50–70 parts per billion by volume (ppbv), a level not uncommon in rural areas of the United States during the summer; and (3) the study indicated that crops are best protected from O3 damage by an air quality standard that limits the integrated exposure over the 3–4 month period that the crop is growing rather than the relatively short-term 1-hr or 8-hr primary NAAQS used to protect human health (EPA 1996b). In part, because of the NCLAN study, EPA recommended an alternative secondary standard in its 1996 staff paper to review the O3 NAAQS. This standard would regulate O3 in a seasonal, cumulative manner and was designed to be more protective of vegetation. However, it was never implemented (Heck et al. 1998).
The Agricultural Research Service (ARS) in the U.S. Department of Agriculture (USDA) has the Air Quality-Plant Growth and Development Research Unit to carry out the following objectives: determine the separate and combined effects of O3 and elevated CO2 on growth and yield of selected agronomic species; determine whether plant, pest, and pathogen interactions are altered by exposure of plants to these pollutants; and develop techniques for mitigating the problems (USDA-ARS 2002). The effects of O3 and CO2 are studied individually, in combination, and in interaction with other factors associated with changes in global climate, and research is conducted under field, greenhouse, and laboratory conditions. The research unit is also working with crop-growth models for evaluation of air quality impacts on production. The future plans of the unit are to investigate the extent to which rooting media affect soybean response to mixtures of O3 and revised CO2 and to investigate the effects of increased concentrations of CO2 and O3 on a few insect pest populations.
A major problem of impacts research is the difficulty of predicting ecosystem-level responses from short-term studies of young trees grown under controlled conditions. More realistic studies, such as FACE experiments, are needed (Karnosky et al. 2001). To assess the effect of AQM on ecosystems, programs are needed that integrate measurements and analysis across terrestrial and aquatic systems, integrate disciplines, and integrate information across vegetation types and climatic and physiographic regions. To date, a number of research programs have been initiated that attempt to achieve this level of integration. However, for the most part, they are focused on unraveling the links between climate change and ecosystem function and are not considering air pollution per se as a perturbing factor. Some studies of note are described below:
AmeriFlux is a network of more than 100 sites dedicated to studying the influence of climate variation, vegetation developmental stage, vegeta-