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World's First Free-Air CO2 Enrichment Study of a Stand of Sweetgum Trees
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.

What was done
The authors worked in a single-species sweetgum (Liquidambar styraciflua L.) plantation at the Oak Ridge National Environmental Research Park in Roane County, Tennessee, USA.  Within this monoculture forest stand, which had been established in the spring of 1988, they constructed three ambient and two CO2-enriched circular FACE plots of 25 meters' diameter.  CO2 enrichment to a daytime average concentration of 533 ppm (as opposed to the ambient concentration of 394 ppm) was begun in the two CO2-enriched FACE rings 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 assessments of tree growth (based on bole circumference measurements at a height of 1.3 m above the surface of the ground) were begun in April of 1997, a full year before the start of differential CO2 treatments.  These measurements were made on every tree within 10 m of the centers of the FACE plots.  Several trees were then sacrificed to determine their aboveground biomass; and a relationship was developed between this parameter and tree basal area derived from the bole circumference measurements.

The purpose of the study was to test the hypothesis that previously observed effects of elevated CO2 on the growth of young and isolated trees "will persist after canopy closure."

What was learned
Based on their 1997 data, the authors first determined "there was no pretreatment bias to confound subsequent effects of CO2 on growth."  Then, from the next two years of differential CO2 exposure, they determined that the 35% 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.

What it means
In the words of the authors, "these results indicate that large trees have the capacity to respond to elevated CO2 just as much as younger trees that are in exponential growth," something that until this time had been highly conjectural.  Hence, it is likely that even mature trees of closed-canopy forests will respond with increasing growth rates and growing appetites for carbon sequestration as the air's CO2 content continues to climb ever higher in the years ahead.