Bulbophyllum romklaoense resembles B. muscarirubrum Seidenf. and B. triste Rchb.f. Bulbophyllum romklaoense differs from both by having 4–6-flowered inflorescences (vs. 10–24(–50)-flowered inflorescences in B. muscarirubrum and B. triste), falcate-subovate lateral sepals (vs. narrowly ovate lateral sepals in B. muscarirubrum and B. triste), petals with erose to fimbriate margins (vs. petals with ± entire margins in B. muscarirubrum and B. triste) and a lip with long cilia in the distal half on the lower surface (vs. lip entirely glabrous in B. muscarirubrum and B. triste). Bulbophyllum romklaoense also differs from B. triste by having a peduncle which is about as long as the rachis (vs. peduncle longer than twice as long as the rachis in B. triste).

Figure 1. 

Bulbophyllum romklaoense Pingyot & Thawara A habit B pseudobulb with inflorescence arising from the base C flower, front view D flower, side view (right petal and right lateral sepal removed) E floral bract F dorsal sepal G lateral sepals (flattened & indumentum removed) H petals J column, top part K lip L anther cap (from Inthakul N887-50). Drawn by T. Pingyot.

Thailand. Phitsanulok Province, Chat Trakan District, Ban Romklao Botanic Garden, ca. 1300 m a.s.l., 15 February 2008, Inthakul N887-50 (holotype QBG!, isotypes QBG! (2 sheets)).

Epiphyte with short rhizome and pseudobulbs close together. Pseudobulbs subglobose, surface slightly bullate, 10.5–25 mm in diameter, 2-leaved, pale green to purplish-green, covered with a thin and translucent-white sheath when young. Leaves shed at flowering time, narrowly ovate to oblong, 3.3–8 cm long, 0.7–1 cm wide, apex acute, base cuneate, thinly herbaceous, glabrous. Inflorescences arising from base of pseudobulb, ca. 2 cm long, prostrate, racemose, 4–6-flowered, flowers in the same inflorescence open simultaneously; peduncle 8–11 mm long, ca. 1 mm in diam., with one peduncle-scale; rachis ca. 10 mm long; floral bracts reddish, broadly lanceolate, 3.5–5.6 mm long, 1.5–2.3 mm wide, 3-veined, apex acuminate, margins entire. Flowers ca. 6 mm wide; ovary ca. 1.6 mm long, ca. 2 mm in diam., pedicel very short, inconspicuous. Sepals greenish-yellow with dense reddish-purple-brown dots especially in upper half; dorsal sepal broadly ovate, 3.7–4 mm long, 2.4–3 mm wide, apex acuminate, margins erose to fimbriate in upper half, 3-veined, adaxially papillose; lateral sepals connate in upper half along interior margins, forming a suborbicular blade in outline, individual sepals falcate-subovate, 6–6.5 mm long, 3.6–3.8 mm wide, 5-veined, adaxially sparsely ciliate in distal part, apex cuspidate, margins entire, outer margins decurved. Petals pale green with reddish-purple dots, ovate, 2.4–3 mm long, 1.7–2 mm wide, apex acuminate, margins erose to fimbriate, except near base, 1-veined, adaxially sparsely papillose and ciliate; lip white with reddish-purple dots and a large purple blotch on epichile, triangular, ca. 2 mm long, 1.3–1.5 mm wide, thickened, entire, adaxially with longitudinal ridges, with long cilia in distal half on lower surface. Column white with faint reddish-purple dots, ca. 1.5 mm long, ca. 1 mm wide, winged along lower margins; stelidia subulate, ca. 0.6 mm long, curved, pointing forwards; anther cap white, sometimes with purple marks, ca. 1 mm wide; pollinia 4; stigma concave, ca. 1 mm long. Fruit not seen.

Figure 2. 

Bulbophyllum romklaoense Pingyot & Thawara in vivo A habit (vegetative stage) B habit (flowering stage) C pseudobulb with inflorescence arising from the base D flower, front view E flowers, side view F lateral sepals. Photographed by P. Suksathan.

Epiphytic on oak trees (Lithocarpus spp.) in open evergreen broad-leaved lower montane forest, ca. 1300 m a.s.l. Fl. January–February.

Northern Thailand. This new species is currently known only from the type locality, which is located less than 7 km from the Lao PDR border. It is possible that this species occurs in Lao PDR or in other areas around the Phu Soi Dao Plateau (Fig. 3).

Figure 3. 

The distribution of Bulbophyllum romklaoense Pingyot & Thawara. The inset figure shows the position of this species on the complete map of Thailand.

Named after its type locality at Ban Romklao (Romklao Village).

This new species is known only from the type locality, situated in the protected area of BRBG. However, the Extent of Occurrence (EOO) and the Area of Occupancy (AOO) are less than 100 km² and 10 km², respectively. The number of mature individuals is less than 50. Moreover, its habitat is frequently threatened by forest fires and climatic changes, such as warmer and drier conditions that increase drought and extend the fire season. These factors have led to significant habitat destruction. Thus, this species is preliminarily assessed as Critically Endangered (CR; B1+B2ab(iii,v)+C2a(i)), based on current information and according to the IUCN Red List Categories and Criteria (IUCN 2022).

Thailand. Phitsanulok Province, Ban Romklao Botanic Garden, ca. 1300 m a.s.l., 15 February 2008, Inthakul N887-50 sub Suksathan 5476 (cultivated plant of the holotype (QBG)).

Vermeulen et al. (2014a) redefined Bulbophyllum sect. Lemniscata Pfitz. by including B. sect. Tripudianthes Seidenf. (except B. blepharistes Rchb.f.) and B. sect. Pleiophyllus J.J. Sm. Bulbophyllum romklaoense also belongs to section Lemniscata, characterised by its two-leaved pseudobulbs, deciduous leaves, elongate racemes, 4 pollinia and connate lateral sepals. This section contains ca. 37 species, mainly distributed in South and South-East Asia (Vermeulen et al. 2014a, 2021; Averyanov et al. 2019; Zhou et al. 2022; Nguyen et al. 2023). Currently, 26 species in this section are known from Thailand. According to Seidenfaden’s key (1979), B. romklaoense would belong to section Pleiophyllus by its 2-leaved pseudobulb and lateral sepals that are not much longer than the dorsal sepal.

Vermeulen et al. (2014b) synonymised Bulbophyllum tripaleum Seidenf. under B. dhaninivatii Seidenf. because the only differentiating character is the presence of palea on the sepal apices, but this character is considered to be variable. We also observed this variability in a population of B. dhaninivatii at Phu Luang in Loei Province (north-eastern Thailand). Therefore, B. tripaleum is here treated as a synonym of B. dhaninivatii and is excluded from the key.