Rare plants typically have narrow distributions and highly specific habitats with limited and/or small populations (Işik, 2011). Many rare plants are vulnerable to habitat loss and/or disturbances by environmental, human, and genetic factors (Lavergne et al., 2005; Işik, 2011). Moreover, the International Union for Conservation of Nature (IUCN) has warned that approximately 13% of plant species in the world are at risk of extinction (Walter and Gillett, 1998). Accordingly, many countries have established a Red List and assessed their threatened species. In Korea, rare plants are mainly organized and protected by the Korea Forest Service and the Ministry of Environment (Korea National Arboretum, 2008; National Institute of Biological Resources, 2012). However, some species, for which little is known about habitat types and population size, are not listed.
Among 15 taxa belonging to Asclepiadoideae in Korea, four in Cynanchum and Marsdenia (C. amplexicaule (Siebold & Zucc.) Hemsl., C. japonicum (C. Morren & Decne.) Hemsl., C. inamoenum (Maxim.) Loes. ex Gilg & Loes., and M. tomentosa C. Morren & Decne.) are on the Korean Red List as Endangered and Vulnerable species (Korea National Arboretum, 2008). Despite the highly narrow distributions of Vincetoxicum glabrum (Nakai) Kitag. (= Cynanchum glabrum Nakai) and V. volubile Maxim. (= C. volubile (Maxim.) Hemsl.), they are not included in the list. To establish a conservation management system, accurate identification and comprehensive analyses of habitats, population sizes, and distributions of Korean Asclepiadoideae taxa are critical.
Cynanchum L. is a large genus in the subfamily Asclepiadoideae (Apocynaceae) with approximately 200 species and a wide distribution throughout the world (Liede and Täuber, 2002; Khanum et al., 2016). The genus is morphologically distinguished from Vincetoxicum Wolf. by twining stems, white latex, and mostly heart-shaped leaves; it also exhibits high diversity in floral structures, especially in corona shapes (Khanum et al., 2016; Nam and Chung, 2018).
In the Korean peninsula, Cynanchum was formerly composed of about 10–12 species, and these included Vincetoxicum species (Lee, 1996; Lee, 2007; Korea National Arboretum, 2017). However, there are now four recognized species based on morphological and phylogenetic studies: C. chinense R. Br., C. purpureum (Pall.) K. Schum., C. thesioides (Freyn) K. Schum., and C. wilfordii (Maxim.) Hemsl. (Khanum et al., 2016; Nam and Chung, 2018). C. purpureum and C. thesioides have been reported in North Korea, C. chinense is restricted to South and North Korea, and C. wilfordii is distributed from the seashore to mountain slopes in the Korean peninsula (Lee, 1996; Im, 1999; Chang et al., 2014).
We report new distribution records of two rare species, C. chinense and C. thesioides, based on a field investigation in South Korea, including the first observation of C. thesioides in the country. The aims of this study are to evaluate the distribution in South Korea of two rare species with careful morphological examination based on the original descriptions and type specimens, and to provide fundamental information for further studies assessing their conservation status.
Taxonomic Treatment
Cynanchum thesioides (Freyn) K. Schum., Nat. Pflanzenfam. 4: 252, 1895; Vincetoxicum thesioides Freyn, Oestrr. Bot. Z. 40: 124, 1890.—TYPE: RUSSIA. Dahuria, Nertschinsk, 1889, Karo, F.K. 145 (syntypes: K, barcode K000872723!; Z, barcode Z-000001810!).
Perennial herbs. Roots long, cylindrical tuberous. Stems erect or upper part twining, ca 25 cm long, branched or not, white latex, densely pubescent. Leaves opposite, rarely 3-whorled, petiole 0.5–1.8mm long or subsessile; blades linear to narrowly lanceolate or sometimes oblong, 2.7–4.8 cm long, 2.0–8.0mm wide, apex acute or rarely rounded, base cordate or truncate, sometimes oblique with colleters at middle part of leaf base, margin entire, ciliolate, midvein elevated abaxially, lateral veins obscure, puberulent on both sides. Inflorescences umbel-like, axillary to terminal; cymules 1–10-flowered, solitary or clustered; peduncles 2.7–4.6mm long, puberulent; pedicels 1.2–3.8mm long, puberulent. Flowers calyx tube very short; lobes 5, triangular 2.3–2.8mm long, puberulent, ciliate, colleters between inside lobes; corollas white-green, glabrous; tube very short; lobes 5, narrowly triangular to oblong, 3.5–4.3mm long, mostly twisted with one direction; coronas consisting of fused staminal and interstaminal parts at base; lobes 5, laminar, triangular-lanceolate, up to middle of anthers, apex acute; gynostegia sessile; anthers 5, square, with white appendages, triangular-ovate, as long as gynostegia; pollinaria 5; corpusculum ovoid, ca. 200 μm long, ca. 100 μm wide; caudicles ca. 150 μm long, attached to apical pollinia; pollinia pendulous, ellipsoid, ca. 250 μm long, ca. 120 μm wide; style heads conical, 2-divided umbonate. Fruits not seen.
Phenology: Flowering from July to August. Fruiting not observed.
Distribution: Kazakhstan, China, Mongolia, Russia, and Korea (Gyeonggi-do) (Fig. 1).
Habitat and ecology: Grows on semi-shady slopes of rocky mountains (elev. 343 m) along with Acer pictum Thunb. var. mono (Maxim.) Maxim. ex Franch., Lindera obtusiloba Blume, Quercus mongolica Fisch. ex Ledeb., Fraxinus rhynchophylla Hance, and Securinega suffruticosa (Pall.) Rehder, among others.
Voucher specimen: KOREA. Gyeonggi-do: Gimpo-si, Wolgot-myeon, Munsusan Mt., 17 Aug 2019, B.-M. Nam & S. Y. Yang 190817-001 (ANH).
Note: Cynanchum thesioides mainly grows in open disturbed areas, sand dunes, grasslands, and roadsides of Kazakhstan, western China, central and Far East of Russia, Mongolia, and North Korea (Hwanghae-do and Pyeongannam-do) (Li et al., 1995; Lee, 1996; Im, 1999; Chang et al., 2014). It is morphologically characterized by linear to narrowly lanceolate leaves, mostly puberulent; an erect stem with twining apical parts; triangular-lanceolate corona lobes; and broadly fusiform follicles (Li et al., 1995) (Fig. 2). During the field survey, we discovered the first population of C. thesioides in Gimpo-si, Gyeonggi-do, South Korea, very close to North Korea (Figs. 1, 3A, B). The population only included 18 individuals along with Acer pictum Thunb. var. mono (Maxim.) Maxim. ex Franch., Lindera obtusiloba Blume, Quercus mongolica Fisch. ex Ledeb., Fraxinus rhynchophylla Hance, and Securinega suffruticosa (Pall.) Rehder. Two individuals flowered, and fruit set was not observed. We believe that this is the only population in South Korea and that the species is dependent mainly on asexual reproduction. The species can be categorized as critically endangered (CR) based on the B2 criterion of IUCN (IUCN Standards and Petitions Committee, 2019) because its area of occupancy in South Korea is smaller than 10 km2. However, to assess the conservation status, an intensive investigation is required, including analyses of population/individual sizes with careful taxonomic revision.
Cynanchum chinense R. Br., Mem. Wern. Nat. Hist. Soc. 1: 44, 1810.—TYPE: CHINA. Provincia Pechiley, Georgius Staunton, Baronettus, Staunton s.n. (holotype: BM, barcode BM001014201!).
C. pubescens Bunge, Enum. Pl. China Bor. 44, 1831; Vincetoxicum pubescens (Bunge) Kuntze, Revis. Gen. Pl. 2: 423, 1891.
C. deltoideum Hance, Ann. Sci. Nat., Bot., sér. 5: 228, 1866.
Perennial herbs. Roots long, cylindrical tuberous. Stems twining, many branched, terete, pubescent, white latex. Leaves opposite, petiole 22.6–41.0 mm long, 1.0–1.6 mm wide; blades triangular or triangular-ovate, 1.6–7.9 cm long, 3.1–7.1 cm wide, apex acute or acuminate, base rounded or cordate rarely truncate, 4–7 colleters at base, margin entire, ciliolate, lateral veins 5–9 pairs, pubescent usually on veins. Inflorescences axillary to terminal, dichasial cyme, 2 to 32-flowered, usually branched at base, 11.7–73.4 mm long; peduncles 1.2–29.2 mm long, 0.4–1.2 mm wide, pubescent; pedicels 1.5–6.0 mm long, 0.2–0.6 mm wide, pubescent; bracts lanceolate, 0.8–1.9 mm long, 0.1–1.1 mm wide. Flowers calyx longer than corolla tube; tube short; lobes 5, lanceolate, apex acute, pubescent, 1–2 colleters between inside lobes; corollas white; lobes 5, linear-lanceolate, rounded, recurved, glabrous; coronas white; tubular, laminar, as long as gynostegium; lobes 5, fused with tube inside, apex linear, longer than gynostegium, usually recurved; corona appendages threadlike on lobes inside, shorter than lobes; gynostegia sessile; anthers 5, triangular-ovate, appendages exceeding gynostegium; lateral 2 anther wings projected on base; anther wings parallel to each other; pollinaria 5; corpusculum ellipsoid, 163–300 μm long, 100–133 μm wide; caudicles 60–70 μm long, attached to apical pollinium; pollinia pendulous, ellipsoid, 233–333 μm long, 100–167 μm wide; style heads conical, 2-divided umbonate. Fruit follicles, usually one per flower, linear, 84.0–129.8 mm long, 4.6–7.5 mm wide, pubescent. Seeds 36–64 per follicle, oblong, 4.3–6.9 mm long, 1.8–3.1 mm wide, winged, brown, glabrous; comas 17.7–40.6 mm long.
Phenology: Flowering in August, fruiting from September to October.
Korean name: Ga-neun-teol-baek-mi (가는털백미) (Chang et al., 2014; Korea National Arboretum, 2017).
Distribution: China, Mongolia, and Korea (Incheon and Gyeonggi-do) (Fig. 1).
Habitat and ecology: In Korea, the species grows in open areas of seashore, swamp edges, and roadsides, along with Carex sp. (dominant species), Phragmites australis (Cav.) Trin. ex Steud., Artemisia lancea Vaniot, Conyza canadensis (L.) Cronquist, etc.
Voucher specimens: KOREA. Gyeonggi-do: Hwaseong-si, Namyang-eup, Munho-ri, 17 Aug 2019, B.-M. Nam & S. Y. Yang 190817-002–005 (KIOM). Incheon: Ganghwa-gun, Samsan-myeon, Seongmodo Isl., 2 Sep 2014, B.-M. Nam & G.Y. Chung 140902-100–109 (ANH); 9 Aug 2016, B.-M. Nam & G.Y. Chung 160809-100–103 (ANH).
Note: Cynachum chinense mainly occurs in thickets, roadsides, seashores, and riverbanks in China, Mongolia, and Korea (Li et al., 1995). It is clearly distinguished from other species within the genus Cynanchum by its triangular leaves with cordate bases and a tubular corona with long and slender appendages (Nam and Chung, 2018) (Fig. 4). Previously, the species has only been observed on the seashore and saltern areas of Is. Seokmodo, Incheon, South Korea. We found an additional natural population in the inland area of Hwaseong-si, Gyeonggi-do (Figs. 1, 3C, D). C. chinense is considered a rare species, found in only two locations in South Korea; however, no attempts have been made to assess its conservation status to date. To determine the correct IUCN category, more data for population size and demographic changes are needed.
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