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Ferns have particular association with man made habitats. Page (1998) has a chapter on the different types of man made structures that ferns make their home, from canal locks and railway ballast to castles and harbour walls.

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Where I live in South Wales fern communities are commonly found on stone and brick walls. They are particularly associated with garden and boundary walls with older types of mortar (cinders or coal dust seem to have been used as an aggregate) possibly predating modern cements. The ferns are not found on structures made with modern cements including recent walls, recently pointed old walls or houses (the latter are generally rendered or pebble dashed).

The two common ferns found on these walls are Asplenium trichomanes and Asplenium ruta-muraria, often in association with various mosses. Together these may form an identifiable plant community under the National Vegetation Classification as OV39 Asplenium trichomanes-A ruta-muraria community.

‘Both A trichomanes and A ruta-muraria, as well as the less common Ceterach, are strongly calcicolous ferns which gain a hold in crevices where other colonisers are limited by the extreme environmental conditions.'(Rodwell, ed 2000 p453)

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