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22 CONCLUSIONS AND RECOMMENDATIONS Used individually, the available biologic markers are insufficient for resolving complex issues of forest decline, but used in selected combinations, they can help to simplify the evaluation of the mechanisms and consequences of environmental stress due to atmospheric pollutants. Markers currently in use constitute a starting point for diagnosis that should be expanded as knowledge of forest ecology, physiology, and responses to natural and anthropogenic stresses increase. One task of the committee was to describe the status of research on markers of forest responses to air pollutants. The committee reviewed the workshop presentations and papers and drew general conclusions intended to synthesize and summarize applications of biologic markers to trees and forests. Conclusions 1. 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. Current markers of forest change typically fail to distinguish the effects of different pollutants. Examples of nonpollution stresses are those caused by drought, insufficiency of nitrogen, pathogens, and insects. In addition, a single air pollutant can cause an array of symptoms in plants. For example, ozone is known to cause chlorosis, stippling, bronzing, flecking, and blackening of broadleaf plants and tip burn, mottling, and banding on conifer needles, as well as reductions in growth and changes in susceptibility to other stresses. 2. 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. Differences among species in sensitivity to air pollution can be exploited by focusing analysis on the most sensitive species. For example, lichens are known to be sensitive to sulfur dioxide. Milkweed, some strains of white pine, and tobacco foliage are known to be sensitive to ozone. A given ambient air-pollution concentration can have different effects on a single species, because of variations in environment, genotype, symbionts, and plant age, if such variations alter absorption or assimilation of air pollutants. Environmental factors that increase stomata! conductance are likely to enhance uptake of gaseous pollutants and therefore increase their effect. Genotypic differences between populations that confer contrasting physiologic adaptations to environmental stresses can result in a physiologic basis for population differences in air-pollution sensitivity. Such genetically based variation is well known in white pine, loblolly pine, and poplar. In addition, symbiotic microorganisms growing in the rhizosphere and on foliage, as well as macroscopic symbionts and changes in age, can alter the functioning of stomata and the metabolic capacity of an entire tree. Such changes are likely to alter sensitivity of a plant to air pollution. Thus, a single air- pollution concentration might or might not elicit a recognizable response. 3. 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.
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23 Most current markers of forest responses to air pollutants measure particular characteristics of plant canopies. These markers cannot be used to characterize pollution-caused changes in the growth of roots, the growth of whole trees, forest productivity, or other forest characteristics. Further research is needed to develop markers of such changes, particularly at the forest-stand and ecosystem levels. 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. Much of the ambiguity associated with interpreting data from biologic markers of stress in forests is due to the potential for interactions between natural and anthropogenic changes in the forest environment. More detailed and precise knowledge of forest responses to drought, low temperatures, and long-term environmental variables is needed to help researchers separate responses to these variables from responses to air 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. Markers of change in forests--such as visible symptoms, gaseous emissions, shifts in nutrient allocation, and other general markers of stress--can be effectively used when they are related to patterns of air-pollution distribution. Sites of greatest interest are those where forest status either is beginning to change or is changing rapidly and where air pollutants are present at concentrations known to affect plants. Rates of change in forest status and correlations with air-pollution concentrations can be assessed only through repeated surveys and analyses. 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. Controlled experimental exposures of forest species to air pollution are necessary to confirm a conclusion that forest changes observed in survey studies are due to ambient exposures. Such experiments can be done on potted plants in greenhouses and other laboratory facilities, or in the field with chambers for seedlings or branches, or in both places simultaneously. Without such experiments, it is difficult to link observed changes in trees, stands, or forests with air pollutants in a cause-and-effect relationship. Recommendations A second task of the committee was to formulate recommendations--on the basis of the workshop presentations and its own conclusions--regarding the use of markers of forest responses to air pollutants. The committee believes that several current efforts to develop and apply markers of air-pollutant effects are promising and should be expanded. The efforts should include the use of newly available technologies in an integrated analytical strategy, along with the development of a protocol for interpreting the relative effects of various air pollutants on the functions and composition of forest ecosystems. The committee's review yielded recommendations for several other activities that should advance the identification and measurement of specific types of air-pollutant damage to trees and forests. The committee believes that such advancement will continue to present challenges, but that the potential results justify the effort.
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24 Increased emphasis should be placed on identifying suites of biologic markers for detecting forest responses to pollutants at various levels of biological organization. This approach should include the application of statistical techniques that allow inferences to be stated in terms of probability. - 3. 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 developed along known gradients of air-pollution intensity. 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. 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.