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

Summary The impacts of elevated atmospheric CO 2 and/or O 3 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, O 3 ‐sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera , and the determinate, late successional, O 3 ‐tolerant species Sugar Maple, Acer saccharum ). 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 O 3 ‐tolerant as well as O 3 ‐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), O 3 offsets or moderates the responses induced by elevated CO 2 . After 3 years of exposure to 560 µmol mol −1 CO 2 , the above‐ground volume of Aspen stands was 40% above those grown at ambient CO 2 , and there was no indication of a diminishing growth trend. In contrast, O 3 at 1·5 × ambient completely offset the growth enhancement by CO 2 , both for O 3 ‐sensitive and O 3 ‐tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO 2 , and global models of forest productivity and climate change are presented.