Effect of elevated CO2 on net ecosystem carbon production: combined eddy covariance and chamber measurements on Florida scrub oak during a nine year study.

Thomas Powell, Smithsonian Environmental Research Center, PO Box 28, Edgewater, MD 21037, USA, powellt@si.edu (Presenting)
Bert Drake, Smithsonian Environmental Research Center, PO Box 28, Edgewater, MD 21037, USA, drakeb@si.edu (Presenting)
David Johnson, LI-COR Biosciences, 4647 Superior Street, Lincoln, NE 68504, USA, johnsondp@si.edu
Troy Seiler, Smithsonian Environmental Research Center, PO Box 28, Edgewater, MD 21037, USA, seilert@si.edu
Ross Hinkle, Department of Biology, University of Central Florida, Orlando, FL 32816, USA, rhinkle@mail.ucf.edu

The effect of elevated atmospheric CO2 on net ecosystem carbon production (NEP) has seldom been determined using gas exchange. Since regeneration after fire in 1997, we used open top chambers to periodically measure NEP of a Florida scrub oak ecosystem exposed to elevated atmospheric CO2 (700 ppm). NEP also was continuously measured since 2000 in the same plant community using an eddy covariance system. Comparison of simultaneous measurements of net ecosystem exchange (NEE) from eddy covariance and chambers at normal ambient CO2 confirmed that the two methods were within +/-10%. We used the chamber data to scale the eddy covariance NEP measurements to elevated CO2 conditions and then compared those results to biomass measurements. After nine years of fumigation, elevated CO2 resulted in a net increase in total above and belowground biomass of 15.4 t C ha-1 compared with gas exchange estimate of 17.2 t C ha-1. After canopy closure in 2000, precipitation was positively correlated with CO2 stimulation of NEP (NEP = NEPe – NEPa, where the subscript ‘a’ represents ambient atmospheric [CO2] and the subscript ‘e’ represents elevated atmospheric CO2). NEP increased by approximately 71 g C m-2 yr-1 per 100 mm increase in precipitation (R2 = 0.82). The CO2 stimulation was most intense during spring, typically the dry period, where the average daily magnitude of NEP nearly doubled per mm increase in precipitation and increased by 70% per mm increase in precipitation during cool winter months. NEP was unaffected by rainfall during the warm wet summer months.