar styraciflua) grown in elevated and ambient carbon dioxide concentrations.  Tree Physiology 23: 109-118.

Norby, R.J., Hanson, P.J., O'Neill, E.G., Tschaplinski, T.J., Weltzin, J.F., Hansen, R.A., Cheng, W., Wullschleger, S.D., Gunderson, C.A., Edwards, N.T. and Johnson, D.W.  2002.  Net primary productivity of a CO2-enriched deciduous forest and the implications for carbon storage.  Ecological Applications 12: 1261-1266.

Norby, R.J., Sholtis, J.D., Gunderson, C.A. and Jawdy, S.S.  2003.  Leaf dynamics of a deciduous forest canopy: no response to elevated CO2Oecologia 10.1007/s00442-003-1296-2.

Norby, R.J., Todd, D.E., Fults, J. and Johnson, D.W.  2001.  Allometric determination of tree growth in a CO2-enriched sweetgum stand.  New Phytologist 150: 477-487.