Introduction

Myanmar is the largest country in continental Southeast Asia, spanning c.20 degrees of latitude and encompassing 14 terrestrial ecoregions (Olson et al., Reference Olson, Dinerstein, Wikramanayake, Burgess, Powell, Underwood, D’Amico, Itoua, Strand, Morrison, Loucks, Allnutt, Ricketts, Kura, Lamoreux, Wettengel, Hedao and Kassem2001), resulting in a great diversity of habitats and an equally rich flora. Although new species continue to be documented, particularly from northern Kachin State (e.g. Yang et al., Reference Yang, Ding, Zhou, Zhu, Li, Maw, Tan, X.-H., Y.-M., Y.-H. and M.2017; Ding et al., Reference Ding, Bin, Zhou, Li, Maw, Kyaw, Tan, X.-H., Y.-M., Y.-H. and M.2018; Tan et al., Reference Tan, Li, Zhou, Chen, Bramley, Li, X.-H., Y.-M., Y.-H. and M.2018), the flora is still relatively poorly known (Hundley & Chit Ko Ko, Reference Hundley and Chit Ko Ko1961; Frodin, Reference Frodin2001). This research is part of an ongoing collaboration between the New York Botanical Garden, the Royal Botanic Garden Edinburgh and the Myanmar Forest Research Institute to document the flora of northern Myanmar.

During a 2001 expedition by the third author, a new lanceolate-leaved Begonia species was noticed (but not collected) growing in a seasonal stream bed between Sinlumdan and Nam Ti, on the Babulongtan mountain trail (at c.27.41°N, 97.62°E). Five subsequent expeditions were made to the region, but this species was not found again until it was collected in approximately the same locality during a June 2017 expedition to the buffer zone of Hkakaborazi National Park (Armstrong 3041 [E, K, NY]). This specimen matched six previous collections from the same area (Toppin 4264 [K]; Kingdon-Ward 5561 [E], 6633 [K], 7355 [K], 9067 [BM(2), NY], 13567 [BM]), some of which were listed as doubtful records of Begonia rhoephila Ridl. from Myanmar in Hughes (Reference Hughes2008), where it is stated, “There are no records of B. rhoephila between northern Myanmar and Selangor in Peninsular Malaysia, to which Kiew (Reference Kiew2005) records it as endemic. Whilst there is a gross similarity of the Burmese specimens to B. rhoephila, they differ in having longer petioles, longer internodes, the occasional appearance of caulescent stems and the presence of curled, fleshy hairs on the veins on the underside of the leaves”.

Since the first checklist for Myanmar listed 57 species of Begonia (Hughes, Reference Hughes2008), several new species and records have been published: B. difformis (Irmsch.) W.C.Leong, C.-I Peng & K.-F.Chung (Leong et al., Reference Leong, Deng, Sun, Peng and Chung2015), B. discreta Craib (Kang et al., Reference Kang, Kyaw, Jung, Shin, Kim and Ong2018), B. foveolata Irmsch. (omitted from Hughes, Reference Hughes2008, in error, as the type is from Yangon [‘Rangoon’]) (Irmscher, Reference Irmscher1959), B. hayamiana Nob. Tanaka (Tanaka & Hughes, Reference Tanaka and Hughes2007), B. hymenophylla Gagnep. (Hughes et al., Reference Hughes, Girmansyah and Ardi2015–), B. kachinensis Nob. Tanaka (Tanaka & Hayami, Reference Tanaka and Hayami2011), B. lushaiensis C.E.C.Fisch. (reinstated from synonymy with B. modestiflora Kurz by Camfield & Hughes, Reference Camfield and Hughes2018), B. macrotoma Irmsch. (Hughes et al., Reference Hughes, Girmansyah and Ardi2015–), B. mariachristinae Wahlsteen (Wahlsteen, Reference Wahlsteen2018), B. minicarpa H.Hara (Hughes et al., Reference Hughes, Girmansyah and Ardi2015–), B. myanmarica C.-I Peng & Y.D.Kim (Tseng et al., Reference Tseng, Kim, Peng, Htwe, Cho, Kono and Chung2017), B. siamensis Gagnep., B. soluta Craib (Kang et al., Reference Kang, Kyaw, Jung, Shin, Kim and Ong2018), B. tenasserimensis Phutthai & M.Hughes (Phutthai & Hughes, Reference Phutthai and Hughes2017), B. togashii Nob. Tanaka & C.-I Peng (Tanaka & Peng, Reference Tanaka and Peng2016) and B. wui-senioris C.-I Peng (a valid name for the taxon B. lacei ined. (Peng et al., Reference Peng, Wang, Kono and Yang2014). The record for Begonia oreodoxa Chun & F.Chun ex C.Y.Wu & T.C.Ku from Myanmar in Hughes (Reference Hughes2008) is probably an error, and we also consider the record of B. incerta Craib in Kang et al. (Reference Kang, Kyaw, Jung, Shin, Kim and Ong2018) to be a misidentification (T. Phutthai, Mahidol University, personal communication). The specimens listed above do not match any of the species currently known from Myanmar, and they also differ from Begonia rhoephila from Malaysia; given the remarkable dispersal limitation and narrow endemism of most Begonia species (Hughes & Hollingsworth, Reference Hughes and Hollingsworth2008), the occurrence of this tropical species in the mountains of northern Myanmar is very unlikely.

Here we describe the new species Begonia rheophytica, bringing the total number of Begonia species known from Myanmar to 73 (listed in the Appendix). The new species belongs in Begonia sect. Platycentrum (Klotzsch) A.DC., as it has fruit with one enlarged wing, and anthers with extended connectives (Doorenbos et al., Reference Doorenbos, Sosef and De Wilde1998; Moonlight et al., Reference Moonlight, Ardi, Arroyo Padilla, Chung, Fuller, Girmansyah, Hollands, Jara-Muñoz, Kiew, Mahardika, Marasinghe, Leong, Liu, O’Connor, Peng, Pérez, Phutthai, Pullan, Rajbhandary, Reynel, Rubite, Sang, Scherberich, Shui, Tebbitt, Thomas, Wilson, Zaini and Hughes2018). We compare our specimens with two other species in Begonia sect. Platycentrum, namely B. caobangensis C.-I Peng & C.W.Lin from Vietnam (Peng et al., Reference Peng, Lin, Yang, Kono and Nguyen2015) and B. yingjiangensis S.H.Huang from Yunnan, China (Shui & Huang, Reference Shui and Huang1999), which have a similar leaf shape and a closer geographical range.

Taxonomic Treatment

Begonia rheophytica M.Hughes, sp. nov. § Platycentrum. – Type: Myanmar. Hills east of the Mali Hka, 2000–3000 ft, xii 1930, Kingdon-Ward 9067 (holo BM [BM000896328]; iso BM [BM000896327], NY [02652766]). Figs 1, 2.

Differs from Begonia caobangensis C.-I Peng & C.W.Lin in having narrower leaves (2–2.5 cm wide, not 3.9–7.3 cm) without winged petioles (versus petioles slightly winged), with the petioles having longer erect hairs (not minutely appressed tomentose to glabrous), and fruits without fleshy bristles at the apex of the wings.

Differs from Begonia rhoephila Ridl. in having shorter, curled hairs on the petioles (not erect, straight hairs), having glabrous bracts and stipules (not bracts with short hairs and stipules with hairs along the keel), and sometimes cauline leaves subtending the inflorescence (not inflorescence always arising directly from the rhizome).

Differs from Begonia yingjiangensis S.H.Huang in having smaller leaves (9–13 × 2–2.5 cm, not 22–29.5 × 5–7.5 cm), with a broadly denticulate margin (not subentire and slightly repand).

Rheophytic herb. Stem rhizomatous, c.5 mm wide when dry, internodes usually 2–5 mm long, sometimes extending to c.7 cm long on the apical flowering portion of the plant. Stipules 15 × 5 mm, glabrous, keeled, triangular, cuspidate, tip extending 2 mm. Leaves: petiole 4.5–10 cm long, slightly grooved or flattened adaxially, giving a D-shaped cross-section, c.1.5 mm wide when dry, with sparse to dense short, thin curly hairs on the abaxial side of the petiole, the adaxial side being glabrous; lamina coriaceous (‘rather thick and leathery’, Ward 5561), 9–13 × 2–2.5 cm, elongate lanceolate, symmetrical, base cuneate, tip acute, glabrous above, with short, thin curly hairs on the main veins only beneath, venation pinnate with c.8 pairs of alternate veins, margin broadly denticulate. Inflorescence axillary, cymose, usually arising from the rhizome, sometimes arising from the axil of an elongated apical portion of the stem and possessing cauline leaves, primary peduncle (5–)10–14 cm long, secondary up to 5 cm long, with up to c.7 flowers; bracts glabrous: first pair subtending the first branching of the inflorescence fused together forming a cup c.10 mm long, subsequent bracts not fused, ovate-lanceolate, c.6 mm long, smaller towards the terminal dichotomy. Male flowers: pedicel c.10 mm long, glabrous; tepals 4, glabrous, white or white suffused with pink, outer pair c.15 × 9 mm, inner pair c.12 × 5 mm; androecium with c.100 stamens on a short column, filaments c.1.5 mm long, anther c.1.5 mm long, oblong, connective extended, rounded in basal anthers and acute in apical anthers, slits lateral, extending most of the length of the anther. Female flowers unknown. Fruit borne in pairs, 2-locular, recurved, glabrous, upper wings rectangular, c.5 × 9 mm, forming a splash cup, lower wing rounded triangular, c.11 × 8 mm.

Fig. 1.

Begonia rheophytica M.Hughes, sp. nov. A, Habit in the field (note water level at the base of the image); B, abaxial surface of petiole, showing short curled hairs; C, inflorescence; D, adaxial (left) and abaxial (right) leaf surface; E, lamina margin; F, unripe fruit. A–E, Armstrong et al. 3041; F, Kingdon-Ward 13567. All scale bars, 1 cm.

Fig. 2.

Distribution of Begonia rheophytica M.Hughes (star) in northern Myanmar, with adjacent towns highlighted for reference (circles).

Additional specimens examined. Myanmar. Kachin State: Putao District, Machanbaw Township, buffer zone of Hkakaborazi National Park, along trail between Nam Ti and Sinlundam, 798 m, 20 vi 2017, Armstrong 3041 with Thet Yu Nwe, Moe Myint Thu, San Naing Dee, Zaw Naing Tun, Hla Naing Htay, Pa Rang Gang Ken Sar (E, NY [02654022], RAF); Kachin Hills, Toppin 4264 (K); Nam Ti, 16 xi 1922, Kingdon-Ward 5561 (E [00300496]); Nam Ti, 2000 ft, 18 iv 1926, Kingdon-Ward 6633 (K); Mountains E of Fort Hertz, 3000 ft, 9 ix 1926, Kingdon-Ward 7355 (K); Hills east of Putao, 3000 ft, 10 xii 1937, Kingdon-Ward 13567 (BM).

Distribution and ecology. Forms large colonies on boulders in streams, a narrow endemic to Nam Ti in the north of Kachin State, Myanmar, at c.600–1200 m elevation (Fig. 2). Kingdon-Ward noted (Kingdon-Ward 7355 [K]) that the plant is ‘found on wet moss covered rocks in jungle torrents. The plant is liable to be more or less submerged during the summer rise’; ‘In massive clumps on stones in perennial streams, now uncovered, but submerged in the rainy season. Only seen in two streams where is was fairly abundant’ (Kingdon-Ward 9067 [BM]).

Etymology. Named after the rheophytic habit of the species (rheo-, pertaining to flowing water [Greek]).

Proposed IUCN conservation status. The only known locality is currently outside the core of the Hkakaborazi National Park. The area is potentially vulnerable to encroachment in the next decade or so, as slash-and-burn agriculture moves further up the mountain. Therefore, we consider a placement in the Vulnerable category under VU D2 (IUCN, 2017) to be appropriate, although a successful outcome for the proposed UNESCO World Heritage Site status for the area would improve the outlook for the species and necessitate a reconsideration of this category.