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 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). 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 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 x ambient, with ambient averaging 34-36 nL L-1 during the summer daylight hours), O-3 offsets or moderates the responses induced by elevated CO2. 3. After 3 years of exposure to 560 mumol 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, O-3 at 1.5 x ambient completely offset the growth enhancement by CO2, both for O-3-sensitive and O-3-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.