Airborne chemicals and forest health Convincing data are lacking to prove the case for injury caused by atmospheric pollutants other than ozone
James N. Woodman EUis B. Cowling North Carolina State University Raleigh, N.C. 27695
I
Over the past few years the possible contribution of acid rain to the problem of forest decline has been a cause of increasing public concern. It bas also become a topic of inteuse scientific, industrial, and government interest in Canada and the United States. Research has begun to determine whether airborne chemicals are causing or contributing to visible damage and mortality in eastern spruce-fir and sugar maple forests and to changes in tree growth, usually without visible symptoms, in other parts of North America. This paper dascribes some of the complex biological relationships that determine health and productivity of forests and that make it difficult to distinguish effects of airborne chemicals from effects of natural stress. It d e scribes four major research approaches for assessment of the effects of airborne chemicals on forests, and it s u m m a r k s current understanding of the known and possible effects of airborne chemicals on forest trees in North America and Europe. It also briefly descrihes the major air quality and forest health research programs in North America, and it assesses how well these programs are likely to meet information needs during the coming decade.
Complicationsin identifition Understanding the effects of airborne chemicals requires some knowledge of how forests grow and how trees respond to stress. Forests are dynamic ecosystems in which only the fittest organisms survive. In every forest some trees will exhibit poor vigor, growth 120 Envimn. Sci. Technol..Vol. 21. NO. 2, 1987
CQ138e6W87/0921-012Ml .5010 @ I987 American Chemical Society
rates that are less than those of other trees, physical deformation, and natural mortality. These natural changes in forest condition make it difficult to detect additional changes induced by airborne chemicals. Most forest management practices are based on ecological and physiological principles that explain the survival and growth of trees in forests ( I , 2). The ecological principle of competition assumes that all organisms in a forest differ genetically in resistance and susceptibility to all types of stress. Because surviving and suppressed trees are distributed randomly within forests, the nutrients, water, and solar energy that would have been used by trees that die will be used by the survivors (3, 4). Forest trees are under continued stress in a number of ways. Natural competitive stress occurs when forest trees vie with one another and with other plants for limited supplies of sunlight, water, and essential nutrients from the same soil and microclimate. This is the main cause of low vigor, poor growth, and death of trees in most forests. Sources of natural climate stress include freezing, drought, flooding, low humidity, and high winds. Lack of water is the most common cause of decreased growth, low vigor, premature loss of foliage, and mortality (5).Natural biological stress is the impairment of normal physiological processes by fungi, bacteria, nematodes, viruses, insects, or other pathogens. In forests, fungi and insects are the most important of these. Natural chemical stress is induced by deficiencies (or occasionally excesses) of essential nutrients or by toxic soil chemicals, such as aluminum. Nitrogen is the most commonly deficient nutrient. Disturbance stress is caused by various human activities, including logging, controlled burning, or physical disturbance of soils leading to compaction, erosion, or leaching of nutrients. Air pollution stress occurs whenever forest trees are exposed to toxic concentrations of gases, such as sulfur or nitrogen oxides, ozone, or fluoride, or when trees are exposed to accumulations of toxic chemicals in soil. Most symptoms attributed to air pollutants are difficult to distinguish from those caused by natural stress ( 6 8 ) . Gaseous pollutants often produce chlorosis or premature shedding of leavessymptoms similar to those induced by drought, frost, foliar diseases, and some nutrient deficiencies. Most postulated effects of acid deposition are indirect and as such are similar to those induced by nutrient deficiencies and by biotic pathogens that attack tree roots, The two other reasons that it is difficult to detect airborne chemical stress
are embodied in the principles of genetic diversity (Figure l) and in the relationship between forest productivity and growing stock in forests (Figure 2). In theory, it is possible that all individuals within a given species or population of trees might be uniformly susceptible or resistant to a given air pollutant. Most evidence from studies of crop plants and forest trees, however, indicates that these populations generally contain substantial genetic diversity in resistance and susceptibility to air pollutants (9). Figure 1 shows the hypothetical extent of this genetically controlled variation. Thus, only a portion is highly susceptible to airborne pollutants, and these trees are distributed at random in most forests. Figure 2 shows that forest productivity (usually measured as the volume of stemwood produced per unit area of land at a given age) is relatively constant over a wide range of tree densities or total forest volumes (10). Thus, pollution-induced changes in growth or mortality rates of randomly distributed genetically susceptible trees are not likely to affect the productivity of a whole forest unless the percentage of affected trees becomes so large that the forest becomes understocked. It is possible that air pollution stress may simply increase mortality of pollution-sensitivetrees enough that resistant trees will grow more rapidly because of their improved competitive advantage. For these reasons it will be difficult to measure an effect on forest productivity even when many individual trees are injured or killed by airborne chemicals.
Scientific approaches The logical methods of determining cause and effect dictate that a causal relationship can be inferred when there is a strong pattern of consistency, responsiveness, and a proven biological mechanism with the suspected causal factors (11). In research on the effects of air pollution on forests, use of the term consistency requires that symptoms of injury or dysfunction be associated consistently with the presence of the suspected causal factor. Dose-response relationships are established through tests in which healthy trees are exposed to various known concentrations of suspected airborne pollutants under controlled conditions that simulate those in the forest. This relationship is called responsiveness. A mechanism is one or a series of biological processes through which the suspected cause is related to the observed effect. In simple systems that involve only one or two causal factors, any two of these three linkage patterns may be sufficient to infer cause. In more complicated
systems, all three may be necessary, This notion of patterns of linkage is derived in part from a series of four rules for proof of causality that were first used by Robert Koch (12) and later by Louis Pasteur to establish the germ theory of disease. For more than a century, Koch’s postulates have defined both the procedural steps and the standard for rigor in proof of causality in the medical, veterinary, and plant sciences. Many scientists have recognized the need for adaptation of Koch’s postulates when they are applied to abiotic stress factors such as air pollution (13-16)These adaptations fulfill the consistency and responsiveness patterns of linkage with the following rules of proof Rule l-The injury or dysfunction symptoms observed in the case of individual trees in the forest must be associated consistently with the presence of the suspected causal factors. Rule 2-The same injury or dysfunction symptoms must be seen when healthy trees are exposed to the suspected causal factors under controlled conditions. Rule 3-Natural variation in resistance and susceptibility observed in forest trees also must be seen when clones of the same trees are exposed to the suspected causal factors under controlled conditions. The first two of these adaptations of Koch’s postulates must be fulfilled to draw a firm inference about cause and effect; the third increases the biological rigor of the second. These rules are useful mainly in studies of relatively simple systems-that is those that involve one, two, or possibly three interacting causal factors. A synoptic approach for diagnosis of more complex problems has been suggested (17). This research involved initial surveys to identify important variables, multipleregression analyses to generate hypotheses, and field tests to verify diagnoses. The four scientific approaches commonly used in research on forest health and air quality are damage surveys, controlled-exposure tests, mechanism tests, and risk assessments. Damage or injury surveys quantify the number of trees involved or the geographic extent of economic damage or visible injury to forest trees (18-20). One common method has been to examine changes in radial growth using increment cores from sample trees across a pollution gradient (21-23). For air pollution research, damage surveys are useful only when they are combined with reliable data on air quality. Rule 1 requires that concentrations of airborne chemicals, sufficient to cause injury, be present consistently at the same time and place in which symptoms are exEnviron. Sci. Technol., Vol. 21, No. 2, 1987 121
c
hibited. Such correlational evidence alone, however, does not constitute proof of cause. Controlled-exposure tests range from simple comparisons of effects on healthy plants exposed to pollutant-free and pollutant-laden air to complex experiments in which healthy plants are exposed to various concentrations of one or more airborne chemicals and other stress-producing factors. Most controlled-exposure tests are performed with immature trees in growth chambers, greenhouses, or open-top chambers in forests; tests with mature trees are rare. Results are often expressed as dose-response relationships or as the minimum exposure (threshold value) needed to induce symptoms of damage or changes in growth. Controlled-exposure tests fulfill the responsiveness requirement (Rules 2 and 3) by demonstrating that the suspected pollutants can induce injury. Before it can be concluded that air pollution does cause injury, however, the test results must be combined with supporting data that demonstrate consistency under forest conditions (Rule 1). Mechanism tests are used to evaluate the hypothetical pathways proposed during the past several years for action of airborne chemicals ( 2 4 2 5 ) . For example, ozone has been pos!dated to inhibit tree growth by damaging cell membranes. This results in decreased photosynthesis, increased leaching of nutrients from foliage by acid rain, or both. Similarly, atmospheric deposition of nitrogen has been suspected of p r e disposing high-elevation red spruce fcliage to injury by frost. Just as correlation alone (Rule 1) is not proof of cause, neither is demonstration of a plausible mechanism unless it is supported by tests for consistency and re122 Environ. Sci. Technol., Vol. 21. No. 2, 1987
sponsiveness conducted under actual forest conditions (Rules 1 and 2 or 3). Risk assessments vary widely in objectives, methods, amount of effort required, and degree of certainty expected. They can be used to estimate the likelihood that one or more airborne chemicals will affect forest health or productivity in a given region.
Effects of airborne chemicals Most European and North American studies of the detrimental effects of air pollutants on forests have been made near metal smelters and other major sources of S q , NO,. and F or F2 (13. 2632). Much less is known conclusively about the effects of acid deposition or regionally dispersed airborne chemicals such as ozone (33). Although nearly 20 regionally important cases of change in condition of forest trees have been reported in Europe and North America (34),in only a few cases are airborne chemicals considered probable or possible causal or contributing factors. Some of the most important cases are described below in order of decreasing quality of evidence that airborne chemicals are liely to be involved.
Rigorous proof of cause The studies that demonstrate the strongest p m f of cause are those that involve white pine forests in the eastern United States and several forests in Southern California. A number of other field and experimental studies have been conducted as well. Eastern white pine. Ozone and other photochemical oxidants have been shown to induce visible damage and decreased growth in randomly distributed white pine trees in various parts of the eastern United States and
Canada (35-41). Injured trees are randomly distributed in forests in part because of genetic diversity in resistance and susceptibility to ozone. Ozone injuries in white pine trees include a variety of symptoms, such as chlorotic mottling and premature loss of needles, decreased growth of individual trees, and mortality. No sNdy has shown a loss in productivity of white pine forests. Proof that these injuries are induced by m n e is based on evidence from injury surveys with concomitant air quality measurements, dostxesponse studies with ozone, and tests with clones of individual trees observed to be resistant or susceptible in forests. Thus, all three adaptations of Koch’s rules have been fulfded in this case. Southern California forests. Ozone and other photochemical oxidants have been shown to cause similar visible injury, to d e c k growth of some trees, and to change the species composition of forests near Los Angeles (42-46). F’remaNre loss of needles leading to decreased photosynthesis is the major visible symptom; mortality of ozoneweakened trees usually is caused by bark beetles and root-rotting fungi. The major species affected include ponderosa pine, Jeffrey pine, white fr, limber pine, incense cedar, and California black oak. Injured trees are distributed randomly in part because of genetically controlled resistance and susceptibility to ozone. No studies have shown a loss in forest productivity. The research approaches used to demonstrate that ozone and other oxidants are involved included injury surveys with concomitant air quality measurements and dose-response studies with m n e . Thus, both Rule 1 and Rule 2 have been fulfilled.
Other field and experimental stud-
fores
Overstock
ies. A number of investigators have made field measurements or controlled-exposure studies that fulfii either Rule 1 or Rules 2 and 3, but not all. Their results apply only to the specific field or experimental situations in which the studies were made. livo such studies serve as examples. In the first, field symptoms of oxidant damage to seedlings of tulip poplar, green ash, hickory, black locust, eastern hemlock, table mountain pine, Virginia pine, and pitch pine have been correlated with high concentrations of ozone in Shenandoah National Park (47). The second study showed that annual dry-matter production increased when ozone was removed from the air surrounding seedlings of four clones of trembling aspen grown in New York State (48).
Cireumstantial evidence of cause There are two regional cases that exhibit only circumstantial evidence of causality: one in central Europe and one in eastern U.S. high-elevation SpNCe f0EStS. Central European forests. Essentially all commercially important tree species in Germany and several other countries of central Europe have shown neueariige Waldschiiden (new types of forest damage) since the late 1970s and early 1980s (32, 49). The major tree species affected include Norway spruce, silver fir, European larch, Scots pine, European beech, and certain species of oak, maple, ash, birch, and dder. The most common symptoms include chlorosis and visible thinning of tree crowns, decrease. in growth, decrease in root biomass, and changes in the shape and size of leaves (25). These symptoms are most commonly seen among trees that are at least 60 years
old (50) that are distributed at random in forest stands. Only a small portion of trees have died, and no studies have shown losses in forest productivity. The major research approaches are forest injury surveys and soil surveys. Very few reliable air quality measurements have been made in injured forests. A few controlled-exposure tests with ozone and acid mists (51)and simulated acid deposition on Norway spruce reproduced only a few field symptoms. Thus, none of the adaptations of Koch’s postulates have been fulfilled. Airborne chemicals are suspected mainly because no single natural factor or combination of natural factors can explain the observed changes in forest condition. High elevation spruce-fir forests. Visible injury, decreased radial growth, and widespread mortality of some highelevation red spruce trees have been observed in the Appalachian Mountains from Vermont to North Carolina (52, 53). Changes in condition include dieback of tree tops and branch tips on saplings and larger trees (54); widespread synchronous decrease in radial growth beginning in the early 1960s (21, 23, 52); and widespread mortality and a decrease in live basal area in forests in New York, New Hampshire, and Vermont (19,55). Current field research includes injury surveys and tree ring studies but few concomitant measurements of air quality. Mechanism tests and controlled-exposure studies have been started recently. The suspicion that airborne chemicals might be involved is based mainly on correlations between observed altitudinal gradients in the abundance of visible symptoms and known altitudinal gradients of several airborne chemicals. None of Koch’s
postulates has been satisfied.
Limited evidence of cause There are several regional cases of forest injury for which there is little or no evidence of the involvement of airborne chemicals. Sugar maple forests. Visible injury and mortality among sugar maple and other hardwood trees have been observed in Quebec, Ontario, New England, New York, and Pennsylvania (56, 57). Changes in forest conditions include branch and top dieback, sap production, and increased mortality. The only research to date has involved the use of damage surveys without concomitant air quality measurements. Air pollution is believed to be a primary or contributing causal factor because no other more plausible explanation has been suggested. Low-elevation coniferous forests. Unexplained decreases in radial growth without visible symptoms have been detected among trees in three areas: low-elevation red spruce in New York and several New England states (58);
pitch pine and shortleaf pines in Pennsylvania and New Jersey (20); and natural stands of loblolly, slash, and shortleaf pines in North and South Carolina, Georgia, Alabama, and Florida (59. Higher than expected mortality rates also have been detected in some southern pine forests. All of these changes were detected in surveys that were not designed to identify possible causal factors and with which there were no concomitant air quality measurements. Several natural stress factors could explain these decreases in growth. Experimental tests Environ. Sci. Technol.. Voi. 21. No. 2. 1987 123
for a possible causal or contributing role of airborne chemicals have only recently started.
North American research The most significant research programs in North America are funded by a handtid of organizations: Federal government. The U S . government’s Forest Response Program is part of the National Acid Precipitation Assessment Program (NAPAP) (24). It is funded by congressional appropriations to EPA and the U S . Forest Service and by voluntary contributions from the forest products industry. Total funding in 1986 was about $20 million. Its major objectives are to determine whether acid deposition alone or in combination with other air pollutants is causing or contributiug to forest d a m age in various regions. This program is designed to determine not only whether it is possible that exposure to pollutants can affect forest trees but also whether pollutants are in fact affecting forests or can be expected to do so in the fume. Table 1 outlines the organization and objectives of the Forest Response Program. The National Park Service also is conducting injury surveys with concomitant air quality measurements and controlledexposure tests. The 1986 budget for this item was about $2.5 million. Electric power industry. Forestry research programs funded by utilities are conducted by the Electric Power Research Institute (EPRI) (60). Volunrary contributions from electric power Droducers came to about $5 million in 1986. The general objectives of industrial research are to develop quantitative models of the influence of acid deposition and airborne sulfur compounds on nutrient cycles in forest emsystems and to model the interactions between air pollution and natural sources of stress. EPRI’s effortsare organized in two major projects. The fmt, the Integrated Forest Study (IFS), is designed to assess the effects of acid deposition on forest canopy and soil processes and on elemental accumulation and losses in selected high- and low-elevation forest ecosystems. The second project, Response of Plants to Integrated Stress (ROPIS), is designed to develop a general mechanistic theory and mathematical model that relate plant response to interacting pollutants and other anthropogenic and natural sources of stress. Canadian programs. The Canadian federal government and the provincial governments of Ontario and Quebec are funding three general risk assessment studies-the Canadian Forestry Service Acid Rain Program (61), the Acid Precipitation in Ontario Study 124 Environ. Sci. Technol..Vol. 21. No. 2. 1987
(a),
and the Quebec research program on sugar maple decline (63).Total 1986 funding for these projects was $7 million Canadian ($5 million U.S.). These programs are organized to achieve four objectives. First is an assessment of the magnitude of current problems that may be caused by air pollution in spruce, maple, and other forests. Another goal is the establishment of baseline damage characteristics and the development of monitoring systems for forests and soils against which future changes can be measured. Third, Canadian researchers hope to determine dose-response relationships for major tree species and forest soils. Finally, the projects aim at estimating safe loadings for various pollutants in several forest regions. Forest products industry. The US. forest products industry currently sup ports studies through the National Council of the Paper Industry for Air and Stream Improvement (NCASI).
Total 1985-86 funding was about $1 million. NCASI’s objective is to obtain information for a general risk assessment of the possibleeffects of air pollution on commercial forests in the United States (64). The project has four specific goals. First, and most important, is determining the concentrations of ozone and other air pollutants in commercial forests in the northeast, south, and western United States. Next is defining threshold concentrations of pollutants for the major commercial tree species in these regions. Third, project researchers hope to define the effect of air pollution on the productivity of commercial forests. Finally, they hope to determine the effect of acid deposition on forest soil quality. Expected results If sufficient time and stability of funding can be provided for the four major programs, significant progress should be made in a number of areas over the next five years. We should know much more than we do now about the chemistry of cloud water and gaseous pollutants, about the distribution of visible symptoms, and about the changes in growth and mortality rates
in high-elevation eastern spruce-fir forests and in moderate-elevation sugar maple forests in the northeastern United States and southeastern Canada. We also should know, from laboratory research, whether ozone, simulated acid rain, and excess nitrogen can induce symptoms of stress in red spruce and sugar maple seedlings. We believe that rigorous proof of the influence of any of these airborne pollutants will require more than five years. We should be able to make better predictions of the influence of airborne sulfur, nitrogen, and acidic substances on nutrient cycling processes. We also should arrive at a better understanding of the roles of these substances in the mobilization of aluminum and in soil leaching, soil weathering, and nutrient transformations in acid-sensitive soils. We should know much more than we do now about the potential effects of regional concentrations of ozone and about the effects of acid deposition on the physiology and growth of red spruce; sugar maple; loblolly, slash, and shortleaf pines; selected western conifers; and eastern hardwoods. This dose-response information will be based mainly on data from controlledexposure studies conducted in greenhouses and laboratory or field exposure chambers using small trees. We should have descriptive models of a few physiological manifestations of stress in seedlings and young trees as a result of controlled exposures to acid deposition, ozone, and excess nitrogen, kith alone and in combination with cerlain climatic and biotic pathogen stress factors. Within five iears,-we also should have improved our understanding of the hypothetical mechanisms of action by which these airborne chemicals might be operating in a few species of trees. In addition, we should have a few models to predict the effects on trees of changes in air quality. The current research programs will not be able to provide all the information that would be desirable. We believe that by 1990 there will still be no rigorous proof that one or several air pollutants cause decreases in the productivity of commercial forests in any region of North America. We also do not expect to have conclusive evidence of which postulated mechanisms of action of airborne chemicals are actually operating in whole forests, even though some mecbanisms may have been demonstrated for individual trees in controlledexposure studies. We also will not have completed thorough studies on many tree species. Most research will concentrate on red spruce, sugar maple, and loblolly pine, for example. The greatest uncertainty for the future of all four major programs has to
TABLE 1
Organization, program, and budget of the U.S. Forest Response Program
Eastern spruce-fir Southern commercial Eastern hardwoods Western conifers
Red spruce LP, SLt? SP To be determined To be determined
AD,O*,EN
AD, Os ADOa AD,Oa.SD
+ -
+ +
-
-
-
-
+ +
-
-
+ -
+
+
-
-
-
-
-
+
+
-
+
+
-
6.0
-
+
+
Supporting activities Identity geographical extent of changes in forest condition and growth Oetermine chemical exposure of Hi h elevation spruce forests-MCCP 08er forest types or regions Deveio data qual ob ect!ves, meth& manuals.%PJ& protocols Facilitateanalysis of data on elleas of acid &pasition and other pollutants in various forest regmns
National Vegetaron Survey Atmosphere Expasure Cwperatlv Quality Assurance and Quality Contml Pmgram synthesis and Integration Team
Total 1986 budpa( = $20 rnllllm U.S.
-
*Ma‘ tree Wcies: LP loblolly pine; SLP = shorUeaf pine; SP = slash pine. b A i gm e chemicals:AD acid depo6ition;0, = omne; EN = excess nilragen; S D = Sulhlr dioxide %?qmptoms suspened to be induced by airbornechemicals: VS = visible symptoms includingvisible in’ury to leaves or premature loss of ieaves. mortaiity; symptoms observed; dieback of shaais from top of lorest canopy; DRG decreased radial growth; MOR = inCr& mptOmS not ObWNed. %search appmaches: OS = Damage surveys without concomitantair quality mwuremnts; AQ = wnwmitant measurement of air qual and damage symptoms in the =ma forests; CE = controlledexposureof healthy plantsto known concentraflonsof airborne chemicals; RA assessment involvingvisible mptoms on major lorest species; MT tests of h patheticalmechanismsby which airborne Chemicals might induce visible symptoms or growvl e%ls: + = damage observed; = damage nM OLeNed.
..
+
-
do with the stability and longevity of funding. A substantial increase in scientific understanding cannot occur unless current levels of financial support are sustained for at least 5-10 years. The greatest scientific limitation in all four programs is the lack of reliable current and historical data on the chemical exposure of forests. The first rule of p m f for determination of causality requires that the symptoms observed in the forest be associated consistently with the presence of the suspected airborne chemical. The second and third rules require that the same symptoms observed in the forest must be replicated in contrnlled-exposure tests. If the symptoms and the chemicals are not associated consistently, or if the conmlled exposures are not similar to the exposures occurring under forest conditions, even high-quality survey data on the visible condition, growth, or mortality of forest trees will be of no value in drawing a scientific conclusion about cause-and-effect nr dose-response relationships. In essence, we may learn that trees are being subjected to stress without learning whether air pollutants are involved. Or we may learn that certain air pollutants can affect some forest trees without learning whether these air pollutants also affect the productivity of the forests containing those trees. The abundance and quality of information about chemical exposure in forests are far less than the abundance and quality of information about visible symptoms of damage or changes in growth of forest trees. Reliable maps are now available to show the chemis-
-
try of wet deposition in low-elevation forests in much of Canada and the United States, but much b e a r information is needed on wet deposition at high elevations and on the concentration of toxic gases at all elevations in both countries. These limitations are attributable in part to methodological problems in the collection and chemical analysis of gases and aerosols. There also is a lack of chemical monitoring data from rural and remote areas and a lack of historical data on air quality to match the historical records of growth measurements in forests. Finally, there are logistical problems in conducting research at high elevations. Some of these difficulties are overcome to some extent by analysis of existing air quality monitoring and emissions data. One recent example is the “Bends Report” published by the National Academy of Sciences (65).Analyses are currently bemg made of regional pollutant concentrations in the eastern United States (669).In addition, there is an effort within NAPAF’ to produce a series of electronic atlases that show the geographical and elevational distributions of airborne chemicals, physical climate factors, tree species, and soil types. Finally, we believe that current North American forest health and air quality research programs should be coordi~ t e to d obtain the most useful answers to the most important scientific and policy questions in the shortest time and at the least cost. We recommend that each research program focus on satisfying the scientific rules for proof
-
-
-
- r.2
of causality outlined in this paper. The problems of forest health and air quality are too complex for any organization to solve alone. The number of skilled scientists and the amount of research funds are too limited to waste on studies that do not directly address the most crucial scientific and policy questions. The current collaborative efforts of university, industry, and government scientists and research leaders must be sustained for at least the next decade if we are to be successful.
Looking ahead Toxic gases are the only airborne chemicals now known to cause damage to forest trees. Most of our current knowledge is based on studies of sulfur dioxide, fluoride, and nimgen oxides near point sources. Ozone is the only regionally dispersed airborne chemical that has been proven to cause visible symptoms, decrease radial growth, or increase mortality of individual forest trees; regional loss in productivity of whole forests bas not been proven. Airborne chemicals make up only one of six general classes of stress-prw ducing factors known to decrease growth, change visible condition, or kill forest trees. Airborne chemicals may act alone, but are much more likely to act in combination with natural causes of stress. Adaptations of Koch’s postulates and the logical principles of consistency, responsiveness, and mechanism provide the most appropriate methods by which cause-and-effea and dose-response relationships can be determined. Controlledexposure tests conducted Environ. Sci. Technol.. MI. 21, NO.2, 1907 125
with known concentrations of specific airborne chemicals, both alone and in combination with natural stress-producing factors, are promising methods by which to learn whether air pollutants can affect individual trees. This information will help researchers to determine whether specific, regionally dispersed air pollutants do affect some forest trees. Much more sophisticated physiological, ecological, and genetic response models must be developed before it will be possible to determine whether there is injury being done now or likely to occur in the future.
Acknowledgment This article was reviewed by the following experts in research o n air quality and forest health: Joe Barnard, Ann Bartuska, David Bennett, James Bennett, Nick Berenyi, Roger Blair, Ann Carey, Jay Garner, Walter Heck, Gerald Hertel, Aloys Huttermann, Tony Janetos, Kim loyner, Lavaille La Pointe, Alan Lucier, Samuel Limon, Louis Pitelka, B a r r y M e l a c , C h a r l e s Philpot, Carol Raulston, Wayne Son, Ben Stout, Jack Winjum, William Winner. This research was supported in part by Champion International Corporation and by a joint program of EPA and the US. Forest Service. T h e Forest Response Program is part of the National Acid Precipitation Assessment Program. This paper has not been subject t o EPA o r Forest Service policy review and should not be construed to represent the policies of either agency.
(19) Weiss. M.J. el 81. “Cooperative Survey of Red Spruce and Balsam Fir Decline and MOrtaliN in New Hamoshire. New York. and Veimont--1984,”’ 1nte;im Report: Northeastern Forest Experimental Station, U.S. Department of Agriculture Forest Service: Broomall. Pa., 1985. (20) Rezabec, el al. “Regional Effects of Sulfur Dioxide and Ozone on Eastern White Pine (Pinus strobus L.) in Eastern WisconSin,’) Report, Wisconsin Department of Natural Resources: Madison. Wis., 1986. (21) Johnson, A. H.; McLaughlin. S. B. In “Acid Deposition Long-Term Trends”: National Academy Press: Washington, D.C., 1986; pp. 200-230. (22) Johnson. A. H. et al. 3. Environ. Quol. 1981,10, 420-27. (23) McLaughlin. S. B. J. Air Polfut. Control Assoc. 1985,35(5), 512-34. (24) “Response of Forests to Atmospheric Deposition: National Research Plan for the Forest Response Program”; EPA and U.S. Forest Service: Washington, D.C.. 1986. (25) Schiitt, P; Cowling. E. B. PIom Dis. 1985.69, 548-58. (26) Woodwell, G. M. Science 1970, 168, 429-33. (27) Carlson. C. E. “Evaluation of Sulfur Dioxide Injury to Vegetation on Federal Lands Near the Anaconda Copper Smelter at Anaconda, Montana,” USDA Insect Disease RePort 74-15: U S . DeDartment of Aericulture: Missoula, Mont., 1974. (28) Thompson, M. A. Environ. Pollut. (Ser. AJ 1981.26,251-66. (29) Legge, A. H. WaterAir SoilPollur. 1980, IS a ?!.. (48) Wang D.; Karnosky. S.F.: Bormann. F. H. Can. J. For: Res. 1986, 16.47-55. ~ Noture 1985.314, 311-14. (49) ~Blank, L.W. (50) Schiitt, P: et al. So Stirbr der Wald; BLV Verlagsegesellschaft: Munich, West Germany, 1983. (51) Prinz, B.; Krause, Q.H.M.: lung. K.A. In Waldschiiden-theorie und Prazis auf der suckenach Antwonen: Olden Bouey Verlag: Munich, West Germany, 1985; pp. 143-93.
James N. Woodman (I.) is a ,,isiring p r w fessor in the School of Foresr Resources, North Carolina Srate University. He is rhe former director of forest environmental affairs of Champion International Corporation and sewed as chairman of the Task Group on Forest Health f o r the Notional Council of rhe Paper Industry f o r Air and Stream Improvement. He holds a Ph.D. in forest ecophysiologyfiom the University of Washington.
EUis B. Cowling ir) is associare dean in the School of Forest Resources and director of rhe acid deposirion program at North Carolina Stare University. He has done research on the ecological effects of airborne chemicals since 1971. He is a scientific a& viser in the area offorestry research ro the National Acid Precipirarion Assessment Program.
