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Air Pollution Impacts on Forests in North America ANN M. BARTUSKA U.S. Forest Service U.S. Department of Agriculture No other impact of air pollution has so mobilized public concern in North America as has the effect on forests. During the 1980s, funding for air-pollution research on forests increased tenfold. "Acid rain" was the primary issue early in the 1980s when effects on lakes and streams were the focus; now, as concern increases about effects on forests, studies are focusing on acidity (sulfur and nitrogen) in association with photochemical oxidants (especially ozone) and other gases. This chapter presents our current understanding of the impact of air pollution on the productivity and health of forests in various parts of North America, and discusses the major uncertainties still remaining in this area of research. This chapter will not review the substantial literature dealing with effects near major point sources of sulfur dioxide or fluoride (e.g., smelters), but instead will concentrate on regional air pollution. EFFECTS OF AIR POLLUTION ON FORESTS: CASE STUDIES Forest decline is a term used to refer to a general decrease in health and vigor leading to tree mortality over a large geographic region. Declines are not new phenomena, and have been widely reported in the literature. Some declines are caused by a complex interaction of multiple abiotic and biotic factors (Marion, 1981~. Others are caused by only one or two ca- sual factors (Benoit et al., 1982~. A classic example was the dieback of birch which occurred between 1930 and 1950 in southeastern Canada and the northeastern United States. In this example, higher than average soil temperatures in summer apparently resulted in extensive root mortality. The weakened trees were subsequently attacked by foliar and bark-boring 141

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142 ECOLOGICAL RISKS ~ -I- ~ ~4.. Expbnation me\ ' pH at Sample Site \ ~ Line of Equal pH Value ~ J FIG-ORE 1 The 1986 distribution of precipitation-weighted pH (NAPAP, 1986~. insects Armillarea mellea and various viruses leading to widespread mor- tality. Clearly, there was a multiplicity of causes. Whether atmospheric deposition plays a role in forest decline can only be assessed within the context of the numerous other stress factors that affect forests. While there have been numerous studies on the effects of a specific air pollutant on a specific plant, the complex nature of declines can only be assessed through an intensive, multilevel investigation of the community at risk. Due to this complexity, it is not surprising that we have few clear-cut examples of regional declines where air pollution has been a contributory factor (Figures 1 and 2~. In only two cases has air pollution been implicated as a cause of damage: the mixed conifer forests of the San Bernardino mountains of southern California and the Eastern white pine (Pinus strobes L.) in eastern North America. A third and more recent decline red spruce (Picea rubens Sarg.) at high elevations in the eastern United States has been suggested as being due to atmospheric deposition in association with natural causes. Finally, recent growth reductions in southern pines has led to speculation that regional air pollution is a primary cause. These four cases will be discussed in detail below (Figure 3~. San Bernardino Mountains Foliar damage to a variety of western conifers was observed in the San Bernardino Mountains of southern California in the early 1970s. These

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HUA"V EFFECTS ON THE TERRESTRIAL ENVIRONMENT 143 , }-~ ~ ~~ ' W~'. ,1 . to l 4 42 Nor 65 i;: ~ ~ Am, FIGURE 2 Average of maximum 7-hour daily mean concentration (ppb) of O3 during 1984 growing season at selected rural sites in the United States (NAPAP, 1987~. Me ~ ~ _SoUthem FIGURE 3 Areas where air pollution is suspected to impact forest trees. Mortem White Poe Spruce-flr effects provoked an extensive interdisciplinary study of this mixed conifer forest ecosystem (Miller et al., 1977; Miller, 1983~. Eighteen vegetation study sites were established along a geographical gradient of visible injury in the San Bernardino Mountains. The frequency and intensity of visible injury in the forest types occupying the transect was

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144 ECOLOGICAL RISES monitored from 1973 to 1976. The five forest types identified along this gradient include: ponderosa pine (fin us ponderosa Dougl. ex Laws.~;'pon- derosa pine/white fir (Abies concolor [Gord. & Glend.] Lindl. ex Hildebr.~; ponderosa pine/Jeffrey pine (R je~reyi Grev. & Balm.; Jeffrey pine/white fir; and Jeffrey pine. The symptoms visible on these species are very characteristic of what is termed "typical symptoms of ozone exposure," and include chlorotic mot- tling, necrotic tipburn progressing from the tip to the base, and premature needle abscission (Miller et al., 1982~. The trees showing the most visible injury were ponderosa -pine and Jeffrey pine. The symptoms on these trees appeared to worsen in degree and magnitude over time, i.e., some of the trees showing chlorotic symptoms in 1973 showed symptoms of necrosis by 1976, and a few of the trees showing necrosis and needle drop in 1973 were dead by 1976. Overall, the accumulated mortality of ponderosa pine and Jeffrey pine varied with forest type, but ranged from 0 to 6% for the 18 plots from 1973 to 1978. Average accumulated mortality rates by type were 4% for ponderosa pine, 4% for ponderosa pine/white fir, 6% for ponderosa pine/Jeffrey pine, 1% for Jeffrey pine/white fir, and 0%~ for Jeffrey pine (Taylor, 1980~. The visible symptoms and mortality were associated with decreased photosynthetic capacity, suppressed radial growth,' and reduced nutrient retention and availability. There was also evidence of decreased cone and seed production, increased litter accumulation, and increased infestation by pests and pathogens such as the pine beetle (Dendroctonus brevicomus) and root rot-caused by Comes annosus. The long-term effects expected for this system result in shifts from a pine overstory to a self-perpetuating ozone-tolerant community of oak and shrub species. The evidence is strong for ozone as the agent causing foliar injury and other types of injury. The San Bernardino National Forest (SBNF), which is located ' et the east end of the South Coast Air Basin (including Los Angeles), has been exposed to increasing annual dosages of air pollutants from heavy urban and industrial development over the last three to four decades. Photochemical oxidants have been carried by marine air currents to the mixed conifer forests since at least the early 1950s, when signs of injury were detected in ponderosa pine. As human population growth in the Basin has continued, both pollutant concentrations and the extent of the affected geographic area has increased (Miller, 1977; Taylor, 1980~. Thirteen air monitoring sites accompanied the vegetation study sites along the geographical gradient of visible foliar injury (Miller, 1983~. The natural background ozone (03) concentration in the study areas was 30 to 40 parts per billion (ppb); this concentration is comparable to that in other mountainous regions in the United States. The first signs of injury-of ponderosa pine needles were associated with a 24-hour average

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HUAL4N EFFECTS ON THE TERRESTRIAL ENVIRONMENT 145 O3 concentration exceeding 50 to 60 ppb during the months of May through September. When the average ranged from 100 to 120 ppb, injury to sensitive species such as ponderosa pine was severe and ecosystem functions were affected (Miller, 1983~. More recently, Peterson et al. (1987) demonstrated a significant decrease in Jeffrey pine growth north of Los Angeles in the Sierra Nevadas of California. This growth decrease has been linked to o7.one exposure and the appearance of visual damage symptoms, and is alarming due to the high aesthetic value of the area. Eastern White Pine Needle-blight injury on Eastern white pine has been reported for decades; however, it was not determined until the 1960s that the injury was caused by ozone (Berry and Hepting, 1964~. The conditions of Eastern white pine in areas of the southeastern United States mimic those of western conifers in the San Bernardino Mountains. In addition to the symptoms being similar, the short-term effects on the biological, chemical, and physical properties of the trees appear to be analogous. Foliar injury was visible on Eastern white pines in the Blue Ridge Mountains of southern Virginia (Skelly et al., 1972; Stone and Skelly, 1974; Phillips et al., 1977a,b; Benoit et al., 1982~. The pines showed typical ozone symptoms which varied in intensity within the species. Trees were classified as sensitive, intermediate, or tolerant based on needle length, needle retention, and presence of foliar injury. Eleven, 67, and 22% of the white pines surveyed were rated sensitive, intermediate, and tolerant, respectively (Skelly et al., 1979~. Nine Eastern white pine trees were sampled in Tennessee, three in each sensitivity class, and cored for radial growth analysis. Similar growth trends were found in the pines of tolerant and intermediate class, while a steady decline in growth rate (approximately 70~o over 15 years) was observed in pines of the sensitive class (McLaughlin et al., 1981~. This study also detected altered allocation of carbon, resulting in a reduction of photosynthate to the boles and roots. Radial growth decline was also evident in the trees sampled in the Blue Ridge Mountains and was corre- lated to changing local air quality (Benoit et al., 1982~. Skelly and Stone (1972) also noted a 66% decrease in height growth. The rate of decline of these white pine will be furthered by bark beetle infestation and root disease such as Verticicladiella procera (Skelly et al., 1983~. McLaughlin et al. (1982) suggest the following sequence of events and conditions afflicting Eastern white pine in the southeastern United States: 1. premature senescence and loss of older needles at the end of the growing season

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146 ECOLOGICAL RISKS 2. reduced storage capacity in the fall and resupply capacity in the spring to support new needle growth; 3. increased reliance of new needles on self-support during growth; 4. shorter new needles resulting in lower gross photosynthetic pro- ductivity; 5. higher retention of current photosynthate by foliage resulting in reduced availability of photosynthate for external usage (including repair of chronically stressed tissues of older needles); 6. premature casting of older needles. The net result of this condition is a reduction in the total amount of photosynthesizing tissue and carbohydrates available for growth and main- tenance. This sequence of events was derived from continuous observation of Eastern white pine (Pious strobes) in the Cumberland Plateau region of eastern Tennessee. Perhaps more importantly, these stages also are appear- ing in loblolly pine following ozone exposure, and may reflect a common pattern of tree response. Concentrations of ozone in the study areas of the Blue Ridge Moun- tains repeatedly reached 40-60 ppb, with peak episodes ranging from 100 to 200 ppb (Skelly et al., 1983~. Foliar injury occurred most frequently on Eastern white pine and milkweed and was absent on characteristically insensitive species. The observations of foliar injury corresponded with periods when ozone concentrations were the highest. The Eastern white pines that did not show visible injury did appear to exhibit growth effects (Skelly et al., 1983~. High-Elevation Red Spruce Although evidence of injurer has been recorded in high-elevation spruce-fir forests throughout the eastern United States, the patterns of symptom expression vary a great deal, and the linkage to air pollution is uncertain. In the northern Appalachian forests, Siccama et al. (1982) noted a reduction in density of all size classes of red spruce in higher elevational zones (>760 m) in the Green Mountains of Vermont. Similar observations on the decline of spruce in the Adirondacks were reported by Raynal et al. (1980), as well as in the White Mountains of New Hampshire, where a decline in number and basal area of this species was noted (Siccama et al., 1982~. The major visible foliar symptoms are loss of needles from the tips of the branches and from the apex of the crown, without the pronounced chlorosis indicating nutrient deficiency and with only slight macroscopic structural damage, i.e., necrotic spotting on the older foliage (Friedland et al., 1984a). Field and microscopic observations indicate that cold and/or winter moisture stress are responsible for the loss of the young foliage

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HURON EFFECTS ON THE TERRESTPL4L ENVIRONMENT 147 (Friedland et al., 1983b). Mature red spruce showed foliar loss from the tips of the crowns down and from the tips of the branches inward, similar to foliar loss exhibited on younger trees (Friedland et al., 1984a). Balsam fir (Abies balsamea [L.] Mill.) appears vigorous in that the symptoms of stress and necrosis exhibited by the spruce have not yet been noted. However, measurement of average annual increment on balsam fir shows a decrease similar to that of spruce (Siccama et al., 1982~. The symptoms cited above are typical of declining red spruce in the northern Appalachians. The spruce-fir forests of Camels Hump in the Green Mountains of Vermont show the decline symptoms typical of the northern region. Camels Hump is comprised of a complex of schist overlain by glacial till. On the lower slopes (< 750 m), northern hardwoods with occasional conifers are found. The upper slopes (750-1,210 m) are primarily red spruce-balsam fir, with occasional white birch (Betula papynfera var. cordifolia). The upper slopes are characterized by shallow acid till and very acidic Spodosols or Histosols (pH in H2O 3.04.5~. Average annual rainfall is 1,200-2,000 mm, increasing with elevation, and there is prolonged contact with cloud moisture (70-120 days per year). Visible characteristics of decline include the loss of younger foliage, severe frost damage (i.e., browning of foliage), and damage to needles at the cellular level, including to the chloroplasts and tonoplasts, which is typical of the damage to high-elevation spruce-fir in the north. The decline of red spruce, which is a long-lived, shade-tolerant species, is not the anticipated pattern based on its known ecological strategies and former abundance. Significant in the assessment of this decline is the fact that the pattern is recorded for both larger trees (30 m in height) as well as saplings (2 m in height in the substrate). Equally important is the fact that these symptoms occur throughout all the elevational gradients and across all age and size classes (Siccama et al., 1982~. In the southern Appalachians (i.e., Virginia, North Carolina, and Tennessee), some of the spruce-fir forests are in a severe state of decline (Bruck, 1984~. Here, symptoms include the loss of older foliage, proceeding from the inner to outer portions of the crowns. Chlorosis on the needles is apparent. Also, the appearance of epicormic branches has been noted on both the red spruce and the Fraser fir (Abies Easers [Pursh] Poir.) on the branches and stems. Mount Mitchell is located in western North Carolina and the first symptoms of decline in the south were noted here (Bruck, 1984~. At 2,038 m, it is the highest peak in the eastern United States and in the Appalachians. The bedrock is Precambrian metamorphic, gneiss and schist, with fine granitoid layers. Slopes of 20-60% are common, but some flat areas occur as draws and on crests. Where soils have coarse textured, parent material and a stable surface, Spodosols may be likely; where soils

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148 ECOLOGICAL RISKS are of medium textured parent material with unstable substrates, Umbrepts are likely. The climate is characterized by high precipitation: 1,800 mm throughout the year with extensive periods of cloud cover and relatively high input of total precipitation from cloud moisture. The survey of Mount Mitchell in 1984 showed that trees aged 45-85 years, primarily red spruce, averaged 75-90% defoliation characterized by loss of the oldest needles, often leaving a chlorotic tuft of needles at the branch ends. Most trees at or above 1,900 m exhibited some form of growth reduction beginning in the early 1960s. The spruce stands below 1,800 m also have begun to show signs of defoliation. Foliar analyses of needles collected during the survey showed no unusual concentration of macro- or micronutrients or trace elements. However, anticipated decreases of nitrogen (N) content with flush age for red spruce were not observed. Instead, N content of the 1983 needles was actually greater than the 1984 or 1982 needles (Bruck, 1984~. More recent surveys completed in 1985 and 1986 have shown contin- ued deterioration of the spruce-fir stands in the Mount Mitchell area. A concomitant study of cloud and rain chemistry in the area is showing sign- ficantly greater deposition of pollutants than at lower elevations; however, cause and effect cannot be attributed to any specific factories) at this time. Another important factor is the extensive Fraser fir mortality through- out the southern Appalachians due to the balsam woolly adelgid (Adelges piceae Ritz). The loss of fir has significantly altered the microclimate of these sites as the crown has broken up, increasing light penetration to the forest floor. Whether this structural change has affected the red spruce is still conjectural; however, it is an important natural stress factor which must be considered (Bartuska and Medlarz, 1986~. Recent hypotheses suggest that air pollution in the region is acting as a predisposing stress on Fraser fir, increasing the fir's susceptibility to adelgid attack (Hain and Arthur, 1985~. However, the link is circumstantial at present, and underscores the point that high elevation ecosystems are subject to many co-occurring stresses. Therefore, it may not be reasonable to expect that one factor can be isolated as the ultimate cause of mortality. Southern Pines Of the various case studies discussed in this chapter, the role of air pollution in the southern commercial forest region (specifically, Piedmont and Coastal Plain) of the United States is the most uncertain, but also the most important with regard to potential economic impact. Data from ten-year remeasurements of Forest Inventory and Assessment (FIA) plots in Alabama, Georgia, South Carolina, and North Carolina indicate that there has been a decrease in the rate of diameter growth in some portions

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HUMAN EFFECTS ON THE TERRESTRIAL ENVIRONMENT 149 of the southern pine region (Sheffield et al., 1985). These U.S. Department of Agriculture Forest Survey data suggest that commercially important loblolly (Pinus taeda L.), shortleaf (Pinus echmata Mill.), and slash pine (Pinus ellipse Engelm.) forests in the Piedmont and low elevation mountain regions of these states grew less rapidly by about 16-20% in diameter during the past ~ to 10 years than during the previous 10 years. Shortleaf pine also showed a decrease in growth in the Coastal Plain, although loblolly and slash pine showed a volume increase of 17% and 49%, respectively. The cause of the growth reduction has been attributed to increased hardwood competition, loss of the `'old field condition," an increase in pine mortality due to pine-beetle outbreaks, and the natural aging of stands. However, in evaluating any of these factors, air pollution can not be ignored. The average growing season rainfall pH is < 4.6, with events as low as pH 3.6 (NADP, 1988~. Perhaps of greater importance with respect to southern forests, the Piedmont and Mountain regions of the southeastern United States showed higher hourly O3 levels than most other regions (Lefohn and Pinkerton, 1988~. The influence of these air pollution factors on tree or forest response is still unknown and is being studied extensively. RECENT FINDINGS AND UNCERTAINTIES Prom 1985 to the present, significant research efforts have been un- derway to study the effects of atmospheric deposition and ozone exposure on forest species, with support from federal and state agencies, forest in- dust~y, and other institutions. While much of the data from this research is still being evaluated, some significant advances in understanding have been made. The purpose of this section is to provide some more recent research results which perhaps bring us closer to evaluating the link between air pollution and forest health. Western Conifers While the San Bernardino forests of southern California have long been known to be impacted by air pollution as described earlier in this chapter, recent research also is focusing on other areas which are, or have the potential to be, at risk. These areas are: Puget Sound in Washington State; the Sierra Nevada in California; and the Front Range of the Colorado Rockies. Sites in the southern Sierra Nevada have shown recent reduced growth rates from increment core analysis and visible ozone damage. Re- duced growth was an isolated occurrence among sites; the region as a whole showed no growth decline for ponderosa pine.

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150 ECOLOGICAL RISKS Comparative analysis of cloud water and rainfall indicates that clouds can have 3-10 times more hydrogen (H+), nitrate, and sulfate than precipitation in many parts of the western United States. These results are consistent with similar observations on cloud water chemistry in the eastern part of the country. Spruce-Fir . Measurement and analysis of increment cores collected from dom- inant and codominant red spruce and balsam fir trees at permanent plot locations throughout New York and New England have documented that there has been a significant decline across the region in the rate of indi- vidual tree growth since about 1960 for red spruce, and since about 1970 for balsam fir; however, all other major forest tree species are currently growing at rates that equal or exceed the rates prior to 1960 (Hornbeck et al., 1987~. While these analyses initially focused on the general question of growth reduction without attempting to answer questions of cause, recent analyses of both red spruce and balsam fir data have indicated that the declines should have been expected due to a natural stand growth phenomena. Stands of red spruce and balsam fir growing at elevations below 700 m are experiencing reduced growth as would be expected because of natural factors of stand development. Although atmospheric deposition impacts on growth rate cannot be completely ruled out by these analyses, the involvement of such pollutants must be considered minimal. This position is now widely accepted by other scientists, many of whom continue to corroborate these findings with other data. A field study at Whitetop Mountain, Virginia, confirmed that wet acidic deposition leaches basic cations from red spruce foliage. A compar- ison of cloud water and throughfall chemistry showed that there was an exchange of incoming H+ ions with foliar cations, mainly Ca2+ and Mg2+. This leaching increased as cloud pH decreased. In addition, a depletion of NH4+ in throughfall appeared to indicate the occurrence of direct foliar nitrogen uptake. The significance of these results is unclear. Foliar leaching is a normal process that usually occurs to some extent. It has not been determined whether the leaching losses observed here are excessive with respect to internal nutrient balances. Additional research is needed to better quantity nutrient leaching and to determine its significance as related to growth and productivity. Controlled laboratory and field studies have not demonstrated a significant ozone effect on red spruce seedlings, although ozone stress may alter winter hardiness. In contrast, exposure to sulfate acidic mist

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HUMAN EFFECTS ON THE TERRESTRI24L ENVIRONMENT 151 produced foliar symptoms rapidly on red spruce seedlings. Results indicate that spruce is particularly vulnerable to injury from acidic mist when sulfate concentrations are high, and there are repeated opportunities for drying of liquid on foliage. Recent hypotheses have suggested that a primary mechanism of pol- lutant impact on red spruce is through an alteration in the cold-hardening process, leading to increased winter injury. Several studies are ongoing, so that these data are only preliminary; however, results to date indicate that most hardiness is strongly negatively influenced by acid mist treatments. Further research is needed to better define the linkage between acidic deposition and winter damage. Southern Pines A dendrochronological analysis of over 2,000 increment cores from dominant and codominant naturally regenerated loblolly pine trees on typical Piedmont sites in North Carolina, South Carolina, and Georgia has led to the conclusion that these trees are growing today at less than two-thirds the annual rate that equivalent trees in stands on the same sites were growing 35 years ago. Analysis of the data using a modeling technique that allowed for the quantitative analysis of the impact of various stress factors on growth, including site, quality, stand-density changes, and climatic impacts revealed that an additional unexplained reduction in radial growth remained. A number of coordinated projects have demonstrated that families of loblolly pine (Pinus taeda) differ significantly in response both to simu- lated acid rain and to ozone, although the differences were most striking for ozone. An examination was made of 100 half-sib families of loblolly pine, selected for superior growth under ambient conditions, from throughout the southern commercial pine region. There were significant differences in response to acidic rain and to ozone among these families. Response to acidic rain treatments (pH 3.5, 4.3, and 5.2) was variable; some exper- iments showed negative effects, others showed positive, while still others showed no effect at all. In a two-year field study, the acid treatment affect did not appear until 18 months with rain exposure, at which point growth stimulation due to the pH 3.5 treatment was measured. These results emphasize the need for long-term research under controlled conditions to better quantify responses to acid deposition. In contrast, loblolly pine response to ozone was significant after only 6 months. Higher concentrations of ozone resulted in decreased height and diameter growth. These same saplings showed marked reduction in photosynthetic capacity and quantum yield. Similar studies with slash (Pinus elliotii) and shortleaf (fin us echinata) pines are showing responses

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152 ECOLOGICAL RISKS after eight months of treatment, consistent with the loblolly pine response. Further evaluation of the mechanism of impact is ongoing and must be expanded to include trees at all stages of development, from seedling to mature tree. There appears to be a strong link between ozone exposure and changes in carbon fixation and allocation. The net effect appears as a reduction in productivity without visible symptoms a response only detectable with long-term repeated measurements. CONCLUSION Our knowledge concerning the impact of regionally dispersed (as op- posed to locally dispersed) air pollutants on forest ecosystems clearly is incomplete. Improving this knowledge base is complicated by an absence of visible symptoms in many cases. The ability to perform research which links physiological or nutritional changes with grown appears to be a more useful approach to evaluating air pollution impacts on trees in the short-term. Ultimately, the response which society should consider is forest growth and productivity changes, which can only be detected with long-term measurements. A major challenge will be in providing sufficient information in the short term to assist policy makers in the decision process, coupled with the understanding that only through long-term studies wid uncertainty be reduced. Because multiple causal factors are probably involved in the response, the general public, policymakers, and scientists must recognize that if a large number of interactive factors contribute to a widespread change in forest conditions, even a well-funded research program may not produce a definite cause-and-effect relationship in forest ecosystems. We may have to be satisfied with a substantial body of circumstantial evidence with which to make decisions. Acknowledgement Sections of this document have been adapted from the Forest Response Program (FRP) Plan, the 1987 FRP Annual Report, the briefing document on the FRP prepared for the EPA Science Advisory Board, and the 1989 FOP Accomplishments Report. The contributions of the FRP management are appreciated. REFERENCES Bartuska, A.M., and S.N Medlarz. 1986. Spruce fir decline: Air pollution related? Pp. 55-73 in the Proceedings of the Appalachian Society of American Foresters Annual Meeting. Raleigh, North Carolina, January 30-31, 1986.

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HUMAN EFFECTS ON THE TERRESTRL4L ENVIRONMENT 153 Benoit, LF., J.M. Skelly, LD. Moore, and US. Dochinger. 1982. Radial growth reductions of Plus scubas L correlated with foliar ozone sensitivity as an indicator of ozone-induced losses in eastern forests. Can. J. For. Res. 12:673-678. Berry, C.R., and G.H. Hepting. 1964. Injury to Eastern white pine by unidentified atmospheric constituents. Forest Science 10:2-13 Bruck, R.I. 1984. Decline of montane boreal ecosystems in central Europe and the southern Appalachian Mountains, TAPPI Proceedings, pp. 159-163. Eagar, C. 1984. Review of the biology and ecology of the balsam woolly aphid in southern Appalachian spruce-fir forests. Pp. 36-50 in The Southern Appalachian Spruce-F~r Ecosystem: Its Biology and Threats, P.S. White, ed. USDI-National Park Service, Research/Resource Management Report SEA-71. Friedland, AJ., R.N Gregory, L Karenlampi, and A H. Johnson. 1984a. Winter damage to foliage as a factor in red spruce decline. Can. J. For. Res. 14:963-965. Friedland, AJ., NH. Johnson, and T.G. Siccama. 1984b. Trace metal content of the forest floor in the Green Mountains of Vermont: Spatial and temporal patterns. Water Air Soil Pollution 21:161-170. Hain, F.P., and F.H. Arthur. 1985. The role of atmospheric deposition in the latitudinal variation of Fraser fir mortality caused by the Balsam woolly adelgid Adelges piceae: A hypothesis. Zeitschrift fur angewand te Entomologie 99145-152. Hornbeck, J.W., R.B. Smith, and CA. Federer. 1987. Growth decline in red spruce and balsam fir relative to natural processes. Pp. 425~30 in Acidic Precipitation: Part II, H.C. Martin, ed. Boston: D. Reidel Publishing Company. Lefohn, AS., and J.E. Pinkerton. 1988. High resolution characterization of ozone data for sites located in forested areas of the United States. J. Air Pollut. Control Assoc. 38:1504-1511. Manion, P.D., 1981. Tree Disease Concepts. Englewood Cliffs, New Jersey: Prentice-Hall, Inc. McLaughlin, S.B., R.K McConathy, D. Duvick, and L.K. Mann. 1982. Effects of chronic air pollution stress on photosynthesis, carbon allocation, and growth of white pine trees. Forest Science 28~1~:60-70. Miller, P.R. 1983. Ozone effects in the San Bernardino National Forest. P. 161 in Air Pollution and the Productivity of the Forest, D.D. Davis, A A. Millen, and L. Dochinger, eds. Izaak Walton League and Pennsylvania State University. Miller, P.R., R.N. Kickert, O.C. Taylor, RJ. Arkley, F.W. Cobb, Jr., D.L" Dahlsten, PJ. Gersper, R.F. Luck, J.R. McBride, J.R Parmeter, Jr., J.R. Wenz, M. White, and W.W. Wilcox, Jr. 1977. Photochemical oxidant air pollutant effects on a mixed conifer forest ecosystem. Annual Progress Report, 1975-1976. EPA 600~77-104. U.S. Environmental Protection Agency. National Acid Precipitation Assessment Program. 1986. Annual Report to Congress. pp. 163. National Acid Precipitation Assessment Program. 1987. Interim assessment: The causes and effects of acidic deposition, Volume III. Atmospheric processes, chapter 5. Acidic deposition and its gaseous precursors. pp. 116. Peterson, D.L^, MJ. Arbaugh, V.N WakeSeld, and P.R. Miller. 1987. Evidence of growth reduction in ozone-injured Jeffrey pine in Sequoia and Kings Canyon National Parks. J. Air Pollut. Control. Assoc. 37:906-912. PhilliDs. S.O.. J.M. Skellv. and H.E. Burkhart. 1977a. Growth fluctuation of loblollv Pine . , , ,, ~ . due to periodic air pollution levels: Interaction ot ralntal1 and age. rny~opamology 67:71~720. Phillips, S.O., J.M. Skelly, and H.E. Burkhart. 1977b. Eastern white pine exhibits growth retardation lay fluctuating air pollutant levels: Interaction of rainfall, age, and symptom expression. Phytopathology 67:721-725. Raynal, D.J., and D.C. LeBlanc. 1985. Acidic deposition and effects on forests in the United States. In the Proceedings of the 12th Energy Technology Conference, Washington, D.C., March 25-27, 1985.

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154 ECOLOGICAL RISES Sheffield, R.M., N.D. Cbst, W.A Bechtold, and J.P. McClure. 1985. Pine growth reductions in the southeast. USDA Forest Semae: Southeastern Forest Expenment Station, Resource Bulletin SE~3:131. Siccama, T.G., M. Bliss, and H.W. Vogelmann. 1982. Decline of red spruce in the Green Mountains of Vermont. Bull. Torrey Bot. Club 109:163-168. Skelly, J.M., LD. Moore, and LL. Stone. 197Z Symptom expression of Eastern white pine located near a source of oxides of nitrogen and sulfur dioxide. Plant Disease Reptr. 56:3. Skelly, J.M., Y.S. Yang, B.I. Chevone, SJ. Long, J.E. Nellessen, and WE. Winner. 1983. Ozone ooncent~ations and their influence on forest species in the Blue Ridge Mountains of Virginia. Pp. 143-159 in Air Pollution and the Productivitr of the Forest, D.D. Davis, Am. Millen, and Lo Dochinger, eds. Izaak Walton League and Pennsylvania State University. Stone, LL., and J.M. Skelly. 1974. The growth of two forest tree species adjacent to a periodic source of air pollution. Phytopathology 64:773-778.