Checklist to the <i>Elatostema</i> (Urticaceae) of Vietnam including 19 new records, ten new combinations, two new names and four new synonyms

<i>Elatostema</i> (Urticaceae) comprises several hundred herbaceous species distributed in tropical and subtropical Africa, Asia, Australia and Oceania. The greatest species richness occurs on limestone karst in Southeast Asia. Taxonomic revisions of <i>Elatostema</i> are largely out of date and contradict each other with respect to the delimitation of <i>Elatostema</i> and <i>Pellionia.</i> Most herbaria in SE Asia and worldwide contain significant amounts of unidentified material. As part of a broader revision of <i>Elatostema</i> in SE Asia, we present an updated checklist for Vietnam based on field visits, a review of specimens in herbaria worldwide, a review of type material and nomenclature. We recognize 77 taxa (75 species and two infraspecific taxa) of <i>Elatostema</i> in Vietnam, 23 of which were previously ascribed to <i>Pellionia</i>. Nineteen of these are new records for the country, i.e., <i>E. attenuatoides</i>, <i>E. austrosinense</i>, <i>E. backeri</i>, <i>E. brunneinerve</i>, <i>E. crassiusculum</i>, <i>E. crenatum</i>, <i>E. fengshanense</i>, <i>E. glochidioides</i>, <i>E. malacotrichum</i>, <i>E. nanchuanense</i>, <i>E. oblongifolium</i>, <i>E. obtusum</i>, <i>E. oppositum</i>, <i>E. pergameneum</i>, <i>E. prunifolium</i>, <i>E. pseudolongipes</i>, <i>E. pycnodontum</i>, <i>E. salvinioides</i> and <i>E. xichouense</i>. We place <i>E. baviensis</i> in synonymy of <i>E. platyphyllum</i>, <i>E. colaniae</i> in synonymy of <i>E. myrtillus</i>, <i>P. macroceras</i> in synonymy of <i>E. hookerianum</i>, and <i>P. tetramera</i> in synonymy of <i>E. dissectum</i> for the first time. Fourteen taxa (18% of all the recognized taxa) are endemic to Vietnam, which makes <i>Elatostema</i> one of the richest genera for endemic species in this country; this level of endemism is comparable to levels observed in Orchidaceae. Our checklist suggests that the highest diversity and endemism of <i>Elatostema</i> occurs in northern Vietnam, and that there is the greatest floristic similarity of northern Vietnam to SW China. The relationship among floristic regions is also investigated. We could find no records of <i>Elatostema</i> for 33 out of 63 provincial units of Vietnam, including all the southernmost provinces. We propose that further studies on the diversity of <i>Elatostema</i> in central and southern Vietnam are severely needed.