Rare ferns re-visited

November 6th 2018

After the hunt for the white-stalked puffball on 22nd August 2018 (‘Counting the white-stalked puffball’), we took ourselves in the afternoon about 1km north along the Stronchrubie cliffs to check out two of the rarest ferns in Scotland.

Both were originally discovered by Alan Stirling, an Ayrshire botanist with a love for the north-west.  In 1951 he came across a stand of the limestone fern or limestone oak fern Gymnocarpium robertianum. This wasat the base of the Creag Sron Chrubaidh (NC250202) and the first time it had been found in Scotland.  It resembles the much commoner oak fern G. dryopteris, but is less markedly tripartite and has a glandular grey-green frond.  It is widespread on limestone in England and Wales, but has only been reported since from two other localities in Scotland, one in Easter Ross (where it has not been seen since 1980) and the other in mid-Perthshire.

The Assynt stand was last visited on 29thAugust 2009, during a field meeting of the British Pteridological Society.  We were able to confirm that it is flourishing, right at the foot of the crag, in a dense patch covering some ten square metres. We did manage to make a photographic record, albeit in deteriorating weather.

The limestone fern occurs, in smaller quantity, lower down the steep slope below the crag, in limestone scree.  Here it is accompanied by the alpine hybrid shield-fern Polystichum x illyricum, whose parents, holly fern P. lonchitis and hard shield-fern P. aculeatum, are both plentiful nearby.  The hybrid can be quite variable, so is easiest to recognise in the presence of its parents.  All are protected from grazing by the unstable nature of the scree.

Alan Stirling found this hybrid over two decades later, in 1973. It is even rarer than the limestone fern, with just one other locality in Assynt, an exclosure behind Inchnadamph (NC2619) where four of us found it on 3rdJuly 1992, four localities in Perthshire and two in Ireland.

Ian Evans and Gwen Richards

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