Tropospheric O3 compromises net primary production in young stands of trembling aspen, paper birch and sugar maple in response to elevated atmospheric CO2

Concentrations of atmospheric CO2 and tropospheric ozone (O-3) 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 O-3 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 O-3 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 O-3 significantly reduce the capacity of NPP to respond to elevated CO2 in some forests.