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

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.