The influence of rising tropospheric carbon dioxide and ozone on plant productivity.

Human activities result in a wide array of pollutants being released to the atmosphere. A number of these pollutants have direct effects on plants, including carbon dioxide (CO ), which is the substrate for photosynthesis, and ozone (O ), a damaging oxidant. How plants respond to changes in these atmospheric air pollutants, both directly and indirectly, feeds back on atmospheric composition and climate, global net primary productivity and ecosystem service provisioning. Here we discuss the past, current and future trends in emissions of CO and O and synthesise the current atmospheric CO and O budgets, describing the important role of vegetation in determining the atmospheric burden of those pollutants. While increased atmospheric CO concentration over the past 150 years has been accompanied by greater CO assimilation and storage in terrestrial ecosystems, there is evidence that rising temperatures and increased drought stress may limit the ability of future terrestrial ecosystems to buffer against atmospheric emissions. Long-term Free Air CO or O Enrichment (FACE) experiments provide critical experimentation about the effects of future CO and O on ecosystems, and highlight the important interactive effects of temperature, nutrients and water supply in determining ecosystem responses to air pollution. Long-term experimentation in both natural and cropping systems is needed to provide critical empirical data for modelling the effects of air pollutants on plant productivity in the decades to come.