Oberonia mahaeliyensis sp. nov. (Figs. 1, 2)

TYPE:— SRI LANKA, Central Province, Nuwara-Eliya District, Horton Plains National Park (N 6 47’ 56.901’’ E 80 49’ 28.661’’), 20 November 2018, PHTL20111801 (holotype PDA!; isotype PDA!: spirit)

Diagnosis: Oberonia mahaeliyensis is similar to O. wightiana distributed in Sri Lanka and southern India. Oberonia wightiana has greenish-yellow color flowers and three-lobed labellum with two orbicular lateral lobes and bifurcated midlobe with two diverging linear lobules. In contrast, O. mahaeliyensis with yellowish-white color flowers and three-lobed labellum with orbicular lateral lobes and quadrate shaped midlobe with distantly and irregularly dentate margin at the apex.

Description: Herbaceous epiphyte. Roots vermiform, 1.0– 1.2 mm thick, up to 10–15 cm long. Stem ca. 5‒10 mm long. Leaves jointed, 5–7, distichous, equitant, laterally compressed, 3.5–5.5 × 0.7–0.9 cm, oblong˗ensiform. Inflorescence many-flowered, drooping; peduncle 4–5 cm long, terete with few sterile bracts towards the apex; rachis 7–9 cm long, ca. 1 mm diameter; floral bract ca. 0.8 × 0.2 mm, acute to sub˗acute, minutely serrate. Flowers 1.2 × 2.1 mm across, widely opening, laxly placed in spiral, yellowish-white, opening progressively upwards. Sepals and petals curved backward; dissimilar, petals much narrower than sepals. Dorsal sepal 1.1 × 0.4 mm, oblong, obtuse to sub˗acute. Lateral sepals obtuse to sub˗acute, 1.2 × 0.6 mm. Petals 1.0 × 0.1 mm. Labellum 1.4 × 1.2 mm, giving a square-shaped look; three lobed labellum with orbicular lateral lobes and quadrate-shaped midlobe with distantly and irregularly dentate margin at the apex. Column short ca. 0.5 × 0.3 mm. Operculum pale yellow; 0.2 × 0.15 mm. Pollinia 4 in two pairs, individual pollinia of each pair unequal, pyriform, larger pollinia ca. 0.1 × 0.08 mm and the smaller ones ca. 0.08 × 0.06 mm. Pedicel and ovary 1.2 mm long. Mature fruit clavate, green in color, 1.7 × 2.4 mm.

Flowering: Flowering and fruiting were recorded in November and December.

Etymology: This species epithet ‘ mahaeliyensis ’ is derived from “Mahaeliya-thenna” (Great open plain), referring to the Horton Plains National Park before the colonial age, where the species was discovered. The specific name is an adjective from the geographical name.

Distribution: Oberonia mahaeliyensis is so far, known only from two localities in Horton Plains National Park, Sri Lanka.

Habitat ecology: Annual precipitation of Horton Plains National Park is 2150 mm (DWC 2007) and daytime temperature indicates a mean annual air temperature of 18.4 ± 3.4 ˚C (Ediriweera et al. 2008) and approximately consisting with 3160 hectares (DWC 2007) and the highest plateau of Sri Lanka between the elevation of 1500 and 2524 m a.s.l. Within the total area of the Horton Plains National Park, 24.5% (654 ha) of forested area, was identified as subjected to the die-back (Adikaram et al. 2006). The adjacent area of the type locality of O. mahaeliyensis was identified as a severely affected area by the canopy die-back (Fig. 3) and this incident in the Horton Plains National Park is one of the major threats to the fauna and flora of this remarkable ecosystem (Perera 1978, Werner 1982; 1988, Hoffmann 1988, Balasubramaniam et al. 1993, Adikaram et al. 1999; 2006, Ranasinghe et al. 2007; 2009, Ranawana 2014, Lakkana et al. 2022).

Oberonia mahaeliyensis was growing on moss-covered tree trunks of Symplocos elegans Thwaites (1860: 185) (Symplocaceae) and Glochidion pycnocarpum (Müller Argoviensis) Beddome (1873: 194) (Phyllanthaceae) with firmly attached roots to the bark. This populations grow in 2‒5 m above the ground level and was found to be associated with other epiphytic orchid species viz. Oberonia wightiana Lindley (1839: 14), Dendrobium heterocarpum (Wallich) Lindley (1830: 78), Cleisostomopsis filiformis (Rchb.f) R. Rice (2019; 184), Porpax braccata (Lindl.) Schuiteman, Y.P. Ng & H.A. Pedersen (2018: 199) and Pinalia bicolor (Lindley) Kuntze (1891: 679).

Conservation assessment: Approximately 25 individuals were observed constituting two separated small subpopulations (in die-backed forest patch along with Farr Inn–Ohiya road; 2159 m a.s.l. and along Farr Inn– Kirigalpotta trail; 2196 m a.s.l.) of Oberonia mahaeliyensis in a montane cloud forest of Horton Plains National Park, Central Highlands of Sri Lanka. The surrounding environment consists of disturbed forest canopy due to die-back and posing an immense threat of habitat loss and degradation in the quality of habitat for the species. Area of occupancy (AOO) and extent of occurrence (EOO) are less than 10 km 2. The two subpopulations constitute two locations as in presence of a threat both subpopulations may not be affected simultaneously. Hence, we assess this species as Critically Endangered (CR C2a(i); D) (IUCN 2019).

Specimens of other allied taxa examined: SRI LANKA. Oberonia wightiana:—NUWARA-ELIYA DISTRICT: Hakgala, Oct. 1960, Jayaweera 2022 (PDA!); Nuwara-Eliya, Westward-Ho, Nov. 1963, Jayaweera 2364 (PDA!); Hatton, Nov. 1959, Jayaweera 2023 (PDA!), Pidurutalagala, Oct. 1973, Sohmer, Jayasuriya & Eliezer 8409 (PDA!), Oct. 1974, Davidse & Sumithraarachchi 8019 (PDA!), Sept. 1969, C.F. & R.J. van Beusekom 1390 (PDA!). KANDY DISTRICT: Adam’s Peak, Dec. 1975, Bernardi 15813 & 15936 (PDA!).