Preliminary checklist of Hoya (Asclepiadaceae) in the flora of Cambodia, Laos and Vietnam

This paper provides new data on 33 new or rare species of Hoya obtained in fieldwork mainly during 2012–2017 in eastern Indochina including Cambodia, Laos and Vietnam. These data include illustrated descriptions of three species new for science, as well as illustrated records of 24 species new for the floras of the studied countries. When these new data are included, the flora of eastern Indochina comprises at least 45 species. Among them eight species are recorded and documented for Cambodia, 21 sp. for Laos and 40 sp. for Vietnam; two new species are described in Vietnam (H. crassipetiolata and H. nutans) and one species is described from Laos (H. uniflora). Eighteen species were recorded newly for the flora of Vietnam (H. arnottiana, H. burmanica, H. chinghungensis, H. erythrina, H. erythrostemma, H. fungii, H. griffithii, H. hainanensis, H. lanceolata, H. linearis, H. loyceandrewsiana, H. mengtzeensis, H. nummularioides, H. pachyclada, H. pandurata, H. parviflora, H. revolubilis, H. vaccinioides), nine for Laos (H. arnottiana, H. carnosa, H. chinghungensis, H. fungii, H. globulosa, H. griffithii, H. linearis, H. pandurata, H. polyneura) and two for Cambodia (H. lobbii, H. multiflora). New localities were discovered for H. bonii, H. lockii, H. lyi, H. micrantha, H. multiflora, H. oblongacutifolia, and H. verticillata known earlier in Cambodia, Laos and Vietnam by few collections. Lectotypes are proposed for the following eight species: H. arnottiana, H griffithii, H. lanceolata, H. lobbii, H. nummularioides, H. oblongacutifolia, H. pachyclada, and H. vaccinioides. Names of five taxa earlier known as separate species (H. villosa, H. pseudovalifolia, H. graveolens, H. balansae) are regarded as synonyms of H. globulosa, H. micrantha, H. oblongacutifolia, H. verticillata respectively.


Materials and methods
Materials suitable for studies were field collected mainly during [2012][2013][2014][2015][2016][2017]. Herbarium studies at IBSC, P and SING and some previously gathered herbarium specimens and living collections provided significant additional information for the current investigation. Fresh inflorescences and flowers were fixed and preserved in 60-70 % ethanol. Measurements of floral parts for descriptions were made on both living and liquid-preserved materials. Fresh flowers or floral parts were found to shrink up to ca 15-25 % in size in the drying process of making herbarium specimens. It is noteworthy that floral parts of Hoya species shrink often disproportionally, in particular the corona contracts less than sepals and petals. This was taken into account when dried herbarium specimens were studied. In describing quantitative characters, infrequent extreme values (i.e. rarely occurring minimal and maximal values) of a variation range are parenthesized respectively before and after a normal variation range.
The annotated list of all discovered species is presented below. Paragraphs for species are arranged in the list according to species name in alphabetic order. The annotation for each species includes following data in separate lines: -valid name and main synonyms, occurring in regional literature; -data about type or original materials; -living form, summarized available data on ecology, habitat elevation, phenology and observed rarity; -distribution in studied countries (mostly by listing of the country provinces) and general distribution; -short notes on taxonomy, expected relations and/or biology; -abbreviated and unified text of herbarium labels including geographical locality, collection date, collectors' names, collection number and acronym of herbarium where mentioned specimen is housed.
Color and line illustrations of voucher specimens are provided for almost all recorded species. Species distribution in countries of eastern Indochina is indicated in the text by mentioning concerned provinces according to the current official administrative division. Online version of the IUCN Red List of Threatened Species (2016) was used for estimation of preliminary species conservation status.
Habitat, phenology and conservation status. Epiphytic semi-woody vine about 3 m long. Primary evergreen seasonal broad-leaved closed submontane forest on basalt, 1500-1550 m. Fl. April -June. Not common. Estimated IUCN Red List status -DD.
Habitat, phenology and conservation status. Lithophytic and occasionally epiphytic undershrub with pendulous shoots to 1 m long. Primary evergreen broad-leaved humid forests on rocky limestone, commonly on shady cliffs at mountaintops, 1350-1500 m. Fl. April -August. Very rare. Estimated IUCN Red List status -EN.
Notes. The discovery of H. burmanica in northwestern Vietnam considerably expands the known distribution area of this species to the east. It grows in Vietnam in humid evergreen forests, a rather different environment from the seasonally dry open woodlands where it can be found in Myanmar. In Natma Taung National Park (Myanmar) it is one of the most common epiphytes at 1500-1800 m, but it was not found to grow as a lithophyte (M. Rodda, pers. obs.).
Notes. This is the first record for Laos although the species is widely distributed in mainland tropical Asia. Vernacular name in Vietnam. Cẩm cù hoa tán. Habitat, phenology and conservation status. Humus epiphytic or occasionally lithophytic undershrub with arching and pendulous shoots to 2 m long. Primary broad-leaved evergreen forests on any kind of soils, but preferably on rocky limestone, commonly on mountaintops, 700-1500 m. Fl. April -August. Locally common. Estimated IUCN Red List status -VU.
Notes. This is the first record of this rare species for the flora of Laos and Vietnam. It is part of a group of rather similar species including Hoya dickasoniana P. T. Li (Li, 1994), H. weebella Kloppenb. (Kloppenburg, 2005), H. lanceolata D. Don (Don, 1825), H. vaccinioides Hook. f. (Hooker, 1883), H. engleriana Hosseus (Hosseus, 1907) and H. kingdonwardii P. T. Li (Li, 1994) that have not been worked out yet but appear to have very similar flower morphology, but a few taxa can be separated based on lamina shape. Hoya chinghungensis is the only species in the complex bearing almost round leaves. In Laos and Vietnam H. chinghungensis is a very sensitive, highly endangered element of the fast decreasing primary woods and desire status of vulnerable. The collecting as ornamental plant due to market demands is additional factor of its extinction. Etymology. Species name refers to the remarkably thick leaf petiole.
Habitat, phenology and conservation status. Creeping epiphytic and lithophytic herbaceous to semi-woody vine to 6 m long. Secondary evergreen broad-leaved lowland forests on sandy soils along seashore and primary broad-leaved evergreen forests on rocky karstic limestone, often on steep slopes and cliffs near mountaintops, 10-1400 m. Fl. May -August. Locally common. Estimated IUCN Red List status -DD.

Endemic.
Notes. This new species is rather similar in leaf and flower morphology to Hoya hainanensis Merr. (Merrill, 1923) because both species have rhomboid elliptic leaves with a decussate base, reflexed corolla and ellipsoid corona lobes. The two species can be separated by the upper surface of the corona lobe that is flat in H. crassipetiolata (vs. sunken) and the inner process of the corona lobe that is obtuse, short (vs. apiculate and recurved upwards). In its floral morphology H. crassipetiolata is also very similar to H. pachyclada Kerr (Kerr, 1939) widely distributed in Thailand and Cambodia. However, it is totally  (2017) different in its vegetative habit. Described plant is slender herbaceous or semi lignified vine (not undershrub with rather robust stems), glabrous in all parts (vs. sparsely pubescent leaves, peduncle, pedicels and calyx), coriaceous (not fleshy) leaves and glabrous corona base (not densely hairy with long hairs). It may be also close to H. verticillata (Vahl) G. Don s. l., including H. acuta Haw. (Haworth, 1821) and H. parasitica Wall. ex Wight (Wight, 1834) widely distributed in tropical Asia, but well differs in rather sparse, not many-flowered umbels, larger flowers with strongly recurved broad petals, broadly ellipsoid corolla lobes with rather obtuse outer angle and prominent, irregularly branching secondary veins on leaf blade. The species superficially resembles the type specimens of H. oblongacutifolia Costantin (Costantin, 1912) [P00639820, P05207933, P00639821, P00639822] in character of leaf size and venation, but quite differs in distinct prominent secondary veins (hardly visible in leaves of H. oblongacutifolia), glabrous pedicels (not hairy) and bigger flower with corolla 1.5-1.6 cm in diameter (not about 1.2 cm across recorded for H. oblongacutifolia). The new species may be also easily distinguished by its unusually thick petioles (twice thicker as stem) and usually quite distinct green/white tessellation of adaxial surface of leaf blade. The species is easily cultivated and may be recommended as a fast growing ornamental plant.
A collection from Khanh Hoa province ( Notes. This is a remarkable discovery of a rare species regarded earlier as occurring in peninsular Malaysia and Thailand only. The distance between its recorded locations in southern Vietnam and the locus classicus in Malaysia exceeds 1400 km. H. erythrina is widely cultivated as an ornamental plant. Its flowers vary in color from white and yellowish to pink.   Kerr, 1939, Bull. Misc. Inf. Kew. 8: 460;id., 1951. Fl. Siamensis Enum. 3, 1: 36. Fig. 6D-F. Described from peninsular Thailand. Lectotype ("Thailand, Tasan, C. B. Kloss 6909") -K [K000895112], lectotype designated by Rodda et al. (2014).

Hoya erythrostemma
Habitat, phenology and conservation status. Creeping epiphytic herbaceous or semi-woody vine to 5 m long. Primary and secondary broadleaved evergreen lowland forests on alluvial soils. Fl. October -November, occasionally year round. Rare. Estimated IUCN Red List status -DD.
Notes. H. erythrostemma was thought to be endemic to Thailand (Rodda et al., 2014) and with its discovery in Vietnam in Dak Lak province its range is extended about 500 km further eastwards.

Hoya fungii
Vernacular name in Vietnam. Cẩm cù nấm. Habitat, phenology and conservation status. Epiphytic and occasionally lithophytic semi-woody vine to 10 m long. Primary and secondary broadleaved evergreen forests on any kind of mother rocks, but preferably on rocky limestone, often on tall trees along streams and small rivers, 0-1200 m. Notes. This species is new for the floras of Laos and Vietnam. It is similar in corolla and corona morphology to H. bonii Costantin (Costantin, 1912) and H. carnosa (L. f.) R. Br. (Brown, 1810). However, H. fungii differs from H. bonii in oblong elliptic (not cordate) glabrous leaves (young leaves and stems may be pubescent), which are bigger, longer and having distinct venation. The calyx lobes of H. fungii are lanceolate and pubescent while H. carnosa has shorter triangular calyx lobes. According to visual not documented observations of Mr Nguyen Van Canh, this species is also common in some areas of Dak Lak province. The plant is fast growing vine desirable for cultivation as ornamental plant. Habitat, phenology and conservation status. Creeping epiphytic vine. Primary and secondary broad-leaved evergreen humid mountain forests, often with bamboo, on any kind of mother rocks, 900-1400 m. Fl. April -May, August -December. Not common. Estimated IUCN Red List status -DD.
Note. The discovery of this rare species in highland areas of northern Vietnam and Laos is not much surprising. It was known before in NE India, S China and Thailand. Records listed here are the first for the flora of Vietnam. The species has large attractive flowers and may be recommended for cultivation as a fast, rather cool growing ornamental vine.
Habitat, phenology and conservation status. Lithophytic and occasionally epiphytic creeping vine. Primary evergreen broad-leaved forests, often on granite outcrops along streams, 800 m. Fl. January -February. Locally common. Estimated IUCN Red List status -DD.
Notes. H. hainanensis was only known from Hainan Island. Its discovery in southern Vietnam considerably extends its known distribution area. This species is known insufficiently in the flora of Vietnam; hence we provide here its description based on Vietnamese material. Habitat, phenology and conservation status. Epiphytic herb or undershrub with creeping or spreading shoots to 0.5 m long. Primary and secondary broad-leaved evergreen submontane forests, 1200-1300 m. Fl. June -July. Very rare. Estimated IUCN Red List status -DD.
Notes. Very rare ornamental plant closely related to the widely cultivated H. bella Hook. (Hooker, 1848, t. 4402), which is sometimes regarded as a subspecies of H. lanceolata D. Don subsp. bella (Hook.) D. H. Kent (Kent, 1981). Habitat, phenology and conservation status. Branch and trunk epiphyte with pendulous shoots to 1 m long. Primary and secondary broad-leaved evergreen humid forests on any kind of mother Notes. Easily distinguished species for its long slender herbaceous drooping shoots bearing densely hairy, almost terete leaves. Collected in Vietnam by Poilane as early as 1926 (Poilane 12932), but the specimen was until recently not identified to species. Distribution. Cambodia (Pursat province). India, Thailand.
Notes. H. lobbii is found in central India and Thailand and its record in neighbouring Cambodia is therefore not surprising. Habitat, phenology and conservation status. Humus epiphytic undershrub. Primary evergreen broad-leaved, mixed and coniferous forests on any kind of mother rocks, commonly on tall trees in sunny canopy zones, 500-1200 m. Fl. May -August. Occasional. Estimated IUCN Red List status -DD.
Notes. This local Vietnamese endemic is close to the widespread species H. multiflora Blume (Blume, 1823). According to available collections, it is not rare in its limited area in central part of Vietnam. Ink and color illustrations of this species were presented in early publication (Pham, Averyanov, 2012). Habitat, phenology and conservation status. Epiphytic creeping semi-woody vine. Primary and secondary evergreen broad-leaved lowland forests. Fl. October -November. Very rare. Estimated IUCN Red List status -DD.
Notes. H. loyceandrewsiana was described based on cultivated material of unknown origin. This is the first discovery of the species in the wild.  H. Lév., 1907, Bull. Soc. Bot. France. 54: 369;Rodda, 2012, Edinburgh Journ. Bot. 69: 85. -H. mekongensis auct. non M. G.   Notes. This miniature ornamental species may be easily recognized in nature even in sterile state by its small oval leaves and dense soft short indumentum on all plant parts. The species was previously recorded from China, Laos and from Vietnam-Chinese border (Rodda, 2012). Presently recorded localities of this species are located almost throughout all mountain areas of Vietnam, and in Xiangkhouang and Houa Phan provinces of Laos. Cultivated specimens often develop much larger leaves to 18 cm long (vs. 2-4 cm in wild collections) with acute apex, longer scape and distinctly longer pedicels (Fig. 11).  Tsiang et P. T. Li, 1974, Acta Phytotax. Sin. 12, 1: 120. Fig. 10G, H. Described (Tsiang, Li, 1974  Distribution. Vietnam (Lam Dong province).

Hoya multiflora
Habitat, phenology and conservation status. Primary broad-leaved forests on rocky karstic limestone, commonly on steep slopes near mountaintops, 500-700 m. Fl. July. Occasional. Estimated IUCN Red List status -DD.
Habitat, phenology and conservation status. Lithophytic and epiphytic semi-woody vine to 8 m long. Primary and secondary evergreen, broadleaved lowland forests on any kind of soils, often on low karstic limestone hills, 50-300 m. Fl. April, January -February. Not common. Estimated IUCN Red List status -DD.
Notes. Original material of H. oblongacutifolia was only indicated as "Mt. Deon-ba, Tay-ninh, Pierre". Five duplicates have been found at P. The one with barcode [P00639820] is fertile and well preserved and is therefore here designated as lectotype. H. graveolens Kerr (1939) was reported as a new record from Vietnam in 2013  and only subsequent examination of original material of H. oblongacutifolia allowed to clarify that the two names should be applied to the same taxon.  Kerr, 1939, Bull. Misc. Inform. Kew. 8: 462. Fig. 14D Notes. This is the first record of the species for Vietnam. It was observed in visited localities as a quite common plant in dry lowland primary and secondary forests. It is also not rare in Cambodia and Laos. The color of the corolla is variable from almost white to purple even within one population (based on observation in Thailand and Vietnam).  Tsiang, 1939, Sunyatsenia 4: 125;Tsiang, Li, 1974, Acta Phytotax. Sin. 12, 1: 125;Li et al., 1995, Fl. China 16: 235;Kidyoo, Thaithong, 2007, Nat. Hist. Journ. Chulalongkorn Univ. 7, 1: 47. Fig. 14G-I. Habitat, phenology and conservation status. Humus epiphytic undershrub with spreading, sparsely branched shoots up to 1 m long. Primary and secondary broad-leaved semideciduous and evergreen dry and humid forests on any kind of mother rocks, often on exposed mountaintops, 800-1200 m. Fl. December -January. Very rare. Estimated IUCN Red List status -DD.
Habitat, phenology and conservation status. Humus epiphytic undershrub with spreading, sparsely branching shoots to 1 m long. Primary and secondary broad-leaved evergreen dry forests on shale and sandstone mountains, commonly on tall trees along ridge edges, 1700-1900 m. Fl. May -June. Rare. Estimated IUCN Red List status -DD.
Notes. The collection of H. polyneura in Laos slightly extends its distribution area to the south, as the species was already known to occur in neighbouring Yunnan. Flowers of cultivated plants have light sweet fragrance.
Habitat, phenology and conservation status. Creeping tiny herbaceous lithophytic and occasionally epiphytic vine to 35 cm long. Primary and secondary broad-leaved evergreen dry forests on rocky karstic limestone, commonly on open mossy rocks and cliffs near mountaintops, 1200-1450 m. Fl. April -June. Locally common. Estimated IUCN Red List status -DD.
Notes. The new species is rather unusual in the genus because of its obligatory 1-flowered inflorescences. No other species from Indochina have this peculiar feature that is otherwise observed in mainland Asia only in H. mappigera Rodda et Simonsson Juhonewe (2012b) and H. wallichii (Wight) C. M. Burton (Burton, 1996;Rodda et al., 2016)