Ecological Applications (2002) 12:1261-1266.

Net Primary Productivity of a CO₂-Enriched Deciduous Forest

and the Implications for Carbon Storage

Richard J. Norby^1,6, Paul J. Hanson¹, Elizabeth G. O'Neill¹, Tim J. Tschaplinski¹, Jake F. Weltzin², Randi T. Hansen³, Weixin Cheng⁴, Stan D. Wullschleger¹, Carla A. Gunderson¹, Nelson T. Edwards¹, Dale W. Johnson⁵

¹Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA; ²Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996, USA; ³Department of Biological Sciences, University of South Carolina, Columbia, SC 29208, USA; ⁴Department of Environmental Studies, University of California, Santa Cruz, CA 95064, USA; ⁵Department of Environmental and Resource Sciences, University of Nevada, Reno, NV 89557, USA

Abstract. A central question concerning the response of terrestrial ecosystems to a changing atmosphere is whether increased uptake of carbon in response to increasing atmospheric carbon dioxide concentration results in greater plant biomass and carbon storage or, alternatively, faster cycling of carbon through the ecosystem. Net primary productivity of a closed-canopy Liquidambar styraciflua (sweetgum) forest stand was assessed for 3 years in a free-air CO₂ enrichment (FACE) experiment. NPP increased 21% in stands exposed to elevated carbon dioxide, and there was no loss of response over time. Wood increment increased significantly during the first year of exposure, but subsequently most of the extra carbon was allocated to production of leaves and fine roots. These pools turn over more rapidly than wood, thereby reducing the potential of the forest stand to sequester additional carbon in response to atmospheric CO₂ enrichment. Hence, while this experiment provides the first evidence that CO₂ enrichment can increase productivity in a closed-canopy deciduous forest, the implications of this result must be tempered because the increase in productivity resulted in faster cycling of carbon through the system rather than increased carbon storage in wood. The fate of the additional carbon entering the soil system and the environmental interactions that influence allocation need further investigation.