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HQThresholds: Establishing critical loads from threshold values for a habitat quality metric for the UK’s response to a data request under the Convention on Long-range Transboundary Air Pollution. - AQ0840

Under the Convention on Long Range Transboundary Air Pollution (CLRTAP), the UK will be requested by the Coordination Centre for Effects (CCE) to respond to a “Call for Data” by March 2015. The HQThresholds project (AQxxxx) will prepare for this response, developing Critical Load (CL) functions which describe the maximum nitrogen and sulphur deposition compatible with long-term protection of habitats. These functions will be based on acceptable limits for biodiversity, defined using a Habitat Quality Index (HQI) developed in Defra project AQ0828. Habitat specialists from the Statutory Nature Conservation Bodies (SNCBs) were consulted, and agreement was reached that the basis for a biodiversity indicator should be habitat suitability for positive indicator-species. UK results were presented at the 24th CCE Workshop in April 2014, and were instrumental in the decision by the CCE to request all Signatory Parties to provide data based on biodiversity indicator of this type.
Threshold HQI values will be developed, below which a habitat should be considered damaged. Clearly there is more to biodiversity assessment than a binary choice (undamaged versus damaged) on a single axis, but this approach is necessary if biodiversity responses are to be explicitly included in CL modelling and integrated assessment of air pollution impacts. The HQI metric was selected as most clearly related to overall assessments of the quality of habitat examples by the specialists. Parallel approaches will be taken to determine the most appropriate threshold values. Firstly, HQI will be calculated for sites already judged to be in favourable or unfavourable condition and for which species composition data are available. If HQI values are distinct for these groups, this will provide a good foundation for a threshold. Secondly, HQI will be calculated for sites with deposition set to the empirical CLs for acidity and nutrient-N. The greater of the two resulting HQI values can also be seen as a good estimate for the threshold. Threshold HQI values obtained using these two approaches will be assessed and a judgement made as to the most suitable value to use. The approach will be illustrated using examples, and SNCB habitat specialists will be asked to review the threshold values chosen.
The advantage of using threshold HQI values then becomes clear. While empirical critical loads are the best available basis for assessing whether a site will become damaged in the long-term, they are less easy to apply to dynamic situations. For example, how long will a site take to recover when deposition is reduced after the CL was exceeded for many years? Dynamic modelling allows such changes to be simulated, and has been successfully applied, e.g. using pH thresholds, to determine target loads for recovery from acidification. Thresholds for a biodiversity metric will enable direct assessment of the impact of pollution scenarios on policy targets such as “no net loss of biodiversity”. The UK response to the Call for Data enabled by this project will be a major contribution to understanding of air pollution impacts and appropriate policy responses.
1. To collate site data held by the Statutory Nature Conservation Bodies (SNCBs), where habitat composition has been assessed concurrently with habitat condition assessment.
2. To assess the relationship between habitat condition status and values of the Habitat Quality (HQ) metric operationalised in the AQ0832 project.
3. To establish threshold values for the HQ metric, such that a habitat may be considered damaged below this value and in favourable / recovered condition above it, using evidence from the above site data together with the expert judgement of SNCB habitat specialists.
4. To select habitats and example sites to illustrate the approach, discuss this selection with Defra, and finalise a list of sites by 31st November 2014.
5. To set up the MADOC-MultiMOVE soil-vegetation model for these sites, by calibrating to current conditions as indicated by abiotic measurements and/or the floristic composition.
6. To run the model forward to equilibrium under a range of S and N deposition rates for these sites, and determine Critical Load functions based on the S and N deposition rates that maintain the HQ metric above the threshold value.
7. To assist the UK National Focal Centre to make a first submission to CCE in the Call for Data template, on 6th March 2015.
8. To describe and justify the approach in a 20-page report to Defra, with a draft to be submitted on 25th March 2015 and a final version on 24th April 2015.
9. To further publicise and disseminate the project and its conclusions.

Interdependencies are illustrated using a flow chart (see attached document "AQxxx HQThresholds - dependencies, Gantt chart & pie chart.docx").
Project Documents
• FRP - Final Report : AQ0840 Final report - Deriving critical load functions for nitrogen and sulphur using threshold values of a habitat quality metric   (1393k)
Time-Scale and Cost
From: 2014

To: 2015

Cost: £34,840
Contractor / Funded Organisations
Centre For Ecology and Hydrology (CEH)
Fields of Study
Air Quality