Tropospheric O3 moderates responses of temperate hardwood forests to elevated CO2: A synthesis of molecular to ecosystem results from the Aspen FACE project

D. F. Karnosky, D. R. Zak, K. S. Pregitzer, C. S. Awmack, J. G. Bockheim, R. E. Dickson, G. R. Hendrey, G. E. Host, J. S. King, B. J. Kopper, E. L. Kruger, M. E. Kubiske, R. L. Lindroth, W. J. Mattson, E. P. Mcdonald, A. Noormets, E. Oksanen, W. F.J. Parsons, K. E. Percy, G. K. PodilaD. E. Riemenschneider, P. Sharma, R. Thakur, A. Sôber, J. Sôber, W. S. Jones, S. Anttonen, E. Vapaavuori, B. Mankovska, W. Heilman, J. G. Isebrands

Research output: Contribution to journalReview articlepeer-review

231 Scopus citations


1. The impacts of elevated atmospheric CO2 and/or O3 have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, O3-sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera, and the determinate, late successionati O3-tolerant species Sugar Maple, Acer saccharum). 2. The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O3-tolerant as well as O3-sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1.5 x ambient, with ambient averaging 34-36 nL L-1 during the summer daylight hours), O3 offsets or moderates the responses induced by elevated CO2. 3. After 3 years of exposure to 560 μmol mol-1 CO2, the above-ground volume of Aspen stands was 40% above those grown at ambient CO2, and there was no indication of a diminishing growth trend. In contrast, O3 at 1.5 x ambient completely offset the growth enhancement by CO2, both for O3-sensitive and O3-tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO2, and global models of forest productivity and climate change are presented.

Original languageEnglish (US)
Pages (from-to)289-304
Number of pages16
JournalFunctional Ecology
Issue number3
StatePublished - Jun 2003


  • Aggrading aspen forest
  • Carbon budgets
  • Carbon sequestration
  • Interacting pollutants


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