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CO2 Enhancement of Forest Productivity Constrained by Limited Nitrogen Availability

Richard J. Norby¹, Jeffrey M. Warren¹, Colleen M. Iversen¹, Charles T. Garten¹, Belinda E. Medlyn² & Ross E. McMurtrie³

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1. Environmental Sciences Division, Oak Ridge National Laboratory
2. Department of Biological Sciences, Macquarie University
3. School of Biological Earth and Environmental Sciences, University of New South Wales

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Document Type:

Manuscript

Date:

Received 11 September 2009 15:42 UTC; Posted 14 September 2009

Subjects:

Earth & Environment, Plant Biology

Tags:

• elevated CO2
• forest ecology
• carbon cycle
• nitrogen dynamics

Abstract:

Stimulation of terrestrial productivity by rising CO₂ concentration is projected to reduce the airborne fraction of anthropogenic CO₂ emissions; coupled climate-carbon (C) cycle models, including those used in the IPCC Fourth Assessment Report (AR4), are sensitive to this negative feedback on atmospheric CO₂¹. The representation of the so-called CO₂ fertilization effect in the 11 models used in AR4 and subsequent models^2,3 was broadly consistent with experimental evidence from four free-air CO₂ enrichment (FACE) experiments, which indicated that net primary productivity (NPP) of forests was increased by 23 +/- 2% in response to atmospheric CO₂ enrichment to 550 ppm⁴. Substantial uncertainty remains, however, because of the expectation that feedbacks through the nitrogen (N) cycle will reduce the CO₂ stimulation of NPP^5,6; these feedbacks were not included in the AR4 models and heretofore have not been confirmed by experiments in forests⁷. Here, we provide new evidence from a FACE experiment in a deciduous Liquidambar styraciflua (sweetgum) forest stand in Tennessee, USA, that N limitation has significantly reduced the stimulation of NPP by elevated atmospheric CO₂ concentration (eCO₂). Isotopic evidence and N budget analysis support the premise that N availability in this forest ecosystem has been declining over time, and declining faster in eCO₂. Model analyses and evidence from leaf- and stand-level observations provide mechanistic evidence that declining N availability constrained the tree response to eCO2. These results provide a strong rationale and process understanding for incorporating N limitation and N feedback effects in ecosystem and global models used in climate change assessments.

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This document is licensed to the public under the Creative Commons Attribution 3.0 License

How to cite this document:

Norby, Richard, Warren, Jeffrey, Iversen, Colleen, Garten, Charles, Medlyn, Belinda, and McMurtrie, Ross. CO2 Enhancement of Forest Productivity Constrained by Limited Nitrogen Availability. Available from Nature Precedings <http://hdl.handle.net/10101/npre.2009.3747.1> (2009)

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