Orchids in the torrent: on the circumscription, conservation and rheophytic habit of E pipactis flava

The systematically difficult genus E pipactis ( O rchidaceae) is often subdivided into two sections: E pipactis and A rthrochilium . 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 A sia ( E . flava , E . atromarginata ), based on morphological examination of live plants in situ in northern T hailand and of herbarium specimens from the entire range of the complex ( T hailand, L aos and V ietnam). 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 T hailand 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 O rchidaceae). 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 S teenis). Combining taxonomic, distributional and ecological data, we discuss the occurrence of this stream‐inhabiting E pipactis in a conservation context. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society , 2013, 172 , 358–370.