Diphasiastrum ×issleri (Lycopodiaceae) in England and Wales

Abstract

Diphasiastrum ×issleri (Rouy) Holub has been much misunderstood and greatly confused in the British Isles, the consequence of both nomenclatural and practical identification issues. Following the clarification of its hybridogenous origin(s) and the parent taxa involved (which had been the source of dispute between European and American authors) the taxonomic treatment of this plant has seen various approaches adopted, some of which have regrettably led to mis-recording and a lack of clarity. In parallel to the taxonomic and nomenclatural issues are the very real difficulties posed by the differentiation of this taxon from its progenitors, a task made difficult by their morphological plasticity, the possibilities of introgression and the formation of triploid as well as diploid primary hybrids. Here we consider the appropriate treatment of this taxon: nomenclaturally as species vs. hybrid and from a conservation viewpoint. We elaborate and re-assess all of its English and Welsh records. As a consequence of this review a revised conservation status is required. We conclude that all Welsh and most English records of this taxon are erroneous, including those for the Northumbrian sites and that therefore it is currently Regionally Extinct (RE) in England and not CR (D) as given in the Red-List for England (as D. complanatum). It, however, remains NT (D) at a GB level because of the presence of the taxon in Scotland. Guidance for the discrimination of D. ×issleri from atypical forms of D. alpinum is given.