CO₂ enhancement of forest productivity constrained by limited nitrogen availability

• Published in: Proceedings of the National Academy of Sciences - PNAS 2010-11, Vol.107 (45), p.19368-19373
• Main Authors: Norby, Richard J., Warren, Jeffrey M., Iversen, Colleen M., Medlyn, Belinda E., McMurtrie, Ross E., Schlesinger, William H.
• Format: Article
• Language: English
• Subjects: Atmospheric models; Atmospherics; Biological Sciences; budgets; Carbon dioxide; Carbon Dioxide - pharmacology; Climate change; Climate models; Deciduous forests; Ecosystem; Ecosystem models; Emissions; Fertilization; Forest ecosystems; Forest soils; Forest stands; Forests; Liquidambar styraciflua; Nitrogen; Nitrogen - pharmacology; Tennessee; Terrestrial environments; Trees; Trees - growth & development; Trees - metabolism
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1006463107

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.