HQ Thresholds: Establishing critical loads from theshold values

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. To determine the most appropriate threshold values, 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 will be assessed and a judgement made as to the most suitable value to use. The approach will be illustrated using examples.