INTRODUCTION
The flora of Laos are among the least known in the region, primarily due to the lack of botanical exploration, expertise, and research and also hampered by the Indochinese wars (Newman et al., 2007). The collection density of plant specimens of Laos is lowest among Southeast Asian countries (Middleton et al., 2019). Recently, efforts to collect plant specimens in Laos have increased, and the resulting publications of new taxa are partly summarized in Lim et al. (2021). In addition, Lim et al. (2021) reported 14 species new to the flora of Laos, collected solely from the Phou Khao Khouay National Biodiversity Conservation Area, located near the capital, Vientiane, in addition to nine rarely collected flowering plant species of Laos.
We have been conducting field surveys in Laos in collaboration with the Department of Forestry, Ministry of Agriculture and Forestry of the Lao PDR since December 2015 (Lim et al., 2016), but the field survey was completely suspended between the years 2020 and 2021 due to the COVID pandemic. We resumed the field survey on Laos in August of 2022. We expanded the survey area further toward the south and north. It now includes the Dong Hua Sao National Biodiversity Conservation Area to the south, Phou Den Din NBCA to the north, and Phou Hin Poun NBCA in the central area. During field surveys in 2022 and 2023, we encountered flowering plants new to the flora of Laos. These are reported here.
MATERIALS AND METHODS
We surveyed several locations in Laos between August of 2022 and October of 2023 and compiled approximately 800 collections. These areas include the Dong Hua Sao National Biodiversity Conservation Area, Phou Xiang Thong NBCA, Phou Hin Poun NBCA, Phou Khao Khouay NBCA, Phou Den Din NBCA, and nearby places along the route from south to north (Fig. 1). The collected specimens have been deposited at the herbarium of the National Institute of Biological Resources (KB), Ministry of Environment, Republic of Korea, and duplicates are at the herbarium of Faculty of Forestry, National University of Laos (FOF) and Daegu University herbarium (DGU).
To identify species and confirm distribution records in Laos, we first checked relevant taxonomic literature regarding plants in the region; in this case, “Flore du Cambodge, du Laos et du Vietnam” (Aubréville et al., 1960–present); “Flora of Thailand” (Smitinand and Larsen et al., 1970–present); and “Flora of China” (Wu et al., 1994–2003), as well as a checklist of the vascular plants of Laos (Newman et al., 2017–present; https://padme.rbge.org.uk/laos/) were consulted. We examined images of type materials and specimens available on the web, specifically JSTOR Global Plants (https://plants.jstor.org), the Plants of the World Online website (POWO, 2024; http://www.plantsoftheworldonline.org), Royal Botanical Garden Edinburgh (E), (http://data.rbge.org.uk), Museum National d’Histoire Naturelle (P), the Arnold Arboretum of Harvard University (A), and the GBIF website. Also, the relevant taxonomic literature with original descriptions and revisions was consulted.
RESULTS AND DISCUSSION
We discovered 12 species of flowering plants new to the flora of Laos. We also collected vouchers for the three species enlisted in Newman et al. (2017–present) that lacked vouchers and two alien species new to the flora of Laos. In total, 17 species are presented here (Fig. 2). Among the 12 newly recorded species, Strobilanthes barbigera, and Argostemma cordatum are endemic to Vietnam, and Ceropegia tenuicaulis and Ornithoboea puglisiae are endemic to Thailand. The remaining species are widely distributed in the countries neighboring Laos. The collection sites of the 17 species are presented in Fig. 1.
New flowering plant records for the flora of Laos
Acanthaceae
1. Strobilanthes barbigera J. R. I. Wood, Nuraliev & Scotland, Kew Bull. 72: 28, 2017.—TYPE: Vietnam (southern part). Dak Lak Province, Lak Distr., Mun. Bong Krang, Chu Yang Sin National Park, 11 km S of Krong Kmar Village, 12°23′15″N, 108°20′45″E, elev. 1,300 m, 13 Apr 2012, M. S. Nuraliev 543 (holotype: FHO; isotype: MW) (Fig. 2A).
Distribution: Laos and Vietnam.
Specimen examined: LAOS. Champasak: Paksong district, at Phu Pong, along a trail from the Nam Phak Dam construction site at valley to the plateau, 15°07′ 42.96″N, 106°05′ 26.07″E, elev. 1,030 m, 15 Jan 2023, H. Won et al. 17837, 17838 (KB, DGU); Paksong district, at Phu Pong, along a trail from the Nam Phak Dam construction site at valley to the plateau, 15°07′38.43″N, 106°05′16.63″E, elev. 1,033 m, 15 Jan 2023, H. Won et al. 17854 (KB, DGU).
Note: While collecting at the Phu Pong region of the Bolaven Plateau, we encountered flowering individuals of S. barbigera along dry streams in shaded forests. While Strobilanthes hypomalla is distributed in open areas of the Phu Pong region, S. barbigera only occurs in shaded forests. These habitat characteristics are in good agreement with those in Wood et al. (2017), for which the description is “located in shaded ravines along small streams.”Strobilanthes barbigera is distinct in having prominent, long white glandular hairs developing on the abaxial surface of bracteoles after anthesis and a small corolla with exerted stamens.
Apocynaceae
2. Ceropegia tenuicaulis Kidyoo, Phytotaxa 162: 93, 2014.—TYPE: Thailand. Ubon Ratchathani Province, Pha Taem National Park, elev. 230 m, 13 Sep 2010, M. Kidyoo 1508 (holotype: BCU; isotype: BKF) (Fig. 2B).
Distribution: Laos, Thailand
Specimen examined: LAOS. Champasak: Sanasomboun district, at forest located north of the road from Xone Phark to Ban Mai Singsamphan, at Phou Xiang Thong National Biodiversity Conservation Area, dry, partly open forest, 15°20′44.24″N, 105°34′ 03.04″E, elev. 301 m, 27 Jun 2023, H. Won et al. 17981 (KB, DGU).
Note: The type locality of C. tenuicaulis, Pha Taem National Park, Thailand, is across the Mekong River from the Phou Xiang Thong NBCA, Laos, suggesting the possibility of the presence of the species around the area. Like the type locality, the collection site in PXTNBCA is at the edge of a forest and on open dry grassy land, with sandy soil. Unlike most other Ceropegia, this species does not climb but remains erect at a height of 30 cm, with linear leaves and linear corolla lobes not united at the tip.
Cucurbitaceae
3. Trichosanthes pilosa Lour. var. roseipulpa W. J. de Wilde & Duyfjes, Reinwardtia 12: 271, 2008. —TYPE: Thailand. Nan, Doi Phu Kha National Park, Km. 32, elev. 1,020 m, 25 Nov 2005, P. Phonsena, W.J.J.O. De Wilde & B.E.E. Duyfjes 4694 (holotype: BKF; isotype: L [2 sheets, photo!]) (Fig. 2C).
Distribution: Australia, China, India, Japan, Laos, Malaysia, New Guinea, Philippines, Taiwan, Thailand, and Vietnam
Specimen examined: LAOS. Phongsaly: Boun Nuea district, along a trail near the road between Boun Nuea and Ban Chicho, Phou Thalen, 21°37′59.07″N, 101°55′53.90″E, elev. 1,116 m, 6 Oct 2023, H. Won et al. 18308 (KB, DGU).
Note: According to the aforementioned checklist for Laos (Newman et al., 2007), there are six Trichosanthes species, T. dunniana H. Lév., T. inthanonensis Duyfjes & Pruesapan, T. kirilowii Maxim, T. rubriflos Thorel ex Cayla, T. tricuspidata Lour., and T. villosa Blume, reported for Laos. Trichosanthes pilosa var. roseipulpa is distinguished from other Trichosanthes by its seed morphology. The seeds of T. pilosa var. roseipulpa are unique in having a pentagonal shape, while most Trichosanthes species have ellipsoid to oblongoid seeds.
Ericaceae
4. Gaultheria discolor Nutt. ex Hook., Bot. Mag. 84: t. 5034, 1858. Diplycosia discolor (Nutt. ex Hook.) C. B. Clarke.—TYPE: Bhutan. Booth s.n. (holotype: K [photo!]). G. longibracteolata R. C. Fang, Novon 9: 166, f. 4, 1999.— TYPE: China. S. Yunnan: Luchun Xian, 1,700 m, 17 Sep 1973, D. D. Tao 68 (holotype, isotype: KUN [not seen]) (Fig. 2D).
Distribution: Bhutan, China, Laos, Thailand, and Vietnam
Specimen examined: LAOS. Vientiane: Kasy district, along a trail east of the Phu Luang hill, along the road #4C from Kasy to Luang Prabang via Ban Pongdong, near the border between the Vientiane and Luang Prabang Province, on open slope, 19°23′24.38″N, 102°08′24.60″E, elev. 1,852 m, 2 Oct 2023, H. Won et al. 18172 (KB, DGU).
Note: Fang (1999) described G. longibracteolata R. C. Fang based on specimens collected from several places in Yunnan, China and one specimen collected from Doi Inthanon, Thailand (J.F. Maxwell 89-1613). After an examination of specimens collected from Chiang Mai (Doi Inthanon, Doi Lang Ka, Doi Pha Hom Pok, Doi Pee Pan Nam, Mae Fang), Watthana (2015) concluded that G. longibracteolata is conspecific of G. discolor, based on morphological variations in the leaf shape, the degree of pubescence of the style (from distinctly pilose to almost glabrous), its anther appendages, and hairiness on the branchlets. The collection site is frequently clouded due to its high elevation (above 1,800 m).
Gesneriaceae
5. Ornithoboea puglisiae S. M. Scott, Gard. Bull. Singapore 66: 112, 2014.—TYPE: Thailand. Nan, Muang Nan, Tham Pha Toop, trail to Phra Cave, elev. 300 m, 16 Aug 2012, D. J. Middleton, P. Karaket, S. Suddee & P. Triboun 5617 (holotype: E [photo! at GBIF]; isotype: BKF [not seen], P [not seen]) (Fig. 2E).
Distribution: Laos and Thailand.
Specimen examined: LAOS. Champasak: Paksong district, Paksong, along a steep trail from the Agriculture Research Center of South Laos (Coffee Research Station) to the valley of Houay Bangliang, Dong Hua Sao National Biodiversity Conservation Area, 15°10′12.39″N, 106°05′52.33″E, elev. 793 m, 10 Aug 2022, H. Won et al. 17574 (KB, DGU).
Note: Ornithoboea puglisiae has been collected and reported only from a few places in Thailand, such as Mae Hong Son and Nan district, thus far (Scott and Middleton, 2014). This is the first time we collected the species in Laos. The collection site is difficult to reach given that it is located near a cliff and a steep slope at the rim of the Bolaven Plateau. Ornithoboea species are distinct from other plants among the Gesneriaceae genera according to its flower morphology, specifically the palatal beard and circlet of hairs around the mouth of the corolla tube. Ornithoboea puglisiae is most similar to O. calcicola but distinct in having shorter fruits with a persistent style, glabrous anthers, and longer petioles by up to twofold.
Lauraceae
6. Beilschmiedia percoriacea C. K. Allen var. percoriacea, J. Arnold Arbor. 23: 450, 1942.—TYPE: China. Hainan, Poting, in forest, elev. 2,800 ft., 21 Jun 1935, F. C. How 72964 (holotype: A [photo!]; isotypes: BM, MO [photo!], P [photo!]) (Fig. 2F).
Distribution: China, Indonesia, Laos, Malaysia, Thailand, and Vietnam
Specimen examined: LAOS. Phongsaly: Phongsaly district, along a trail of the Phu Fa Mountain, near the summit of the Phu Fa Mountain, 21°41′19.74″N, 102°06′29.53″E, elev. 1,640 m, 5 Oct 2023, H. Won et al. 18253 (KB, DGU).
Note: Beilschmiedia percoriacea is collected near the summit of Phu Fa Mountain, Phongsaly. The species is a tall evergreen tree with coriaceous leaves and ellipsoid fruits. In a revision of the Beilschmiedia taxa in Thailand and Indochina, de Kok (2021) recognized two varieties in B. percoriacea, var. percoriacea and var. glaucoides. Based on the inflorescence and fruit morphology, the collection from Laos belongs to the var. percoriacea.
Malpighiaceae
7. Aspidopterys glabriuscula A. Juss., Ann. Sci. Nat., Bot., sér. 2, 13: 267, 1840. Aspidopterys roxburghiana var. glabriuscula (A. Juss.) Hook. f., Fl. Brit. India 1: 420, 1874.— TYPE: “Sylhet,” W. Gomez s.n., Wall. Cat. no. 6626 (type: K [photo!]) (Fig. 2G).
Distribution: India, Bangladesh, China (east Himalaya, Hainan), and Vietnam.
Specimen examined: LAOS. Phongsaly: Phongsaly district, along a trail of the Phu Fa mountain, near the top of the Phu Fa mountain, 21°41′19.74″N, 102°06′29.53″E, elev. 1,640 m, 5 Oct. 2023, H. Won et al. 18252 (KB, DGU); Phongsaly district, along a trail of the Phu Fa mountain, 21°41′03.23″N, 102°06′41.41″E, elev. 1,523 m, 5 Oct 2023, H. Won et al. 18277 (KB, DGU). Vientiane: Kasy district, along a trail east of the Phu Luang hill, along road #4C from Kasy to Luang Prabang via Ban Pongdong, near the border between the Vientiane and Luang Prabang Province, 19°23′21.19″N, 102°08′35.63″E, elev. 1,865 m, 2 Oct 2023, H. Won et al. 18172 (KB, DGU).
Note: In the “checklist of the vascular plants of Lao PDR,” three Aspidopterys species—A. nutans A. Juss., A. thorelii Dop, and A. tomentosa A. Juss.—are enlisted among the flora of Laos. These species are easily distinguished by their samara shape. We collected A. glabriuscula from two different places in Laos, one at Phu Luang hill located at the border between the Luang Prabang and Vientiane provinces, the other on Phu Fa Mountain in Phongsaly in Phongsaly province. The one collection made at Phu Luang hill had fully ripened samara, while those at Phu Fa Mountain had flower buds, flowers and young samara. All collected plants presented the typical samara shape of A. glabriuscula.
Malvaceae
8. Cenocentrum tonkinense Gagnep., Notul. Syst. (Paris) 1: 79, 1909.—TYPE: Vietnam. Tonkin, entre Phuong-lam et Cho-bo, 12 Nov 1887, B. Balansa 3712 (holotype: P [photo!]; isotype K [photo!], P [photo!]) (Fig. 2H).
Distribution: China, Laos, Thailand, and Vietnam
Specimen examined: LAOS. Phongsaly: Phongsaly district, along a trail of the Phu Fa Mountain, 21°41′05.71″N, 102°06′ 37.22″E, elev. 1,496 m, 5 Oct 2023, H. Won et al. 18279 (KB, DGU).
Note: Cenocentrum tonkinense is an erect, herbaceous plant and is covered with long, dense spiny hairs.
Piperaceae
9. Peperomia tetraphylla (G. Forst.) Hook. & Arn., Bot. Beechey Voy. 97 (1832). Piper tetraphyllum G. Forst., Fl. Ins. Austr.: 5, 1786.—TYPE: French Polynesia, J.G.A. Forster s.n. (lectotype: K [photo!]; isolectotype: C [photo!]) (Fig. 2I).
Distribution: Widely distributed in the tropical and subtropical regions of the Asian, African, and American continents.
Specimen examined: LAOS. Phongsaly: Phongsaly district, along a trail of the Phu Fa Mountain, epiphytic on tree trunk, 21°41′05.54″N, 102°06′ 34.60″E, elev. 1,448 m, 5 Oct 2023, H. Won et al. 18230 (KB, DGU). Note: Peperomia tetraphylla is widely distributed in tropical and subtropical regions of the Asian, African, and American continents. This is the first instance of a report of P. tetraphylla in Laos. Peperomia tetraphylla is unique in having elliptic to round leaves in whorls of 3 to 4.
Polygalaceae
10. Polygala karensium Kurz, J. Asiat. Soc. Bengal, pt. 2, Nat. Hist. 41: 292, 1872. Chamaebuxus karensium (Kurz) Kurz, Forest Fl. Burma 1: 79, 1877.—TYPE: Myanmar (Birma). Karen country and hills, S. Kurz s.n. (K [photo! K000592227]) (Fig. 2J).
Distribution: India, China, Laos, Myanmar, Thailand, and Vietnam
Specimen examined: LAOS. Vientiane: Kasy district, along a trail east of the Phu Luang hill, along the road #4C from Kasy to Luang Prabang via Ban Pongdong, near the border between the Vientiane and Luang Prabang Province, on open slope, 19°23′22.60″N, 102°08′29.99″E, elev. 1,859 m, 2 Oct 2023, H. Won et al. 18176 (KB, DGU).
Note: The page number containing the original description of Polygala karensium Kurz is in most cases incorrectly cited as page “392,” but it is in fact page “292.” Polygala karensium is distributed in evergreen or montane forests at elevations in the range of 950–2,300 m. We collected the species together with Gaultheria discolor (Ericaceae) and Koenigia mollis (Polygonaceae), where the area is humid and cool with frequent fog and clouds due to its higher elevation. Polygala karensium is distinct from other Polygala spp. in having a shrub habit and relatively large flowers with 2–2.5 cm long corolla and caducous sepals.
Polygonaceae
11. Koenigia mollis (D. Don) T. M. Schust. & Reveal, Taxon 64: 1201, 2015. Polygonum molle D. Don, Prodr. Fl. Nepal. 72, 1825; Persicaria mollis (D. Don) H. Gross, Bot. Jahrb. Syst. 49: 269, 1913; Ampelygonum molle (D. Don) Roberty & Vautier, Boissiera 10: 31, 1964.—TYPE: Nepal, in 1819, N. Wallich s.n. (Lectotype: BM [photo! BM000521971]) (Fig. 2K).
Distribution: Assam, China, India, Indonesia, Malaysia, Myanmar, Nepal, Pakistan, Sri Lanka, Thailand, and Vietnam
Specimen examined: LAOS. Vientiane: Kasy district, along a trail east of the Phu Luang hill, along the road #4C from Kasy to Luang Prabang via Ban Pongdong, near the border between the Vientiane and Luang Prabang Province, on open slope, 19°23′23.53″N, 102°08′21.17″E, elev. 1,847 m, 2 Oct 2023, H. Won et al. 18170 (KB, DGU).
Note: Koenigia mollis is a perennial shrub species growing up to 1.5 m tall. It grows in hilly areas with frequent fog and clouds due to the higher elevations of these areas. It has paniculate inflorescence with persistent perianths and fleshy fruit.
Rubiaceae
12. Argostemma cordatum Nuraliev, Phytotaxa 317: 43.— TYPE: Vietnam. Dak Lak Prov.: Lak district, Bong Krang municipality, Chu Yang Sin National Park, 11 km SSE of Krong Kmar village, in mixed forest, elev. ca. 1,250 m a.s.l., 12°24′ 53″N, 108°22′ 56″E, 22 May 2014, M. S. Nuraliev, A. N. Kuznetsov, S. P. Kuznetsova 960 (holotype: MW0595623) (Fig. 2L).
Distribution: Laos and Vietnam.
Specimen examined: LAOS. Champasak: Pathoumphone District, along a newly built trail for Nam Phak Dam construction, to west from Nam Phak Bridge, ca. 25 km east from the road #13, Dong Hua Sao National Biodiversity Conservation Area, in thick lowland forest along a trail, 15°03′25.95″N, 106°05′34.65″E, elev. 249 m, 11 Aug 2022, H. Won et al. 17596 (KB, DGU); Paksong district, Paksong, at the top of the Tad Kamued Waterfall, in the middle of stream, 15°04′10.6″N, 106°12′19.8″E, elev. 1,064 m, 26 Jun 2023, H. Won et al. 17939 (KB, DGU).
Note: Argostemma cordatum is characterized by a single large leaf created due to the reduction of one of two opposite leaves and a cordate leaf base of the larger leaf. The plant possesses small tubers and is a perennial.
Voucher specimens collected for the flora of Laos
Myrtaceae
1. Baeckea frutescens L., Sp. Pl. 358, 1753.—TYPE: Anonymous s.n. (lectotype: LINN [photo!]; isolectotype: S) (Fig. 2M).
Distribution: Australia, Cambodia, China, Indonesia, Laos, Malaysia, Myanmar, New Guinea, Philippines, Thailand, and Vietnam
Specimen examined: LAOS. Champasak: Paksong district, along a trail to the Phu Pong, at open sandstone rock area, 15°07′17.94″N, 106°05′00.06″E, elev. 1,163 m, 15 Jan 2023, H. Won et al. 17851 (KB, DGU).
Note: Baeckea frutescens is a shrub species distributed on a sandstone flat of the Bolaven Plateau. It has small and needle-like leaves and flowers with five white petals.
Polygonaceae
2. Fagopyrum cymosum (Trevir.) Meisn., Pl. Asiat. Rar. 3: 63, 1832. Polygonum cymosum Trevir., Index Seminum (WROCL, Wratislaviensi) 1823: 2, 1824. Phegopyrum cymosum (Trevir.) Peterm., Pflanzenreich 740 (1841-5).—TYPE: Nepal, in 1821, N. Wallich 1869 (holotype: K [photo!]; isotypes: LE [photo!]). (Fig. 2N). Fagopyrum dibotrys (D. Don) H. Hara, Fl. E. Himal. 69, 1966. Polygonum dibotrys D. Don, Prodr. Fl. Nepal. 73, 1825.
Distribution: China, India, Laos, Myanmar, Nepal, Thailand, and Vietnam
Specimen examined: LAOS. Phongsaly: Phongsaly district, Phongsaly, along the lake at Phongsaly, 21°40′ 48.41″N, 102°06′ 04.23″E, elev. 1,352 m, 5 Oct 2023, H. Won et al. 18219 (KB, DGU).
Note: Fagopyrum cymosum is a wild species of Fagopyrum that is utilized as forage and as a source of pharmaceutical drugs.
Potamogetonaceae
3. Sagittaria guayanensis Kunth, F. W. H. von Humboldt, A. J. A. Bonpland, & C. S. Kunth, Nov. Gen. Sp. 1: 250, 1816. Echinodorus guyanensis (Kunth) Griseb., Fl. Brit. W. I. 505, 1862. Lophiocarpus guyanensis (Kunth) Micheli, Monogr. Phan. 3: 62, 1881. Lophotocarpus guyananesis (Kunth) T. Durand & Schinz, Consp. Fl. Afric. 5L 487, 1894.—TYPE: Venezuela, “Crescit in paludibus Provinciae Guyanensis propter El Trapiche de Don Felix Farreeras, et urbem Angosturae”, Jun 1800, F. W. H. A. Humboldt & A. J. A. Bonpland s.n. (lectotype: B) (Fig. 2O).
Distribution: Tropics and Subtropics
Specimen examined: LAOS. Vientiane: Xaythany district, Ban Na Monh, lake along the road, 18°03′18.49″N, 102°44′20.25″E, elev. 175 m, 30 Jun 2023, H. Won et al. 18053 (KB, DGU).
Note: Sagittaria guayanensis is a perennial aquatic plant native to both the old- and new-world tropics and subtropics. It grows in wet areas near a village, together with other aquatic plants such as Nymphaea nouchali.
Alien species
Acanthaceae
1. Ruellia simplex C. Wright, Anales Acad. Ci. Méd. Habana 6: 321, 1870 (Fig. 2P).
Distribution: The native distribution range is Mexico to tropical parts of the American continents.
Specimen examined: LAOS. Champasak: Pakse district, Pakse, at guest house, cultivated, 15° 38′12.42″N, 105°48′ 10.73″E, elev. 132 m, 27 Jun 2023, H. Won et al. 17991 (KB, DGU).
Note: Ruellia simplex is frequently planted as an ornamental garden plant in Laos. It is an erect herb growing up to 1 m tall. In some introduced areas, R. simplex has become invasive and has spread widely.
Polygalaceae
2. Polygala paniculata L., Syst. Nat., ed. 10, 2: 1154, 1759. Senega paniculata (L.) J. F. B. Pastore & J. R. Abbott, Ann. Missouri Bot. Gard. 108: 208, 2023 (Fig. 2Q).
Distribution: The native distribution range is Mexico to tropical America
Specimen examined: LAOS. Champasak: Paksong district, along a trail to the Phu Pong, 15°07′36.19″N, 106°05′ 50.84″E, elev. 916 m, 15 Jan 2023, H. Won et al. 17835 (KB, DGU).
Note: Polygala paniculata has been introduced and has spread widely in SE Asia. Recently, all New World Polygala species were transferred from Polygala to Senega (Pastore et al., 2023).