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chronic exposure. The permanent-plot system used by Forest Inventory and Assessment
personnel of the U. S. Forest Service and tree-ring analysis are particularly useful for
integrating long-term effects of pollutant stresses on trees and forests.
THE WORKSHOP
The committee's workshop was held on April 25-27, 1 98S, at Little Switzerland,
North Carolina. A fundamental question discussed at the workshop was whether there are
any unequivocal markers of the effects of air pollution on trees. Can changes in the
structure, color, metabolism, or reproductive success of trees and associated plants
or changes in whole forest stands or systems unequivocally indicate effects of specific
air pollutants not caused by any other factors? Or are the effects of individual
pollutants so confounded with the effects of other stresses that they are impossible to
identify without extensive experimentation?
Introductory Papers
Several papers at the workshop provided overviews of current programs for
monitoring forest conditions and air-pollutant concentrations. Husar described a
large-scale program for monitoring chemical air pollutants over North America and
Europe. He provided, in effect, a distributional atlas of atmospheric pollutants, such
as sulfur and nitrogen oxides, including their emissions, deposition, and concentra-
tions. This atlas, which can be examined with microcomputers, is intended to reveal
patterns of pollutant occurrence and of forest change. On a smaller scale, Mohnen
examined elevational gradients of air pollutants based on data accumulated by the
Mountain Cloud Chemistry Program (MCCP) in the eastern United States. At MCCP sites,
chemical and physical measurement of selected acidic compounds and associated oxidants
are being taken annually; the data will be useful for detecting attitudinal and
latitudinal variations. Barnard reviewed the Forest Response Program of the National
Vegetation Survey (NVS), a program begun in 1985 to determine the extent and magnitude
of forest conditions in the United States. Several NVS projects, such as one on
dendrochronology, give promise of a broad-scale monitoring program for the identifica-
tion of visible symptoms in forest stands. In the southern United States, for example,
a geographic information network will provide data on forest properties and on
atmospheric deposition of pollutants. According to Cape, monitoring European forest
conditions using routine visual assessments of leaf loss and discoloration has
presented problems of interpretation. He suggested the use of alternative methods, such
as comparison of visibly damaged versus undamaged trees, comparison of trees at
different sites exposed to different pollution regimes, and controlled-exposure
experiments intended to improve the validity of large-scale field surveys.
Tingey examined general applications and constraints in the use of biologic
markers for air pollution research. Because markers usually are relatively nonspecific
indicators of a problem, they must be used with caution in drawing conclusions about
cause-and-effect relationships. Ideally, markers are easily measurable, are responses
to specific pollutants, and produce distinctive symptoms that are not confused with
those caused by other environmental stresses. Sharpe and Spence examined several new
and emerging technologies for potential use in the detection of responses to stress,
including short-lived radioisotopes, nuclear magnetic resonance, infrared reflectance,
fiber optics, and semiconductors. Successful implementation of these tools would not
be easy or inexpensive, but preliminary information suggests that these new and emerging
methods would be remarkably cost-effective, compared with traditional experimental
protocols, for detecting some types of stress.
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Regional Applications of Biologic Markers
The next group of papers covered the use of biologic markers at forest sites in
different regions. A. H. Johnson reported on abiotic stresses and air pollution in the
decline of northern Appalachian red spruce forests. Multiple stress factors in winter,
coupled with pathogen and insect injuries, have been associated with declines in these
forests. Air-pollution stress (e.g., related to ozone) might increase the effects of
winter injury or affect energy balance as trees deplete resources in response to
pathogens and insect attacks. Anderson reviewed surveys showing relationships between
airborne pollutants and symptoms in several sensitive plants in southeastern U.S.
forests: eastern white pine, milkweed, and lichens. The use of those plants as markers
has provided information on extent of injury, type of associated pollutant, and temporal
variations. However, limitations in their use were found to be related to mimicked
symptoms and genetic, as well as climatic, variations. For western coniferous forests,
Miller considered the relation of biologic markers at tissue and whole-tree levels to
air-pollution exposure. An array of markers (foliage injury and changes in needle
tissues and elemental contents of leaves) was found to be more effective than a single
marker in suggesting the cause of observed effects. The evaluation of air-pollution
effects is improved by measuring the concentration of suspected pollutants along
gradients of decreasing pollutant deposition in combination with controlled exposures
of seedlings or small trees in enclosures that compare ambient air with carbon-filtered
air. More specific markers are needed to distinguish air-pollution effects from those
of other abiotic stress factors. Schutt discussed some problems in understanding forest
decline in Europe, such as the presence of multiple air pollutants and possible
synergistic effects. The detection of reliable specific symptoms is complicated by the
large number of tree species and wide-ranging variations in climate, soil, ecology, and
altitude. The existence of all those variations and gaps in knowledge makes it difficult
to determine whether different groups of scientists in different locations are working
on similar or different sets of forest decline problems.
Physiologic, Morphologic, and Ecologic Markers
The authors of this group of papers examined physiologic processes and structural
features as potentially useful markers of air pollution in trees. Waring focused on
resource allocation in various parts of a tree as an aid to interpreting signals of
environmental change. For example, nutritional imbalances brought about by environmen-
tal changes can alter the ratios of essential minerals to one another in foliage and
roots. Comparing changes in selected structural ratios (e.g., the leaf-to-bole ratio)
might make it possible to distinguish specific causes of stress and to correlate them
with changes in pollution loads on forests. D. W. Johnson, Van Miegroet, and Swank noted
how air pollution might affect cycles of forest nutrients, particularly sulfates and
nitrates in natural waters. Although nutrient pools might be affected by air pollution,
their relatively large size makes them rather insensitive markers. Even so, well-
documented long-term elemental budgets should be useful for determining changes in
ecosystem nutrient pools caused by air pollution and other factors. Fry looked at the
degree to which stable isotopes (in tree rings, lake sediments, and the atmosphere) can
serve as historical tracers of anthropogenic pollutants. Large isotopic changes in
sulfur were found in recent lake sediments in response to relatively large changes in
sulfate loading. Historical elemental records were compared with records of regional
and national emission patterns.
Cook and Innes described the use of tree-ring analysis to assess the impact of air
pollutants on forests and pointed out that anomalous tree-ring behavior could be
important in understanding pollution effects on forests. However, inferring a causal
link between air pollution and forest declines from tree-ring analyses alone requires
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a better understanding of air-pollution effects on tree growth under natural
conditions. Richards evaluated root growth and function after pollution-caused
reduction in carbon allocation. Four techniques showed particular promise for
assessing root-system damage caused by air pollution. As pointed out in several other
papers given at the workshop, the use of multiple measurements will yield more reliable
information from biologic markers than will attempts to relate air pollution to tree
damage on the basis of a single measurement.
Rock, Vogelmann, and Defeo discussed the use of remote-sensing techniques for
monitoring air-pollution effects in several forests. Remote sensing has shown changes
in the health of portions of spruce-fir forests in the eastern United States that are too
large to attribute to typical trends and natural variability. Some spectral signatures
are characteristic of forest damage and can be used on a large scale to identify various
specific symptoms of damage. Weinstein and Laurence evaluated the use of indigenous and
cultivated plants as biologic indicators of air pollution. Carefully selected
indicator species not only can identify some pollutants but also can provide estimates
of pollutant distribution and a rough approximation of source strength. Of the many
plant species that are sensitive to air pollution, a large number are low in cultivation
cost, require little maintenance, and can be grown over wide geographic areas, including
remote sites where the absence of electric power precludes many on-site measurements.
Scott and Hutchinson explored the suitability of using epiphytic lichens at high
altitudes as early indicators of forest decline. Preliminary work indicated that growth
abnormalities and metal accumulation can be demonstrated in some lichens growing on
healthy conifers. Although epiphytic lichens appear to have potential as biologic
markers of forest decline, their use as early indicators requires the identification of
specific structural and chemical predictors of severe tree dieback. Little-inves-
tigated microbial markers were the subjects of papers by Marx and Shafer, who used fungal
and bacterial symbiosis, and by Antibus and Linkens, who examined rhizosphere
activities. In the absence of baseline data on and adequate techniques for assessing
symbiosis in forests, it is doubtful whether microbial associations have potential as
biologic markers of air-pollution effects in forests. Similarly, data are too sparse
to suggest that any specific rhizosphere organisms or functions can serve as simple
biologic markers of air-pollution effects. However, measurements of root and
rhizosphere enzyme activities do offer a simple approach to the study of air-pollution
effects on rhizosphere physiology.
Biochemical, Cellular, and Tissue-Level Markers
The final group of workshop papers focused on potential biologic markers at the
biochemical, cell, and tissue levels. Norby concentrated on foliar nitrate reductase,
an enzyme that responds to gaseous nitrogen pollutants, can be measured in the field, and
thus might be a useful marker of the effects of nitrogen pollution on trees. Richardson,
DiGiulio, and Tandy looked at the potential use of free-radical-mediated processes as
early biologic markers of stress in trees. The authors established dose-response
relationships between ozone and acid-rain exposures and growth, photosynthesis, and
other biochemical activities in loblolly pines. Mechanisms of action and interspecific
variations are poorly understood, but it appears that antioxidants can be used as early
markers of oxidative stress in trees. Jones and Coleman also investigated the use of
biochemical indicators to predict changes in secondary metabolites and nitrogen in
leaves; their models, supported by preliminary data, indicate that two or more of these
characteristics will help to identify air-pollution stress or damage in trees.
Shortle was concerned with changing concentrations of metals in various tree
tissues. Some soil-related effects of acidic deposition might be linked to specific
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adverse plant conditions (e.g., suppression of cambial growth). The mechanism is
believed to involve the release of aluminum cations into solution in acidic soils (pH 3-
4.5) followed by the gradual replacement of essential metal cations, such as calcium and
magnesium, with aluminum in the rooting zone of spruce-fir stands. Bondietti, Baes, and
McLaughlin focused on changes in aluminum mobilization. The trend in Al:Ca cation
ratios in a tree growing on a relatively undisturbed site is a sensitive marker of
aluminum mobilization caused by atmospheric acid cleposition. The use of correlations
between Al:Ca ratios and radial growth rates is a promising approach to evluating the
linkage between aluminum mobilization (from atmospheric deposition) and growth-rate
declines.
McLaughlin examined the allocation and partitioning of carbon among various
physiologic processes as indicators of pollutant impacts on forest trees. The use of
carbon allocation pathways to identify and diagnose responses of trees to air pollutants
can include (1) identification of stress-induced shifts in patterns of carbon
production, storage, and use; (2) quantification of changes in the amount, timing, and
distribution of growth; and (3) evaluation of alterations in susceptibility to other
stresses. A whole-tree perspective is recommended as the best approach for detection
of changes in carbon allocation.
Winner dealt with the use of photosynthesis and transpiration changes as biologic
indicators in forests. Although those processes are affected by pollutant exposures,
nonpollution environmental factors, as well as seasonal variations, can produce similar
effects or confounding problems. Luxmoore was concerned with nutrient-use efficiency
(NUE) as a biologic indicator of air-pollution stress in trees. A review of reported
experiments led Luxmoore to suggest that NUE can be an insensitive indicator of stress.
He concluded that the use of NUE as an indicator of stress is impractical on a whole-
plant basis and is not definitive on a tissue basis. The possibility of using changes
in leaf cuticle structure as a marker of air pollution was discussed by Berg, who used
field and laboratory studies to demonstrate pollution-caused changes in ultrastructure
and physiologic properties.
Alscher, Cumming, and Fincher focused on ozone effects in red spruce during
different seasons. Results from a dose-response experiment on red spruce seedlings
suggest that ozone exposure during the summer and fall leads to changes in carbohydrate
metabolism associated with winter hardening and to cell damage during the late fall and
indicator
of leaf stress; the loss of chlorophyll from plants has long been used to assess injury
induced by air pollutants. Changes in chlorophyll fluorescence kinetics, although
indicative of pollutant injury, might not be specific to air-pollutant injury.
Therefore, several markers should be correlated before air pollutants alone can be
identified as causing stresses observed in plants. Tyree's paper discussed drought as
a factor that might interact with or be confused with stresses from atmospheric
pollutants. After reviewing the relevant literature on drought and related stress
mechanisms, Tyree pointed out forest decline symptoms caused by drought that could be
confused with symptoms caused by air pollutants.
early winter. Heath examined the use of chlorophyll fluorescence as an early
ESTABLISHING CAUSE- AND- EFFECT RELATIONSHIPS
Where should researchers look to detect early stages of the effects of air
pollution on forests and trees, and how might they address the classical problem of
establishing relationships between causes and effects? The answers are complicated and
somewhat unsatisfactory, but, in view of the importance of this issue for understanding
and controlling air pollution in the United States and elsewhere, are worthy of review.
Representative terms from entire chapter:
air pollutants
