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6 associated unequivocally with one pollutant, given that several pollutants are commonly present. The NRC's Committee on Biologic Markers of Air-Pollution Damage in Trees was convened by the Board on Environmental Studies and Toxicology of the Commission on Life Sciences at the request of the EPA. This committee was asked to conduct a workshop to review, summarize, and evaluate current and potential uses of a broad range of biologic markers of stress and damage in trees and forests that might be caused by air pollutants. In a related effort, the NRC,s Committee on Biologic Markers has been examining the use of markers in environmental (human) health research. That committee has defined biologic markers as "indicators of variation in cellular or biochemical components or processes, structure, or function that are measurable in biologic systems or samples" (NRC, 1989) and is considering biologic markers of exposure, effect, and suscep- tibility. A marker of effect is defined as "a measurable biochemical, physiologic, or other alteration within an organism that, depending on magnitude, can be recognized as an established or potential health impairment or disease. This committee has used similar definitions. However, to deal with forests and trees, it concentrated on biologic markers of effect and considered all measurable variations in structure or function to be potential markers. Stresses can often cause effects for which trees have compensatory mechanisms that prevent measurable damage or dysfunction; the term "effects," as used in this publication, is intended to include measurable compensatory responses as well as damage. USING MARKERS IN COMBINATION The application of biologic markers of air-pollution effects in forests reflects two related objectives: documenting that changes have occurred in natural systems exposed to high levels of stress and demonstrating under controlled conditions that a given pollutant dose can elicit a measurable response in some specific biologic or chemical process. The ultimate aim of developing markers is to integrate the two objectives to provide a set of markers that will point toward primary mechanisms and likely consequences of stress. Biologic markers can be used in forests at several organizational levels, including watersheds, forest stancis, individual trees, and metabolic processes. The workshop emphasized responses that indicate dysfunctions in single trees and at lower organizational levels, but useful indicators of ecosystem dysfunction were also reviewed. Such indicators include chemical attributes of soils and streams, hydrologic budgets (including evapotranspiration and water yield), and nutrient budgets (based on fluxes and pool sizes) that describe the net movement of water and nutrients through forest ecosystems. For example, despite the difficulties of documenting the specific process alterations and assigning them to specific causes, the integration of system function reflected in stream chemistry could turn out to provide a useful index of cumulative effects of some air pollutants on forest systems (see D.W. Johnson et al. in Part II). Time--a fundamental variable--often is inadequately addressed. Biologic processes are strongly influenced by daily and seasonal cycles and by changes in surrounding plant communities that take place annually or over longer periods. The consideration of time is particularly important in selecting markers to evaluate forest responses to chronic low-level pollution, which can take months or years to be expressed. For example, a change in net photosynthesis might be a useful marker of short-term high-concentration exposure to pollutant gases, but be less useful than a change in carbohydrate storage reserves or loss of membrane integrity as a marker of