United States Science Advisory Board EPA-SAB-EPEC-95-019
Environmental 1400 September 1995
Protection Agency Washington, DC
AN SAB REPORT: REVIEW
OF THE ACID DEPOSITION
STANDARD FEASIBILITY
STUDY REPORT TO
CONGRESS
PREPARED BY THE ACID
DEPOSITION EFFECTS
SUBCOMMITTEE OF THE
ECOLOGICAL PROCESSES AND
EFFECTS COMMITTEE
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Septembers, 1995
EPA-SAB-EPEC-95-019
Honorable Carol M. Browner
Administrator
U.S. Environmental Protection Agency
401M Street, S.W.
Washington, DC 20460
Subject: Review of the Acid Deposition Standard Feasibility Study Report to
Congress
Dear Ms. Browner:
On April 12, 1995, the Acid Deposition Effects Subcommittee of the Ecological Processes
and Effects Committee of the Science Advisory Board met to review the draft Acid Deposition
Standard Feasibility Study Report to Congress (the Study). The Subcommittee concurs with the
conclusion of the Study that the current state-of-the-science with regard to acid deposition effects
on terrestrial and aquatic ecosystems does not support an acid deposition standard at this time.
We compliment the Agency for the significant effort expended to develop an approach for
assessing acid deposition and the need for an acid deposition standard. The Study relates many
complex scientific issues in a generally clear and concise manner. While the Congress explicitly
directed the Agency to assess the implications of an acid deposition standard for ecological
resources, a more detailed evaluation of human health and other possible benefits would be
important were the Agency to develop an acid deposition standard at some future time.
The Subcommittee had a number of reservations, however, about the modeling approach
employed by the Agency to project future effects of acid deposition under various emission
scenarios, as well as the conclusions which are drawn in the Study. While our report contains a
number of specific recommendations for improving the Study, we would like to emphasize the
following general concerns:
1) The Study should contain a clear statement of the ecological resources and
resource endpoints to be protected and the level of protection desired. In the
absence of such a statement, it is difficult, if not impossible, to determine the
relevant science questions, assumptions and assessment methods which should be
pursued by the Agency.
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2) The models utilized for the Study do not include the biological processes which
control nitrogen cycling in ecosystems; in our report, we recommend a number of
alternative models that simulate nitrogen cycling in a more thorough and realistic
manner.
3) While the Study frequently alludes to the uncertainties associated with model
outputs, the Study should better characterize and quantify these uncertainties to
allow policy makers to judge whether the differences in surface water acidification
under different emission scenarios are significant.
4) The Executive Summary, which may be the most widely read part of the Study, is
misleading in that it contains a number of summary conclusions that are
inconsistent with language in the body of the report or are unsupported by current
science.
We appreciated the opportunity to review the draft Acid Deposition Standard Feasibility
Study Report to Congress. We recognize the difficulty in developing such a report and we hope
that our comments will be helpful to the Agency.
Sincerely,
Dr. Genevieve Matanoski, Chair
Executive Committee
Dr. Mark A. Harwell, Chair Dr. William H. Smith, Ohair
Ecological Processes and Acid Deposition Effects
Effects Committee Subcommittee
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U.S. Environmental Protection Agency
NOTICE
This report has been written as part of the activities of the Science Advisory Board, a
public advisory group providing extramural scientific information and advice to the Administrator
and other officials of the Environmental Protection Agency. The Board is structured to provide
balanced, expert assessment of scientific matters related to problems facing the Agency. This
report has not been reviewed for approval by the Agency and, hence, the contents of this report
do not necessarily represent the views and policies of the Environmental Protection Agency, nor
of other agencies in the Executive Branch of the Federal government, nor does mention of trade
names or commercial products constitute a recommendation for use.
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ABSTRACT
The Acid Deposition Effects Subcommittee of the Ecological Processes and Effects
Committee met on April 12, 1995, to review the draft Acid Deposition Standard Feasibility Study
Report to Congress (the Study). The Subcommittee reviewed material in the Study relating to
scientific aspects of an acid deposition standard, but did not evaluate portions of the Study dealing
with non-ecological benefits, implementation options, and compliance costs. The Subcommittee
agreed with the conclusion of the Study that development of an acid deposition standard is not
recommended at this time because scientific uncertainties are too great. The models utilized for
the Study, the Regional Acid Deposition Model (RADM) and the Model of Acidification of
Groundwater in Catchments (MAGIC), are relevant models but have significant limitations. The
Subcommittee recommended that the Agency utilize an alternative acid deposition model that
more thoroughly and realistically simulates nitrogen cycling in ecosystems. The Subcommittee
also recommended that the Agency develop an overall conceptual framework identifying relevant
science questions for the broader set of acid deposition effects, clarify what ecological resources
and resource endpoints are to be protected and the level of protection desired, better characterize
and quantify uncertainties in model projections of acid deposition effects, and place greater
reliance on references from peer-reviewed literature published in science journals. The
Subcommittee also noted the importance of environmental monitoring of deposition and effects to
complement modeling efforts.
KEYWORDS: Acid Deposition, Ecological Modeling, Environmental Monitoring, Peer Review
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US ENVIRONMENTAL PROTECTION AGENCY
SCIENCE ADVISORY BOARD
ECOLOGICAL PROCESSES AND EFFECTS COMMITTEE
ACID DEPOSITION EFFECTS SUBCOMMITTEE
CHAIR
Dr. William H. Smith, School of Forestry and Environmental Studies, Yale University, New
Haven, CT
MEMBERS
Dr. William Adams, ABC Laboratories, Columbia, MO
Dr. Virginia Dale, Environmental Sciences Division, Oak Ridge National Laboratory, Oak
Ridge, TN
Dr. Jay S. Jacobson, Boyce Thompson Institute, Cornell University, Ithaca, NY
Dr. Frederick K. Pfaender, Carolina Federation for Environmental Programs, University of
North Carolina, Chapel Hill, NC
CONSULTANT
Dr. Robert A. Goldstein, Electric Power Research Institute, Palo Alto, CA
INVITED EXPERT
Dr. Jill S. Baron, National Biological Survey, Natural Resource Ecology Laboratory, Fort
Collins, CO
SCIENCE ADVISORY BOARD STAFF
Ms. Stephanie Sanzone, Designated Federal Official, US EPA, Science Advisory Board
(1400F), 401 M Street, SW, Washington, DC 20460, (202) 260-6557
Ms. Connie Valentine, Staff Secretary, US EPA, Science Advisory Board (1400F), 401 M
Street, SW, Washington, DC 20460, (202) 260-6552
111
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TABLE OF CONTENTS
1. EXECUTIVE SUMMARY 1
2. INTRODUCTION 3
3. GENERAL COMMENTS 4
3.1 The Interdependence of Policy and Science 4
3.2 Scientific Uncertainty and an Acid Deposition Standard 5
3.3 Unsupported or Inconsistent Conclusions 6
4. MODELING NITROGEN DEPOSITION AND EFFECTS 9
4.1 Alternative Models for Nitrogen Cycling 9
4.2 Nitrogen Saturation Potential 10
5. MONITORING 11
6. CONCLUSIONS 11
7. REFERENCES CITED 13
IV
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1. EXECUTIVE SUMMARY
The Acid Deposition Effects Subcommittee of the Ecological Processes and Effects
Committee met on April 12, 1995, to review the draft Acid Deposition Standard Feasibility Study
Report to Congress (the Study). The Subcommittee reviewed material in the Study relating to
scientific aspects of an acid deposition standard, but did not evaluate portions of the Study dealing
with non-ecological benefits, implementation options, and compliance costs.
The Subcommittee compliments the Agency on the significant effort expended to develop an
approach for assessing acid deposition and the need for a national standard. The Study relates many
complex scientific issues in a generally clear and concise manner. However, the complexity of the
approach used to model acid deposition and effects results in uncertainties in the model outputs.
Thus, the Subcommmittee agrees with the conclusion of the Study that development of an acid
deposition standard is NOT recommended at this time because scientific uncertainties are too great.
The models utilized for the Study, the Regional Acid Deposition Model (RADM) and the Model of
Acidification of Groundwater in Catchments (MAGIC), are relevant models but have significant
limitations. For example, an important shortcoming of the Study is its inappropriate treatment of
nitrogen dynamics; alternative models are available to better characterize nitrogen dynamics in
ecosystems. Further, the Subcommittee concluded that scientific uncertainties have not been
adequately characterized in the Study.
Our specific recommendations for improving the Study include the following:
a) The Agency should develop an overall conceptual framework which identifies the
relevant science questions for the broader set of acid deposition effects (human health,
ecological resource health, visibility, materials erosion, atmospheric chemistry, and
other socioeconomic effects). While the Congress explicitly directed the Agency to
assess the implications of an acid deposition standard for ecological resources, a more
detailed evaluation of human health and other possible benefits would be important
were the Agency to develop an acid deposition standard at some future time.
b) The Study should contain a clear statement of the ecological resources and resource
endpoints to be protected by an acid deposition standard and the level of protection
desired.
c) The Study should better characterize and quantify the uncertainties in model
projections of acid deposition effects.
d) The Agency should carefully review the Executive Summary of the Study for two
types of misstatements: 1) summary conclusions which are inconsistent with the
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wording of the body of the report, and 2) conclusions which are not adequately
supported by available scientific evidence.
e) The Study should clearly emphasize the distinction between sensitive (at risk)
ecological resources (the focus of the Study) and the general populations of
ecological resources which are more resistant or fully resistant to adverse impact via
acid deposition.
f) Technical conclusions in the Study should be based primarily on references from the
peer-reviewed science-journal literature.
h) The Agency should utilize acid deposition models that include the biological processes
controlling nitrogen dynamics; MAGIC does NOT include these processes.
i) The Study should more clearly characterize the scientific uncertainty regarding
terrestrial ecosystem nitrogen saturation.
j) The Study should identify and emphasize the importance of environmental monitoring
of deposition, ecological indicators, and ecological endpoints as a parallel and
complementary strategy to modeling in order to assess ecological resource risk from
acid deposition.
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2. INTRODUCTION
The Acid Deposition Effects Subcommittee of the Ecological Processes and Effects
Committee of the Science Advisory Board met on April 12, 1995, in Washington, DC to review the
draft Acid Deposition Standard Feasibility Study Report to Congress (the Study). The Study was
prepared by the Agency in response to section 404 of the Clean Air Act Amendments (C AAA) of
1990, which required the Agency to evaluate the feasibility and effectiveness of an acid deposition
standard to protect sensitive aquatic and terrestrial resources. The Study utilizes the Regional Acid
Deposition Model (RADM) and the Model for Acidification of Groundwater in Catchments
(MAGIC) to project possible future effects of sulfur and nitrogen deposition under a range of
emission scenarios, including scenarios with emission reductions beyond those required by the CAAA
of 1990.
The Charge to the Subcommittee from the Office of Air and Radiation's Acid Rain Division
included the following questions:
a) Have the models (RADM, MAGIC) been applied in a credible manner and/or within
the bounds of applicability for the scientific analyses and assessments in the feasibility
study?
b) Have the modeling results been appropriately integrated? In what ways might the
integration be improved?
c) Are the conclusions drawn in the study consistent with the state of the science and the
state of modeling? What conclusions are insupportable or weak? For what reasons?
How might they be improved?
d) Are there important conclusions from other published modeling studies that should
be included in the feasibility study?
e) Have the scientific uncertainties associated with the conclusions drawn in the
feasibility study been adequately characterized?
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3. GENERAL COMMENTS
3.1 The Interdependence of Policy and Science
The Study includes, by design, a combination of scientific and policy issues. The portions
of the report dealing with non-ecological benefits, implementation options for an acid deposition
standard, and compliance costs of various options were not reviewed by the Subcommittee. The
primary focus of the Subcommittee was on the utility and application of the models, the quality of
peer review which they received, and the extent to which the technical findings in the Study can
be supported by current science. The overall policy goals must be considered in order to evaluate
whether the models are providing useful simulations and predictions.
Recommendation 1: While the Congress explicitly directed the Agency to assess the
implications of an acid deposition standard on ecological resources, policy questions will
change as decision makers and societal priorities change. Thus, the Agency should develop
a conceptual framework which identifies the relevant science questions for the broader set
of acid deposition effects (human health, ecological resource health, visibility, materials
erosion, atmospheric chemistry, and other socioeconomic effects).
Policy questions will influence the science questions that should be addressed by the acid
deposition research. However, the policy questions will change as decision makers and societal
priorities change. Therefore, the Agency should develop a conceptual framework that shows how
the different elements of the acid deposition issues interact (e.g., air quality, ecological effects,
socioeconomic effects, human health effects and material effects) and how they contribute to
addressing particular science questions. Then, the scientific assumptions and questions of the
current approach can be specified. The framework will also help determine the choice of
particular endpoints for the research (e.g., water chemistry as compared to aquatic life). The
clarification provided by the framework will give the reader a context for the Study. The
framework will also provide the means to interpret the cost-benefit analysis of the different
components of the research and will provide a basis for setting future research priorities.
In the event that the Agency were to develop an acid deposition standard at some future
time, a more detailed evaluation of human health and other possible benefits would be important.
These issues have been dealt with more fully in Agency documents prepared to support National
Ambient Air Quality Standards for NOX and SO2 and reviewed by the SAB's Clean Air Science
Advisory Committee (CASAC) (EPA-SAB-CASAC-LTR-94-007, EPA-SAB-CASAC-LTR-93-
015, EPA-SAB-CASAC-LTR-92-017).
Recommendation 2: The Study should contain a clear statement of the ecological resources
and resource endpoints to be protected by an acid deposition standard and the level of
protection desired.
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The Study deals only minimally with the subjects of "ecosystem effects" and "resources at
risk." Instead, it focuses almost exclusively on protecting the environment from further
degradation of acid neutralizing capacity (ANC). Refining models for projecting ANC will not
necessarily lead to better information on the effects of acid deposition on key species or species
diversity in sensitive ecoregions. In addition, the Study points to the importance of episodic
acidification in causing biological effects, both through mobilization of aluminum and through
abrupt changes in pH during critical biological events such as spawning. However, the design of
an acid deposition standard to protect sensitive aquatic resources from episodic acidification
would likely differ from a standard designed to protect sensitive resources from chronic
acidification.
3.2 Scientific Uncertainty and an Acid Deposition Standard
The Study sends a mixed message on the question of whether or not it is feasible to set an
acid deposition standard; both the Executive Summary and Chapter 6 conclude that "it would be
feasible to set sulfur and nitrogen deposition standards to protect aquatic resources," whereas
Chapters 1 and 6 include statements that "scientific uncertainties associated with the response of
specific sensitive regions to acid deposition" and "scientific uncertainty regarding watershed
nitrogen saturation makes determining a standard difficult at this time."
At the meeting of April 12, the Agency presented as a "key conclusion":
"Although developing an acid deposition standard may be feasible, based on the
scientific uncertainty in determining the level [of the standard], an acid deposition
standard is not recommended at this time."
We agree that the scientific uncertainty in the models used is too great to support an acid
deposition standard. This conclusion should be clearly stated in the Executive Summary of the
Study.
Although the impact of sulfur on aquatic systems is clear, there are unknown levels of
scientific uncertainty associated with impacts of nitrogen and sulfur deposition on terrestrial
resources and of nitrogen deposition on aquatic resources. As noted in the Study, an important
source of uncertainty is that associated with effects of nitrogen deposition and the potential for
watershed nitrogen saturation. This topic is dealt with in section 4 of this report.
Recommendation 3: The Study should better characterize and quantify the uncertainties
in model projections of acid deposition effects.
There are multiple sources of uncertainties that contribute to the estimation of the extent
of surface water alkalinity under different emission scenarios. These include uncertainties in
emission projections, in the RADM and MAGIC formulations, and the calibration and
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parameterization of the models. Estimates of these uncertainties and calculation of their
propagation through the models is needed to judge whether the differences in estimates of the
extent of acidification of surface waters under different deposition scenarios are significant. This
will allow the placement of confidence levels around the predictions. With regard to uncertainties
in emission projections, the Agency should ensure that the emission scenarios bracket the
possible emission levels; although the Study includes scenarios with nitrogen emissions either
increasing or held constant at 1985 levels, reductions in nitrogen emissions resulting from
replacement of the current motor vehicle fleet with lower NOx-emitting vehicles and from state
efforts to meet ozone attainment goals contained in the CAAA of 1990 may result in nitrogen
emissions lower than 1985 levels. In fact, initial state inventories of nitrogen emissions indicate
that this is already occurring.
Aluminum mobilization due to acid deposition is known to cause toxic effects on sensitive
fish in freshwater lakes with depressed pH levels. However, as acknowledged in the Study, the
results of the model (MAGIC) analysis of aluminum mobilization and transport are highly
uncertain and further improvements in the model would be needed if the Agency decides to use
MAGIC to predict aluminum mobility.
Application of the three effects models evaluated by the Agency (the Trickle-Down
Model, the Integrated Lake Watershed Acidification Study-ILWAS—Model, and MAGIC) gave
comparable results for Northeast lakes, but inconsistent results for Southern Blue Ridge streams.
It is critically important to determine the source/cause of these differing results in order to select
the most appropriate model for the Study. Sensitivity analyses of the models would be
appropriate and may reveal the reasons for differences in model projections.
3.3 Unsupported or Inconsistent Conclusions
A number of the conclusions in the Executive Summary are not consistent with language
in the body of the report or are not supported by current scientific studies. Since the Executive
Summary may be the most widely read part of the Study, it is particularly important that the
conclusions drawn are accurate and supported by the remainder of the document.
Recommendation 4: The Agency should carefully review the Executive Summary for two
types of misstatements: a) summary conclusions which are inconsistent with the wording of
the body of the report, and b) conclusions which are not adequately supported by available
scientific evidence.
In some cases, conclusions in the Executive Summary are stated without the caveats that
accompany the conclusions in the body of the report. For example, section headings such as
"CAAA Provides Clear Benefits To Surface Waters; Further Reductions May Be Necessary For
Full Protection; and Emission Trading Is Cost Effective And Maintains Environmental Benefits"
imply greater certainty in model projections than can be justified by current science. Further,
statements in the Study that describe high-elevation red spruce forests as being among the systems
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most at risk from acidic deposition are not justified by the available scientific evidence and should
be deleted.
In contrast to the case for aquatic effects, the evidence linking acid deposition to effects
on red spruce is far from conclusive. This conclusion is based on a chain of inferences that
involves indirect and correlative evidence and the lack of other explanations for the decline of red
spruce. Several explanations for the occurrence of winter injury are being investigated. There is
no agreement on the components of acidic deposition that are involved in reductions of cold
tolerance of red spruce foliage found in controlled experiments. The chain of events between
deposition of sulfur and nitrogen compounds in high-elevation forests and mortality of trees is not
understood. Consequently, benefits, if any, to red spruce forests cannot be identified.
Recommendation 5: The Study should clearly emphasize the distinction between sensitive
(at risk) ecological resources (the focus of the Study) and the general populations of these
ecological resources which are more resistant or fully resistant to adverse impact via acid
deposition.
The National Surface Water Survey (NSWS) was designed to provide assessment of the
chemical status of aquatic systems in regions of the U.S. thought to be at high risk to acid
deposition effects due to various sensitivity factors. The NSWS sample, therefore, represents
only a small fraction of all lakes and streams in the U.S. (the general population). Subsequent
reference to NSWS lakes, without reference to the fact that this is a small portion of all lakes, can
mislead the reader regarding the general significance of adverse effects to these high-risk lakes.
The graphic developed for Exhibit 6 in Chapter 2 (p. 29) is useful to make this important
distinction. If possible, a similar graphic should be developed based on actual proportional
differences and be moved forward in the document to both the Executive Summary and
Introduction (Chapter One).
Recommendation 6: Technical conclusions in the Study should be based primarily on
references from the peer-reviewed science-journal literature.
Many technical statements in the report are not specifically referenced; instead, sections
are broadly referenced by summary documents. Also, the sections on episodic acidification
include estimates of the extent of episodic acidification which are not referenced. In general, the
Agency should focus more attention on citing primary sources in peer-reviewed science-journals
rather than referring to "gray" literature; i.e., Agency documents which have not received
independent peer review as journal articles. In this case, the Nitrogen Bounding Study (NBS),
which forms much of the technical basis of the Study, did receive some level of peer review.
However, as a yet-to-be-published Agency document, the NBS has not had the benefit of review
by the broader scientific community.
A precedent for relying exclusively on published literature can be found in the 1990
decision by the Intergovernmental Panel on Climate Change (IPCC) to cite only peer-reviewed
publications in its state-of-the-science assessments. By encouraging the publication of Agency
science in peer-reviewed science-journals, a similar commitment by the Agency would do much to
improve the scientific credibility of EPA documents.
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4. MODELING NITROGEN DEPOSITION AND EFFECTS
4.1 Alternative Models for Nitrogen Cycling
Recommendation 7: The Agency should utilize acid deposition models that include the
biological processes controlling nitrogen cycling; MAGIC does not include these processes.
By treating NO3" strictly as a strong acid anion moving through watersheds, one can derive
a worst case scenario for the effect of nitrate, in addition to sulfate, on surface water acid
neutralizing capacity (ANC). However, nitrogen cycling is inherently a biological process, with
many alternative pathways through an ecosystem. Because the availability of nitrogen to aquatic
systems is tightly linked with terrestrial processes of the surrounding catchment, it is unrealistic to
attempt to model nitrogen without taking terrestrial biological processes into account.
A number of empirical studies in natural and agricultural systems over the past 30 years
have identified important pathways, controls, and pools of terrestrial nitrogen, all of which
influence the amount of potential leakage, and thus nitrogen saturation potential, of terrestrial N
into surface waters (Agren and Bosatta, 1988; Kahl et al., 1993; Matson et al., 1992). These
include nitrification, ammonification, uptake and storage in woody biomass, burial in soil organic
matter with varying turnover times, emissions as N2O, or leaching into soil and surface waters. In
some ecosystems, organic nitrogen, which will not contribute to surface water acidification, is the
major nitrogen species leaching into surface waters (Hedin et al., 1995). Land use and land use
history, vegetation type and maturity, phenology, and climate, all contribute to the degree to
which terrestrial systems are able to sequester nitrogen. These biological processes must be
reflected in any model chosen to project effects of nitrogen deposition.
There are a number of models that simulate nitrogen cycling in a more thorough and
realistic manner than does MAGIC. These models include the Terrestrial Ecosystem Model
(TEM) developed at Woods Hole (Melillo, 1995), the CENTURY model (Parton et al., 1987;
1988, 1993; Sanford et al., 1991; Schimel et al., 1994), the Nutrient Cycling Model(NuCM: Liu
et al., 1992), and the RHESSys model (Regional Hydro-Ecosystem Simulation System: Band et
al., 1993). RHESSys is a watershed based model that uses TOPMoDEL to route water through
soils, similar to MAGIC, and a lumped, 2-soil compartment for moving NO3" from soils to surface
waters. Many of these models are either developing or have developed capability for spatial
simulation. The Agency should incorporate the ecological processes included in these models
and, because of the many different ways in which hydrologic and biogeochemical processes are
treated in each, should consider comparing model outputs against each other and observed data.
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4.2 Nitrogen Saturation Potential
Forest ecosystems are generally recognized as nitrogen-limited; i.e., the biological sink
capacity for nitrogen exceeds nitrogen availability. Nitrogen is tightly regulated and recycled in
undisturbed forest ecosystems. The Study discusses the hypothesis that forests can become
nitrogen saturated if atmospheric deposition of nitrogen causes nitrogen supply to exceed the
biological sink capacity. The result of nitrogen saturation could be the export of nitrogen largely
in the form of NO3" and NH4+, which has the potential for numerous adverse effects (e.g., cation
depletion, perturbation of nitrogen mineralization or nitrification processes, disturbance of
mycorrhizal relationships, and acidification and eutrophication of associated aquatic ecosystems).
Recommendation 8: The Study should more clearly characterize the scientific uncertainty
regarding terrestrial ecosystem nitrogen saturation.
The Study considers emission scenarios for a range of times to watershed nitrogen
saturation and acknowledges the uncertainties associated with this variable. However, although
one of the scenarios assumes that nitrogen saturation does not occur, the Study focuses more on
those scenarios where nitrogen saturation occurs. Some European studies suggest that nitrogen
saturation is occurring in areas receiving much greater nitrogen deposition than presently
documented in North America. Although there is evidence that nitrogen saturation may occur in
the U.S. at high elevations (e.g., McNulty et al., 1990; Johnson and Lindberg, 1992), there is no
science community consensus on the extent of and degree to which nitrogen saturation caused
primarily by atmospheric deposition does and can occur.
Another factor influencing nitrogen saturation potential is the potential for vegetation to
respond to increasing nitrogen availability. This possibility has been observed in nitrogen
fertilization studies, where the response of the ecosystem to added nitrogen was a shift in species
and community composition to vegetation more competitive under increased nitrogen availability
(Aber, 1992; Bowman et al., 1993). Carbon dioxide fertilization will also stimulate biomass
production and as a consequence increase the nitrogen sink.
Some ecosystems, such as those of the Sierra or Rocky Mountains, have the potential to
saturate with nitrogen at much lower nitrogen deposition levels than the three provinces the
Agency evaluated (Baron et al., 1994). Some of these systems are not forested, and may be more
sensitive to nitrogen deposition because of seasonal hydrologic processes not addressed by the
MAGIC model.
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5. MONITORING
Recommendation 9: The Study should identify and emphasize the importance of
environmental monitoring of deposition, ecological indicators, and ecological endpoints as a
parallel and complementary strategy to modeling in order to assess ecological resource
issues.
The National Surface Water Survey (NSWS) of lakes and streams conducted in the mid-
1980's sampled "at risk" freshwater ecosystems using a standard protocol. Repeat of this survey
ten years later would provide assessment of current status (e.g., ANC, SO4=,NO3") and permit a
direct assessment of trends. Modeling and monitoring should be viewed as parallel activities that
interact and support one another. Monitoring will allow model validation. Modeling will allow
more focused monitoring. Decision-makers need models for predictions and monitoring for
evaluation of regulations, risk assessment, and priority-setting. Absent direct monitoring evidence
for declining ecosystem quality, it is doubtful that incremental regulations, especially if costly,
would be implemented based on modeling evidence alone.
6. CONCLUSIONS
The Subcommittee compliments the Agency on the significant effort expended to develop
an approach for assessing acid deposition and the need for a national standard. The Study relates
many complex scientific issues in a generally clear and concise manner. We concur with the
Agency that the current state-of-the-science with regard to acid deposition effects on terrestrial
and aquatic ecosystems does not support an acid deposition standard.
The Subcommittee has the following summary responses to the Charge Questions:
a) Have the models (RADM, MAGIC) been applied in a credible manner
and/or within the bounds of applicability for the scientific analyses and
assessments in the feasibility study?
Not in all cases. The models are appropriate for assessing future effects of sulfur
deposition, but lack realistic biological processes necessary for assessing effects of
nitrogen deposition.
b) Have the modeling results been appropriately integrated? In what ways
might the integration be improved?
Yes, with regard to sulfur dynamics. The modeling results for nitrogen dynamics,
however, are not appropriately synthesized. The Subcommittee recommends that
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the Agency consider several models which more adequately describe the biological
controls on nitrogen dynamics.
c) Are the conclusions drawn in the study consistent with the state of the
science and the state of modeling? What conclusions are insupportable or
weak? For what reasons? How might they be improved?
Not in all cases. A number of the conclusions in the Executive Summary are not
consistent with language in the body of the report or are not supported by current
scientific studies.
d) Are there important conclusions from other published modeling studies that
should be included in the feasibility study?
Yes. A number of alternative nitrogen models are suggested that treat biological
nitrogen cycling in a more thorough and realistic manner than does MAGIC.
e) Have the scientific uncertainties associated with the conclusions drawn in the
feasibility study been adequately characterized?
No. Uncertainties arising from emission projections, model formulations, and
model calibration and parameterization have not been adequately characterized.
The lack of adequate documentation of the uncertainty associated with use of the
models is a major deficiency of the Study.
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7. REFERENCES CITED
Aber, J. 1992. Nitrogen cycling and nitrogen saturation in temperate forest ecosystems. Trends in
Ecol. and Evol. 7:220-223.
Agren, G.I. andE. Bosatta. 1988. Nitrogen saturation of terrestrial ecosystems. Environmental
Pollution 54:185-197.
Band, L.E., P. Patterson, R. Nemani, and S.W. Running. 1993. Forest ecosystem processes at
the watershed scale: Incorporating hillslope hydrology. Agric. For. Meteorol. 63:93-126.
Baron, J.S., D.S. Ojima, E.A. Holland, and WJ. Parton. 1994. Analysis of nitrogen saturation
potential in Rocky Mountain tundra and forest: implications for aquatic systems. Biogeochem.
27:61-82.
Bowman, W.D., T.A. Theodose, J.C. Schardt, andR.T. Conant. 1993. Constraints of nutrient
availability on primary production in two alpine tundra communities. Ecology 74:2085-2097.
Hedin, L.O., JJ. Armesto, and A.H. Johnson. 1995. Patterns of nutrient loss from unpolluted,
old-growth temperate forests: evaluation of biogeochemical theory. Ecology 76:493-509.
Johnson, D.W. and S.E. Lindberg. 1992. Atmospheric deposition and forest nutrient cycling: a
synthesis of the integrated forest study. Springer-Verlag, New York.
Kahl, J.S., S.A. Norton, I.J. Fernandez, K.J. Nadelhoffer, C.T. Discoll, and J.D. Aber. 1993.
Experimental inducement of nitrogen saturation at the watershed scale. Environ. Sci. Technol.
27:565-568.
Liu, S., R. Munson, D.W. Johnson, S. Gherini, K. Summers, R. Hudson, K. Wilkenson, and L.F.
Pitelke. 1992. The Nutrient Cycling Model (NuCM): overview and application, pp. 583-609 In,
D.W. Johnson and S.E. Lindberg (eds), Atmospheric deposition and forest nutrient cycling: a
synthesis of the integrated forest study. Springer-Verlag, New York.
Matson, P.A., S.T. Gower, C. Volkmann, C. Billow, and C.C. Grier. 1992. Soil nitrogen cycling
and nitrous oxide flux in a Rocky Mountain douglas fir forest: effects of fertilization, irrigation,
and carbon addition. Biogeochemistry 18:101-117.
McNulty, S.G., J.D. Aber, T.M. McLellan, and S.M. Katt. 1990. Nitrogen cycling in high
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