Tropospheric O3 compromises net primary production in young stands of trembling aspen, paper birch and sugar maple in response to elevated atmospheric CO2 (vol 168, pg 623, 2005)

• Concentrations of atmospheric CO2 and tropospheric ozone (O3) are rising concurrently in the atmosphere, with potentially antagonistic effects on forest net primary production (NPP) and implications for terrestrial carbon sequestration. • Using free-air CO2 enrichment (FACE) technology, we exposed north-temperate forest communities to concentrations of CO2 and O3 predicted for the year 2050 for the first 7 yr of stand development. Site-specific allometric equations were applied to annual nondestructive growth measurements to estimate above- and below-ground biomass and NPP for each year of the experiment. • Relative to the control, elevated CO2 increased total biomass 25, 45 and 60% in the aspen, aspen-birch and aspen-maple communities, respectively. Tropospheric O3 caused 23, 13 and 14% reductions in total biomass relative to the control in the respective communities. Combined fumigation resulted in total biomass response of -7.8, +8.4 and +24.3% relative to the control in the aspen, aspen-birch and aspen-sugar maple communities, respectively. • These results indicate that exposure to even moderate levels of O3 significantly reduce the capacity of NPP to respond to elevated CO2 in some forests. © New Phytologist (2005).