Results:

Findings from the workshop are synthesized into four categories:

• Sulfur Deposition;
• Nitrogen Deposition;
• Acid Neutralizing Capacity and Base Cation Cycling; and
• Response and Recovery of Ecological Systems.

Conclusions:

• In some regions of the United States, such as the Midwest and Northeast, emission reductions of sulfur dioxide correspond to decreases in the concentrations and fluxes of sulfate in both precipitation and surface waters;
• In some regions and ecosystems, current reductions of sulfur emissions may be insufficient for ecological recovery to occur;
• Factors limiting the recovery of ecosystems from acidification include nitrogen deposition, depletion of exchangeable base cations, unaccounted for inputs of sulfur (which might result from underestimates of total deposition or internal sources of sulfur such as weathering of minerals or mineralization of organic sulfur in soil);
• Given current knowledge of nitrogen deposition and its impact on ecological systems, the importance of nitrogen must be fully considered in formulating future air quality policies;
• Acidic episodes can have significant biological effects (e.g., loss of biodiversity; changes in community structure), and these effects are more widespread geographically than chronic acidification;
• Monitoring the chemistry of atmospheric inputs and surface waters should be continued and enhanced, and include efforts to improve measurements of total (wet and dry) deposition; and
• Current monitoring efforts should be maintained and expanded through inclusion of biological indicators of ecosystem response to acidifying deposition.

Monitoring

In order to evaluate the effectiveness of environmental policies and programs, a firm commitment is needed to long-term monitoring programs that help in assessing the status and trends of ecological systems and ecosystem response to historic and future changes in stress (i.e., deposition), as well as discerning when, where and why recovery is occurring.

Research

Much has been learned over the last two decades as a result of research regarding the sources, mechanisms, and effects of acid deposition on ecological systems. Our understanding of the impacts of sulfur, nitrogen, base cation depletion, and the response and recovery of ecological systems has been refined. Still, many questions must be answered to fully assess whether the environmental goals of the 1990 CAAA are being met as a result of sulfur and nitrous oxide emission reductions. Future research should focus on these particular areas: nitrogen deposition surface water chemistry, watershed/soil chemistry, and forest ecosystems.

*Bolded words indicate terms defined in glossary

This document reports the results of a workshop attended by over forty leading academic and Federal agency researchers from the United States and Canada (Appendix 2). The workshop was held on March 1-3, 1999 in Washington, D.C. and was sponsored by the Ecological Society of America and several Federal agencies, including the Environmental Protection Agency (EPA), U.S.D.A Forest Service (USFS), U.S. Geological Survey (USGS), and the National Acid Precipitation Assessment Program (NAPAP). The workshop goals were to: 1) evaluate the status and trends of various types of ecosystems in response to acid deposition; and 2) determine whether and how the extent of ecological damage from this disturbance has evolved since observed, projected, and reported in the 1990 NAPAP Report.

Scientific evaluation of these patterns is timely due to the availability of long-term data sets developed by Federal agency monitoring programs, investigations initiated by EPA and the National Science Foundation's Long-Term Ecological Research Network, especially the Hubbard Brook data base that was collected and assembled by scientists, and monitoring and research programs sponsored by a variety of other agencies and organizations. Moreover, this is a timely opportunity to assess the response of ecological systems to Title IV emissions reductions called for in the 1990 amendments to the Clean Air Act. It is important to note that the workshop was not intended as a rigorous review of the science and data presented. Instead, the workshop accepted, at face value, peer-reviewed data and information in an effort to assess the current state of the science regarding acid deposition.

Workshop Framing Questions 1. What have been the ecological responses to chronic deposition? Trends in surface water chemistry and biology Trends in watershed/soil chemistry and biology Trends in forest responses Where have responses been observed? 2. What is the capacity of our monitoring systems to track our expectations? What are the uncertainties? 3. What will be the ecological response to projected future deposition rates? What determines the sensitivity of ecosystems to atmospheric deposition effects? What is the potential for recovery of sensitive ecosystems? What are the uncertainties? What future research is needed? 4. What are our priority gaps in understanding?

Policy Context

In 1980, Congress passed the Acid Precipitation Act (PL 96-294, Title VII) to develop information regarding the formation and environmental effects of acid deposition. The Act mandated creation of a Federal interagency group referred to as the National Acid Precipitation Assessment Program (NAPAP). Through NAPAP, federal agencies gained a forum to define common research interests, develop coordinated research programs, submit coordinated research budgets to Congress, and conduct assessments. Beginning in 1980, NAPAP examined the relationships among fossil fuel combustion, pollutants resulting from emissions and the effects of these pollutants (particularly sulfur dioxide and nitrogen oxides) on the environment, economy, and human health. Results of that research were published as the NAPAP 1990 Integrated Assessment Report.

In 1990, the Clean Air Act Amendments (CAAA) (PL 101-549, Title IV) were passed to reduce the adverse effects of acid deposition through phased reductions of annual emissions of its precursors, sulfur dioxide (SO₂) and nitrogen oxides (NO_X), in the forty-eight contiguous states and the District of Columbia. The Acid Deposition Control Program created by Title IV is being implemented in two phases, with two major goals, to reduce total SO₂ and NO_X emissions.

When Congress enacted acid rain provisions in the 1990 CAAA, it developed an approach specifically designed to address long-range air pollution problems having regional environmental and human health effects. Title IV also introduced market-based mechanisms to implement emissions reductions at lower cost while capping the overall level of SO₂ emissions. The depth of research on cause and effects, the existence of baselines established through environmental monitoring, and the use of innovative policy approaches all led to the acid rain control program being referred to as the "big experiment" by the scientific and policy communities. Evaluating the ecological response to changes in emissions of SO₂ and NOx is an important link between scientific research and environmental policy in the context of implementing the 1990 Clean Air Act Amendments.

Understanding the current quantities and trends in emissions and deposition of sulfur (S) and nitrogen (N), and the extent and rate of ecological responses to these changes, is important for other reasons as well. Through the Government Performance and Results Act (GPRA) and the National Performance Review (NPR), both Congress and the White House have directed all Federal agencies to initiate performance-based management programs. Performance-based management entails establishment of justifiable goals or objectives, and a periodic assessment of progress toward those goals. In the case of environmental policy, a performance-based system requires a common metric of environmental or biological relevance that is measurable over time. Taken together, GPRA requirements, NPR mandates, and the Congressional directive to prepare a NAPAP 2000 Integrated Assessment Report create the need for a scientific evaluation of current issues and data related to acid precipitation and patterns of ecosystem response.

1990 Clean Air Act Amendments, Title IV Statutory Provisions SO2 emissions--Reduce by 10 million tons below 1980 levels by 2010 (roughly a 50% reduction below 1980 levels); Phase I initiated in January 1995 and Phase II began in 2000; utility SO2 emissions are limited, or "capped," at 8.95 million tons per year; non-utility (i.e., industrial) emissions of SO2 capped at 5.6 million tons per year beginning in 1995.  NOx emissions-- Reduce by 2 million tons below projected levels from coal-fired boilers by 2000 (roughly a 10% reduction off of levels without Title IV); Phase I began in January 1996, and Phase II became effective in 2000; no cap on NOx emissions. Current Emission Levels SO2 emissions -- During the 1995-1998 period, SO2 emissions from all utility units (including substitution and compensating units) affected by Phase I averaged 5.33 million tons/year, an average annual decrease of approximately 51% from those sources from 1980 levels.  From 1995-1997, total national SO2 emissions from all sources averaged 19.8 million tons/year.  NOx emissions-- During the 1996-1998 period, NOx emissions from the 265 utility units (including substitution and compensating units) affected by Phase I averaged 923,516 tons/year, an average annual decrease of 31% from those sources from 1990 levels.  From 1995-1997, total national NOx emissions from all sources (stationary, mobile, other) averaged 23.6 million tons/year.

Current Scientific Understanding

The 1980s was a decade in which knowledge of acid deposition increased dramatically. Scientists developed greater understanding regarding patterns of acid deposition, the mechanisms through which it affects aquatic and terrestrial ecosystems, and the ecological response to acid deposition. These advances in scientific knowledge and understanding built on research, observations and hypotheses developed in the 1970s, and reflected the substantial policy attention and funding that the issue of acid deposition received during the 1980s. The understanding developed in the 1980s was summarized in the 1990 State of the Science and Technology and 1990 Integrated Assessment reports published by NAPAP in 1991. Research has continued into the 1990s, although at reduced levels of funding.

The following broad messages developed by workshop participants summarize the current state of our knowledge of acid deposition and patterns of ecosystem response:

          Ecosystem responses to acid deposition: non-linear, case specific, and sub-lethal

This section of the Report summarizes workshop discussions on current scientific understanding of the effects of sulfur and nitrogen emissions and deposition on surface water chemistry and biology, watershed/soil chemistry and biology, and forest ecosystems. In addition, the Report describes issues involved in recovery of these systems as a result of decreases in deposition of sulfur and potential future decreases in deposition of nitrogen. The statements reported below represent the conclusions reached during workshop plenary and breakout sessions, and reflect the general sense of workshop participants.

The majority of the workshop conclusions reflect a high degree of agreement. The high degree of agreement by no means indicates that we have a complete understanding of acid deposition, rather the conclusion section of this Report calls for continued and new research. Scientific participants at the workshop used existing resources to generate understanding and certainty regarding scientific conclusions reached through earlier research. As mentioned, this Report identifies research and monitoring needs that should be considered in order to continue to achieve the purpose and goals of the Clean Air Act Amendments of 1990.

The ranking system reflects the amount of agreement among workshop participants. Rankings are as follows: high, moderate, little, and no agreement. "No agreement" statements are included as they were important issues raised by participants, or were added to the Report after the workshop during the peer review process.

Sulfur Deposition

The current human impact on the sulfur cycle is unprecedented in the geologic record. The use of fossil fuels to meet energy demands for electricity, industrial combustion, transportation, home heating, and other needs has added a large flux of gaseous sulfur to the atmosphere, more than doubling the rate at which sulfur was mobilized from the Earth's crust 200 years ago. The net effect of these activities is to increase the pool of "oxidized" sulfur (sulfate) in the global cycle, while decreasing the storage of "reduced" sulfur in the Earth's crust. Although these activities have resulted only in a small change in the global pool of sulfur in the lithosphere, they have caused enormous changes in the annual flux of sulfur through the atmosphere and, thus, the amount and rate of atmospheric sulfur deposition to aquatic and forest ecosystems (see box on workshop findings for sulfur deposition).

Sulfur in the atmosphere reaches the Earth's surface and ecological systems through wet deposition (e.g., rain, snow), dry deposition (e.g., particles and gases), and in some ecosystems through fog or cloud water deposition. When SO₂ is emitted from human activities, it may form sulfuric acid (H₂SO₄) through reactions in the atmosphere. Sulfuric acid falls to Earth as precipitation, or is deposited as cloud or fog water in some high-elevation or coastal areas. Dry deposition involves deposition of sulfate particles and gases to the land and vegetation surfaces (e.g., leaves) during periods without precipitation. Another mechanism of dry sulfur deposition is the absorption of sulfur-containing gases directly from the atmosphere by vegetation or other surfaces, a process that is particularly important in humid regions.

Atmospheric sulfur enters ecosystems as sulfur dioxide and sulfate. Both forms can generate acidity directly. The leaching of sulfate from soil may result in the depletion of nutrient cations (e.g., calcium, magnesium, and potassium) or the release of acid cations to surface waters (e.g., hydrogen, aluminum). The depletion of nutrient cations from soils may decrease the productivity of terrestrial ecosystems. Furthermore, high concentrations of acid cations in the soil may be toxic to terrestrial biota and be linked to forest decline under some conditions. The transport of acid cations to surface waters contributes to surface water acidification and may be toxic to aquatic biota, such as fish.

Thus, reducing SO₂ emissions and sulfur deposition should enhance the health of both terrestrial and aquatic ecosystems. Phase I of the Acid Deposition Control Program has been successful in effecting a decrease in SO₂ emissions in some regions. Emission reductions began on time in 1995 and SO₂ reductions were greater than projected by the Phase I implementation schedule. Although sulfur deposition has been reduced, it is unclear whether reductions in SO₂ emissions have been, or will be, sufficient for recovery of terrestrial and aquatic ecosystems to occur. Moreover, the level of recovery depends on the fate of sulfur that has accumulated in soils during periods of high sulfur deposition, as well as on the amount delivered directly to trees, surface waters, and other ecosystem components. The leaching of accumulated sulfur will slow the recovery of both terrestrial and aquatic ecosystems due to the deleterious effects of sulfate mobilization on acidification and the release of cations to soil water and, ultimately, surface waters.

Workshop Findings for Sulfur Deposition Patterns in surface water chemistry In the northeast and upper Midwest, there is a direct relationship between decreases in atmospheric wet deposition of sulfur and decreases in concentrations and fluxes of sulfate in surface waters (high agreement);  In many ecological systems in the mid-Appalachians and Southern Blue Ridge Province, sulfate concentrations in surface waters are increasing, even as rates of sulfur deposition are decreasing (high agreement); and  In the mountainous West, no consistent changes in sulfate concentrations in surface waters have been observed in the absence of large changes in sulfur deposition (little agreement). Patterns in watershed/soil chemistry In many glaciated forest soils in the Northeast that have previously accumulated both inorganic and organic forms of sulfur from atmospheric sulfur deposition, this accumulated sulfur currently is being released to drainage waters (high agreement). Other discussion Current decreases in atmospheric sulfur deposition will: Expose the relative importance of nitrogen in acidification (moderate agreement); Make episodic acidification more important relative to chronic acidification (moderate agreement); and Make acidification less widespread and less predictable (moderate agreement).

Nitrogen Deposition

Nitrogen is one of the four most common chemical elements in living tissue, and is an essential component of key organic molecules. In short, all organisms require nitrogen in order to live. However, most plants and animals cannot use nitrogen directly from the air, which is composed of 78% nitrogen gas (N₂). Instead, they require nitrogen to be bonded to hydrogen molecules through a process called "fixation," creating biologically usable compounds. Generally, this short supply of bio-available nitrogen plays an important role in controlling productivity, biodiversity, and nutrient cycling in terrestrial and aquatic ecosystems. For example, estuarine ecosystems normally are characterized by low nitrogen availability. In estuarine ecosystems along the Atlantic and Gulf of Mexico coasts of the United States, greater amounts of available nitrogen have led to changes in aquatic communities and losses in biological diversity.

The 1990 NAPAP Report and Title IV of the CAAA concentrated principally on atmospheric sulfur deposition. Concerns over nitrogen emissions focused specifically on nitrate deposition under Title IV. Research on nitrogen as a component of acid deposition is less mature than our understanding of the role and effects of sulfur deposition. Moreover, the role of nitrogen in acid deposition is much more complicated because of biological mediation. Consequently, the effects of nitrogen deposition in relation to soil and surface water acidification is not fully understood. Scientists in the United States and elsewhere are concerned about the long-term effects of nitrogen deposition on forests, grasslands, streams, and estuaries. These concerns involve deposition of organic nitrogen; oxides of nitrogen (NO_X, HNO₃, and NO₃^-) formed as a consequence of fossil fuel combustion; and, reduced forms of nitrogen, such as ammonium (NH₄⁺) resulting from intensive animal husbandry, fertilizer use, and other activities. The effects of NO_X emissions and nitrogen deposition include:

• Increased regional concentrations of oxides of nitrogen (including nitric oxide, NO) that drive the formation of photochemical smog;
• Degraded drinking water quality due to elevated nitrate levels in streams and groundwater from forested watersheds supplying water for human consumption (e.g., San Bernardino and San Gabriel Mountains and Los Angeles metropolitan area water supply);
• Increased atmospheric concentrations of greenhouse gases. A process whereby nitrogen deposition leads to nitrogen accumulation in soil and the forest floor, with a portion being re-emitted to the atmosphere from soil as trace gases (e.g., NO and N₂O);
• Losses of soil nutrients through nitrate leaching. These nutrients include calcium and potassium that are essential for long-term soil fertility;

• Acidification of soils, streams and lakes in several regions of the United States, Canada, and Europe; and

• Increased transport of nitrogen by rivers into estuaries and coastal waters leading to eutrophication.

As reflected in the workshop findings below, a decade of research and scientific work has increased our understanding of the impact of nitrogen on surface water chemistry, soil chemistry, and forest ecosystems, but significant uncertainties remain (see box on workshop findings for nitrogen deposition). For example, there are significant questions regarding the fate of ammonium deposition in the environment, and its role in contributing to acidification.

Workshop Findings for Nitrogen Deposition Patterns in surface water chemistry Nitrate (NO3-) can be a significant contributor to surface water acidification in regions of high nitrogen deposition, but the processes controlling nitrate leaching are more complex than for sulfate leaching. Regional decreases in wet nitrogen deposition have not been observed in any region of the United States; in some areas, nitrogen deposition has increased (high agreement); Ammonium deposition can result in the acidification of soil and water if this input is oxidized, but this process is highly variable geographically (high agreement); and  Nitrogen deposition can lead to biological changes in aquatic systems before nitrogen-caused acidification occurs (moderate agreement). Patterns in watershed/soil chemistry Total atmospheric inputs of nitrogen to forests above a value of around 10 kg N/ha-yr show leaching losses of nitrate at some but not all sites (high agreement). In the East, watersheds with low atmospheric nitrogen deposition (< 5-10 kg/ha) show low leaching losses while watersheds with high atmospheric nitrogen deposition (> 5-10 kg/ha) demonstrate variable leaching losses of nitrate (high agreement); and In forest and chaparral watersheds surrounding the Los Angeles Basin, the deposition threshold at which high leaching losses of nitrogen are observed in streamwater is approximately 15-20 kg N/ha/yr.  For some high elevation watersheds in the West, this threshold may be lower (no agreement). Atmospheric inputs of nitrogen to forest soils: More nitrogen is accumulated in sites that were highly disturbed through activities that resulted in nitrogen losses in the past, such as agriculture and fire (high agreement); and Areas with low soil pools of nitrogen leach less nitrogen (moderate agreement). Soil emissions of greenhouse gases (NO, N2O, CH4) change in response to increasing atmospheric nitrogen deposition (little agreement). Patterns in forest response There is a need for measurements of total (wet, dry, and in some locations, cloud or fog) atmospheric nitrogen deposition over space and time (high agreement).  However, some advances have been made in the past ten years: We now have models that predict patterns of wet and dry (net) nitrogen deposition for the northeastern United States (high agreement); Organic nitrogen has been identified as a component of total nitrogen deposition (high agreement); and We have better estimates of total nitrogen deposition to specific forest sites (Integrated Forest Study sites) (high agreement). Other discussions Deposition of nitrogen affects terrestrial ecosystem processes and plant dynamics, and the effects are consistent with scientific understanding of nitrogen cycling. A. Terrestrial Processes: For example, nitrogen addition and exclusion experiments (Harvard Forest, Bear Brook, NITREX, EXMAN, Mt. Ascutney, Rocky Mountains) demonstrated "nitrogen saturation" as a continuous, non-linear response to nitrogen deposition (high agreement); Responses of forest systems to nitrogen deposition are influenced by previous land use and current stand history (high agreement); and  With chronic nitrogen deposition, forests and chaparral watersheds in the Mediterranean climate of southern California are prone to high nitrogen losses, B. Plant Community Dynamics: Experimental studies in alpine and grassland systems have shown changes in above and below ground community dynamics.  For example, nitrogen deposition affects forest understory, soil mycorrhizae, and alpine community composition (high agreement); Descriptive studies in European forest and grasslands have shown community shifts related to differences in nitrogen deposition (high agreement).

Acid Neutralizing Capacity and Base Cation Cycling

Rain water is normally weakly acidic (pH level of 5.2 to 5.4) due to the presence of naturally occurring oxides of nitrogen and sulfur. However, with the addition of strong acids from human activities, the mean pH of rain is often in the range of 3.5 to 5. The degree to which acid deposition results in the acidification of surface waters greatly depends on processes occurring in the surrounding watershed. Greater acid inputs to a watershed result in increased leaching of base cations through acid neutralizing reactions in the soil. As water moves through a watershed, two important chemical processes act to neutralize acids through the release of base cations, positively charged ions such as calcium (Ca²⁺), potassium (K⁺), sodium (Na⁺), and magnesium (Mg²⁺). The first process is mineral weathering. Most calcium in soil is bound within the mineral structure of rocks, and must be released by weathering, a process whereby minerals gradually breakdown and dissolve over long time periods. As calcium is released by weathering, it neutralizes acidity and increases the pH level in soil water and surface waters. The second reaction involves cation exchange in soils, a process by which hydrogen ions displace other cations absorbed to the surface of soil particles, releasing these cations to soil and surface water. The degree of acid neutralization that has occurred can be determined by measuring the acid neutralizing capacity (ANC) of surface waters (see box on ANC).
 
 

Acid Neutralizing Capacity:  Measuring the Acid Status of Lakes and Streams Acid neutralizing capacity (ANC) is a measure of the ability of a water or soil sample to chemically neutralize acid inputs.  Water with a low ANC has little ability to neutralize acids, and extremely acidic waters can have a negative ANC.  A change in ANC alone does not necessarily signal harmful effects on aquatic life because aquatic organisms are affected primarily by changes in pH, aluminum, and calcium caused by acid precipitation.  Decreases in ANC are generally associated with decreases in pH and base cation concentrations, and increases in concentrations of acid anions (sulfate, nitrate and organic anions) and aluminum.   Soil bases initially act to buffer the ANC of soil water; however, their depletion can eventually lead to decreases in soil ANC and soil and surface waters.  ANC, therefore, provides a useful index of acidification status, although it reflects both natural acidification processes as well as acid deposition effects.

Base cation depletion is a cause for concern because of the role these ions play in acid neutralization, and in the case of calcium, magnesium and potassium, their importance as essential nutrients for tree growth (see box on workshop findings for ANC and base cation response). Depletion of base cations from the soil is the process that leads to aluminum mobilization, which can have deleterious effects on fish and may interfere with the uptake of calcium by tree roots in forest soils. Aluminum mobilization may have contributed to the widespread decline of red spruce trees in the Northeast and deficiency of calcium and magnesium in soils has been linked to high mortality rates of sugar maple in western and central Pennsylvania. Research results from the 1990's indicate that documented decreases in soil calcium in the northeastern and southeastern United States are at least partially attributable to acid deposition.

Workshop Findings for ANC and Base Cation Response Some forest sites show depletion of labile pools of calcium and magnesium due to acid deposition (high agreement). Some cation deficiencies are associated with increased mortality in sugar maples and red spruce, as has been demonstrated in western Pennsylvania, where soils are highly weathered and have low cation exchange capacity (moderate agreement). Aluminum mobilization may have contributed to the widespread decline of red spruce trees in the Northeast (moderate agreement).

Response and Recovery of Ecological Systems

Research regarding recovery of ecological systems remains in a relatively nascent stage, particularly as recovery relates to changes in ANC, base cation depletion, and aluminum concentration. Some workshop participants debated the term "recovery," questioning whether ecosystems can fully recover, whether recovery can be measured, how one judges when recovery is achieved, and whose standards should be used in making such judgments. Other workshop participants defined recovery as involving changes in chemical or biological indicators consistent with a return to pre-disturbance conditions, including:

• Increases in ANC or pH in acidified systems;
• Decreases in concentration of inorganic aluminum;
• Re-establishment of native fish species; and
• Increased biological integrity.

As discussed above, implementation of Title IV has resulted in reductions of SO₂ emissions in the United States. These emission reductions have decreased the concentrations and fluxes of sulfate in precipitation and surface waters in some regions, such as the northeastern United States. Decreases in sulfate concentrations resulting from Title IV Phase I and II emissions reduction programs are expected to lessen the deleterious impacts of acidification on surface water chemistry and aquatic ecosystems. It should be noted that even with the current reduction in SO₂ emissions, acid sensitive regions such as the Adirondack and Catskill Mountains of New York, and some areas of Maine, demonstrate little change in the ANC of surface waters, possible continued depletion of base cations in soil, and mobilization of inorganic aluminum into soil water. Recovery of aquatic ecosystems also has not been observed in these regions.

Workshop Findings for Ecological Response and Recovery Biological Effects Acidic episodes can have significant biological effects (e.g., loss of biodiversity; changes in community structure), and these effects are more widespread geographically than chronic acidification (high agreement). Significant sublethal effects on fish have been observed in areas where data exist, indicating that the extent of affected ecosystems is broader than previously recognized (high agreement). Widespread response to atmospheric deposition has not been detected in all ecosystems.  This may be due to a lack of data, or it could be that data have not been synthesized in a way that allows evaluation of hypothesized effects. However, interactions of various stressors and atmospheric deposition have demonstrated effects for certain species and site combinations in forest ecosystems (high agreement): The physiological mechanisms by which acid deposition reduces the cold tolerance of red spruce are better understood now than in 1990 (high agreement); There are examples where atmospheric deposition predisposes plants to disease and insects.  For example, field experiments demonstrate that dogwood anthracnose may be favored by acid deposition (little agreement); and Acidic deposition can have interactive effects with other stressors, such as, ozone, climate warming, and water scarcity.  For example, interactive effects have been documented between sulfur and nitrogen deposition and ozone in southeastern forests (although species specific responses have not been documents) and mixed conifer forests in the San Bernadino Mountains of southern California (no agreement). Ecosystem Recovery Biological monitoring data in the United Sates are inadequate to track regional biological response to changes in emissions and deposition of sulfur and nitrogen (high agreement). Biological recovery has been observed in heavily impacted areas outside of the U.S. (e.g., Sudbury, Canada), but neither large chemical changes nor biological recovery has been observed yet in the United States (high agreement). Time to recovery is uncertain, and further reductions in acid deposition and/or longer response periods may be required to achieve chemical recovery (high agreement). When acid deposition is decreased, chemical changes indicating recovery may be delayed in areas where cations have been depleted in watershed soils (high agreement). Presently used ecological indicators of acidification and recovery are adequate to measure aquatic responses to changes in deposition (high agreement). Such indicators include:    Chemical indicators (pH, ANC, sulfate, nitrate, calcium, inorganic aluminum); Indicators focusing on groups, or assemblages, of species (e.g., Index of Biological Integrity); and Indicators of sublethal effects to organisms. However, it should be stressed that chemical indicators are the only widely used indicators. Other discussions Physical climatic variability can complicate our ability to detect ecological changes that may result from declines in atmospheric deposition: (high agreement) Variations in precipitation from year to year, and in relation to topography, can result in uncertainty regarding deposition rates and trends. Changes in rates of ecological processes can be affected both by climate and acid deposition.

• Reducing sulfur emissions was a major focus of the 1990 CAAA, Title IV. Although the results only reflect the first phase of the Acid Deposition Control Program, which began in 1995, implementation of Title IV has resulted in marked reductions of SO₂ emissions in the United States. During the 1995-1998 period, for example, SO₂ emissions from all utility units (including substitution and compensating units) affected by Phase I averaged 5.33 million tons/year, an average annual decrease of approximately 51% from those sources from 1980 levels. In some regions, such as the upper Mid-west and northeastern United States, these emissions reductions have decreased the concentrations and fluxes of sulfate (SO₄^2-) in both deposition and surface waters.

• This reduction of sulfate in precipitation and surface waters, as a result of reduced SO₂ emissions is linked with the recovery of ecosystems from the deleterious impacts of acidification. However, in some regions and ecosystems, current reductions in sulfate may be insufficient for recovery to occur. In regions such as New York State's Adirondack Mountains, the White Mountains of New Hampshire, and some sensitive lakes in Maine, the ecosystem response has been less than might have been expected as a result of SO₂ emissions reductions and decreased sulfate wet deposition. In some locations, in fact, additional acidification has occurred in spite of decreasing sulfate concentrations in surface waters.

• Factors limiting the recovery of ecosystems from acidification include: 1) atmospheric deposition of nitrogen that increases the leaching of nitrogen as nitrate (NO₃^-) from ecosystems; 2) depletion of exchangeable base cations (calcium, magnesium, sodium, potassium); 3) contributions of natural acidity; and 4) unaccounted for inputs of sulfur (which may result from underestimates of dry deposition, or internal sources of sulfur such as weathering of minerals or mineralization of organic sulfur in soil). Given current knowledge of nitrogen deposition and its impact on ecological systems, the importance of nitrogen must be fully considered in formulating future air quality policies

• Monitoring the chemistry of atmospheric inputs and surface waters should be continued and enhanced, and include efforts to quantify dry deposition. After nearly twenty years of research and monitoring activities, there is still a lack of reliable estimates on total deposition on a regional basis. The inability to quantify total acid deposition to ecosystems limits the ability to measure progress towards the goals of the 1990 CAAA.

• Current monitoring efforts should be expanded through inclusion of biological indicators of ecosystem response. We can more fully judge ecosystem condition and evaluate the effectiveness of past and future clean air legislation only by undertaking selected biological assessments.

Exploring statistical relationships between multiple interacting environmental factors and long-lived organisms requires controlled experimentation and gathering of large numbers of observations over time across broad geographic regions. These requirements necessitate studying ecosystem responses to air pollutants over decades, rather than years, and across an array of ecosystem types. While some research sites and the efforts of research teams predate the NAPAP efforts of the 1980s, the data required for ecosystem studies at various temporal and spatial scales were largely unavailable at the time the NAPAP Report was published in 1991. Now, thanks in part to a decade of monitoring network implementation and long-term experimentation, there are more long-term data available with which to assess the effects of acid deposition on terrestrial and aquatic ecosystems.

Combining newly acquired data with long-term datasets developed over decades at individual research sites (e.g., Hubbard Brook, New Hampshire), incremental advances have been made over the last ten years in refining our understanding of the ecological effects of acid deposition, and the chemical and biological mechanisms through which those effects manifest themselves. However, as discussed in earlier sections of this Report, we do not fully understand acid deposition and its impacts on soils, surface water chemistry and biology, and forest ecosystems. As with most complex interactions, at least as many questions remain as have been answered. Workshop participants identified needs in the two broad areas of monitoring and research.

In order to evaluate the effectiveness of environmental policies and programs, a firm commitment is needed to long-term monitoring programs that help in assessing the status and trends of ecological systems, as well as discerning when and where recovery is evident. Monitoring data allow evaluation of the effectiveness of emission controls, exploration of dose-response relationships, validation of predictive models, and understanding of watershed processes. Recognizing that long-term monitoring data are essential and fundamental to future acid deposition research, several workshop participants noted during this discussion that "monitoring is research" (see box on workshop recommendations for monitoring needs). Thus, monitoring the chemistry of atmospheric inputs and surface waters must be continued and expanded. Continued long-term monitoring of wet deposition (including cloud and fog), dry deposition, streamflow, water chemistry, soil chemistry, and biological condition are essential for understanding and evaluating the response of ecological systems to acid deposition.

In addition, however, biological monitoring capabilities must be expanded. We are developing a record of chemical response of surface waters and watersheds to acid deposition, but we have little or no foundation to determine if long-term changes in acid deposition are leading to long-term biological responses. Present understanding of biological "trends" is not based on biological data because sustained biological monitoring rarely occurs. Instead, scientists must extrapolate biological effects from monitoring of chemical indicators combined with an understanding of the ecological effects of chemical changes. Based on their understanding of the reaction of fish species to pH levels and inorganic aluminum concentrations, for example, scientists might expect recovery of aquatic ecological systems if they detect increasing pH and decreasing inorganic aluminum concentrations. Chemical and biological interactions within the ecosystems, however, may complicate simple cause and effect relationships like these. Moving beyond such indirect analyses demands developing and refining biotic indicators for evaluating acidification and recovery of aquatic ecosystems. Therefore, including biological resources in monitoring activities is critical if we are to truly advance our understanding of ecological effects of acid deposition.

Workshop Recommendations for Monitoring Needs Minimum monitoring needs: Regional monitoring of biological condition must be developed and integrated with chemical monitoring efforts; Current levels of wet and cloud deposition monitoring must be maintained; Dry deposition monitoring must be refined and expanded, particularly in complex terrain where many acid sensitive systems are located. Surface water monitoring must be maintained or restored in acid sensitive regions across the United States. Future monitoring needs that need to be addressed: 1. Monitoring that is multi-scale, linked, and integrated; 2. Utilization of linked process sites and survey sites that are representative of affected areas; 3. Multiple variables: Total deposition Hydrological fluxes Elemental fluxes in drainage waters Water chemistry indicators Soil indicators Forest indicators Aquatic indicators 4.     Integration of existing programs, including forest monitoring, and their synthesis.

One of the most important refinements of our understanding involves the significance of nitrogen. Put simply, scientists are much more certain than they were in 1991 that nitrogen plays an important role in acid deposition and its ecological effects. However, the scientific community also realizes that it is much more difficult to "close" the nitrogen budget than the sulfur budget. In other words, discovering and accounting for sources, sinks, and flows of nitrogen is a difficult and complex problem. In particular, the inability to measure significant components of nitrogen deposition (e.g., dry deposition of ammonia) is a basic and critical issue. These questions can be addressed at long term and experimental research sites already involved in this type of research (e.g., Hubbard Brook Experimental Forest, Harvard Forest, Bear Brook). Thus, basic questions regarding nitrogen deposition and its effects remain at the top of the research agenda.

Workshop Recommendations for Research Needs Nitrogen deposition What are the rates of total (dry and wet, and where appropriate, cloud) atmospheric nitrogen deposition, including inorganic and organic nitrogen species? How do rates of nitrogen deposition change geographically and over time? What is the relative contribution of anthropogenic sources of nitrogen to regional nitrogen loading? What percentage of the nitrogen leached from terrestrial systems to surface waters comes from anthropogenic sources and how much comes from natural sources? What interactions among climate, a landscape, and other environmental features must we understand if we are to better characterize the nitrogen budget and, therefore, strengthen our ability to predict ecological responses to various nitrogen loadings? Surface water chemistry Why are nitrate concentrations in stream water not increasing in some regions where strong concerns for nitrogen saturation exist? To what extent are stream water responses to atmospheric deposition of nitrate being influenced by other phenomena, such as Gypsy moth defoliation, ozone, and climatic events (e.g., hurricanes, ice and wind storms)? Sulfur deposition has decreased and sulfate concentration in surface waters has decreased in the northeastern U.S, so why has ANC not recovered? Potential explanatory factors include: Base cation supply Role of organic acids Fate of stored N and S Vegetation/biological response Interactions with climate Changes in dissolved organic carbon character or quantity Watershed/soil chemistry What is the fate of sulfur that has accumulated in soil from atmospheric sulfur deposition and what controls the leaching of sulfate following decreases in atmospheric sulfur deposition? What is the fate of atmospheric nitrogen that has accumulated in soil? How much of the deposited nitrogen is denitrified, versus being accumulated in the watershed? What factors (e.g., watershed hydrology) are controlling nitrogen retention and release from forested watersheds? To what extent are historical and current land use patterns contributing factors in determining the spatial variability of nitrogen leaching? Can nitrogen retention be modeled on a regional scale to characterize regions sensitive to nitrogen loadings? What are the possible complicating interactions with climate change (e.g., increased temperatures, CO2 increases and variations)? What is the ecological response to anthropogenic acidification? If so, what is the rate of recovery and how is this coupled with chemical recovery? What are the ecological effects of depletion of labile pools of calcium and magnesium from soil? What is the rate of recovery of labile pools of calcium and magnesium in soil following decreases in acid deposition? Forest ecosystems Increases of forest productivity in some ecosystems (not net growth) have been detected.  Are increases due to fertilization with carbon and nitrogen? Do increases in forest productivity explain decreases in cations and continued retention of nitrate within terrestrial ecosystems? How are the normal processes of stand dynamics and genetic selection in forest ecosystems affected by atmospheric deposition? What is the role of nitrogen fixation in nitrogen budgets of forest ecosystems? What mechanisms control retention of nitrogen in terrestrial ecosystems? Are there upper limits (i.e., saturation) and how can we detect when and at what scale limits have been reached? What are the biological consequences of nitrogen deposition? What are the consequences of soil base depletion on forest ecosystems? What indicators of stress are found in low elevation forests? How stressor specific are these stress indicators? How is the cycling of base cations affected by nitrogen saturation? To what extent has forest function been altered by acid deposition?

Anions - Negatively charged molecule such as sulfate (SO₄^2-) and nitrate (NO₃^-). In combination with hydrogen (H⁺), these molecules act as strong acids.

Acid neutralizing capacity (ANC) - A measure of the ability for water or soil to neutralize added acids. This is done by the reaction of hydrogen ions with inorganic or organic bases such as bicarbonate (HCO₃^-) or organic ions.

Acidification - refers to the loss of ANC or the lowering of pH.

Adsorb - To take up and hold (a gas, liquid, or dissolved substance) in a thin layer of molecules on the surface of a solid substance.

Atmospheric deposition - The processes by which chemical constituents move from the atmosphere to the Earth's surface. These processes include precipitation (wet deposition, such as rain, cloud or fog), as well as particle and gas deposition (dry deposition).

Biological mediation- A biological activity that counters the effects of another biological event.

Buffering capacity - The resistance of water or soil to changes in pH.

Base cations - Positively charged ions such as magnesium, sodium, potassium, and calcium that increase pH of water when released to solution through mineral weathering and exchange reactions.

Chronic acidification - Generally refers to surface waters that remain acidified (ANC<0) regardless of variations in hydrologic conditions (precipitation, stream flow, etc.).

Critical thresholds - The point at which a system switches to another state.

Dry deposition - The portion of total atmospheric deposition that is deposited on dry surfaces during periods of no precipitation as particles or in a gaseous form.

Labile - Chemically reactive or bioavailable element.

Leaching - Process by which water removes chemical solutes from soil through chemical reactions and the downward movement of water.

Mineral weathering - The physical and chemical breakdown of rocks that releases ions such as calcium and aluminum.

Nitrogen fixation - The process in which bacteria convert biologically unusable nitrogen gas (N₂) into biologically usable ammonia (NH₃) and nitrates (NO₃^-).

Non-Linear - A relationship between two factors that does not vary on a 1 to 1 basis.

Precipitation - Water in the form of rain, sleet, or snow (wet deposition).

Sublethal - Insufficient to cause death.

Total Deposition - Total atmospheric deposition is determined using both wet and dry deposition

measurements.

Wet deposition - Wet deposition is the portion of total atmospheric deposition dissolved in cloud droplets and is deposited during rain or other forms of precipitation.

Al - Aluminum

Ca²⁺ - Calcium

CH₄ - Methane

H⁺ - Hydrogen ion

HCO₃^- - Bicarbonate

HNO₃ - Nitric acid

Mg²⁺ - Magnesium

K⁺ - Potassium

Na⁺ - Sodium

N₂ - Diatomic nitrogen

N₂O - Nitrous oxide

NH₃ - Ammonia

NH₄⁺ - Ammonium

NO - Nitric Oxide

NO₂ - Nitrogen dioxide

NO_X - Sum of NO and NO₂

NO₃^- - Nitrate

O₃ - Ozone

SO₂ - Sulfur Dioxide

SO₄^2- - Sulfate

Monday, March 1

I. Setting Policy, Management, and Scientific Context

12:30-12:45pm Introduction and Opening Remarks

Welcome-Mary Barber (Ecological Society of America)

Scope and purpose of meeting-Rich Pouyat (US Forest Service)

12:45-1:15pm Acid Rain Policy and Initial Results of Acid Rain Program-

Brian McLean (EPA)

Brief acid rain policy history

Current policy and management needs/questions

Emission trends (historic, future projections)

1:15-1:35pm Wet and dry deposition trends-

James Lynch (Pennsylvania State University)

1:35-2:00pm Historical Perspective On Research-

James Galloway (University of Virginia)

2:00-2:30pm Break

II. Atmospheric Deposition: Evaluating the Ecological Response

2:30- 2:45pm Present workshop framing questions

2:45-4:15pm Ecological Responses to Atmospheric Deposition

Trends in surface water chemistry and biology-

John Stoddard (EPA)

Trends in watershed/soil chemistry and biology-

Greg Lawrence (USGS)

Trends in forest response (USFS)

Walter Shortle (USFS)

4:15-4:30pm Integrated Assessment-Putting the Pieces Together

Steve Kahl (University of Maine)

A multi-ecosystem assessment of acid-base changes in Maine aquatic systems since 1982.

4:30-5:30pm Breakout session I: What have we learned about the effects of atmospheric deposition since the 1990 synthesis? How do the scientific conclusions of the 1990 NAPAP Report hold up in light of new data?

Trends in surface water chemistry and biology?

Facilitator-Michael Uhart

Rapporteur-John Stoddard

Trends in watershed/soil chemistry and biology?

Facilitator-Greg Lawrence

Rapporteur-Charles Driscoll

Trends in Forest Response?

Facilitator-Kathie Weathers

Rapporteur-Rich Pouyat

Tuesday, March 2

8:00-8:30am Continental Breakfast

8:30-10:30am Breakout session I: continued

10:30-11:30am Plenary: Breakout group I summary presentations and discussion

11:30-1:00pm How well can we predict ecological response to changes in atmospheric deposition?-

Experimental approaches- John Aber (University of New Hampshire)

Modeling approaches- Jack Cosby, Jr. (University of Virginia)

1:00-2:00pm Lunch

2:00-5:00pm Breakout Session II: What determines sensitivity of ecosystems to changes in atmospheric deposition? What is the potential for recovery of sensitive ecosystems?

Aquatic breakout group

Facilitator-Mark Nilles (USGS)

Rapporteur-Timothy Sullivan (E&S Environmental Chemistry)

Terrestrial breakout group

Facilitator-Rona Birnaum (EPA)

Rapporteur-Gary Lovett (IES)

5:00-5:30pm Plenary

Breakout group II summary presentations and discussion

General Discussion

Wednesday, March 3

8:30-12:30pm Plenary

1 ) Trends in coastal watersheds (15 min). Report from ESA Narragansett workshop

(objectives, format, outcomes to provide model for workshop Report)

2) Highlights from breakout group summary and presentations

3) Develop workshop recommendations and report

Workshop Planning Group

Stephanie Benkovic (EPA)
Rona Birnbaum (EPA)
Greg Lawrence (USGS)
Mark Nilles (USGS)
Richard Pouyat (USFS)
Paul Ringold (EPA)
Doug Ryan (USFS)
Michael Uhart (NOAA)
Kathie Weathers (Institute of Ecosystem Studies)

Workshop Participants

John Aber (University of New Hampshire)
Jill Baron (USGS, Natural Resource Ecology Lab)
Douglas Burns (USGS)
Don Cambell (USGS)
Robbins Church (EPA)
Jack Cosby (University of Virginia)
Ellis Cowling (North Carolina State University)
Brian Cumming (Queens University)
Mark David (University of Illinois)
Charley Driscoll (Syracuse University)
Keith Eshlemann (University of Maryland)
Jennifer Kramer (EPA)
Kathy Fallon -Lambert (Hubbard Brook Research Foundation)
Ivan Fernandez (University of Maine)
Guy Fenech (Environment Canada)
Mark Fenn (USFS)
James Galloway (University of Virginia)
Robert Goldstein (Electric Power Research Institute)
John Hom (USFS)
Rick Haeuber (Ecological Society of America)
Dean Jeffries (National Water Research Institute, Environment Canada)
Art Johnson (University of Pennsylvania)
Steve Kahl (University of Maine)
Kim Kirchner (University of California, Berkeley)
Dennis Lemly (USFS)
Orie Loucks (Miami University)
James Lynch (Pennsylvania State University)
Gary Lovett (Institute for Ecosystem Studies)
Brian McLean (EPA)
John Melack (University of California, Santa Barbara)
Myron Mitchell (SUNY-Syracuse)
Peter Murdoch (USGS)
Steve Norton (University of Maine)
Ellen Porter (USFW)
Walter Shortle (USFS)
John Stoddard (EPA)
Tim Sullivan (E&S Environmental Chemistry)
Helga Van Miegroet (Utah State University)
Ann Watkins (EPA)
Jim Wigington (EPA)

The Ecological Society of America
1990 M Street, NW
Washington, DC 20036
Tel. 202-833-8773