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No. 69: Critical loads of Danish nature

Bak, J.L. 2013. Tålegrænser for dansk natur. Opdateret landsdækkende kortlægning af tålegrænser for dansk natur og overskridelser heraf. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 94 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 69




The latest national calculations of critical loads and exceedances were made in 2006 as part of the preparatory work for the new approval act for livestock farms (Nielsen et al., 2006). Since then, significant progress in method development has been made. In recent years, the development has particularly focused on critical loads based on biodiversity endpoints, as biodiversity loss has had a major policy focus, in particular as a result of the EU Nature Directives and the CBD. Since 2006, there has been a significant improvement in the data available. Among other things, data from the national monitoring program for the aquatic and terrestrial environment (NOVANA), where the terrestrial part started in 2004, have become available, and national deposition monitoring and modeling has been developed, including local-scale calculations with a resolution of 400x400 m (Ellermann et al, 2011). This development has made it relevant to update the national mapping of critical loads and exceedances and to include indicators and criteria concerning biodiversity.

Despite the fact that there at European level has been substantial progress in the development of methods for the calculation of critical loads based on biodiversity endpoints, there is not yet scientific consensus or solid, international recommendations for the choice of methods, indicators and criteria at different scales. Calculations have been made on a European scale, but these calculations include only a limited number of habitats and have, so far, been based on relatively arbitrary criteria. The work presented here, therefore, attempts to clarify which methods, indicators and criteria there in a Danish context can be used for calculation and mapping of critical loads based on objectives for biodiversity and available data.

The calculations presented are based on current best practices and best available data. The selected methods are a coupling of dynamic soil chemical models with empirical / statistical-based plant occurrence models. It has, thus, been possible both to calculate critical loads for the main Danish Natura 2000 habitats and to make scenario analyses to identify plant species that have declined as a consequence of air pollution and can expect further decline if the current deposition continues. In addition to these analyses, the report includes an update of the recommendations concerning the use of empirically based critical loads and an analysis of the level of deposition changes that can be detected with statistical certainty on a single site with the use of data as collected in the NOVANA monitoring program.


Method Development

Biodiversity can be defined and measured in many ways at different scales and, similarly, a number of different ways exist to set targets for biodiversity. However, presently no authoritative national or international recommendation of indicators and criteria for an acceptable impact on biodiversity exists that can be used in calculating critical loads for biodiversity. A part of this study has therefore been an analysis aiming at finding an operative combination of indicators and criteria. The premise has been that the calculation has to be implemented with available and accepted tools and should be based on available data from the NOVANA program. Furthermore, Indicators and criteria should be related to relevant policy objectives.

This analysis resulted in the use of a set of indicators and criteria related to the objective of halting the decline of biodiversity and, thereby, related to the targets of the CBD and the Habitats Directive. It has been a criterion for the choice of indicator, that the indicator must be based on a large number of species in order to reduce uncertainties in model calculations. In practice, the indicator used was, therefore, based on the occurrence probability of all nitrogen-sensitive (higher plant) species. Nitrogen-sensitive species are found as species where the calculated occurrence probability has been reduced by more than 5% from 1950 to 2010 due to air pollution. In practice, these species are found in a scenario calculation, where the trend in air pollution is based on time series data, while all other influences are held constant.

Development of a method for simulating the botanical sampling in the NOVANA program has also been included in the project in order to quantify the uncertainties in sampling and, hence, the thresholds of (botanical) change that can be measured in an area (Appendix 1).

Method of calculating the effect scenarios, critical loads and exceedances of critical limits

It was decided to conduct scenario and critical load calculations with a model system that is a further development of the previously developed EUDANA model system (Bak and Ejrnæs, 2004). The model system consists of the dynamic soil chemical model VSD and an empirical / statistical based plant occurrence model, MOVE. The MOVE model describes the occurrence probability of a number of plant species as a function of environmental parameters described by Ellenberg indicator values (de Wries et al., 2010). The use of Ellenberg indicator values was necessary in order to also have adequate data for more rare species, as the number of plots where only plant occurrence has been monitored far exceeded the number of plots with paired observations of plant abundance and soil chemistry. The link between the model components is based on empirical / statistical-based transfer functions between chemical soil parameters and Ellenberg indicator values. The establishment of the empirical / statistical-based relationships between Ellenberg indicator values and probability occurrence and the chemical parameters and Ellenberg indicator values are based on an assumption of equilibrium, which is not likely to be met in, for example, the Danish monitoring data from the NOVANA program. Therefore, the model used is based on Dutch and British work, where it has been possible to integrate older data from less polluted times and data from background areas. The British and Dutch data cover a climate area where it is justified to extrapolate the results to Denmark. The model system describes the temporal development of soil chemistry, but not the possible time lags between changes in soil chemistry and plant occurrence. Due to limitations in data and the technical implementation of the of the VSD model version used, it has been necessary to limit the total number of calculations. Data has, therefore, been aggregated by nature type and a calibration of the VSD model made, so that the model for each habitat type reproduces the values of soil pH and C / measured in the NOVANA program.

A number of scenario analyses have been made for individual habitats ranging back from 1900 onwards. The purpose of the calculations was to isolate the effect of changes in air pollution impacts, and all other influences have therefore been kept constant in the calculations. This also means that for the semi-natural habitats, where management is necessary, a management regime with constant, low nitrogen removal giving a realistic average for the period was used.

The calculations of critical loads and exceedances have been made for individual nature types based on a criterion of halting the loss of biodiversity caused by air pollution in relation to different reference years. The selected reference years are 1950, 1992 and 2010. Reference years prior to 1950 have not been used, since the uncertainties in the calculations increase significantly when going further back in time. 1992 corresponds to the year the Habitats Directive entered into force and 2010 to the original target year for the CBD.


Critical loads and exceedances

The calculated critical load ranges between 7-12, 7-11 and 3-10 kg N ha-1 yr-1, respectively, for 2010, 1992 and 1950 as reference year. The intervals indicate the spread between the habitat with the lowest, respectively highest tolerable limit. The variation of critical loads within a certain habitat type has not been quantified, but is expected to be at least equal to the variation between habitats. It is expected that a calculation with a more detailed use of local data will give a higher median value for each habitat than the calculated values. The calculated exceedances of critical loads for each habitat type lie between 0 and 5.6 kg N ha-1 yr-1 with 1992 as reference year.

An update of recommended values for empirically based critical loads has been made based on the latest international recommendations. This update resulted in a reduction of critical load for seven Habitat Directive habitat types.

The calculated critical loads for biodiversity are in line with, or below the low range of, empirically based critical loads. This is especially true for some types of grassland. In this analysis, dune types have noticeably exceedances of the critical load. Previous analyses based on empirically based critical loads have shown relatively small exceedances for dune habitats. The latest update of empirically based critical loads has reduced the level for two of the dune habitats. The calculated critical loads for salt marsh is significantly lower than the empirically based critical loads, but this calculation is very uncertain, partly due to a lack of good data sources for nitrogen input from other sources, such as surface near runoff, partly because salinity is not included as a plant distributing factor in the model system used.

An analysis has been made of the species that are expected to have declined as a result of atmospheric nitrogen deposition. There is a total of 179 species, or approx. 11 % of the species observed in the NOVANA program (Appendix 1). 101 of these species are also Danish indicator species, Habitat Directive typical species, Annex 1 species and / or red-listed.

Scenarios, trends

Scenario calculations indicate that atmospheric deposition of nitrogen and sulfur has had a significant effect on soil chemistry and plant communities on Natura 2000 habitats and thereby resulted in a dramatic decline for a number of nitrogen-sensitive species. A continuation of depositions at the current level will, according to scenario calculations, result in a continued decline of these species and possibly the loss of some species, partly as a result of continued exposure of part of the natural area to deposition exceeding critical loads, partly as a result of the cumulative effect of previous impact.

The status and future development of a nature area affected by air pollution depend on the future load in relation to critical loads and the integrated effects of past impacts on soil chemistry and plant communities. There is no direct correlation between change in load and change in the rate of change of load and ecosystem status in the long term, although these parameters sometimes are used as a basis for regulation. Furthermore, no correlation was found between an area's critical load or exceedance of critical load and the level of deposition changes that will lead to measurable changes in the field. The analyses show that relatively large deposition changes are needed before a change in plant community at a single site can be measured with certainty using conventional monitoring methods.

There are likely significant time delays (decades) between changes in deposition, soil chemistry, plant communities and soil structure. Therefore, the currently observed plant communities cannot, in general, be expected to be in balance with the current impacts.

Implications for regulation and management

The results of the analysis underpins the fact that there is a continuing need for reduction of nitrogen deposition through both national and international regulations, if the objective of halting the loss of biodiversity or a the objective of a constant or increasing share of Natura 2000 areas with favorable conservation status are to  be met. The calculated levels of exceedances of critical loads for nitrogen make it possible to have an integrated approach to emission abatement and removal of nitrogen from natural habitats by nature management, at least for some habitat types.

It is worth noting that even with the relatively low critical loads presented here, 100 % of natural areas will not have exceedances, and this might be important to local regulation, e.g. livestock farm approvals. A potential for a differentiated regulation that does not place restrictions on agricultural operations on single properties where this cannot be justified, based on the exceedance of critical loads, therefore exists. This could contribute to a more cost-effective achievement of overall environmental goals.

The methods and tools used in the nationwide study presented here could also be used in the calculation of site-specific critical loads. Use of local soil and vegetation data will give a more precise determination of nitrogen processes such as fixation, denitrifikation and immobilization, which have a significant impact on the critical load, thus reducing the uncertainty in the assessments. Local-specific critical loads may be higher if long-term management plans are in place that remove more nitrogen than assumed in the national scenarios without causing other adverse effects, and / or if less nitrogen sensitive species occur on the site.

The presented methods with coupled soil geochemical models and plant occurrence models follow the latest international recommendations and developments of methods for calculating critical loads based on biodiversity endpoints. The set of indicators and criteria used can be linked to relevant objectives, and the calculations are largely based on data. The methods are, however, still under development and subject to different sources of uncertainty, and there is no scientific consensus or authoritative recommendations concerning the choice of indicators and criteria. The calculations must, therefore, provisionally be seen as a supplement to previously used methods (Ministry of Environment Forest and Nature Agency, 2003). The findings support earlier recommendations that the use of empirically based critical loads should be based on the low end of the range, unless specific assessment supported by local data allows the use of a higher value.

Research, development, and data needs

Because of the large time delay between exposures and changes in soil chemistry, plant abundance and soil structure, it will in practice be necessary to use models to make predictions or assessments that have a time horizon beyond a few years.

However, the problem in relation to the use of these modeling techniques is that only a very limited set of physical / chemical parameters has been included in monitoring programs for terrestrial nature. The model system used can undoubtedly be improved by involving more of the body of data from the NOVANA program and older data, e.g. from the DANVEG database. This will, however, require further analysis and development.

In the absence of historical data, the credibility of modeled scenarios could best be supported by comparison with observed trends. It is, therefore, necessary to involve a relatively responsive indicator of nitrogen status in the monitoring. Generally measurements are more difficult / expensive / more uncertain the farther one get from exposure to effects. Deposition indicators, such as N concentrations in mosses and lichens, can be relatively cheap and robust, while indicators, such as changes in fluxes, can be expensive and difficult to determine. It should also be kept in mind that the models do not describe the delay in biological effects. One possibility is a chemical indicator that reacts faster than C/N and not only responds after the system is saturated with nitrogen. One option could be mineralizable nitrogen, which has been tested in the UK monitoring program.