Interest features and linked critical load/level values for UK protected sites

This dataset contains the critical load and level values for designated habitats and species (where there is an appropriate match) at protected sites across the UK. The data is available for designated interest features for the following designations:
(i) Special Areas of Conservation (SAC)
(ii) Special Protection Areas (SPA)
(iii) Sites of Special Scientific Interest (SSSI) - England only.

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 the assign the relevant critical load to Annex I features, 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.

本数据集涵盖英国全境各保护地内经合理匹配的指定生境与物种的临界负荷及临界水平数值。本次数据适用于以下指定保护类别对应的目标保护特征：
(i) 特殊保护区（Special Areas of Conservation，SAC）
(ii) 特殊保护地（Special Protection Areas，SPA）
(iii) 具有特殊科学价值的地点（Sites of Special Scientific Interest，SSSI）——仅覆盖英格兰地区。

临界负荷与临界水平的制定依据联合国欧洲经济委员会（United Nations Economic Commission for Europe，UNECE）《远距离跨界空气污染公约》开展。

针对营养态氮的临界负荷，其制定基于实证证据，主要来源于实验观测与针对性梯度研究。这类实证临界负荷被归类至欧洲自然信息系统（European Nature Information System，EUNIS）的生境类别中，以保障全欧洲范围内生境术语使用与认知的统一性。为将相关临界负荷匹配至附件I保护特征、具有特殊科学价值的地点（SSSI）生境特征，或附件II/特殊保护地（SPA）的生境特征，研究人员借助生境对应表，明确了已设定氮临界负荷的EUNIS类别与目标保护特征之间的对应关系。

酸性物质的临界负荷则基于土壤与生境类型制定，共涵盖6类宽泛生境：酸性草原、钙质草原、矮灌丛荒地、沼泽、山地生境以及非经营针叶林与阔叶林。

空气污染物的临界水平并非针对特定生境，其设定覆盖了宽泛的植被类型（如森林、农田、半自然植被），通常还为敏感地衣与苔藓类植物设定了临界值。这类临界水平源自实验与观测研究，这些研究揭示了空气污染对植被的多样影响，包括暴露后的可见损伤症状以及半自然植被的物种组成变化。