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This book provides a unique overview of research methods over the past 25 years assessing critical loads and temporal effects of the deposition of air pollutants. It includes critical load methods and applications addressing acidification, eutrophication and heavy metal pollution of terrestrial and aquatic ecosystems. Applications include examples for each air pollution threat, both at local and regional scale, including Europe, Asia, Canada and the US. The book starts with background information on the effects of the deposition of sulphur, nitrogen and heavy metals and geochemical and biological indicators for risk assessments. The use of those indicators is then illustrated in the assessment of critical loads and their exceedances and in the temporal assessment of air pollution risks. It also includes the most recent developments of assessing critical loads and current and future risks of soil and water chemistry and biodiversity under climate change, with a special focus on nitrogen. The book thus provides a complete overview of the knowledge that is currently used for the scientific support of policies in the field of air pollution control to protect ecosystem services.

Inhaltsverzeichnis

Frontmatter

1. The History and Current State of Critical Loads and Dynamic Modelling Assessments

Abstract
This book focuses on knowledge and methods for the assessment of indirect, soil mediated effects of the deposition of sulphur dioxide, oxidized nitrogen and reduced nitrogen on terrestrial and aquatic ecosystems. The emphasis is on the science behind no-effect deposition thresholds (critical loads) and methods to understand future consequences of atmospheric depositions that exceed these thresholds. First, background information is given on drivers and impacts of air pollution and the philosophy behind the critical load approach is pointed out. Then, the history and current state of critical load assessments for sulphur (S), nitrogen (N) and metals is presented. This is followed by recent developments and use of dynamic models, for the assessment of future impacts of excessive deposition and the chapter finalizes with a reading guideline to this book and the logic of its organization.
Wim de Vries, Jean-Paul Hettelingh, Maximilian Posch

Assessment of Indicators for Air Pollutant Impacts

Frontmatter

2. Geochemical Indicators for Use in the Computation of Critical Loads and Dynamic Risk Assessments

Abstract
This chapter provides an overview of geochemical indicators for nitrogen (N), acidity, and metals in soil and water (soil solution, ground water and surface water) in view of their impacts on different endpoints (tree growth/health, human health, soil biodiversity etc.). Relevant indicators for N are the soil C/N ratio, nitrate (NO3) concentration in ground water and total N concentration in soil and surface water. For acidity the most relevant endpoint indicators are the exchangeable base cation pool or base saturation in the soil, the ratio of aluminium (Al) to base cation (Bc) in soil solution, the total Al concentration in ground water and the acid neutralizing capacity (ANC) in surface water. Relevant indicators for metals are the total or reactive metal concentration in the soil and the free or total metal ion concentration in water. Using critical limits for those endpoint indicators, it is possible to assess critical loads for both terrestrial and aquatic ecosystems based on geochemical modelling. An overview is given of the derivation of those limits, mostly under laboratory circumstances, and a critical evaluation of their relevance in the field situation.
Wim de Vries, Maximilian Posch, Harald U. Sverdrup, Thorjørn Larssen, Heleen A. de Wit, Roland Bobbink, Jean-Paul Hettelingh

3. Plant Species Diversity Indicators for Use in the Computation of Critical Loads and Dynamic Risk Assessments

Abstract
Soil models can be used to derive critical loads by computing the deposition that leads to critical limits for abiotic conditions, i.e. conditions that are just tolerated by an ecosystem. In this chapter various approaches are discussed to assess these critical limits for plant communities and plant species. Species diversity indicators have an important role in many of these approaches and such indicators may be based on species numbers, intrinsic values of species or the desirability of certain species or communities to be present in certain locations. Such desired or ‘target’ species or communities are often derived from concepts regarding the ‘pristine’ or ‘natural’ state of an ecosystem. For a diversity indicator the similarity of the actual or modelled state and the ‘target’ state has to be quantified, and various methods for quantification are discussed. Finally, a step-by-step approach is discussed to arrive at critical limits using niche models, based on various concepts regarding plant species diversity.
Han F van Dobben, Maximilian Posch, G. W. Wieger Wamelink, Jean-Paul Hettelingh, Wim de Vries

Empirical and Model-Based Critical Loads and Target Loads

Frontmatter

4. Effects and Empirical Critical Loads of Nitrogen for Europe

Abstract
Empirical critical loads of nitrogen (N) were first presented in a background document for a workshop in 1992 in Sweden. Since their first presentation, the critical loads of N have been updated at regular intervals and for a large number of habitats. This chapter presents a brief history of the empirical critical loads and explains the process of determination of empirical critical loads for nitrogen and their reliability. For European habitats (defined as EUNIS and Natura 2000 habitat classes), current empirical critical loads for nitrogen are presented. For each of these habitats, the main effects of enhanced nitrogen inputs are discussed that have formed the basis for the determination of the empirical critical loads. Factors other than nitrogen, that may affect ecosystem processes or ecosystem functioning, are discussed as these may modify the nitrogen critical load under specific conditions.
Roland Bobbink, Hilde Tomassen, Maaike Weijters, Leon van den Berg, Joachim Strengbom, Sabine Braun, Annika Nordin, Kirsten Schütz, Jean-Paul Hettelingh

5. Effects and Empirical Critical Loads of Nitrogen for Ecoregions of the United States

Abstract
Human activity in the last century has increased nitrogen (N) deposition to a level that has caused or is likely to cause alterations to the structure and function of many ecosystems across the United States. We synthesized current research relating atmospheric N deposition to effects on terrestrial and freshwater ecosystems in the United States, and estimated associated empirical critical loads of N for several receptors: freshwater diatoms, mycorrhizal fungi, lichens, bryophytes, herbaceous plants, shrubs, and trees. Biogeochemical responses included increased N mineralization and nitrification, increased gaseous N losses, and increased N leaching. Individual species, population, and community responses included increased tissue N, physiological and nutrient imbalances, increased growth, altered root-shoot ratios, increased susceptibility to secondary stresses, altered fire regime, shifts in competitive interactions and community composition, changes in species richness and other measures of biodiversity, and increases in invasive species. The range of critical loads of nutrient N reported for U.S. ecoregions, inland surface waters, and freshwater wetlands is 1–39 kg N ha−1 yr−1, spanning the range of N deposition observed over most of the country. The empirical critical loads of N tend to increase in the following sequence: diatoms, lichens and bryophytes, mycorrhizal fungi, herbaceous plants and shrubs, trees.
Linda H. Pardo, Molly J. Robin-Abbott, Mark E. Fenn, Christine L. Goodale, Linda H. Geiser, Charles T. Driscoll, Edith B. Allen, Jill S. Baron, Roland Bobbink, William D. Bowman, Christopher M. Clark, Bridget Emmett, Frank S. Gilliam, Tara L. Greaver, Sharon J. Hall, Erik A. Lilleskov, Lingli Liu, Jason A. Lynch, Knute J. Nadelhoffer, Steven J. Perakis, John L. Stoddard, Kathleen C. Weathers, Robin L. Dennis

6. Mass Balance Models to Derive Critical Loads of Nitrogen and Acidity for Terrestrial and Aquatic Ecosystems

Abstract
This chapter describes the standard approaches (mass balance models) to calculate critical loads of nutrient nitrogen (N) as well as for sulphur (S) and N acidity for both terrestrial and aquatic ecosystems. The description focuses on the so-called Simple Mass Balance (SMB) model for nutrient nitrogen and acidity for terrestrial ecosystems and on the First-order Acidity Balance (FAB) model for aquatic ecosystems. The model descriptions are in accordance with the methods for calculating critical loads under the LRTAP Convention. For both types of models, a discussion is presented on the required input data, data sources and standard model parameter values used in their application. For acidity, the chapter elaborates on the critical load function as there is no unique critical load of S and N acidity, and on the approach to assess critical load exceedances. The chapter ends with a discussion on the possible formulation of critical loads based on biodiversity criteria.
Maximilian Posch, Wim de Vries, Harald U. Sverdrup

7. Mass Balance Approaches to Assess Critical Loads and Target Loads of Metals for Terrestrial and Aquatic Ecosystems

Abstract
Critical loads of heavy metals address not only ecotoxicological effects on organisms in soils and surface waters, but also food quality in view of public health. A critical load for metals is the load resulting at steady state in a metal concentration in a compartment (e.g. soil solution, surface water) that equals the critical limit for that compartment. This chapter presents an overview of methods to derive critical loads of metals, with a focus on cadmium (Cd), lead (Pb), mercury (Hg), copper (Cu) and zinc (Zn), using critical limits for dissolved total metal concentrations based on impacts on food crops, soil organisms and aquatic organisms. Unlike nitrogen and sulphur, the time to reach steady state can be very long and therefore dynamic models are needed to estimate the times involved in attaining a certain chemical state in response to heavy metal inputs. Therefore, simple approaches are also presented to calculate target loads, i.e. deposition levels at which a critical limit is attained within a predefined time horizon. Results are presented for critical loads of Cd in view of food quality, critical loads and target loads of Cd, Pb, Cu and Zn in view of soil biodiversity and a critical limit for Hg in precipitation in view of impacts on fish and soil organisms. Results are discussed in view of the uncertainty and potential applicability for policy support.
Wim de Vries, Jan E. Groenenberg, Maximilian Posch

Dynamic Modelling for the Assessment of Air Pollution Impacts at Site Scale

Frontmatter

8. Dynamic Geochemical Models to Assess Deposition Impacts and Target Loads of Acidity for Soils and Surface Waters

Abstract
This chapter presents four geochemical dynamic models (VSD, MAGIC, ForSAFE and SMARTml) that have been used to assess impacts of nitrogen and acidity inputs on soil and soil solution chemistry. These models differ in their complexity and description of some processes. Some models can be used to calculate effects on surface waters as well. For all models this chapter shows examples of site-scale applications at intensively monitored forested plots in the UK, Germany, Switzerland and Norway, illustrating the adequacy of the model behaviour. Impacts of legislated emission reductions and forest harvest scenarios on soil solution chemistry are illustrated with a MAGIC model application. Besides scenario analyses, dynamic models can also be used to determine target loads, i.e. the deposition to reach a prescribed condition within a given time frame. This chapter introduces the target load concept and presents target load calculations with the MAGIC and the VSD model.
Luc T. C. Bonten, Gert Jan Reinds, Jan E. Groenenberg, Wim de Vries, Maximilian Posch, Chris D. Evans, Salim Belyazid, Sabine Braun, Filip Moldan, Harald U. Sverdrup, Daniel Kurz

9. Dynamic Geochemical Models to Assess Deposition Impacts of Metals for Soils and Surface Waters

Abstract
This chapter describes the use of geochemical models to assess the impacts of the deposition of metals on the concentrations of metals in soils and surface waters. We describe three dynamic models: SMART2-metals, SMARTml and CHUM-AM, each with their specific purpose and geographical scale of application. All three models include the most relevant metal fluxes and soil chemical processes, but with various level of detail related to their specific aim and scale. The ability of the models to simulate the long-term trends of metal fate was assessed by comparing model results and observations of either the present metal status, using hind cast simulations with historical deposition trends, or metal pools in chronosequences of afforested agricultural land of different stand age, or metal concentrations observed in a long-term monitoring study. The model simulations show the long times needed to approach equilibrium concentrations of metals due to changes in the atmospheric deposition of metals, sulphur and nitrogen. Dynamic models are therefore indispensable tools for the assessment of metal concentrations at changing levels of metal inputs to soil-water systems.
Jan E. Groenenberg, Edward Tipping, Luc T. C. Bonten, Wim de Vries

10. Use of Combined Biogeochemical Model Approaches and Empirical Data to Assess Critical Loads of Nitrogen

Abstract
Empirical and dynamic biogeochemical modelling are complementary approaches for determining the critical load (CL) of atmospheric nitrogen (N) or other constituent deposition that an ecosystem can tolerate without causing ecological harm. The greatest benefits are obtained when these approaches are used in combination. Confounding environmental factors can complicate the determination of empirical CLs across depositional gradients, while the experimental application of N amendments for estimating the CL does not realistically mimic the effects of chronic atmospheric N deposition. Biogeochemical and vegetation simulation models can provide CL estimates and valuable ecosystem response information, allowing for past and future scenario testing with various combinations of environmental factors, pollutants, pollutant control options, land management, and ecosystem response parameters. Even so, models are fundamentally gross simplifications of the real ecosystems they attempt to simulate. Empirical approaches are vital as a check on simulations and CL estimates, to parameterize models, and to elucidate mechanisms and responses under real world conditions. In this chapter, we provide examples of empirical and modelled N CL approaches in ecosystems from three regions of the United States: mixed conifer forest, desert scrub and pinyon-juniper woodland in California; alpine catchments in the Rocky Mountains; and lakes in the Adirondack region of New York state.
Mark E. Fenn, Charles T. Driscoll, Qingtao Zhou, Leela E. Rao, Thomas Meixner, Edith B. Allen, Fengming Yuan, Timothy J. Sullivan

11. Field Survey Based Models for Exploring Nitrogen and Acidity Effects on Plant Species Diversity and Assessing Long-Term Critical Loads

Abstract
Empirical critical loads are based on current evidence for relationships between the rate of pollutant deposition and changes to ecosystems observed in experiments and surveys. When considering longer-term change and effects of changes in deposition rate after periods of deposition in excess of the critical load, dynamic modelling approaches are useful. This chapter describes two soil-vegetation-floristics model chains, similar in concept, that are being applied in the Netherlands and the UK to explore pollution scenarios and calculate long-term critical loads for acidity and nutrient-N. These model chains consist of dynamic models of soil and vegetation biogeochemistry, combined with environmental suitability models that define the realised niche for the species or assemblage. The environmental suitability models described in this chapter are based on empirical relationships between species (MOVE, PROPS, MultiMOVE) or assemblage (NTM3) occurrence and environmental conditions, defined on multiple axes. They are driven by different biogeochemical models, forming the model chains SMART2-(SUMO2)-PROPS/NTM3 and MADOC-MultiMOVE. In this chapter these model chains are described in detail, and applications to scenario exploration and setting critical loads are demonstrated.
Ed C. Rowe, G. W. Wieger Wamelink, Simon M. Smart, Adam Butler, Peter A. Henrys, Han F. van Dobben, Gert Jan Reinds, Chris D. Evans, Johannes Kros, Wim de Vries

12. Use of an Integrated Soil-Vegetation Model to Assess Impacts of Atmospheric Deposition and Climate Change on Plant Species Diversity

Abstract
To realistically simulate possible future changes in plant species diversity, it is imperative to include the effects of both climate change and atmospheric deposition. Both factors have direct effects on the vegetation, such as the favouring of nitrophilous plants under elevated nitrogen (N) deposition or the elimination of drought-sensitive plant species under dry climatic conditions. To account for these confounding or reinforcing effects, integrated models are needed. ForSAFE-Veg is a dynamic model integrating the different forest ecosystem processes and feedbacks that make up the web of interactions and responses to climatic conditions and atmospheric deposition. The model was used to simulate forest ecosystems at 48 sites in Sweden and Switzerland. The evaluation of the model performance was satisfactory in regard to soil chemistry and plant community composition. The model was then used to simulate the separate and combined effects of the foreseen climate change and scenarios of N and sulphur (S) deposition levels. According to the simulations, the future change in plant community composition due to climate change alone will be bigger than the change avoided by the current decrease in atmospheric depositions.
Salim Belyazid, Harald U. Sverdrup, Daniel Kurz, Sabine Braun

13. Evaluation of Plant Responses to Atmospheric Nitrogen Deposition in France Using Integrated Soil-Vegetation Models

Abstract
The aim of this chapter is to give an overview of plant responses to nitrogen (N) deposition by using two dynamic biogeochemical models coupled with a vegetation module: VSD+-VEG and ForSAFE-VEG. The biogeochemical models were first validated on some French forest sites from the ICP-Forests network. A French vegetation table (which is now part of a European database) containing 230 species with their appropriate ecological environmental parameters, was set up. The outputs of each model in terms of plant response to atmospheric nitrogen deposition were compared to measured values for one forest stand. The two models underestimated the occurrence of certain herbs and grasses and overestimated (ForSAFE-VEG) or underestimated (VSD+-VEG) the presence of certain mosses. This allowed us to improve the validation and thus the calibration of some parameters. For the simulated period ForSAFE-VEG indicated some variations in the occurrence of the plant groups, the mosses group showing the highest increase and indicating a high sensitivity to atmospheric N deposition. No significant changes in the occurrence percentage of plant groups were observed by running the VSD+-VEG model, this model being less sensitive than ForSAFE-VEG to simulate tenuous vegetation changes. The observed changes over time in the dominant ground plant groups using ForSAFE-VEG could be related to changes in site environmental conditions, but only the influence of the maximum N deposition was obvious. Further investigations are needed to compare the performance of the two models on other sites, but these tests of the ForSAFE-VEG and VSD+-VEG models showed promise for simulating the link between N deposition and vegetation diversity.
Anne Probst, Carole Obeidy, Noémie Gaudio, Salim Belyazid, Jean-Claude Gégout, Didier Alard, Emmanuel Corket, Jean-Paul Party, Thierry Gauquelin, Arnaud Mansat, Bengt Nihlgård, Sophie Leguédois, Harald U. Sverdrup

14. Use of an Empirical Model Approach for Modelling Trends of Ecological Sustainability

Abstract
The BERN (Bioindication for Ecosystem Regeneration towards Natural conditions) model was designed to integrate empirical ecological cause-effect relationships into environmental assessment studies including the derivation of critical loads.
Plant species in a natural or semi-natural ecosystem adapted to essential nutrients, water supply and climate conditions via long-term evolutionary development. Therefore changes in vegetation composition and structure may serve as an indicator for alterations of these parameters. Natural plant communities that were observed on reference sites in a reference year, e.g. before major air pollution impact, can be defined as reference communities. They represent the current solution of long-term interaction between their species and the environment. In order to model reactions of plant communities to changes in the environment, the BERN model derives the reference realized niches of plant species (currently 1940) and of plant communities (688 communities in Europe) with their fuzzy (blurred) thresholds of the suitable site parameters. These actually existing combinations of site parameters are identified as in a dynamic nutrient balance and therefore classified as reference site types.
Model results show the current deviation of site condition in relation to a reference type, the ability of plant species to recover and potential natural communities in the future. Underlying drivers are future scenarios of land use, geochemical changes (e.g. derived from geochemical models) and climate change. BERN can be used to derive critical limits in order to compute critical loads of eutrophying and acidifying depositions, e.g. for Natura 2000 habitat types.
Angela Schlutow, Thomas Dirnböck, Tomasz Pecka, Thomas Scheuschner

Critical Loads and Dynamic Model Applications on a Regional Scale

Frontmatter

15. Assessment of Critical Loads of Sulphur and Nitrogen and Their Exceedances for Terrestrial Ecosystems in the Northern Hemisphere

Abstract
In this chapter an assessment of critical loads of sulphur and nitrogen for forests and (semi-)natural vegetation and their exceedances in the boreal and temperate region of the Northern Hemisphere (excluding the contiguous USA) is reported. Critical loads were estimated using steady-state mass balance methods (see Chap. 6). The influence of different chemical criteria on critical loads and their exceedances was also evaluated.
Gert Jan Reinds, Maximilian Posch, Julian Aherne, Martin Forsius

16. Critical Load Assessments for Sulphur and Nitrogen for Soils and Surface Waters in China

Abstract
As a widely accepted scientific basis for guiding emission abatement strategies, the critical load concept was applied in China, with the prospect of a wider application in policy-making in the future. The aim of this chapter is to give an overview of critical load assessments for China. It includes a regional-scale assessment for soils, based on an extended Steady-State Mass Balance (SSMB) method, and a site-scale assessment for some surface waters in southern and north-eastern China, based on the MAGIC model. The soil critical load maps were derived based on thorough investigations of weathering rates of soils, uptake rates of vegetations, and depositions of base cations. Combining the emission inventory, chemistry transport model, and critical loads, the impacts of China’s emission control policy on soil acidification were evaluated, based on estimated exceedances of critical loads. The increasing emissions of atmospheric nitrogen and the decreasing emissions of particulate matter were estimated to largely counteract the benefits of SO2 emission reductions in the near future. The results implied that a “multi-pollutant control strategy”, combing measures for S, N and PM reduction, is the way forward for future Chinese acidification mitigation.
Lei Duan, Yu Zhao, Jiming Hao

17. Assessment of Critical Loads of Acidity and Their Exceedances for European Lakes

Abstract
Lake acidification in northern Europe provided some of the key impetus for the development of the critical loads approach during the 1980s. While major reductions in acidic deposition have been achieved during the last 20 years, through the application of this approach, regions with continued acidification and critical load exceedance persist around Europe. This chapter describes regional applications of the First-order Acidity Balance (FAB) model in five European countries, highlighting national approaches to lake surveys and regional representation, and how the model has been adapted in each of these countries. We discuss the implications of interpreting critical load exceedances, and provide an overall synthesis of freshwater exceedance in Europe using common European deposition data. Despite uncertainties within the FAB model, such as the parameterisation of nitrogen immobilisation and denitrification, a coherent picture of the spatial extent of acidification within European lakes is evident. The ongoing failure to meet critical loads by 2020 demonstrates that lake acidification is still a current, not a historical, problem in Europe, and under current legislation many lakes will remain more acidic than their pre-industrial reference condition.
Chris J. Curtis, Maximilian Posch, Julian Aherne, Jens Fölster, Martin Forsius, Thorjørn Larssen, Filip Moldan

18. National-Scale Dynamic Model Applications for Nordic Lake Catchments

Abstract
This chapter presents an overview of national-scale dynamic hydro-geochemical model applications to lake catchments in Norway, Sweden and Finland. Model simulations have been used to predict the recovery of soil and surface water chemistry in response to (1) decreased deposition of acidifying compounds, (2) the combined impacts of acidifying and climate change processes, and (3) the impacts of increased removal of forest harvesting residues for bioenergy production.
Martin Forsius, Filip Moldan, Thorjørn Larssen, Maximilian Posch, Julian Aherne, Espen Lund, Richard F. Wright, B. Jack Cosby

19. Critical Load Assessments and Dynamic Model Applications for Lakes in North America

Abstract
Critical loads were first discussed by Canada and the United States during the early 1980s in the Memorandum of Intent on Transboundary Air Pollution—the earliest bilateral acid rain assessment. A legacy of this assessment is the specification of critical loads of acidity for surface waters in Canada.
This chapter reviews the application of steady-state and dynamic models to assess the impacts of acidic deposition on surface waters in North America. It describes the historic development of critical loads in Canada and the United States, and provides a broad overview of wide-scale regional applications of steady-state and dynamic models. Furthermore, the chapter presents a national application of the First-order Acidity Balance (FAB) model for surface waters in Canada, with reference to similar assessments in Europe.
Julian Aherne, Dean Jeffries

20. Critical Loads and Critical Limits of Cadmium, Copper, Lead and Zinc and Their Exceedances for Terrestrial Ecosystems in the United Kingdom

Abstract
Critical loads have been calculated and mapped for cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn) for six habitat types in the UK. This chapter focuses on updates to the methods since 2006 and the results of critical load and critical limit exceedance. The critical limits are expressed as free-metal ion concentrations in soil solution. Transfer functions are used to convert free-ion critical limits to total soil metal concentrations for comparison with current observed values. Critical limit exceedance provides an indication of current soil condition while critical load exceedance provides an indication of areas potentially at risk when steady-state is reached. As it is possible for the critical load to be exceeded but not the critical limit, it is important to look at the exceedance of both, because if deposition continues at current levels, critical limit exceedance may occur in the future.
Results for Cu and Zn are similar, with exceedance of the critical load and critical limit occurring across ~ 20 % of the same habitat areas for managed broadleaved woodland and unmanaged woodland. There are few areas with critical limit exceedance for Pb for any habitat, but significant areas of woodland habitats with critical load exceedance, due to higher deposition scavenging by trees. For Cd there are no habitat areas with critical load exceedance and only small areas with critical limit exceedance. The main reason for the different patterns of critical limit and critical load exceedance is the different rates of change in the soil metal pools in relation to changes in metal deposition.
Jane Hall, Edward Tipping, Stephen Lofts, Michael Ashmore, Laura Shotbolt

21. Critical Loads of Cadmium, Lead and Mercury and Their Exceedances in Europe

Abstract
In this chapter information is summarized on the assessment of the risk of impacts of cadmium, lead and mercury emissions and related depositions of these metals, with an emphasis on natural areas in Europe. Depositions are compared to critical loads to identify areas in Europe where critical loads are exceeded. Critical loads of cadmium, lead and mercury were based on (i) computations by 18 Parties to the Convention on Long-range Transboundary Air Pollution (LRTAP) and (ii) computations from available data on soil chemistry, meteorology and land cover for the other Parties. Two target years are considered, i.e. 2010 and 2020. Emissions for these years have been assessed in support of the negotiations for the review and possible revision of the Heavy metal protocol (Aarhus 1998). The relationship between emissions, depositions and critical load exceedances is analysed assuming the implementation of abatement techniques under Current LEgislation in 2010 (CLE2010) and in 2020 under Full Implementation of the Aarhus protocol (FI2020). Comparing the critical loads to atmospheric depositions in these years, shows that cadmium deposition is not a widespread risk in either years, that the computed risk of lead deposition affects about 22 and 16 % of natural European area in 2010 and 2020, respectively, and that mercury deposition is computed to affect an area of more than 74 % in both years.
Jean-Paul Hettelingh, Gudrun Schütze, Wim de Vries, Hugo Denier van der Gon, Ilia Ilyin, Gert Jan Reinds, Jaap Slootweg, Oleg Travnikov

22. Derivation of Critical Loads of Nitrogen for Habitat Types and Their Exceedances in the Netherlands

Abstract
A method is presented to simulate nitrogen (N) critical load values per vegetation type in the Netherlands following the method outlined in Chap.  3 and to integrate these simulated critical loads with empirical values to unique values per habitat type as defined in the European Habitats Directive. In this way critical loads are generated that (a) can be used as local deposition targets to comply with the Habitats Directive, and (b) have a broad international support.
Han F. van Dobben, Arjen van Hinsberg, Dick Bal, Janet P. Mol-Dijkstra, Henricus J.J. Wieggers, Johannes Kros, Wim de Vries

23. Assessing the Impacts of Nitrogen Deposition on Plant Species Richness in Europe

Abstract
Dose-Response (D-R) relationships derived from nitrogen (N) addition experiments and N deposition gradient studies are extrapolated over natural and (semi-)natural grasslands in Europe, using a European land cover map. Based on emissions of oxidized and reduced N in 2000 and 2020, ecosystem-specific depositions of total N are computed over Europe. For 2020 two scenarios are applied, i.e. one according agreed emission reductions under national and European legislation and the other based on the (hypothetical) application of best available emission control techniques. Results show that the impact of N deposition on plant species diversity computed over European (semi-)natural grasslands is less when N addition based dose-response relationships are used than when a N deposition gradient based alternative is applied. Using the latter approach, the species richness is computed to be reduced by more than 40 % in about 5 % of European grasslands in 2000. This reduction in species richness becomes less than 25 %, when N emissions are cut back using maximum feasible abatement technologies.
Jean-Paul Hettelingh, Carly J. Stevens, Maximilian Posch, Roland Bobbink, Wim de Vries

Integrated Assessment, Policy Applications and Synthesis

Frontmatter

24. Integrated Assessment of Impacts of Atmospheric Deposition and Climate Change on Forest Ecosystem Services in Europe

Abstract
Important forest ecosystem services are pollutant filtering relevant for an adequate water quality (regulating service), wood production (provisioning service) with related carbon (C) storage (regulating service) and the provision of a habitat for a diversity of plants and animals (supporting service). Nitrogen (N) and sulphur (S) deposition affect these ecosystem services. In this chapter, we describe the application of the soil model VSD, in combination with the forest growth model EUgrow and the plant species occurrence model PROPS to quantify the impact of N and S deposition on: (i) changes in soil buffering, in terms of accumulation or depletion of the pools of N, base cations (BC) and aluminium (Al), and changes in nitrate (NO3) and Al concentration in soil water, (ii) forest growth and carbon sequestration, and (iii) plant species diversity. Results showed that the depletion of Al and BC pools and the soil water concentrations of NO3 and Al increased strongly between 1950 and 1980, followed by a decrease between 1980 and 2010, reflecting the strong initial increase and subsequent decrease in N and S deposition in both periods, respectively. The impact of future emission reductions on the various parameters in the period 2010–2050 was larger than the climate change impact. Unlike soil and water quality, both N deposition and climate change had on average a positive impact on carbon sequestration. N deposition was calculated to be the dominant driver of changes in forest growth in the past (period 1900–2000) and climate change for the future (period 2000–2050). Plant species diversity changed hardly in scenarios assuming constant climate and low N deposition reduction, significantly at constant climate and strongly decreasing N deposition, and sharply when both climate and N deposition changed, especially in areas with a pronounced temperature change.
Wim de Vries, Maximilian Posch, Gert Jan Reinds, Luc T.C. Bonten, Janet P. Mol-Dijkstra, G.W. Wieger Wamelink, Jean-Paul Hettelingh

25. Effects-Based Integrated Assessment Modelling for the Support of European Air Pollution Abatement Policies

Abstract
Critical load and exceedance based indicators for effects of air pollution are used to define and compare air pollution abatement scenarios, thus assisting in the framing of policies and strategies, of emission abatement options. In this chapter the effects-based support of European air pollution abatement policies since the early 1990s is described. The systematic use of computed as well as empirical critical loads and other impact assessment methodologies, such as dynamic modelling, are addressed. Computed impacts of policy alternatives that have been considered to alleviate acidification and eutrophication are compared, including the relative robustness of the magnitude and location of these impacts in Europe. It is concluded that policies have led to significant reductions in the acidification over the whole of Europe, such that expected impacts are currently minimal. With respect to eutrophication it is concluded that the excessive atmospheric deposition of nitrogen compounds will continue to have detrimental impacts on plant biodiversity and ecosystems, unless emissions of oxidized and reduced nitrogen are further reduced.
Jean-Paul Hettelingh, Maximilian Posch, Jaap Slootweg, Gert Jan Reinds, Wim de Vries, Anne-Christine Le Gall, Rob Maas

26. Synthesis

Abstract
This chapter first presents an overview of findings described in this book. This includes a summary of the combined use of empirical and model-based approaches, main results of these analyses, and their relevance in view of impacts on ecosystem health, human health and effects on ecosystem services. This is followed by a discussion of the uncertainties involved in critical load assessments and applications. Finally, it presents an outlook on the future of critical loads and dynamic model applications to support assessments of trade-offs between policies in the field of air pollution, biodiversity and climate change. The importance of analysing multiple rather than single effects in these fields, including their interactions, is emphasized as the way forward for the further development and application of critical thresholds.
Jean-Paul Hettelingh, Wim de Vries, Maximilian Posch

Backmatter

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