Bulbophyllum Thouars is the largest genus in the orchid family in the subfamily Epidendroideae, with ca. 2 200 species^[1–3]. It is widely distributed in tropical and subtropical regions of the Americas, Africa, Madagascar, Asia, and Australasia^[4]. Most species within the genus Bulbophyllum are epiphytic or lithophytic with long, creeping or dangling rhizomes. Secondary stems grow as pseudobulbs with one or two usually fleshy or leathery leaves at their apices. The scape arises from the base of the pseudobulb producing a racemose or umbellate inflorescence. The dorsal sepal is free from the lateral sepals while the lateral sepals are free or connate. The petals are also free from each other and smaller than the lateral sepals. The lip (labellum) is often fleshy, curved and hinged at the base of the column. Variation in floral modifications based on size, organ shape, pigmentation patterns and epidermal sculpturing makes Bulbophyllum one of the most complicated genera within the family^{[2, 5]}.

Approximately 110 species of the genus Bulbophyllum are distributed through tropical and subtropical forests in southern China^[6] and this includes several new taxa described in recent years^[7–9]. Ongoing biogeographic analyses of Bulbophyllum s.s. with ancestral area reconstructions identify the Asian-Pacific region as the ancestral area of the lineage suggesting an early-to-late Miocene dispersal from east to west with subsequent speciation events^[10].

During fieldwork in Mojiang County, Yunnan Province, we found some small plants of an unidentified Bulbophyllum species growing on the trunks of native Quercus trees in February and March 2017. We collected some plants for cultivation in the nursery of Yunnan Academy of Forestry. Three plants flowers in March 2017 providing us with sufficient material for comparative morphological analyses and a subsequent description of a new species. The surviving plants showed vegetative and floral traits indicative to Section Tripudianthes Seidenf.

Section Tripudianthes remains a small section within genus Bulbophyllum. Almost all members of this section are deciduous, dropping their leaves at the onset of winter. The section consists currently of 13 species^[11–12] distributed from India to Indochina, Thailand, Java, and China. Seven species are distributed in Yunnan, China^[13]. Therefore, we needed to compare these Yunnan specimens with previously described taxa within section Tripudianthes focusing on four congeners with distributions in southern Asia. This permitted segregating and describing a new species in section Tripudianthes.

1 Materials and methods

All morphological traits and measurements of the three plants in the vegetative state were taken from living specimens in situ. Measurements and observations continued using the same plants as they flowered in the greenhouse of the Yunnan Academy of Forestry. Flowering plants were photographed using Nikon D300. Floral traits were recorded using a fresh flower from a plant designated as the holotype. Floral organs were carefully dissected, photographed and measured with digital calipers to an accuracy of 0.1 mm, prior to pressing them with the holotype. We pressed other plants in flowers as isotypes.

We developed a data set based on vegetative and floral traits using our living plants in and ex situ. We also checked additional herbarium sheets of Bulbophyllum species belonging to the Yunnan Academy of Forestry (YAF), the Kunming Institute of Botany, the Chinese Academy of Sciences (KUN) with additional sheets from Chinese collections at PE and IBSC. Unfortuately, there were no specimens available of the four species in section Tripudianthes considered most closely allied to our plants (Table 1). Therefore, to compare floral traits of our plants to B. dickasonii Seidenf., B. kanburiense Seidenf., B. rugosisepalum Seidenf., and B. tripudians CSP Parish & Rchb. f., we used photos of flowers of online herbarium sheets including types from AMES, MNHN, NL and K. We also used Seidenfaden^[11] to compare previous descryptions of related species in section Tripudianthes to our own measurements and descriptions.

2 Results

Bulbophyllum dresslerianum Z. D. Han & H. Wang sp. nov. Figs. 1, 2

Diagnosis: Bulbophyllum dresslerianum is similar to B. dickasonii, B. kanburiense, B. rugosisepalum, and B. tripudiansin terms of morphological structure of flowers. This new species can be distinguished from B. rugosisepalum by dorsal sepal ovate to narrowly ovate, lateral sepals absent rugose-papillae, and the margins and adaxial side of epichile with sparely short glandular hairs. It can be distinguished from B. tripudians with the epichile of lip with globular papillae. In addition, the new species can be distinguished from B. dickasonii and B. kanburiense with the margins of dorsal sepals irregularly fimbriated, the petal ovate-lanceolated and the margins of petal fimbriated.

Type: CHINA. Yunnan: Kunming, voucher from the cultivated plant at Yunnan Academy of Forestry, 15 March 2016, H. Jiang et al. 07360 (Holotype: KUN!; Isotypes: YAF!), collected originally in CHINA. Yunnan: Mojiang County, Mengnong Town, Bingman village, in mixed broadleaf-conifer forest. Alt. 1 400 m, 23°45′ N, 101°30′ E.

Description: Epiphytic herb. Pseudobulbs clumped and congested, brownish-green, depressed ovoid, 1.5– 2.0 cm in diameter, with 2 leaves at each apex. Leaves deciduous prior to anthesis, oblong, ca. 13.0 cm× 2.0 cm, leaf lamina with an acute apex. Inflorescence racemose and nodding arising from the base of the current year's pseudobulb, terminating in ca. 20 overlapping flowers, 20 cm long, with 3 tubular sheaths, ca. 6.0 cm. Floral bracts yellowish, ovatelanceolate, ca. 5.0 mm, apex acuminate. Dorsal sepal brownish yellow, wide ovate-lanceolate, ca. 5.0 mm× 2.0 mm, margins irregular and minutely dentate, apex long acuminate. Lateral sepals brownish yellow with purple spots, narrowly oblong, 27.0 mm×3.5 mm, glabrous, twisted near bases, and connate on lower edges, margins entire, apex obtuse. Lateral petals with whitish centers and purple-red margins, obliquely ovate-triangular, ca. 2.0 mm×1.0 mm, margins irregular fimbriate-ciliate, apex long acuminate. Labellum/Lip surface deep purple-maroon with a white abaxial surface, subligulate, ca. 2 mm long, fleshy, basal part attached to a column foot by a mobile hinge, margins and abaxial side of epichile sparsely decorated with short hairs, epichile densely ornamented with shiny globular papillae, apex obtuse. Column white and reduced, ca. 2.0 mm long, column wings triangular-falcate, ca. 0.5 mm long with a white, short, column foot. Anther cap subglobose, apex densely papillate. Pollinia 4, in 2 pairs.

Etymology: The species is named for the late Robert Louis Dressler (1927—2019), an authority on the taxonomy of Neotropical Epidendroideae (including the Malaxideae) and his contributions to the evolutionary ecology of the Orchidaceae.

Chinese vernacular name: 斯勒卷瓣兰(sī lè juǎn bàn lán)

Phenology: This new species was observed flowering in March, and fruiting from May to June.

Distribution and habitat: Bulbophyllum dresslerianum is currently found only in one location in southern Yunnan. We did investigate other possible locations near Mojiang in 2017, but without success. It is an epiphytic species growing on oak tree trunks in the mixed coniferous forest at an elevation of 1 400 m.

Specimens of related species examined using herbarium sheet photos: Bulbophyllum dickasonii: Burma, Ranpetlet, Chin Hills, 7 500 ft (c. 2 300 m), Dickason 8 444 (holotype AMES!); Manipur, Chandel Distr., Chakpikarong, 24°23.484′ N, 94°07.088′ E, alt. 1 434 m, in subtropical rain forest, 25 Jan. 2013, Chowlu 00333 (CAL); Chowlu 000325 (Herbarium of Centre for Orchid Gene Conservation Eastern Himalayan Region, Hengbung). Bulbophyllum tripudians: Vietnam, Annam: un peu au sud de la station agricole de Blao, prov. du Haut Donaï, 750 m, Poilane, E. 22157 (MNHN-Paris); Thailand, Province: Chiang Mai, District: Chiang Dao, Doi Chiang Dao Animal Sanctuary, near Huay Mae Gawk Station, 1 450 m, 4 March, 1995, Maxwell, J F95-200 (NL); Myanmar, Parish, C. S. P. 308, K000829091 (K).

Conservation status: The new species is kown only from the type locality. It is restricted to one population of ca. 20 individual plants discovered during our field trips. As more populations may be found with additional field surveys in these underexplored regions we regard this species as Data Deficient (DD: IUCN 2019)^[14].

3 Discussion

Orchids in this section are unique among the Asian Bulbophyllum species as they have two leaves per pseudobulb and leaf like foral characteristics similar to those described in of section Cirrhopetalum (Lindl.) Rchb. f.^[5]. Most plants in section Tripudianthes are deciduous, the leaves dropping in the winter, and then the inflorescence arises from the leafless pseudobulbs during the winter or in the following spring. These distinct characters made this section easy to distinguish from other in the genus. Section Tripudianthes occurs through subtropicaltropical regions with B. dickasonii is distributed from India to Myanmar, B. kanburiense found in Myanmar and Thailand, B. rugosisepalum in Thailand, while B. tripudians occurs in Myanmar, northern Thailand, and Vietnam. Our new species expanded the distribution of this section further north into southern Yunnan.

The new species, Bulbophyllum dresslerianum, does not have vegetative characters distinct from B. dickasonii, B. kanburiense, B. rugosisepalum. and B. tripudians. It does not have unique floral traits absent in all, four, allied, species either (Table 1). Instead, the new species is unique due to a linked suite of labellum and lateral petal traits that do not link in the remaining four species. It is the only species in which the epichile surface is densely decorated with globular papillae while its abaxial surface wears a few, short glandular hairs. In turn, these labellum traits link with the flower's ovate lateral petals with fimbriate margins. A key to distinguish B. dresslerianum with other four allied species was provided.

The transition from tropical, to subtropical, to temperate into montane vegetation in sourthern Yunnan allows some orchid lineages, including the genus Bulbophyllum, to colonize new niches based on extensive variations in topology and climate^[15]. With the description of B. dresslerianum, Section Tripudianthes extends northward into a cooler, coniferousoak ecosystem. It is not surprising that new plant taxa are found continuously in this biodiversity hotspot^[16]. With further field investigations, we expect to find more populations of this new species and perhaps more new Bulbophyllum species considering the high vagility of orchid seeds and the speciation model proposed by Trenblay et al.^[17].

Key to the four species related to Bulbophyllum dresslerianum

1. Lateral sepals with densely rugose-papillose papillae surface……………………………………………………… B. rugosisepalum

1. Lateral sepals with smooth suface

2. Dorsal sepal with entire margin

3. Lip with very sparsely and short hair…………………………………………………………………………………… B. kanburiense

3. Lip with densely fine hair………………………………………………………………………………………………… B. dickasonii

2. Dorsal sepal with irregularly fimbriate margin

4. Lip without globular papillae……………………………………………………………………………………………… B. tripudians

4. Lip with globular papillae…………………………………………………………………………………………… B. dresslerianum

Acknowledgements We are grateful to Mr. Changfei Peng for his help in the field, Mr. Jidong Ya for checking a part of specimens of related species, Ms. Lan Yan and Lin Wang for the illustration. We would also like to thank Prof. Peter Bernhardt of the Missouri Botanical Garden for his suggestions on improving the manuscript.