HR: 11:20h
AN: B32B-05 INVITED
TI: Nitrogen Limitation is Reducing the Enhancement of NPP by Elevated CO₂ in a Deciduous Forest
AU: * Norby, R J
EM: rjn@ornl.gov
AF: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States
AU: Warren, J M
EM: warrenjm@ornl.gov
AF: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States
AU: Iversen, C M
EM: iversencm@ornl.gov
AF: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States
AU: Medlyn, B E
EM: bmedlyn@bio.mq.edu.au
AF: School of Biological Sciences, Macquarie University, Sydney, NSW 2019, Australia
AU: McMurtrie, R E
EM: r.mcmurtrie@unsw.edu.au
AF: School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia
AU: Hoffman, F M
EM: forrest@climatemodeling.org
AF: Computer Science & Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States
AB: Accurate model representation of the long-term response of forested ecosystems to elevated atmospheric CO₂ concentrations (eCO₂) is important for predictions of future concentrations of CO₂. For biogeochemical models that predict the response of net primary productivity (NPP) to eCO₂, free-air CO₂ enrichment (FACE) experiments provide the only source of data for comparison. A synthesis of forest FACE experiments reported a 23% increase in NPP in eCO₂, and this result has been used as a model benchmark. Here, we provide new evidence from a FACE experiment in a deciduous forest in Tennessee that N limitation has significantly reduced the stimulation of NPP by eCO₂, consistent with predictions from ecosystem and global models that incorporate N feedbacks. The Liquidambar styraciflua stand has been exposed to current ambient atmospheric CO₂ or air enriched with CO₂ to 550 ppm since 1998. Results from the first 6 years of the experiment indicated that NPP was significantly enhanced by eCO₂ and that this was a consistent and sustained response. Now, with 10 years of data, our analysis must be revised. The response of NPP to eCO₂ has declined from 24% in 2001-2003 to 9% in 2007. The diminishing response to eCO₂ since 2004 coincides with declining NPP in ambient CO₂ plots. Productivity of this forest stand is limited by N availability, and the steady decline in forest NPP is closely related to changes in the N economy, as evidenced by declining foliar N concentrations. There is a strong linear relationship between foliar [N] and NPP, and the steeper slope in eCO₂ indicates that the NPP response to eCO₂ should diminish as foliar N declines. Increased fine-root production and root proliferation deeper in the soil have sustained N uptake, but not to an extent sufficient to benefit aboveground production. The mechanistic basis of the N effect on NPP resides in the photosynthetic machinery. The linear relationships between J_max and V_cmax with foliar [N] did not change from 1998 to 2008 or in response to eCO₂; hence, lower foliar [N] resulted in significant reductions in J_max, V_cmax, and photosynthesis over time and in eCO₂. It is not yet clear whether foliar [N] and NPP will continue to decline or have reached a new steady state indicative of long-term forest response to eCO₂. These results are consistent with ecosystem models, which suggest that the NPP response to eCO₂ will include a transient increase in NPP followed by a decline to a lower level when fast C and N pools reach quasi-equilibrium at eCO₂. Our results also are consistent with optimization models of carbon-water-nitrogen economy, which suggest eCO₂ leads to increased fine-root production, declining foliar [N], and diminishing enhancement of aboveground production. At a larger scale, a model incorporating N feedbacks (CLM3-CN) predicts a much smaller enhancement of NPP in eCO₂ than the CLM3- CASÁ model without such a feedback. When applied across the terrestrial biosphere, the smaller CO₂ fertilization effect has important implications for the pace of climate change.
UR: http://face.ornl.gov
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0428 Carbon cycling (4806)
DE: 0469 Nitrogen cycling
DE: 1630 Impacts of global change (1225)
SC: Biogeosciences [B]
MN: 2008 Fall Meeting