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17 specific physical or biologic process. Experiments are most valuable if the mechanisms can be demonstrated both in field studies and in carefully controlled experiments. Demonstration of a physical or biologic mechanism of action is an important criterion of causality, as discussed above. Experiments to determine mechanisms go beyond the measurement of given characteristics of markers to the determination of the specific processes by which pollutants or other stress factors might be linked to the effects measured by those markers. As is discussed below, studies of the physiology, pathology, and biochemistry of individual trees, tissues, and cells are more likely to be useful in elucidating mechanisms of action than are studies of changes in natural systems at higher organizational levels. Nevertheless, coordinated measurements of higher-level effects are needed to determine the ultimate scope of expression and impact of the mechanisms in question. Developing a Diagnostic Approach Anyone attempting to use biologic markers in forest ecosystems should recognize that the current array of atmospheric pollutants includes agents that might affect forests at many physiologic and biogeochemical loci. It is therefore essential to select an array, or suite, of markers of effects on several metabolic pathways and structural features to aid in the cause-and-effect analysis described above. In developing a diagnostic approach, it is important to test various hypotheses that are formulated to explain the changes observed. If the hypotheses are organized around known or suspected changes in uptake and use of carbon, water, and nutrients, the analysis of forest responses becomes relevant to a broader range of natural and anthropogenic stresses that are known to affect resource availability. Critical points and mechanisms that determine pollutant effects on forest systems often are biochemical. Therefore, markers related to changes in growth and structure alone are insufficient; they must be used in combination with biochemical markers related to metabolic processes that reflect responses to stress, such as compensation and dysfunction. The next section presents a framework for the integrated application of biologic markers to analyze the effects of air pollutants and other stresses in forest systems. A STRATEGY FOR USING BIOLOGIC MARKERS OF STRESS IN FORESTS A strategy for the effective use of markers of forest damage should involve classifying markers into functional groups and then using the groups in a systematic, diagnostic way. One approach to the integration and interpretation of biologic markers of forest responses to air pollutants is presented below. Other approaches are needed and can be developed. Foresters and resource managers need biologic markers that are specific for evaluating changes in forest health due to air pollutants. Air pollution is one of the few environmental features that, if stressful, could be corrected by human interven- tion. Thus, simply using biologic markers to locate unhealthy forests is not particularly useful, whereas locating and diagnosing forests damaged by air pollution could provide the basis for constructive regulatory and management responses. Suites of markers can be developed to help foresters and resource managers to detect effects specific to air pollutants. The committee suggests classifying markers into the categories described below and using them in a sequential diagnostic pattern to identify forest regions suffering from air-pollution stress. The committee then

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18 suggests an outline for the application of a suite of markers to detect stresses in forest systems. 1. Biologic Markers of Response to Environmental Change. The responses of plants to environmental change are continuous and are evident from the use of an array of biologic markers (see Table 3~. An example of biologic markers of environmental change at the ecosystem level is a dramatic shift in nutrient leakage; this can sometimes be determined by measurements of stream chemistry. Biologic markers of environmental change at the tree level might include sudden changes in tree-ring size or abnormal rates of photosynthesis and other metabolic processes. 2. Biologic Markers of Compensation to Stress. A useful way to evaluate the importance of environmental stresses for trees is to determine the degree to which the trees have developed compensatory responses to them. Compensation maximizes productivity and the likelihood of survival. It also has potential diagnostic value; compensation mechanisms can be specific to particular environmental stresses. Table 4 lists some potentially useful markers of compensation that are discussed in the workshop papers. Compensation by plants in response to environmental stresses can take place at the biochemical, physiologic, and ecosystem levels. For example, trees can compensate for drought by closing stomata to conserve water and by shifting resources to foster greater root growth to enhance acquisition of water from soils. Those forms of compensation differ in important ways from simpler responses of trees to drought; they specifically enhance the acquisition of resources that are most limiting. The growth, survival, and perhaps fitness of compensating plants are depressed, compared with those of unstressed plants, but greater than those of stressed plants that did not compensate. If distinct compensation responses of trees to various air pollutants can be described, and biologic markers of them defined, it should be possible to identify trees that have compensated in response to specific air-pollution stresses. 3. Biologic Markers of Toxicity. Toxic effects of air pollutants occur in plants when absorption of toxic chemicals exceeds the capacity to compensate. In such cases, plants are unable to maintain themselves in a healthy state. There are important biologic markers of such events (see Table 5~. Air pollutants, like other types of stresses, can damage or even kill individual cells. Pollutants can also damage membranes, rendering them less able to select against toxic substances and allowing concentrations of heavy metals or other pollutants to increase in tissues. Cellular damage can become so widespread that the tree is compromised and dead or dying cells become visible. Such visible injury is known to be associated with gaseous pollutants such as ozone and sulfur and nitrogen oxides. Visible injury typical of these pollutants includes foliar chlorosis, necrosis, stippling, and needle banding. Those visible symptoms can be specific for key pollutants, but some nonpollution stresses can induce similar symptoms. The loss of leaf area due to air-pollution toxicity can occur via mechanisms other than cellular death. For example, air pollutants are known to cause premature senescence and casting of deciduous and evergreen foliage. That process can give the canopy of affected trees the appearance of being transparent--a symptom widely used in air-pollution damage surveys. Direct absorption of gaseous pollutants--such as ozone and sulfur dioxide--might eventually overcome the repair capacity of a plant and initiate injury that not only kills individual cells, but also induces production of plant resins and phenols that fill dead cells or spread and wall off surrounding tissue. Damage can also result in formation of visibly thickened cells.

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19 Table 3. Some important biologic markers of environmental change that can be determined by assessing trees, as presented in Part II of this publication. MARKER Tree--Stand level: Nutrient cycling Stable isotopes of carbon, nitrogen, and sulfur Tree ring analysis Canopy spectral analysis Shifts in phonology Root growth Symbiotic rhizosphere fungi Symbiotic rhizosphere bacteria Biochemical--Tissue level: Foliar nitrate reductase Free-radical processes Photosynthesis and transpiration Nutrient-use efficiency Carbon partitioning Cuticular competence Secondary metabolites Chlorophyll content WORKSHOP -PAPER Johnson et al. Fry Cook and Innes; Johnson Rock et al. Rock et al., Barnard; Schutt; Anderson; Miller; Cape Richards Marx and Shafer Antibus and Linkens Norby Richardson et al. Winner Luxmoore McLaughlin Berg Jones and Coleman Heath

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20 Table 4. Some important biologic markers of compensation to air-pollution stress, as presented in Part II of this publication. MARKER Tree-Stand Level: Allocation of internal resources Leaf retention and crown density i Bud damage Biochemical-Tissue Level: -Carbon partitioning WORKSHOP PAPER Waring Miller; Cape; Schutt; Barnard; Anderson Johnson McLaughlin Table 5. Some important biologic markers of air-pollution-caused toxicity and absorption, as presented in Part II of this publication. MARKER Tree-Stand Level: Indigenous and cultivated plants Epiphytic cryptogams Foliar damage Biochemical-Tissue Level: Loss of membrane integrity and selectivity Pollutant content in tissues Foliage histology Phloem damage WORKSHOP PAPER Weinstein and Laurence Scott and Hutchinson Miller Alscher Shortle; Bondietti et al. Rock; Miller Sharpe and Spence

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21 Foliar discoloration, canopy thinning, and tissue isolation can be caused by air pollution as well as other stresses. Consequently, analysis using biologic markers of air-pollution absorption can help to link putative air-pollution toxicity to a specific chemical agent. Such markers might include the presence of sulfur compounds in leaves exposed to sulfur dioxide and abnormal concentrations of heavy metals in plant tissues. The three types of biologic markers discussed above--of response to environmental change, of compensation to stress, and of toxicity--can be used in a systematic, phased manner to elucidate air-pollution-caused changes in forest health. For any given research location, the process requires evaluation of air-quality data and assembly of an appropriate suite of biologic markers that is based on an understanding of air- pollution deposition and site-specific biologic processes. The process should not only reveal the nature of air-pollution-caused changes in forest health, but also identify other key stress factors that influence trees and forests. To identify changes in forest health caused by air pollution, biologic markers in the three categories described can be evaluated in a systematic fashion. If the biologic markers of response to environmental change suggest a stable environment, air-pollution effects have probably not occurred recently. If, however, they indicate significant shifts in environmental factors, recent changes in air quality could be important. Analysis of air-quality data can be useful in showing whether air-pollution deposition at the site has taken place over a long period or has changed sharply recently. Once analysis of biologic markers of response to environmental change and air- quality data suggest that air pollutants are important, markers of compensation and acute toxicity that are specific for air pollution can be evaluated. Because compensation for air-pollutant damage takes time, detection of compensation with biologic markers implies that air pollutants with chronic impacts are present. Such impacts might include a reduction in capacity to compensate for nonpollution stresses the result might be decreases in growth, productivity, and reproduction. Detection of compensation markers also justifies analysis with markers of pollution toxicity and absorption. In this case, foliar injury and other such indicators of pollution damage can be interpreted with more certainty. The sequence of analysis can be reversed. If biologic markers of pollution toxicity and absorption at a site become apparent, air quality can be evaluated to determine whether ozone, sulfur dioxide, acid deposition, or other pollutants are likely causes of observed symptoms. If so, analysis of markers of compensation and environmental change can help to put air-pollution stress in perspective with other stresses. Because the nature of compensation and toxicity responses to each air pollutant can differ, monitoring pollution concentration and deposition is essential for interpreting the effects of pollutants on forest health. Without monitoring, interpretation of information from any set of biologic markers is unreliable. Use of biologic markers will never completely answer questions of cause and effect, nor will markers provide all the information needed by foresters, resource managers, and regulators about the role of air pollutants in forests. That requires supplementing the use of markers with studies of air-pollution response mechanisms of trees and an analysis of ecosystem processes. Only the combination of those approaches can yield all the information necessary to manage air quality and forest resources.