Following Gagnepain (1942), the genus Rhaphidophora Hassk. was represented in Indochina by 9 species. With the discovery of R. megaphylla in 1998 (Nguyen 1998), the checklist of plant species of Vietnam (Pham 2000, Nguyen 2005), now documents 10 species of Rhaphidophora. While carrying out fieldwork in Lac Duong (Lam Dong prov.) and Cam Lam (Khánh Hòa prov.), Vietnam, in 2013, the first author and his group collected some interesting specimens of Rhaphidophora that did not match any known species by having a unique feature in a strongly concave stylar region. After a thorough scrutiny of the relevant literature and herbarium specimens, the specimens were determined to represent an unnamed species; described here as Rhaphidophora lacduongensis V.D.Nguyen & B.H.Quang.
Rhaphidophora lacduongensis resembles R. megaphylla by having a large (18-30 cm wide) entire leaf blade and straight peduncle. Rhaphidophora lacduongensis differs from R. megaphylla by its much smaller spadix (size c. 9 by c. 1.7 cm), a pistil with the surface of stylar region concave (not pointed), and the leaf blade base rounded, not cordate.
Type: Vietnam, Lam Dong, Lac Duong, Da Chais, nearby the road from Nha Trang to Da Lat, N12°7'45.86" E108°42'29.18', altitude c. 1700 m, 15 Aug. 2013, Nguyen Van Du & Bui Van Huong 721 (holotype HN; isotype HN, L).
Etymology: The specific epithet refers to Lac Duong district of Lam Dong province, Vietnam, the type locality.
Vernacular name: Trâm đài lạc dương (in Vietnamese).
Distribution & Habitat: Known only from Lam Dong and Khanh Hoa Province, Vietnam. Rhaphidophora lacduongensis climbs on medium height trees of Lauraceae in evergreen montane forest at 1500-1700 m altitude.
Phenology: Flowering (possibly) from March to May; fruiting from May to August.
Conservation status: The species occurs on two sites that are less than 40 km distant, giving an extent of occurrence of less than 100 km. Both sites are very restricted areas (each less than 10 km2). In Lac Duong (Lam Dong prov.), the plant grows nearby the road from Khanh Hoa to Da Lat, and would be threatened by any road construction programme. In Hon Ba Nature Reserve (Khanh Hoa prov.), the plant is very rare (fewer than 50 mature individuals observed). It is vulnerable to anthropogenic threats such as forest fire, forest clearance, etc. According to IUCN (2012) criteria, R. lacduongensis is provisionally categorized as VU B1,2 D2 (Vulnerable).
Rhaphidophora lacduongensis V.D. Nguyen & B.H. Quang
Other specimens examined: Vietnam, Khanh Hoa, Cam Lam, Hon Ba Nature Reserve, N12°07'04.00" E108°56'46.00", altitude c. 1520 m, 10 Aug. 2013, Nguyen Van Du & Bui Van Huong 550 (HN).