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Aboveground biomass stimulation is sustained over 11 years of CO2 enrichment, mediated by contrasting species responses.

Troy J Seiler, Smithsonian Environmental Research Center, Edgewater, MD, USA, seilert@si.edu
Jiahong Li, Smithsonian Environmental Research Center, Edgewater, MD, USA, lij@si.edu
Paul Dijkstra, Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ, USA, paul.dijkstra@nau.edu
Has P Anderson, Smithsonian Environmental Research Center, Edgewater, MD, USA, andersonh@si.edu
David P Johnson, Smithsonian Environmental Research Center, Edgewater, MD, USA, johnsondp@si.edu
Charles Ross Hinkle, Department of Biology, University of Central Florida, Orlando, FL, USA, rhinkle@mail.ucf.edu
Bert G Drake, Smithsonian Environmental Research Center, Edgewater, MD, USA, drakeb@si.edu (Presenting)

Terrestrial ecosystems may mitigate rising atmospheric carbon dioxide concentration (CO2) through increased carbon uptake and sequestration in plant biomass. Elevated CO2 commonly produces initial stimulation of photosynthesis and growth, but due primarily to complex interactions with limiting environmental factors (i.e. water, light and nutrients); uncertainty regarding long-term biomass response persists. After 11 years of CO2 enrichment using open-top chambers, aboveground biomass stimulation was sustained in a Florida scrub-oak ecosystem, yielding a 67% increase at final harvest in June 2007. The scrub oaks Quercus geminata and Quercus myrtifolia represented 85% of total ecosystem biomass and displayed contrasting responses to elevated CO2. Q. myrtifolia showed consistent biomass response over the course of the study, yielding 128% stimulation of aboveground biomass under elevated CO2 by 2007. CO2 did not significantly affect Q. geminata biomass, which by 2007 displayed only 6% difference between treatments. Both species displayed long-term mean stimulation of net leaf photosynthesis under saturated light conditions, however, stimulation in Q. myrtifolia was nearly twice as much as Q. geminata (63% and 35%, respectively). Over the course of the study, Q. geminata consistently displayed photosynthetic acclimation via reductions in maximum carboxylation rate (Vcmax) and maximum rate of electron transport (Jmax). Q. myrtifolia photosynthesis generally did not acclimate to elevated CO2. Additionally, inter-annual variation in rainfall correlated with Q. myrtifolia annual biomass increment, hence relative stimulation of biomass increment was larger in dry years, although absolute biomass accumulation was greater in wet years. This effect was masked at the ecosystem level because Q. geminata, which utilizes the water table to a greater extent, showed no relationship with rainfall. These relative advantages afforded to Q. myrtifolia by elevated CO2 produced a significant change in ecosystem composition, a trend which may be further compounded over time by this system’s short fire return cycle.

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