Forest fine-root production and nitrogen use under elevated CO2: contrasting responses in evergreen and deciduous trees explained by a common principle

Authors

  • OSKAR FRANKLIN,

    1. School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia,
    2. International Institute for Applied Systems Analysis (IIASA), A-2361 Laxenburg, Austria,
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  • ROSS E. McMURTRIE,

    1. School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia,
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  • COLLEEN M. IVERSEN,

    1. Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996, USA,
    2. Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6422, USA,
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  • KRISTINE Y. CROUS,

    1. School of Natural Resources and Environment, University of Michigan, 440 Church Street, Ann Arbor, MI 48109-1115, USA,
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  • ADRIEN C. FINZI,

    1. Department of Biology, Boston University, Boston, MA 02215, USA,
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  • DAVID T. TISSUE,

    1. Department of Biological Sciences, Texas Tech University, Flint and Main Street Lubbock, TX 79409-3131, USA,
    2. Centre for Plant and Food Science, University of Western Sydney, Penrith South DC, NSW 1797, Australia,
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  • DAVID S. ELLSWORTH,

    1. Centre for Plant and Food Science, University of Western Sydney, Penrith South DC, NSW 1797, Australia,
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  • RAM OREN,

    1. Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708-0328, USA
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  • RICHARD J. NORBY

    1. Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6422, USA,
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Oskar Franklin, International Institute for Applied Systems Analysis (IIASA), A-2361 Laxenburg, Austria, tel. +43 2236 807251, fax +43 2236 807599, e-mail: franklin@iiasa.ac.at

Abstract

Despite the importance of nitrogen (N) limitation of forest carbon (C) sequestration at rising atmospheric CO2 concentration, the mechanisms responsible are not well understood. To elucidate the interactive effects of elevated CO2 (eCO2) and soil N availability on forest productivity and C allocation, we hypothesized that (1) trees maximize fitness by allocating N and C to maximize their net growth and (2) that N uptake is controlled by soil N availability and root exploration for soil N. We tested this model using data collected in Free-Air CO2 Enrichment sites dominated by evergreen (Pinus taeda; Duke Forest) and deciduous [Liquidambar styraciflua; Oak Ridge National Laboratory (ORNL)] trees. The model explained 80–95% of variation in productivity and N-uptake data among eCO2, N fertilization and control treatments over 6 years. The model explains why fine-root production increased, and why N uptake increased despite reduced soil N availability under eCO2 at ORNL and Duke. In agreement with observations at other sites, the model predicts that soil N availability reduced below a critical level diminishes all eCO2 responses. At Duke, a negative feedback between reduced soil N availability and N uptake prevented progressive reduction in soil N availability at eCO2. At ORNL, soil N availability progressively decreased because it did not trigger reductions in N uptake; N uptake was maintained at ORNL through a large increase in the production of fast turnover fine roots. This implies that species with fast root turnover could be more prone to progressive N limitation of carbon sequestration in woody biomass than species with slow root turnover, such as evergreens. However, longer term data are necessary for a thorough evaluation of this hypothesis. The success of the model suggests that the principle of maximization of net growth to control growth and allocation could serve as a basis for simplification and generalization of larger scale forest and ecosystem models, for example by removing the need to specify parameters for relative foliage/stem/root allocation.

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