DOI: 10.5285/c37cb567-6afc-4cf8-b0bc-8ec054346a6f
title: Critical load/level linkages for interest features at UK protected sites

description: This dataset contains the linkages between designated features and their relevant critical loads or levels. The designated features come from the UK network of protected nature sites including:
 (i)	Special Areas of Conservation (SAC)
(ii)	Special Protection Areas (SPA)
(iii)	Sites of Special Scientific Interest (SSSI) - England, Scotland, Wales
(iv)   Areas of Special Scientific Interest (ASSI) - Northern Ireland.

Critical loads and levels are set under the auspices of the United Nations Economic Commission for Europe (UNECE) Convention on Long-Range Transboundary Air Pollution. 

For nutrient nitrogen critical loads are based on empirical evidence, mainly observations from experiments and targeted gradient studies. These empirical critical loads are assigned to habitat classes of the European Nature Information System (EUNIS) to enable consistency of habitat terminology and understanding across Europe.  In order to assign the relevant critical load to Annex I features, A/SSSI habitat features, or habitats of Annex II/SPA features, habitat correspondence tables are used to determine the relationship between the EUNIS classes for which nitrogen critical loads are set and the interest features.

Critical loads of acidity are based on soil and habitat types.  They are set for six Broad Habitats; acid grassland, calcareous grassland, dwarf shrub heath, bogs, montane, unmanaged coniferous and broadleaved woodland.

Critical Levels for air pollutants are not habitat specific and have been set to cover broad vegetation types (e.g. forest arable, semi-natural), often with critical values set for sensitive lichens and bryophytes. They have been derived from experiments and observation that show varied effects on vegetation including visible injury symptoms of exposure and species composition changes in semi-natural vegetation.

description: To be able to estimate the impacts of air pollutants on designated sites, interest features need to be assigned the most relevant critical load or critical level. Site-relevant critical loads and levels is the name given to the process of allocating the most relevant critical load or level to every interest feature at a designated site (SAC, SPA or A/SSSI). 

To provide relevant critical loads and levels for interest features (habitats) links were made using the European Nature Information System (EUNIS) where possible. Critical load and level values are set out in the 'Manual on methodologies and criteria for Modelling and Mapping Critical Loads & Levels and Air Pollution Effects, Risks and Trends (CLRTAP, 2004 to 2016). 

Empirical critical loads for nutrient nitrogen are assigned to a habitat using habitat correspondence tables to determine the relationship between the EUNIS classes for which nitrogen critical loads are set and the interest features. Justifications were provided for each match between an interest feature and its relevant critical load.

Critical loads of acidity are based on soil and habitat types and were based on six Broad Habitats; acid grassland, calcareous grassland, dwarf shrub heath, bogs, montane, unmanaged coniferous and broadleaved woodland (UK Biodiversity Action Plan, 1994).

For species, relevant critical loads were applied to the species' habitat and not to the species itself. For example, for SPA bird features an assessment was made on the sensitivity of the habitat and how that would affect the viability of the breeding, feeding or roosting of that species.

Critical level values are not habitat specific, but have been set to cover broad vegetation types (e.g. forest arable, semi-natural), often with critical values set for sensitive lichens and bryophytes as in the case of ammonia. Linkages were made between habitat and species where possible to provide site relevant critical levels for ammonia, nitrogen dioxide, and sulphur dioxide. 

This work falls under the Air Pollution Information System (APIS) and is jointly funded between the Centre for Ecology & Hydrology and the UK pollution and conservation agencies including Natural Resources Wales (NRW), the Environment Agency, Northern Ireland Environment Agency, Natural England, the Joint Nature Conservation Committee (JNCC), Scotland and Northern Ireland Forum for Environmental Research (SNIFFER), the Scottish Environment Protection Agency (SEPA), and Scottish Natural Heritage (SNH).