There is an extensive literature on ecological effects of sulfur oxides, and a number of reviews have been published (Heggestad and Heck 1971, Brandt and Heck 1968, Wood 1968, Webster 1967, Smith 1974, Naegele 1973, Rennie and Halstead 1973, USDHEW 1969, Hindawi 1970, Halstead and Rennie 1973, Delisle and Schmidt 1973, Stokinger and Coffin 1968, Hobbs et al. 1974), including some preliminary attempts to assess economic losses (Rennie and Halstead 1973, Waddell 1974, USDA 1965, Benedict et al. 1971). In the limited time available, no attempt has been made to repeat these views. The primary purpose of this survey is to assess which of the various reported effects is likely to be sufficiently important to influence the choice of a strategy for emission control.
The procedure for decision analysis outlined in Figure 1 of Chapter 13 includes the specification of models for ecological damage and ecological costs associated with various levels and patterns of occurence of sulfur oxides The purpose of these models is to estimate the marginal costs imposed through ecological effects by additional emissions of sulfur oxides. Although Chapter 13 suggests that such estimates should be made separately for each power plant, this survey will consider only the two extreme cases: nationwide adoption of either (a) an emissions control strategy, or (b) a dispersal strategy (burning high sulfur fuel in rural areas and using tall stacks and/or intermittent
control to meet ambient SO2 air quality standards). Under the emissions control strategy, it is assumed that ambient levels of SO2 would not increase on the average, and would probably continue to decrease in urban areas. Under the dispersal strategy, the analysis in Chapters 6 and 7 suggests that there would be a small general increase in ambient SO2 levels in urban areas, a substantial increase in ambient SO2 levels in rural areas,* and a larger (40–175 percent) increase in the acidity of precipitation.
To place the problem in economic perspective, the dispersal strategy would result in an increase in annual emissions of about 20 109 pounds of sulfur (19 106 tons SOx) in the U.S., according to Table V in Chapter 6. Accordingly, an effect which leads to environmental costs of about $200 million per year would correspond to an average incremental cost of 1 cent per pound of sulfur emitted. Since other effects (e.g. those on human health and materials) are thought to involve costs substantially larger than 1 cent per pound of sulfur emitted (Chapter 13), the only tangible ecological effects that merit detailed consideration are those that involve nationwide economic costs of at least this amount.** To anticipate the results of this survey, none of the known tangible effects would involve costs of more than a few cents per pound of sulfur. However, some weight should be given
It is assumed that a well-conducted dispersal strategy would relieve the damage now being caused by existing low-level point sources, but would lead to a general increase in ambient concentrations, roughly in proportion to the increase in emissions (i.e. by about 65 percent according to Table V in Chapter 6).
This statement is not intended to deny the importance of intangible effects, such as effects on aesthetics, recreational opportunities, or unique natural assets; such effects need to be weighed together with tangible costs and benefits Nor is it intended to deny the importance of local damage around individual point sources, which may justify specific local measures to limit emissions or to provide compensation.
to certain hypothetical effects which would impose significantly larger costs if they actually occur.
There are five major obstacles to assessing the ecological costs of a change in sulfur oxide emissions:
Most investigations and reviews have been concerned exclusively with direct effects of sulfur dioxide (Heggestad and Heck 1971, Brandt and Heck 1968, Wood 1968, Webster 1967, Smith 1974, Naegele 1973, Rennie and Halstead 1973, USDHEW 1969, Hindawi, 1970); effects of acid rain have received attention only very recently (Bolin et al. 1971, Likens and Bormann 1974), and there is extremely little information on effects of suspended particulate sulfates.
Most experimental studies of effects of SO2 on plants have investigated only acute effects at relatively high exposures; effects on growth and productivity at lower exposure levels have been reported but have not been fully investigated (Heggestad and Heck 1971, Brandt and Heck 1968, Wood 1968, Webster 1967, Smith 1974, Naegele 1973, Rennie and Halstead 1973, USDHEW 1969).
Other sublethal effects of air pollutants on plants included enhancement of nutrient stresses, increased susceptibility to insect attack or disease, and effects on soil microorganisms; some experts consider that these are potentially much more important than acute injury (Smith 1974).
Synergistic effects of SO2 and ozone on various plants have been observed and may occur at pollutant concentrations well below levels that are of concern for human health (Tingley et al. 1973, Menser and Heggestad 1966)
Sublethal effects, such as reduced growth or increased susceptibility to disease, are very difficult to measure in agricultural crops or wild plant populations because of the simultaneous effects of other variables such as weather and other air pollutants.
EFFECTS OF SULFUR DIOXIDE ON VEGETATION
After reviewing studies of direct effects
of air pollution on vegetation, Waddell (1974) adopted a figure of about $200 million as a best estimate of its annual cost in the U.S.; of this, however, only about 5 percent was attributed to effects of sulfur dioxide. These figures, derived primarily from the work of Benedict et al. (1971) are likely to be low, for several reasons: (a) losses resulting from reduction in yield were largely ignored; (b) ornamental plants were under-valued, in that only replacement costs were used as a proxy for aesthetic values; (c) some of the damage attributed exclusively to oxidants may well have been caused by synergism between oxidants and SO2; (d) no figures appear to have been included for damage to pines, which are very sensitive to SO2 and to SO2/ozone combinations and have been extensively damaged around a number of point sources (Rennie and Halstead 1973, USEPA 1971, Costonis 1971, Linzon 1971).
Whatever value is assigned to pines and ornamental vegetation, damage to them is likely to be reduced by any emissions control strategy designed to achieve compliance with ambient air quality air standards for SO2. There is some evidence that white pines are damaged even at SO2 levels below the U.S. primary annual standard (Linzon 1971); such damage may continue to be of local significance and may justify more strigent control in areas where pines are of economic importance. However, it is unlikely that the total impact of this damage will be increased above the present level. Accordingly, unless there are large effects of SO2 on crops at levels a little higher than those now prevailing in non-urban areas, the effects of the projected increase in sulfur emissions by 1980 are likely to be small.
However, it should be noted that one expert in the field has recently expressed a contrary opinion (Heck 1973, pp. 128–129):
“The potential effect of an increase in oxidant and/or sulfur dioxide concentration is difficult to forecast. At some level the genetic resistance within a species is not sufficient to cope with a pollution insult. This level varies for both native and cul-
tivated species. Once a given pollution level is reached, the effect may increase rapidly with only slight increases in pollution. An educated guess suggests that a doubling of present pollution concentrations on the East Coast could, under otherwise favorable environmental conditions, produce from 25 to 100 percent loss of many agronomic and horticultural crops and severe injury to many native species. Several growing seasons at these higher pollution levels could result in the temporary or permanent loss of native species and major changes in many ecosystems. We are not far from pollution levels which could cause precipitous effects on agricultural production in the more humid areas of the U.S. However, an important variable must be considered in making any predictions based on increased pollution levels. This is that the capability of the living organism to respond and adapt to changes in its environment, within specific ranges of an adverse insult, has not been adequatley determined.”
Accordingly the possibility of large adverse effects cannot be dismissed.
EFFECTS OF ACID PRECIPITATION ON TREES AND FOREST PRODUCTIVITY
A number of studies in Scandinavia have suggested a progressive adverse effect of acid precipitation on the growth of coniferous forest trees (Bolin et al. 1971, Jonsson and Sundberg 1972, Marlmer 1973, Overrein 1972, Dahl and Skre 1971). In much of the glaciated parts of northern Europe, soils are naturally acid and deficient in nutrients. The productivity of forest land is closely correlated with the soil levels of calcium, which is subject to leaching by acid precipitation. A study in Sweden showed a significant reduction in growth between 1945 and 1965 in the stands most subject
to soil acidification (Jonsson and Sundberg 1972). The extent of the reduction in growth has been estimated to be of the order of 0.3 percent per year in Sweden (Bolin et al. 1971), but perhaps as much as 1 percent per year in Norway (Dahl and Skre 1971). Projecting ahead on the assumption of a continued increase of SO2 emissions in western Europe, it has been estimated that the overall reduction in growth in Swedish commerical forests might be as much as 15 percent by the year 2000 (Bolin et al. 1971).
In North America, the principal commercial forestry based on coniferous trees in the Northeast is in Maine, Ontario, southern Quebec and the Maritime Provinces: these lie in the same geographical relationship (500–1500 km downwind) to the major SO2 emitting regions in the U.S. as the Scandinavian forests to the major emitting regions in western Europe. No measurements of the acidity of rain have been traced for these areas of Canada, but high rates of sulfate deposition and acidic precipitation have been recorded in northern Maine (Chapter 7). Since the forest types and soils in this region are generally similar to those in Scandinavia, similar effects on forest growth would be anticipated.
In a study of a hardwood forest in New Hampshire, Whittaker et al. (1974) found that an “abrupt and striking” decrease in volume growth and productivity had taken place about 1960. The change (an 18 percent decrease) was unprecedented in the history of wood volume growth for the forest; the authors tentatively suggested that it might be related to the effects of acid rain and/or drought. The change is not necessarily comparable with that observed in Scandinavian coniferous forests, because soil nutrients were maintained at a fairly high level (Likens et al. 1971); however, leaching of nutrients from the leaves was measured (Eaton et al. 1973). Foliar leaching has been observed in experimental studies with birches exposed to acid mists at pH 4.0, and tissue damage was observed at pH 3.0, but not at pH 3.3 (Wood and Bormann 1975, Wood and Bormann 1974). Growth
reduction was not significant even at pH 3.0, however (Wood and Bormann 1974).
Growth abnormalities and tissue damage in various species of pines have been associated with acid rain with pH in the range 4.0–4.5 under field conditions (USEPA 1971, Gordon 1972, Hindawi and Ratsch 1974, Gordon 1974) and with simulated acid rain at pH 3.3 and 4.0 under experimental conditions (Gordon 1974, Shriner and Decot 1974). However, the results have been disputed (Wood 1975) and details of the experiments have not yet been published. Pines are especially sensitive to air pollutants (Heggestad and Heck 1971, Brandt and Heck 1968, Wood 1968, Webster 1967, Smith 1974, Naegele 1973, Rennie and Halstead 1973, USDHEW 1969, Hindawi 1970, Gordon 1972), and the results cannot necessarily be extended even to other coniferous trees.
In the present early stage of investigation, with many studies still unpublished, it is difficult to assess the significance of the various effects summarized above. However, the weight of evidence suggests a strong possibility that acid rain at present and projected concentrations may have widespread effects upon trees in the northeastern U.S. and eastern Canada. The industry at risk is substantial: the total stumpage value of forest harvested annually in Ontario, Quebec, and the Maritime Provinces is about $600 million, and the total value of shipments (lumber, paper, etc.) is 4–5 times larger than this (Rennie 1974, Env. Canada 1973). Except in northern Maine, the forest industry in the northeastern U.S. is relatively small, but there is another substantial industry (annual cut over $800 million) based on pines from eastern Virginia and southern Tennessee southwards (Stat. Abstract U.S. 1973), which would be affected if the recent southward extension of the area subject to acid rainfall (Chapter 7) is continued. Without more direct evidence for the nature and magnitude of damage it would be idle to speculate further, but clearly the problem is not negligible and needs further investigation.
In several areas of the Northeast, especially near the larger cities, the average pH of
rain is now around or below 4.0, and locally the average pH of summer rains is as low as 3.5 (Chapter 7). The projected increase in acidity of precipitation if emissions are uncontrolled (Chapter 7) would bring these levels close to those (3.3 and lower) at which acute injury to plants has been recorded under experimental conditions. Accordingly there is a substantial possibility that major damage to vegetation may take place in the most affected regions, especially in urban and suburban areas. The primary concern would probably be for ornamental and garden plants, which are a major recreational resource and contribute substantially to residential property values. The apparent sensitivity of pines is of special significance in this regard, because of their importance in ornamental plantings and for screening.
EFFECTS OF ACID RAIN ON AGRICULTURE
There are very few studies of the effects of acid rain on crop plants, and apparently none has investigated the pH range of greatest interest, 3.5–4.5. Seedlings of kidney beans and soy beans are reported to have shown signs of acute injury when exposed to simulated acid rain at pH 3.2 (Shriner and Decot 1974). A variety of physiological effects was observed in beans exposed to sulfuric acid mist at pH 3.0, but not at pH 3.5 (Ferenbauch 1974). Indirect effects of acid rain on plants include effects on soil micro-organisms, especially those responsible for nitrogen fixation (Rennie and Halstead 1973, Bolin et al. 1971, Shriner 1974), interactions with bacterial and fungal pathogens (Shriner 1974), effects on reproduction (Kratky), possible interactions with herbicides (Gordon 1974), and enhancement of the uptake of soil cadmium (Andersson and Nilsson 1974). None of the studies reported to date indicates a substantial effect on a crop plant at the level of acidity now encountered in precipitation. However, more investigation is needed, because even a small reduction in growth of a major agricultural crop, for example, could have a
major economic impact.
The only clearly identifiable effect of acid rain on agricultural systems is the acidification of soils (Bolin et al. 1971). Present-day agricultural practices already tend to acidify soils, so that lime is routinely used to maintain soil pH. The deposition of sulfuric acid and acid sulfates would require use of additional lime; direct absorption of SO2 would have a similar effect because it is oxidized to sulfuric acid in the soil and similarly contributes to acidification. If it is assumed, as a rough approximation, that half the sulfur oxides emitted are deposited as acid-forming substances on agricultural land, the projected emission of 19 million tons additional SO2 in 1980 would require the use of about 12 million tons additional lime. At a current cost of $14–18 per ton, including apreading, this would involve additional costs approaching $200 million annually.* Probably only part of this cost would be felt immediately, because some of the soils involved are now reasonably well buffered.
EFFECTS OF ACID RAIN ON FISH AND AQUATIC ECOSYSTEMS
Investigators in Scandinavia have reported major changes in the flora and fauna of acidified lakes and streams (Bolin et al. 1971, Almer et al. 1974, Jensen and Snekvik 1972, Grahn et al. 1973, Johansson et al. 1973). The most striking effects were those on fresh-water fish, expecially salmon and trout, which are progressively eliminated as the pH of the water falls to 5 and below (Almer et al. 1974, Jensen and Snekvik 1972). A similar phenomenon has
The offsetting value of the sulfur as a nutrient (Grennard and Ross 1974) is probably of negligible significance in the northeastern U.S., where soil sulfur concentrations are generally adequate to maintain productivity and current rates of deposition are already very high (see Chapter 7).
been recorded in acidified lakes in Ontario, where most fish species failed to reproduce after the pH fell into the range 4.7–5.2 (Beamish 1974, Beamish et al.). Disappearance of the fish followed cessation of reproduction, which involved failure of the females to spawn (Beamish et al.) and failure of eggs to hatch (Johansson et al. 1973).
As discussed in Chapter 1, continuation or increase in current emissions of sulfur oxides is expected to lead to similar pheonomena in the northeastern U.S. and in eastern Canada. The rate of acidification is difficult to predict in individual cases and it may take years or decades before some marginal lakes and streams become critically acidified. However in Massachusetts, for example, many lakes and streams already show peaks of acidity in early spring (with pH often in the range 4.5–5.5) and stocking of trout has to be delayed for 2–8 weeks (Cronin and Dixon). The problem can be ameliorated somewhat by addition of lime to managed ponds, but this does not always work, is expensive or impracticable for running waters, and is not recommended as a long-term solution (Bolin et al. 1971, Cronin and Dixon). The primary consequence of losses in fresh water fish populations would be restriction of recreational opportunity, but the economic aspects should not be ignored: there are some 12 million fresh water fisherman in the northeastern U.S., and their annual expenditure on their sport is of the order of $150–400 per capita (Stat. Abstract U.S. 1973, USDI 1967, Bridges and Sendak 1968: figures prorated to 1974).
In addition to effects on economically important fish, changes in populations of a wide variety of aquatic plants and animals have been recorded in acidified lakes (Almer et al. 1974, Grahn et al. 1973). In general, the biological productivity of lakes and marshes is related to concentrations of bicarbonates and calcium in the water. For example, Patterson (1974) has found a close relationship between the reproductive success of waterfowl and the concentrations of calcium and bicarbonates in the ponds where they breed. Continued acidification is
therefore likely to lead to a general reduction in biological productivity, except in fresh waters that are well supplied with calcium.
In a broad sense, human health and welfare are dependent ultimately on the maintenance of the functioning of natural ecosystems. Direct effects of sulfur oxides on human health have been considered in Chapters 1–4; indirect effects are difficult to assess except by consideration of specific components of natural ecosystems, as in the preceding paragraphs. Woodwell (1970) has raised the possibility that the prolonged occurrence of acid rain in the northeastern U.S. may have long-term adverse effects at the ecosystem level. Woodwell showed that the general effect of physical and chemical stresses is to impair the structure and functioning of ecosystems. A specific relevant example is the profound changes induced in fresh water lake ecosystems by acidification (Almer et al. 1974, Grahn et al. 1973). If such broad effects were starting to take place in terrestial ecosystems they would ultimately have major effects on human welfare and would be difficult to reverse.
A recent study in St. Louis has shown that sulfates (sulfuric acid, ammonium sulfate and ammonium bisulfate) are the predominant constituents in the fine particulate aerosols which form visible, light-scattering hazes in eastern Missouri (Charlson et al. 1974). Such visible, turbid air is noted in summer through the eastern U.S. south and west to Arkansas and Kansas (Flowers et al. 1969), and only really disappears with massive intrusions of Canadian air in winter (Charlson et al. 1974). Munn (1973) reported a progressive increase in the frequency of summer hazes in the Canadian Atlantic Provinces in the period 1953–71: the
hazes were associated with south and southwest winds and were attributed to transport of aerosols from the eastern U.S. A similar increase in haziness has been recorded in Ohio, Kentucky and Tennessee (Miller et al. 1972). The immediate effect of these changes, in visibility is primarily aesthetic, but the possibility of an effect on weather and climate cannot be dismissed, since the aerosols interact with visible light and have the potential for influencing cloud processes (Hobbs et al. 1974).
SUMMARY AND CONCLUSION
The direct effects of sulfur dioxide on vegetation are difficult to assess quantitatively and may have been under-estimated in post studies. However, they are probably unlikely to increase, and may decrease, if an increase in emissions is accompanied by well-managed dispersal.
Effects of acid rain on natural systems are probably of greater consequence than effects of sulfur dioxide, especially if emissions are permitted to increase. However, studies of the effects of acid rain are in their infancy: several important studies are in progress or as yet unpublished.
Identifiable effects of acid rain include acidification of soil, reduction in forest productivity, and depletion of fresh water fish populations. The full impact of these effects may be delayed for years or even decades. Rough estimates of the likely magnitude of these effects suggest that they are relatively modest in economic terms, each involving external costs of the order of 1 cent per pound of sulfur emitted. However, they may also involve loss of recreational opportunity and aesthetic values
The possibility of large additional effects, such as extensive injury to valuable ornamental plants or reduction in agricultural productivity, cannot be dismissed, especially if the acidity of precipitation is permitted to increase.
Atmospheric hazes, attributable in
large part to fine sulfate particles, are widespread in summer in the eastern U.S. Their frequency appears to be increasing as emissions increase. The possibility of effects on weather and climate cannot be dismissed.
Effects of sulfur oxides and acid rain on man-made materials have not been considered in this chapter.
Almer, B. W.Dickson, C.Ekstrom, E.Hornstrom, and U.Miller (1974) Effects of acidification on Swedish lakes. Ambio 3:30–36.
Andersson, A. and K.O.Nilsson (1974) Influence of lime and soil pH on Cd availability to plants. Ambio 3:198–200.
Beamish, R.J. (1974) Loss of fish populations from unexploited remote lakes in Ontario, Canada, as a consequence of atmospheric fallout of acid. Water Research 8:85–95.
Beamish, R.J., W.L.Lockhart, J.C.Van Loon, and H.H.Harvey (1975) Long-term acidification of a lake and resulting effects on fishes. Ambio, in press.
Benedict, H.M., C.J.Miller, and R.E.Olson (1971) Economic impact of air pollutants on plants in the United States. Ann. Rep. SRI Project LSD-1056. Stanford Research Inst., Menlo Park, California.
Bolin, B, et al. (1971) Air pollution across national boundaries. The impact on the environment of sulfur in air and precipitation. Sweden’s case study for the United Nations conference on the human environment. Stockholm: Royal Ministry for Foreign Affairs and Royal Ministry of Agriculture.
Brandt, C.S. and W.W.Heck (1968) Effects of air pollutants on vegetation, pp. 401–443 in: Air Pollution (ed. A.C.Stern) Vol. 1. Academic Press, New York.
Bridges, C.H. and P.E.Sendak (1968) Fish and Wildlife: $110,000,000 a year in Massachusetts. Massachusetts Division of Fish and Game, Boston, Massachusetts.
Charlson, R.J., A.H.Vanderpol, D.S.Covert, A.P.Waggoner, and N.C.Ahlquist (1974) Sulfuric acid-ammonium sulfate aerosol: optical detection in the St. Louis region. Science 184:156–158.
Costonis, A.C. (1971) Effects of ambient sulfur dioxide and ozone on Eastern White Pine in a rural environment. Phytopathology, 61:717–720.
Cronin, A. and J.Dixon (Massachusetts Division of Fish and Game), personal communications.
Dahl, E. and O.Skre (1971) (An investigation of the effects of acid precipitation on land productivity.) Konferensomavsvavling, Stockholm 1969. Vol. 1:27–39. Nordforsk, Miljovardsserkretariatet.
Delisle, C.E. and J.W.Schmidt (1973) Effects of sulphur on water and aquatic life in Canada. Draft chapter for monograph Environmental Effects of Sulphur in Canada prepared for National Research Council of Canada. MS.
Eaton, J.S., G.E.Likens, and F.H.Bormann (1973) Throughfall and stemflow chemistry in a northern hardwood forest. J. Ecol. 61:495–508.
Environment Canada (1973) Forestry Service. Canada’s Forests, 1972. (Statistical leaflet). Ottawa.
Ferenbaugh, R.W. (1974) Effects of simulated acid rain on vegetation. Ph.D. thesis, University of Montana.
Flowers, E.C., R.A.McCormick, and K.R.Kurfis (1969) J. Appl. Meteorol. 8:955.
Gordon, C.C. (1972) Mount Storm Study. Report to Environmental Protection Agency under contract 68–02–0229. University of Montana, Environmental Studies Laboratory.
Gordon, C.C. (1974) Unpublished manuscript. This manuscript was distributed as pp. 712–737 in Draft Environmental Impact Statemement: Colstrip Eelctric Generating Units 3 and 4, 500 Kilowatt Transmission lines, and associated facilities. Helena, Montana: Montana State Department of Natural Resources and Conservation.
Grahn, O., H.Hultberg, and L.Landner (1973) Oligotrophication—a self-accelerating process in lakes subjected to excessive supply of acid substances. Ambio 3:93–94.
Grennard, A. and F.Ross (1974) Progress report on sulfur dioxide. Combustion 4:4–9.
Halstead, R.L. and P.J.Rennie (1973) The effects of sulphur on soils in Canada. Draft chapter for monograph Environmental Effects of Sulphur in Canada prepared for National Research Council of Canada. MS.
Heck, W.W. (1973) Air pollution and the future of agricultural production, pp. 118–129 in Air Pollution Damage to Vegetation (ed. J.A.
Naegele). Advances in Chemistry Series 122. American Chemical Society, Washington, D.C.
Heggestad, H.E. and W.W.Heck (1971) Nature, extent and variation of plant response to air pollutants. Adv. Agronomy 23:111–145.
Hindawi, I.J. (1970) Air pollution injury to vegetation. U.S. Dept. of Health, Education, and Welfare, NAPCA. Raleigh, N.C.
Hindawi, I.J. and H.C.Ratsch (1974) Growth abnormalities of Christmas trees attributed to sulfur dioxide and participate acid aerosol. U.S. Environmental Protection Agency, APCA Paper 74–252.
Hobbs, P.V., H.Harrison, and E.Robinson (1974) Atmospheric effects of pollutants. Science 183:909–915.
Jensen, K.W. and E.Snekvik (1972) Low pH levels wipe out salmon and trout populations in southernmost Norway. Ambio 1:223–225.
Johannson, N., J.E.Kihlstrom, and A.Wahlberg (1973) Low pH values shown to affect developing fish eggs (Brachydanio rerio Ham.-Buch). Ambio 2:42–43.
Jonsson, E. and R.Sundberg (1972) Has the acidification by atmospheric pollution caused a growth reduction in Swedish forests? A comparison of growth between regions with different soil properties. Royal College of Forestry, Stockholm: Dept. of Forest Yield Research, Research Note 20.
Kratky, B.A., Unpublished studies cited in Likens and Bormann 1974.
Likens, G.E. and F.H.Bormann (1974) Acid rain: a serious regional environmental problem. Science 184:1176–1179.
Likens, G.E., F.H.Bormann, R.S.Pierce, and D.W.Fisher (1971) Nutrient-hydrologic cycle interaction in small forested watershed ecosystems. In Productivity of Forest Ecosystems (P.Duvigneaud, ed.), pp. 553–563. Proc. Brussels Symposium, 1969. UNESCO, Paris.
Linzon, S.N. (1971) Economic effects of sulphur dioxide on forest growth. J. Air Poll. Control Assoc. 21:81–86.
Malmer, N. (1973) On the effects on water, soil and vegetation from an increasing
atmospheric supply of sulfur. Statens Naturardsverk PM 402.
Menser, H.A. and H.E.Heggestad (1966) Ozone and sulfur dioxide synergism: injury to tobacco plants. Science 153:424–425.
Miller, M.E., N.L.Canfield, T.A.Ritter, and C.R.Weaver (1972) Visibility changes in Ohio, Kentucky and Tennessee from 1962 to 1969. Mon. Weather Rev. 100:67–71.
Munn, R.E. (1973) Secular increases in summer haziness in the Atlantic provinces. Atmosphere 11:156–161.
Naegele, J.A. (ed.) (1973) Air Pollution Damage to Vegetation. Advances in Chemistry Series 122. American Chemical Society, Washington, D.C.
Overrein, L.N. (1972) Sulfur pollution patterns observed; leaching of calcium in forest soil determined. Ambio 1:145–149.
Patterson, J.H. (1974) The role of environmental heterogeneity in the regulation of duck population. MS. Canadian Wildlife Service, Edmonton.
Rennie, P.J. and R.L.Halstead (1973) The effects of sulphur on plants in Canada. Draft chapter for monograph Environmental Effects of Sulphur in Canada prepared for National Research Council of Canada. MS.
Rennie, P.J., personal communication.
Shriner, D.S. and M.E.Decot (1974) Effects of simulated acid rain acidified with sulfuric acid on host-parasite interactions. MS.
Shriner, D.S. (1974) Effects of simulated rain acidified with sulfuric acid on host-parasite interactions. Ph.D. thesis, N. Carolina State University.
Smith, W.H. (1974) Air pollution—effects on the structure and function of the temperate forest ecosystem. Environ. Pollution 6:111–129.
Statistical Abstract of the United States (1973) 94th edition, Washington, D.C.: U.S. Bureau of the Census.
Stockinger, H.E. and D.L.Coffin (1968) Biological effects of air pollution. In: Air Pollution (ed. A.C.Stern). Vol. 1. Academic Press, New York.
Tingley, D.T., R.A.Reinert, J.A.Dunning and W.W.Heck (1973) Foliar injury responses of eleven plant species to ozone/sulfur dioxide mixtures. Atmos. Environ. 7:201–208.
U.S. Department of the Interior (1967) National Survey of Fishing and Hunting 1965. Resource Publication 27, Washington, D.C.: U.S. Government Printing Office.
U.S. Department of Agriculture, 1965 Agricultural Research Service Losses in Agriculture. USDA Handbook 291. Washington, D.C.
U.S. Department of Health, Education, and Welfare (1969) Air quality criteria for sulfur oxides. NAPCA Publication AP-50. Washington, D.C.
U.S. Environmental Protection Agency (1971) Mount Storm, West Virginia-Gorman, Maryland, and Luke, Maryland-Keyser, West Virginia, air pollution abatement activity. Technical Report APTD-0656. Washington, D.C.
Waddell, T.E. (1974) The economic damages of air pollution. U.S. Environmental Protection Agency, Report EPA-600/5–74–012. Washington, D.C.
Webster, C.C. (1967) The effect of air pollution on plants and soil. Agricultural Research Council, London.
Whittaker, R.H., F.H.Bormann, G.E.Likens, and T.G.Siccama (1974) The Hubbard Brook ecosystem study; forest biomass and production. Ecol. Monogr. 44:233–252.
Wood, A., (1975) Personal communication, has described experiments conducted at the University of Minnesota in which growth abnormalities of a similar type were observed in seedling pines not exposed to acid mist.
Wood, F.A. (1968) Sources of plant-pathogenic air pollutants. Phytopathology 58:1075–1084.
Wood, T. and F.H.Bormann (1974) The effects of an artificial acid mist upon the growth of Betula alleghaniensis Britt. Environ. Pollution 7:259–268.
Wood, T. and F.H.Bormann (1975) Increases in foliar leaching caused by acidification of an artificial mist. Ambio, in press.
Woodwell, G.M. (1970) Effects of pollution on the structure and function of ecosystems. Science, Vol. 168, pp. 429–433.
ASSEMBLY OF ENGINEERING
Robert H.Cannon, Jr., Chairman, California Institute of Technology
William C.Ackerman, Chief, Illinois State Water Survey
Holt Ashley, Stanford University
Raymond L.Bisplinghoff, University of Missouri-Rolla
Henri Busignies, International Telephone and Telegraph Corporation
George F.Carrier, Harvard University
Edward N.Cole, International Husky, Inc.
Edward J.Gornowski, Exxon Research & Engineering Company
Arthur G.Hansen, Purdue University
S.W.Herwald, Westinghouse Electric Corporation
Alfred A.H.Keil, Massachusetts Institute of Technology
Bruce S.Old, Arthur D.Little, Inc., Cambridge
Courtland D.Perkins, Princeton University
Allen E.Puckett, Hughes Aircraft Company
William E.Shoupp, Consultant, Pittsburg
George E.Solomon, TRW, Inc., California
Mac E.Van Valkenburg, University of Illinois
Edward Wenk, Jr., University of Washington
Micah Naftalin, Executive Director
COMMITTEE ON PUBLIC ENGINEERING POLICY
Edward Wenk, Jr., Chairman, University of Washington
Myron Tribus, Vice-Chairman, Massachusetts Institute of Technology
Vinton W.Bacon, University of Wisconsin
Samuel Baxter, Consulting Engineer, Philadelphia
Raymond Bowers, Cornell University
William D.Carey, American Association for the Advancement of Science
Charles S.Dennison, International Consultant, New York
Daniel Drucker, University of Illinois
Hazel Henderson, Private Consultant, Princeton, New Jersey
Walter R.Hibbard, Jr., Virginia Polytechnic Institute in State University
Herbert Hollomon, Massachusetts Institute of Technology
W.Deming Lewis, Lehigh University
Julius Margolis, University of Pennsylvania
Milton Pikarsky, Chicago Regional Transit Transportation Authority
Nelson Polsby, University of California, Berkeley
Louis H.Roddis, Jr., John J.McMullen Associates,,, New York
Abe Silverstein, Consultant, Fairview Park, Ohio
Abel Wolman, Johns Hopkins University, Emeritus
L.F.(Barry) Barrington, Executive Director
REVIEW COMMITTEE ON AIR QUALITY AND POWERPLANT EMISSIONS
Donald L.Katz, Chairman, University of Michigan
Morton Corn, University of Pittsburgh
Vladimir Haensel, Universal Oil Products Company
George N.Hatsopoulos, Thermo Electron Corporation
Alfred Kahn, NY State Public Service Commission
James P.Lodge, Private Consultant
John H.Ludwig, retired
Ian Nisbet, Massachusetts Audubon Society
John H.O’Leary, The Mitre Corporation
Arthur Squires, New York, New York
Laurence I.Moss, Staff Officer and Executive Secretary of Committee on Public Engineering Policy
Ronald J.Tipton, Staff Officer