What was done
The authors used "models that performed well in a multiyear simulation of current carbon and water budgets of an upland-oak forest (Hanson et al., 2004) to evaluate the influence of single and multifactor environmental change scenarios projected for 2100 ... with and without modifications to account for physiological and growth responses 'learned' from long-term field experimental studies (Winnett, 1998)," where the environmental changes to be evaluated were a 385 ppm increase in CO2, a 20 ppb increase in O3, a 4°C increase in temperature, and a 20% increase in winter precipitation, and where the responses to those changes "were derived primarily from field experimental studies on deciduous trees and forest systems."

What was learned
Initial simplistic model projections of annual net ecosystem carbon exchange (NEEa) for the single-factor change scenarios yielded NEEa responses of +191% for CO2, -206% for temperature, 0% for precipitation, and -35% for O3, such that the combined influence of the four environmental changes yielded a 29% reduction in mean NEEa.  However, as Hanson et al. report, "when experimentally observed physiological adjustments were included in the simulations (e.g. acclimation of leaf respiration to warming), the combined influence of the year 2100 scenario resulted in a 20% increase [our italics] in NEEa, not a decrease."  In addition, they say that "consistent with the annual model's predictions, simulations with a forest succession model run for gradually changing conditions from 2000 to 2100 indicated an 11% increase in stand wood biomass in the future compared with current conditions."

What it means
Even with the unrealistically extreme temperature change investigated in this study, which came from the IPCC's Third Assessment Report (Houghton et al., 2001) and the US National Assessment Synthesis Team's report on climate-change impacts (NAST, 2000), knowledge gained from real-world experiments demonstrated that desirable plant responses to atmospheric CO2 enrichment were sufficient to override the huge negative influence of the inflated warming and produce a significant enhancement in NEEa.

References
Hanson, P.J., Samuelson, L.J., Wullschleger, S.D. et al.  1994.  Seasonal patterns of light-saturated photosynthesis and leaf conductance for mature and seedling Quercus rubra L. foliage: differential sensitivity to ozone.  Tree Physiology 14: 1351-1366.

Houghton, J.T., Ding, Y., Griggs, D.J., et al.  2001.  Climate Change 2001.  In: Maskell, K. and Johnson, C.A. (Eds.).  The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC).  Cambridge University Press, Cambridge, United Kingdom.

National Assessment Synthesis Team (NAST).  2000.  Climate Change Impacts on the United States: The Potential Consequences of Climate Variability and Change.  US Global Change Research Program, Washington, DC.

Winnett, S.M.  1998.  Potential effects of climate change on US forests: a review.  Climate Research 11: 39-49.