Abstract, Ecological Society of America Annual Meeting, Madison, WI, August, 2001.
NORBY* R. J., E.G. O'NEILL, C.A.GUNDERSON, & P. J. HANSON. Oak Ridge National Laboratory. Net primary productivity increases but not biomass increment in a closed-canopy forest stand exposed to elevated CO2.
There is ample evidence that the rate of carbon uptake in forests will increase as the atmospheric CO2 concentration rises, but it is not yet clear whether this will result in increased carbon sequestration. We estimated net primary productivity in a closed-canopy sweetgum (Liquidambar styraciflua) stand during three years of exposure to elevated CO2 in a FACE experiment. NPP was calculated using allometric expressions for aboveground and belowground woody increment, litter traps for leaf and branch return, and minirhizotrons for fine root production. NPP was 15% higher in elevated CO2, and the enhancement was maintained each year, consistent with the sustained response of leaf photosynthesis. The increased NPP resulted in a 33% increase in the aboveground woody biomass increment in year 1, but in subsequent years there was a declining effect of CO2 enrichment. Most of the extra NPP was instead accounted for by a small increase in leaf mass and a large increase (33% in year 2) in fine root production. Increased fine root production was matched by root mortality and a proportionate increase in soil CO2 efflux. These results indicate that the additional carbon taken up by this forest stand in response to atmospheric CO2 enrichment is being partitioned preferentially to fast-turnover pools (leaves and fine roots) rather than to slower-turnover pools (wood), resulting in faster cycling of carbon but a limited potential for increased carbon storage.