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Keywords:

  • adaptation;
  • distribution;
  • ecology;
  • Epipactis atromarginata;
  • rarity;
  • taxonomy;
  • typification

The systematically difficult genus Epipactis (Orchidaceae) is often subdivided into two sections: Epipactis and Arthrochilium. Until now, the latter has attracted much less taxonomic attention than the former, but here we reassess the alleged distinction of two rare and nationally endangered taxa from tropical Asia (E. flava, E. atromarginata), based on morphological examination of live plants in situ in northern Thailand and of herbarium specimens from the entire range of the complex (Thailand, Laos and Vietnam). As the variation in vegetative and floral dimensions, flower colour and labellum morphology broadly overlaps between the two taxa, we merge them under the oldest valid name at species level, E. flava. Geomorphological features of our three study sites in Thailand and high shared occurrence of previously confirmed rheophytes among the accompanying species indicate that E. flava is itself rheophytic (a very rare feature in the Orchidaceae). Possible adaptations of E. flava to its rheophytic lifestyle are discussed. Among these, the formation of pure stands through dense clonal growth involving a creeping rhizome places E. flava as a ‘mat-rooted rheophytic landplant’ (sensu van Steenis). Combining taxonomic, distributional and ecological data, we discuss the occurrence of this stream-inhabiting Epipactis in a conservation context. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 172, 358–370.