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


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  • 1The 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 successional, O3-tolerant species Sugar Maple, Acer saccharum).
  • 2The 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 × ambient, with ambient averaging 34–36 nL L−1 during the summer daylight hours), O3 offsets or moderates the responses induced by elevated CO2.
  • 3After 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 × 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.