Air Pollution as a Climate Forcing: A Workshop
Day 4 Presentations
Effects of Acid Deposition on Natural Ecosystems - A Short Summary
Hans M. Seip*+ and Fredric Menz+°
* Dept. of Chemistry, University of Oslo, Oslo, Norway
+ CICERO (Center for International Climate and Environmental Research , Oslo) ° : Clarkson University, Potsdam, NY
Causes. The term acid rain was first used by Robert Angus Smith in 1872. Acid deposition is caused by emissions of SO2 (power stations, metal smelters) and NOx (traffic, industry, power stations) forming sulfuric and nitric acid in precipitation. The gases (or aerosols) may also be deposited as dry deposition. Acidic mist or clouds may have ecological effects and contribute to acid deposition. Rainwater in equilibrium with the CO2 in air (and with no other species affecting pH) is slightly acidic with pH about 5.6. However, also under pristine conditions, the rainwater is often more acidic, i.e. pH is lower. Typical pH values of acid precipitation caused by human emissions may be 3.5-5.0. Ammonia emissions will neutralize the precipitation, but may cause soil acidification through nitrification.
Areas affected. The most affected areas are Europe, eastern North America, and South East Asia, especially central and southern China (Kuylenstierna et al., 2001). Sulfur emissions have played by far the dominant role. However, there have been large reductions in SO2 emissions in Europe and North America during the last two decades, about 65% in Europe and 40% in the USA from 1980 to 1999. NOx emissions in Europe increased from 1980 to 1990, but decreased by about 25% from 1990 to 1999 (EMEP, 2001). The US emissions have been quite stable from 1980 to 1999 (USEPA, 2002). This has led to increased importance of nitrogen emissions relative to sulfur emissions. Sulfur emissions in China have apparently also been reduced over the last few years, although they are still very dominant in most areas. NOx emissions are more difficult to curb than SO2 emissions, and reduction of ammonia emissions is a real challenge (Kaiser, 2001). Increased use of fertilizer has led to large increase in ammonia emissions, especially in some Asian countries (Galloway, 2000).
Effects on soils. Since soil acidification may in turn affect vegetation and acidification of water is strongly related to soil properties, it is important to understand the effects of acid deposition on soils. These effects depend strongly on the fate of sulfate and nitrate. If these anions leach out of the soil they must necessarily be accompanied by cations. If the soil is acidic, a substantial fraction of cations in soil water and leachate is Al ions and H+; a less acidic soil will leach more base cations (Ca2+ and Mg2+). In the former case, acidification of surface waters may occur. The soil water may also become so acidic and contain so much aluminum that the vegetation is affected. Loss of base cations results in soil acidification if it is not compensated by cations in the deposition or released through weathering.
In many young soils, such as those found in the Nordic regions, sulfate is fairly mobile, although some sulfur accumulation probably occurred during the long period of increasing deposition. Old soils, containing a large fraction of secondary clay minerals, such as those found further south, strongly adsorb sulfate. In such soils, a sulfate front may be created, and the concentration in leachate may remain relatively unchanged for many decades until the front has penetrated the soil profile. Nitrate is quite mobile in soils, but is taken up by vegetation. However, when the N deposition is sufficiently large, a considerable part of the nitrate may leach out.
Soil acidification, which is usually related to reduced amounts of exchangeable base cations in the soil, has occurred in Europe, North America and likely also in China. Watmough and Dillon (2001) in a study in central Ontario estimated losses of calcium and magnesium over a 16-year period (1983 to 1998) representing 37% and 59% of their respective exchangeable pools measured in soils in 1983. Since a number of factors may cause soil acidification, e.g. vegetation changes, it is often difficult to determine the contribution from acid deposition.
Water acidification. Since most of the precipitation falls on the terrestrial parts of the catchment, soil properties strongly affect the percolate before it enters a stream or lake. Therefore, water acidification resulting from acid deposition occurs in areas with acidic soils. In many areas, the upper soils are more acidic than deeper soils. During periods with high discharge, a large part of the water will pass through only the upper soils, which may result in critically acid episodes. Water acidification is most serious in Scandinavia where bodies of water with pH below 5 are common. In Norway thousands of lakes have lost their fish populations. The problem occurs also to some extent in other parts of Europe and in parts of the eastern United States and Canada. There have been considerable improvements (increased alkalinity) in acidified bodies of water in Europe as a result of reduced emissions in recent years. However, this is generally not seen in North America (Stoddard et al., 1999). The difference seems to be related to considerable reductions in the leaching of base cations from soils and therefore reduced concentrations in most of the studied bodies of water in North America.
Vegetation. Possible effects of acid precipitation (and its precursors) on forests have been the topic of intensive research primarily in Europe (UN/EC, 2001) and the United States (Driscoll et al., 2001; NAPAP, 1998), and recently also in China. In spite of this effort, quantitative relationships between pollution factors and forest damage have been difficult to obtain. Vegetation damages may be caused by direct exposure to gaseous or particulate air pollutants or indirectly through soil acidification. Direct damage due to SO2 is very likely in some areas. Other possible mechanisms include effects of high concentrations of ozone and other photooxidants and, in some areas, hydrogen fluoride in the air. Indirect effects of elevated levels of toxic aluminum in soil water, leaching of plant nutrients (particularly Mg) from soils, or reduced availability of phosphorus may also be responsible for reduced forest vitality. Acidic mist or acidic cloud water may decrease the cold tolerance. In most pristine forests, increased deposition of available nitrogen will increase growth rate, but if the N deposition becomes too high it may result in damage due to soil acidification, lack of other nutrients or increased sensitivity to other stress factors. Most likely vegetation damage often is a combined effect of man-made and natural stress (e. g. drought, frost, and insects).
In Europe, assessment and monitoring of effects of air pollution on forest have been carried out in a joint UN-EC program since the late 1980s (UN/EC, 2001). Trends in defoliation for some countries are depicted in Figure 1. Except for some areas in eastern Europe, where direct effects of SO2 probably have played an important role, the dramatic forest dieback feared by some scientists in the 1980s never materialized. Recent improvements in the tree vitality in some areas (Poland, the Czech Republic) have been related to decreased pollution and favorable weather conditions, but solid scientific evidence is lacking.
The latest report from the UN-EC program (UN/EC, 2001) states:
Deteriorated crown condition observed in many parts of Europe is the result of a complex of natural and anthropogenic factors. When statistically analyzed crown condition can be explained by stand age, weather conditions, biotic factors, soil condition and air pollution stress... Heavy defoliation in parts of Europe can be explained by high long-term depositions.
There are clear indications that deposition of atmospheric pollutants has changed physical and ecological conditions of forest ecosystems in Europe in recent decades. Furthermore extreme weather, with a series of warm and dry periods as well as heavy storm events in recent years, also has an impact on forest condition.
To date, investigations of possible effects of acid deposition on trees in the northeast United States and in Canada have focused primarily red spruce and sugar maple. There is strong evidence that acid deposition has caused dieback of red spruce by decreasing cold tolerance (Driscoll et al., 2001). Damage to sugar maple may in some areas be caused, at least partly, by loss of base cations (Ca2+, Mg2+) from the soil (cf. section on soil acidification).
In the United States, the National Acid Precipitation Assessment Program published a report in 1998 (NAPAP, 1998). The section on forest ecosystems states:
Sulfur and nitrogen depositions have caused adverse impacts on certain highly sensitive forest ecosystems in the United States. High-elevation spruce-fir forests in the eastern United States are the most sensitive. Most forest ecosystems in the East, South, and West are not currently known to be adversely impacted by sulfur and nitrogen deposition. However, if deposition levels are not reduced in areas where they are presently high, adverse effects may develop in more forests due to chronic, multiple decade exposure.
Matson et al. (1999) assessed the likely direct and indirect effects of increasing anthropogenic N inputs on tropical ecosystem processes. They concluded that anthropogenic inputs of N into tropical forests are unlikely to increase productivity and may even decrease it due to indirect effects on acidity and the availability of phosphorous and cations.
Severe acid rain is found in central, south, southwest and east China. According to Wang et al. (1996), about one third of the forested area in Sichuan province (including Chongqing) is influenced by air pollutants and acid rain. Studies of Masson pine in 11 provinces showed differences in growth between supposedly clean and polluted areas. However, there is a lack of well-documented studies of effects of acid deposition (and precursors) in China except near and close to cities (Larssen et al., 1999).
In 1998 the Chinese State Council approved the plan for an "acid rain control area" and an "SO2 control area," setting up targets and policies. The total control area covers 1.09 million square kilometers, representing 11.4% of total land area; the "acid rain control area" being the largest (8.4% of total land area). In 1995 the total emissions in the two control areas were about 60% of the total national emissions (SEPA, 2001).
Critical loads 1. The concept of critical loads is used to map areas sensitive to acid deposition and to illustrate where acid deposition exceeds what forests or surface waters can tolerate and hence links the emissions to ecosystem effects. Critical loads have played an important role in developing emissions reduction policies in Europe. The scientific basis is disputed, particularly regarding effects on forests which are based on the assumption that high aluminum concentration (or rather high Al/Ca ratio) in soil water is the main cause of forest damage. Since this assumption is dubious, and the aluminum chemistry in soils is very complex, the critical load values for forests are very uncertain.
1 The critical load is the highest deposition of a compound that will not cause chemical changes leading to long-term harmful effects on ecosystem structure and function.
Interaction with climate change. Effects of acid deposition may be amplified or mitigated by climate change in several ways. Changes in precipitation amounts and intensities will affect concentration and deposition of acidifying species and also affect acidification of soils and water through changes in water pathways. Where natural stress increases, interaction with pollution stress may become particularly serious. However, reduced cold stress should mitigate effects on red spruce and perhaps on other species. Increased temperature will increase the rate of many processes important in acidification, such as nitrification and other microbiological processes in soils.
Economic valuation. Lower ambient concentrations of SO2 and NOx yield a variety of environmental benefits to natural ecosystems, including forests and freshwater systems, but these effects have been very difficult to monetize. Interestingly, while damages to aquatic and, in particular, forest ecosystems sparked initial interest and scientific research in acidic deposition in both Europe and North America during the 1970s and 1980s, most of the economic benefits from reduced acid-precursor emissions are now linked to health improvements rather than ecological improvements. In the United States, this is possibly related to EPA's traditional focus on human health impacts of pollution control regulations, but it is also due to the complexity of quantifying linkages between anthropogenic emissions, air quality, and ecological service flows, and the time periods involved in the processes. In addition, there are difficulties in valuing ecological damages despite significant efforts by economists in developing valuation methods — particularly stated preference or contingent valuation surveys — suitable for assessing such values (Cropper 2000). In contrast to revealed preference methods (such as travel cost or hedonic pricing) which capture only the direct use portion of total economic value, contingent valuation can determine non-use values, which may form a considerable portion of the value of acidification damages.
While the interactions between acidic deposition and natural ecosystems can be extensive, there are only a few categories where there is a defensible link between emissions, the quality of the ecological service flow, and economic models to value the e changes. Thus, economic valuation of the effects of SO2 and NOx on ecosystems is very limited and some early numbers concerning the economic magnitude of losses are not credible. The category that has been most extensively studied is acidification damages to freshwater recreational fisheries, and most of the studies use a travel-cost methodology to estimate economic welfare losses. The EPA's recent cost-benefit assessments of the Clean Air Act (USEPA 1997; USEPA 1999) state that reductions in adverse air pollution impacts on wetland, forest, and aquatic ecosystems are possibly important but uncertain and difficult to quantify. Losses of commercial timber have also been estimated but these estimates are of little relevance.
Ecosystem impacts that have been most recently quantified in official US assessments of acidic deposition include reduced effects of acid rain on recreational fishing in the Adirondacks and reduced effects of nitrogen deposition in coastal estuaries. The estimated economic benefits of avoided acidification of the Adirondack fishery — representing the gain in economic welfare to current anglers alone — are about $50 million annually (USEPA 1999, p 97). Burtraw et al. (1997) find that the benefits of the U.S. acid rain control program exceed costs but were able to quantitatively model only recreational fishing benefits, which are of much lower magnitude than all other benefit categories. The relatively low damage estimates for recreational fisheries probably can be explained by the fact that they represent only angler use values and by the presence of substitute angling sites.
Economic valuation has been applied as part of the preparations for the Protocol to abate acidification, eutrophication and ground level ozone in Europe which was signed in 1999 (Holland et al., 1999; European Commission, DG Environment 1999). Based on 1990 emissions, health effects dominated the economic losses. Effects of acidification on forests and on other ecosystems were not quantified because of a lack of dose-response data for forestry and valuation data for ecosystems. Rather than monetizing the ecosystem effects, effects were characterized in terms of the change in the percentage of ecosystems that experienced acidic deposition in excess of critical loads for acidification. Damage to materials from SO2 and NOx was estimated to 1800 Meuro/year. In 1996 Navrud (2001), using contingent valuation, estimated a Norwegian willingness-to-pay of 80 - 154 Meuro/year to lime surface waters in Norway to get the same increment in fish stocks as estimated for fulfillment of the Second Sulphur Protocol. (The reduction in SO2 emissions in Europe would be close to 60% in 2010 compared to 1980.)
Environmental changes affect other ecosystem use values and nonuse values, but the magnitudes of these values for acidic deposition are not available. Effects of acidification that have not been quantified include aesthetic degradation of forests, effects on recreation in terrestrial ecosystems, reduced values to nonusers (option, existence, and bequest values), and reduced biodiversity. In the case of acidic deposition damages, the magnitude of these values could be significant.
As mentioned above, the critical loads have been extensively used in Europe in negotiations of emission reductions, apparently because it was less controversial than using cost-benefit analyses. This implies that expected ecosystem damages are playing a great role even if they are extremely difficult to quantify in monetary terms (cf. Patt, 1998).
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