As new technologies—such as stable isotope analysis and remote sensing from aircraft and satellites—develop, continued studies are needed to identify biological indicators for detecting responses to pollutants at various levels of biological organization. This understanding is particularly important for terrestrial ecosystems, as relatively little progress has been made in this area compared to that in aquatic ecosystems.

4. Improve methods for monitoring ambient air quality in ecosystems, and ecosystem response.

Continue to conduct studies to determine appropriate suites of measurements, sample design, and sampling intensity to detect changes in ecosystem condition in response to pollutants. This is particularly important for terrestrial ecosystems, where little progress has been made. Biogeochemistry, habitat and biodiversity, and the linkage between diversity and productivity, are important factors for which a comprehensive suite of indicators should be developed. Indicators should include intermediate variables (for example, leaf area index and the foliar chemistry used to model productivity) as well as final variables (for example, mortality). Examine the possibility of using critical loads to quantify impacts on terrestrial ecosystems.

5. Conduct risk assessment research.

Develop methods for quantifying susceptibility of ecosystems to multiple stressors at multiple scales. See for example Linthurst et al. (2000).

IMPLEMENTATION RECOMMENDATIONS

There has been a lack of coordination in research on measuring the responses of ecosystems to pollutants, and in implementation of monitoring programs that use the new knowledge gained. Likewise, responsibility for monitoring of ecosystem conditions has been divided among agencies, and offices within agencies, so that a cohesive long-term program on monitoring terrestrial and aquatic ecosystem conditions does not exist. Analysis and reporting of results have been spotty, and very much delayed (for example, U.S. Department of Agriculture Forest Service Forest Health Monitoring), and data and reports are not easily located.

Key elements for implementation of measurements to determine status and trends in air pollution exposure and ecosystem condition are:

1. The institutional framework for monitoring exposure and ecosystem response.

True coordination between federal and state agencies is necessary to implement a unified cohesive program for monitoring air quality and ter-



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