( CO2 fertilization |
global change |
leaf area index |
net primary productivity )
aEnvironmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6422; cDepartment of Plant Biology and iProgram in Ecology and Evolutionary Biology, University of Illinois, Urbana, IL 61801; dDepartment of Biology, University of Antwerp, B-2610 Wilrijk, Belgium; eDepartment of Forest Environment and Resources, University of Tuscia, I-01100 Viterbo, Italy; fU.S. Department of Agriculture Forest Service, North Central Research Station, Houghton, MI 49931; gDepartment of Forestry and Environmental Resources, North Carolina State University, Raleigh, NC 27695; hNicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708-0328; jDepartment of Biology, Boston University, Boston, MA 02215; kEcosystem Science Center, School of Forest Resources and Environmental Science, Michigan Technological University, Houghton, MI 49931; lU.S. Department of Agriculture Forest Service, North Central Research Station, Rhinelander, WI 54501; mSchool of Agricultural and Forest Sciences, University of Wales, Bangor LL57 2UW, United Kingdom; and nInstitute of Agro-Environmental and Forest Biology, Consiglio Nazionale delle Ricerche, 05010 Porano (TR), Italy
Contributed by William H. Schlesinger, November 1, 2005 Climate change predictions derived from coupled carbon-climate models are highly dependent on assumptions about feedbacks between the biosphere and atmosphere. One critical feedback occurs if C uptake by the biosphere increases in response to the fossil-fuel driven increase in atmospheric [CO2] ("CO2 fertilization"), thereby slowing the rate of increase in atmospheric [CO2]. Carbon exchanges between the terrestrial biosphere and atmosphere are often first represented in models as net primary productivity (NPP). However, the contribution of CO2 fertilization to the future global C cycle has been uncertain, especially in forest ecosystems that dominate global NPP, and models that include a feedback between terrestrial biosphere metabolism and atmospheric [CO2] are poorly constrained by experimental evidence. We analyzed the response of NPP to elevated CO2 (
Ecology
Forest response to elevated CO2 is conserved across a broad range of productivity
550 ppm) in four free-air CO2 enrichment experiments in forest stands. We show that the response of forest NPP to elevated [CO2] is highly conserved across a broad range of productivity, with a stimulation at the median of 23 ± 2%. At low leaf area indices, a large portion of the response was attributable to increased light absorption, but as leaf area indices increased, the response to elevated [CO2] was wholly caused by increased light-use efficiency. The surprising consistency of response across diverse sites provides a benchmark to evaluate predictions of ecosystem and global models and allows us now to focus on unresolved questions about carbon partitioning and retention, and spatial variation in NPP response caused by availability of other growth limiting resources.
Author contributions: R.J.N., E.H.D., R.C., A.C.F., D.F.K., K.S.P., G.E.S.-M., W.H.S., and R.O. designed research; R.J.N. and E.H.D. wrote the paper; R.J.N., E.H.D., B.G., C.C., C.P.G., J.S.K., J.L., H.R.M., D.J.P.M., and R.O. participated equally in the data synthesis workshop; and P.D.A., A.C.F., M.E.K., M.L., and K.S.P. contributed data.
Conflict of interest statement: No conflicts declared.
bTo whom correspondence may be addressed:
Richard J. Norby, E-mail: rjn{at}ornl.gov
www.pnas.org/cgi/doi/10.1073/pnas.0509478102
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