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Executive SummaIy Forests are subject to stress from natural causes and human activities. The natural causes include competition, predation, pathogens, extremes of weather, and climatic cycles. The human activities include physical disturbance and air pollution, including acids, oxidants, toxic organic compounds, and trace metals. Stresses often occur in combination, and effects of particular stresses on trees and forests are difficult to recognize. Biologic markers (i.e., indicators) associated with specific air pollutant stress and damage in forests have been sought to improve discrimination among the many anthropogenic and natural causes of stress. To summarize the current state of knowledge and research and to identify and evaluate promising techniques that might lead to the development of new markers, the Environmental Protection Agency (EPA) asked the National Research Council (NBC) to arrange a workshop to bring together researchers in forest science and complementary disciplines. The Board on Environmental Studies and Toxicology of the NRCts Commission on Life Sciences appointed the Committee on Biologic Markers of Air-Pollution Damage in Trees, which organized a workshop held in Little Switzerland, North Carolina, in April 1988. The workshop brought together over 40 experts in forestry, ecology, plant pathology, physiology and biochemistry, and a broad range of other scientific and engineering disciplines. The group reviewed the current and potential uses of biologic markers at a variety of organizational levels, from the subcellular to the ecosystem level. This publication is in two parts. Part I, the committee's report, provides a summary of the workshop, a review of criteria for the establishment of cause and effect in complex relationships, a discussion of the use of biologic markers to identify stress and damage in trees and forests, and the committee's conclusions and recommendations for further development and application of biologic markers. The workshop on which the committee based its report produced 32 papers by individual participants; those papers are included as Part II of this publication. One task of the committee was to describe the status of work on markers of forest responses to air pollutants. A consensus emerged from the workshop that, although the currently available array of biologic markers is insufficient for resolving complex 1
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2 issues of forest decline, when used in combination these markers can facilitate evaluation of the mechanisms and consequences of environmental stress and damage from atmospheric pollutants. The use of biologic markers should be expanded as basic knowledge of forest ecology and physiology increases to allow more discriminating measurements of responses to natural and anthropogenic stresses. The committeets general conclusions follow. Conclusions No readily detectable, pollutant-specific single marker for identifying the effects of air pollution on forests or trees has been identified. Other stresses can produce symptoms in plants that mimic or conceal damage caused by air pollution. Plants differ within and between species in their genetic capacities to absorb, assimilate, and respond to air pollutants; therefore, they exhibit different sensitivities to air pollutants via markers of pollution-caused damage. Dose-response relationships derived from air- pollution exposure experiments can also vary within a single species in response to environmental conditions. Such variations add to the complexity of marker analysis. Most current biologic markers of responses of trees to stress measure changes in plant canopies; additional markers are needed to measure effects on roots and shoots and to provide greater specificity in relating effects to causes of stress and damage. 4. A better understanding of spatial and temporal variations in natural processes that affect forests is needed to establish baselines against which to measure effects of pollutants. 5. Markers of forest-level effects can best identify sites of possible air-pollution damage when analyzed in conjunction with spatial and temporal patterns of air-pollution distribution. 6. The most useful analyses of the effects of air pollutants on forests combine surveys with controlled-exposure studies of potential cause-and-effect relationships. A second task of the committee was to formulate recommendations on the basis of the workshop presentations and the committee's own deliberations for the further development and use of markers of tree and forest damage from air pollution. The committee believes that several current efforts to develop and apply markers of air- pollutant effects are promising and should be expanded. Those efforts should include the use of new technologies in an integrated analytic strategy, and they also should include the development of a protocol for interpreting the relative effects of various air pollutants on the function and composition of forest ecosystems. The committee's recommendations follow. Recommendations 1. Increased emphasis should be placed on identifying suites of biologic markers for detecting forest responses to pollutants at various levels of biologic organization. The approach should include the application of statistical techniques that allow inferences to be stated in terms of probability. Further development of markers of forest-level responses to air pollutants is especially needed. Examples of promising forest-level marker techniques include stream chemistry analysis and remote sensing from aircraft and satellites. Such techniques will be particularly useful if applied along known gradients of air-pollution intensity.
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3 Because markers currently used to survey forest and tree responses to ambient concentrations of air pollutants cannot by themselves delineate cause-and-effect relationships, they must be used in concert with controlled-exposure techniques that involve monitoring and experimentation. 5. Ongoing regional surveys of forest responses to air pollution should be used to help identify specific areas for detailed studies of air-pollution effects. Government and industry should continue supporting the development and use of biologic markers of air-pollutant stress and damage in forests; this work should be coordinated with continuing efforts to identify and model air-pollution distribution patterns.
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