Abstract
Elatostema (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 Taxonomic revisions of Elatostema are largely out of date and contradict each other with respect to the delimitation of Elatostema and Pellionia. Most herbaria in SE Asia and worldwide contain significant amounts of unidentified material. As part of a broader revision of Elatostema 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 Elatostema in Vietnam, 23 of which were previously ascribed to Pellionia. Nineteen of these are new records for the country, i.e., E. attenuatoides, E. austrosinense, E. backeri, E. brunneinerve, E. crassiusculum, E. crenatum, E. fengshanense, E. glochidioides, E. malacotrichum, E. nanchuanense, E. oblongifolium, E. obtusum, E. oppositum, E. pergameneum, E. prunifolium, E. pseudolongipes, E. pycnodontum, E. salvinioides and E. xichouense. We place E. baviensis in synonymy of E. platyphyllum, E. colaniae in synonymy of E. myrtillus, P. macroceras in synonymy of E. hookerianum, and P. tetramera in synonymy of E. dissectum for the first time. Fourteen taxa (18% of all the recognized taxa) are endemic to Vietnam, which makes Elatostema 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 Elatostema 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 Elatostema for 33 out of 63 provincial units of Vietnam, including all the southernmost provinces. We propose that further studies on the diversity of Elatostema in central and southern Vietnam are severely needed.
Highlights
Elatostema JR Forst. & G Forst. (Urticaceae) comprises several hundred species of succulent herbs and subshrubs that grow in shade in forests, gorges, stream sides and caves (Fu et al, 2017; Monro et al, 2018)
We document 77 taxa of Elatostema in Vietnam, 14 taxa (18%) of which are endemic to the country
We found 19 new records of Elatostema for Vietnam, i.e., E. attenuatoides W.T.Wang, E. austrosinense Y.H.Tseng & A.K.Monro nom. nov., E. backeri H.Schroet., E. brunneinerve W.T.Wang, E. crassiusculum W.T.Wang, E. crenatum W.T.Wang, E. fengshanense W.T.Wang & Y.G.Wei, E. glochidioides W.T.Wang, E. malacotrichum W.T.Wang & Y.G.Wei, E. nanchuanense W.T.Wang, E. oblongifolium Fu, E. obtusum Wedd., E. oppositum Q.Lin & Y.M.Shui, E. pergameneum W.T.Wang, E. prunifolium W.T.Wang, E. pseudolongipes W.T.Wang & Y.G.Wei, E. pycnodontum W.T.Wang, E. salvinioides W.T.Wang, and E. xichouense W.T.Wang
Summary
Elatostema JR Forst. & G Forst. (Urticaceae) comprises several hundred species of succulent herbs and subshrubs that grow in shade in forests, gorges, stream sides and caves (Fu et al, 2017; Monro et al, 2018). Elatostema is distributed throughout tropical and subtropical Africa, Madagascar, Asia, Australia and Oceania, with highest diversity on limestone karst in China and Southeast Asia (Yahara, 1984; Lahav-Ginott & Cronk, 1993; Wang & Chen, 1995; Lin, Friis & Wilmot-Dear, 2003; Wang, 2014). The previous studies suggest that this diversity has been driven by the challenges of colonising karst substrates, which once overcome leads to species radiations (Chung et al, 2014; Fu et al, 2017), and that past temperature fluctuations and East Asian monsoons have driven the rates of plant diversification in karst ecosystems by accelerating the rate of karstification (Kong et al, 2017)
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