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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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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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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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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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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.
Representative terms from entire chapter:
air pollutants
