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CO₂ enhancement of forest productivity constrained by limited nitrogen availability

Author:
Norby, Richard J., Warren, Jeffrey M., Iversen, Colleen M., Medlyn, Belinda E., McMurtrie, Ross E.
Source:
Proceedings of the National Academy of Sciences of the United States of America 2010 v.107 no.45 pp. 19368-19373
ISSN:
0027-8424
Subject:
Liquidambar styraciflua, carbon dioxide, climate change, ecosystems, forest stands, forests, free air carbon dioxide enrichment, greenhouse gas emissions, models, nitrogen, nitrogen cycle, primary productivity, tree growth, trees, Tennessee
Abstract:
Stimulation of terrestrial plant production by rising CO₂ concentration is projected to reduce the airborne fraction of anthropogenic CO₂ emissions. Coupled climate-carbon cycle models are sensitive to this negative feedback on atmospheric CO₂, but model projections are uncertain because of the expectation that feedbacks through the nitrogen (N) cycle will reduce this so-called CO₂ fertilization effect. We assessed whether N limitation caused a reduced stimulation of net primary productivity (NPP) by elevated atmospheric CO₂ concentration over 11 y in a free-air CO₂ enrichment (FACE) experiment in a deciduous Liquidambar styraciflua (sweetgum) forest stand in Tennessee. During the first 6 y of the experiment, NPP was significantly enhanced in forest plots exposed to 550 ppm CO₂ compared with NPP in plots in current ambient CO₂, and this was a consistent and sustained response. However, the enhancement of NPP under elevated CO₂ declined from 24% in 2001-2003 to 9% in 2008. Global analyses that assume a sustained CO₂ fertilization effect are no longer supported by this FACE experiment. N budget analysis supports the premise that N availability was limiting to tree growth and declining over time --an expected consequence of stand development, which was exacerbated by elevated CO₂. Leaf- and stand-level observations provide mechanistic evidence that declining N availability constrained the tree response to elevated CO₂; these observations are consistent with stand-level model projections. This FACE experiment provides strong rationale and process understanding for incorporating N limitation and N feedback effects in ecosystem and global models used in climate change assessments.
Agid:
2370777