Editing Askham Bog (section)

==Ecology and habitats==
[[File:Askham Bogs diverse habitats (geograph 2061172).jpg|thumb|Habitat in Askham Bog]]
Askham Bog is regarded as one of the most ecologically diverse sites in Northern England;<ref>Usher M.&nbsp;B. 1979. Changes in the species-area relations of higher plants on nature reserves. ''Journal of Applied Ecology'' '''16''':213–215.</ref> especially regarding plants, invertebrates and birds.<ref name = "FitterSmith"/> The ecological diversity is due to a wide range of habitats ranging from [[fen-meadow]] with base-rich soils on the periphery to [[carr (landform)|carr]] woodland and raised bog with acid soil in the centre, interspersed with [[ditch|dykes]] and stands of open water. The large habitat diversity may be partly explained by the many successional stages present which generally become more advanced towards the centre, in turn related to spatial differences in [[topography]] and [[soil pH]].  Askham Bog is one of the few northern examples in England to show such an array of habitat transitions.<ref name = "FitterSmith"/>

Although the raised bog originally largely replaced the original fen, medieval peat cutting brought the vegetation back under the influence of base-rich groundwater. This extensive peat cutting, in turn, led to a return to fen conditions around some of the previously acid soils.<ref name = "LP">LP Authority. 1984. Askham Bog Status – Natural England’s SSSI information. https://designatedsites.naturalengland.org.uk/PDFsForWeb/Citation/1000196.pdf</ref>

Askham Bog is a heavily wooded area, with [[birch]] being widespread throughout. [[Willow]] also grows extensively throughout.<ref name = "Fitter1980">Fitter A.&nbsp;H., Browne J., Dixon T., Tucker J.&nbsp;J. 1980. Ecological studies at Askham Bog Nature Reserve 1. Interrelations of vegetation and environment. ''Naturalist''.</ref> [[Alnus glutinosa|Alder]] and [[Salix cinerea|grey sallow]] are locally dominant on the fen margins, while [[Quercus robur|oak]] is dominant in the acid centre. Other tree species growing alongside oak in the acid centre include [[Corylus avellana|hazel]], [[Frangula alnus|alder buckthorn]] and [[Sorbus aucuparia|rowan]]. Birch has declined in favour of oak, alder and ash,<ref name = "Pogg">Pogg P., Squires P., Fitter A.&nbsp;H. 1995. Acidification, nitrogen deposition and rapid vegetational change in a small valley mire in Yorkshire. Biological Conservation 71: 143–153.</ref> especially where much of the wood in the north central part has been clear-felled.<ref name = "FitterSmith"/> The reduction in canopy cover in the centre of the bog appears to have had a negative effect on peat growth here, probably through reduced shading and hence increased desiccation of the peat and easier influx of atmospheric [[pollutants]].<ref name = "Pogg"/> In recent decades, sulphur deposition on the bog has increased due to polluted air moving downwind from a large industrial area with coal-fired power stations.<ref name = "Pogg"/>