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Duke Forest Trees Exposed to Elevated Atmospheric CO2 Continue to Sop Up Carbon at Greatly Enhanced Rates
Schafer, K.V.R., Oren, R., Ellsworth, D.S., Lai, C.-T., Herrick, J.D., Finzi, A.C., Richter, D.D. and Katul, G.G.  2003.  Exposure to an enriched CO2 atmosphere alters carbon assimilation and allocation in a pine forest ecosystem.  Global Change Biology 9: 1378-1400.

What was done
The authors linked a leaf-level CO2 assimilation model (Katul et al., 2000) with a light attenuation model (Campbell and Norman, 1998; Stenberg, 1998) and measurements of sap-flux-based canopy conductance (Kostner et al., 1992; Ewers and Oren, 2000) to create what they call a canopy conductance-constrained CO2 assimilation (4C-A) model.  They then tested the new model with measurements of net ecosystem exchange (NEE) and net ecosystem production (NEP) in the ambient and CO2-erniched plots of the Duke Forest FACE study, after which they used it to asses the effects of elevated CO2 on carbon uptake and allocation to different components of the forest's C budget under ambient and CO2-enriched conditions.

What was learned
During the third and fourth years of the study, the extra 200 ppm of CO2 supplied to the CO2-enriched FACE plots increased the uptake of CO2 (NEE) by 39% in the dominant Pinus taeda L. trees, while it increased CO2 uptake by 47% in the mix of broadleaf understory species (Liquidambar styraciflua L., Acer rubrum L., Ulmus alata Michx. and Cornus florida L.), for a mean enhancement of the entire canopy's CO2 uptake of 41%.  NEP increased slightly more, rising by 44% in the CO2-enriched FACE rings.  [Linearly extrapolating the latter result to what might be expected for a 300 ppm increase in the air's CO2 concentration, which is a common enrichment increment used in many open-top chamber experiments, we get a CO2-induced NEP enhancement of 66%.]  The authors report that fully 87% of this extra net ecosystem production "was sequestered in a moderately long-term C pool in wood, with the remainder in the forest floor-soil subsystem."

What it means
The results obtained from the Duke Forest FACE study over the past five years are most impressive.  However, many of the scientists associated with it remain pessimistic about the future.  Schafer et al., for example, suggest that "if nutrient limitation imposes a constraint on future productivity, it is likely that C allocation to the production of wood will decrease in favor of the allocation to fine root production, rhizodeposition, and mycorrhizal symbionts," citing Norby et al. (1992, 2001), which they further suggest "will likely result in a rapid return of fixed C to the atmosphere (Merbach et al., 1999)."  The bottom line of this chain of events would be that "high rates of C fixation under elevated CO2 will result in an acceleration of the C cycle through the forest ecosystem with little of the C remaining in long-term storage pools."

So far, however, there are no indications that such is about to happen anytime soon, or ever, for that matter.  Hence, it is essential that this important study be continued for as long as possible to ultimately provide a definitive answer to this worrisome hypothesis.

Campbell, G.S. and Norman, J.M.  1998.  An Introduction to Environmental Biophysics.  Second Edition.  Springer Verlag, New York, NY.

Ewers, B.E. and Oren, R.  2000.  Analysis of assumptions and errors in the calculation of stomatal conductance from sap flux measurements.  Tree Physiology 20: 579-589.

Katul, G.G., Ellsworth, D.S. and Lai, C.-T.  2000.  Modelling assimilation and intercellular CO2 from measured conductance: a synthesis of approaches.  Plant, Cell and Environment 23: 347-353.

Kostner, B.M.M., Schulze, E.-D., Kelliher, F.M. et al.  1992.  Transpiration and canopy conductance in a pristine broad leafed forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements.  Oecologia 91: 350-359.

Merbach, W., Mirus, E., Knof, G., Remus, R., Ruppel, S., Russow, R., Gransee, A. and Schuize, J.  1999.  Release of carbon and nitrogen compounds by plant roots and their possible ecological importance.  Zeitschrift fur Pflanzenerna'hrung und Bodenkunde 162: 373-383.

Norby R.J., Gunderson, C.A., Wullschleger, S.D., O'Neill, E.G. and McCracken, M.K.  1992.  Productivity and compensatory response of yellow poplar trees in elevated CO2Nature 357: 322-324.

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

Stenberg, P.  1998.  Implications of shoot structure on the rate of photosynthesis at different levels in a coniferous canopy using a model incorporating grouping and penumbra.  Functional Ecology 12: 82-91.

Reviewed 12 November 2003