Nymphanthus namkadingensis Tk.Yamam., Tagane & Soulad., sp. nov.

Type:— LAOS. Bolikhamxai Province: Pakkading District, Nam Aan River, Nam Kading National Protected Area, 18.3460167°N, 104.279817°E, 325 m elev., 2 September 2023, S. Tagane, P. Souladeth, T. Vongthavone, P. Phonepaseuth, T.B. Vuong, T. Yamamoto, D. Kongxaisavath, P.Q. Trong, S. Lorvanhkham Z74 [fl. & fr.] (holotype FOF!, isotypes KAG!, VNM!). Figures 1 & 2.

Nymphanthus namkadingensis is distinguished from the other members of Nymphanthus by having combination of following features: branches and branchlets glabrous, stipules of 3–9 × 1–2 mm, ovate-lanceolate laminae of 14–25 × 5–8 mm with 8–12 pairs of lateral veins, reddish purple sepals with slightly undulate (non-fimbriate) margins, annular disc glands of pistillate flowers, and glabrous ovaries.Among Nymphanthus species distributed in Laos and its adjoining countries, the new species is most similar to N. rubescens distributed in Laos and Vietnam in having floriferous branchlets rather congested near the top of main branches, ovate-lanceolate laminae 1–2.5 cm long, reddish purple sepals with non-fimbriate margins in staminate and pistillate flowers, and glabrous ovaries, but clearly distinguished by its larger stipules (3–9 × 1–2 mm in N. namkadingensis vs. 0.5–1 × 0.5–0.7 mm in N. rubescens), longer pedicels (staminate flowers 4–8 mm long and pistillate flowers 10–16 mm long vs. ca. 1.5 mm and ca. 2.5 mm, respectively), and connate and annular disc glands of pistillate flowers (vs. free, in the form of rectangular tongues) (Table 1).

Description:—Subshrubs up to 40 cm tall. Branches and branchlets reddish light green when young, later becoming brown, glabrous, longitudinally corrugated by outgrowth of superficial tissue; floriferous branchlets congested at upper end of branches. Stipules subulate to lanceolate, 3–9 × 1–2 mm, glabrous on both surfaces, reddish brown when young, later becoming dark brown, acute to acuminate at apex, rounded to cordate at base, sessile, peltate, margin entire. Leaves alternate, 50–60 per branchlet, glabrous throughout; petioles up to 1.2 mm long; laminae ovate-lanceolate, 14–25 × 5–10 mm, thinly coriaceous, reddish gray when young especially on abaxial surface, later becoming green, lustrous on adaxial surface, slightly glaucous on abaxial surface, acute to acuminate at apex, obtuse to rounded, slightly oblique at base, margin entire, revolute, midveins prominent on abaxial surface, lateral veins 8–12 pairs, not prominent on both surfaces. Inflorescences axillary, unisexual; staminate inflorescences 1-flowered or fascicle with 2–3(–4) flowers on proximal to middle axils of branchlets; pistillate inflorescences 1-flowered in distal axils. Bracts ovate, 1.2–1.5 × 0.6–0.8 mm, glabrous on both surfaces, acute to acuminate at apex, cuneate at base, margin entire. Staminate flowers: pedicels 4–8 mm long, slightly dilated acropetally, glabrous, reddish purple; sepals 4, triangular to rhomboidal, 1.2–1.4 × 1.1–1.3 mm, glabrous, reddish purple, with white banded periphery, obtuse to rounded at apex, margin almost entire, weakly undulate; disc glands 4, elliptic to reniform, alternisepalous; stamens 2, glabrous, filaments united throughout their length, forming a staminal column ca. 0.4 mm long, anthers 0.2 × 0.6 mm, thecae 2 per anther, transversely dehiscent, slightly apart each other by horizontal elongation of connective tissue; pistillode absent. Pistillate flowers: pedicels 10–16 mm long, slightly dilated acropetally, glabrous, reddish purple; sepals (5–)6, narrowly elliptic to rhomboidal, 1.5–2 × 0.8–1.2 mm, glabrous, reddish purple, with white banded periphery, obtuse to rounded at apex, margin almost entire, weakly undulate; disc glands connate, annular, forming a ring around lower third to half of ovary, margins undulate to crenate; ovary superior, subglobose, 0.7–0.9 × 0.8–1 mm, 3-locular, glabrous, sparsely verrucose; styles 3, free, up to 0.1 mm long, glabrous; stigmas almost completely bifid to base, 0.2–0.3 mm long, glabrous; staminodes absent. Capsule ellipsoid, 1.6–2.8 × 1.2–1.8 mm, glabrous, smooth to verrucose, reddish black. Seeds trigonous, 2 per locule, light brown, 1.5 × 0.9 mm, smooth; fruiting pedicels 12–19 mm long.

Additional specimen examined:— LAOS. Bolikhamxai Province: Viengthong District, Ban Hin Ngon, Nam Kading National Protected Area, in seasonal broad-leaved evergreen forest mixed with bamboo, on rocks close to a stream, 18.3793833°N, 104.434917°E, 378 m elev., 3 September 2023, S. Tagane et al. Z128 [fl. & fr.] (FOF, KAG, VNM); Bolikhan District, Ban Nam Tek, in broad-leaved evergreen forest, 18.485217°N, 104.126761°E, 427 m elev., 4 November 2023, P. Souladeth et al. Z595 [fr.] (FOF, KAG, VNM); Khamkeuth District, near Ban Kaengbid, in broad-leaved evergreen forest, on rocks along a stream, 18.22244°N, 104.50158°E, 537 m elev., 15 March 2024, S. Tagane et al. Z837 [fl.] (FOF, KAG, VNM).

Distribution: — LAOS. Bolikhamxai Province (so far four localities from Bolikhan District, Khamkeuth District, Pakkading District, and Viengthong District are known, in Nam Kading National Protected Area).

Habitat and ecology: — Nymphanthus namkadingensis is rheophytic on rocks along streams (Fig. 2A), in seasonal broad-leaved evergreen forests, at 300–550 m elev. Flowering specimens were collected in March and September, and fruiting specimens in September and November.

Etymology: —The specific epithet namkadingensis is derived from Nam Kading National Protected Area, in which the species was first discovered.

Vernacular name:— [Mak yom noy namkading (“Mak yom” means Nymphanthus species in general in Lao; “noy” means small fruit), suggested here].

Preliminary conservation assessment:— Endangered (EN). During a total of 22 days of our field surveys conducted in/around the Nam Kading National Protected Area in 2016, 2023, and 2024, Nymphanthus namkadingensis was found in four localities at the streamside of the seasonal broad-leaved evergreen forest at 300–550 m elev., with each 40–60 mature individuals. Based on the four locations, the area of occupancy (AOO, defined in IUCN 2024) is 16 km 2 and the extent of occurrence (EOO) is calculated as 313 km 2 using GeoCAT (Bachman et al. 2011). However, given that N. namkadingensis grows only open to semi-shaded streamside at elev. 300–550 m, the exact EOO should be more narrowly restricted. Although the area is encompassed with the Nam Kading National Protected Area, several dams have been constructed in the protected area, which must have already lost a large area of the habitat of N. namkadingensis. In addition, some of the vicinities are being converted to agricultural fields, which may also endanger the habitat of the species. Thus, we propose Endangered (EN) according to the IUCN criteria of B1a & b (i, ii, iii), B2a & b (i, ii, iii), and D (IUCN 2024).

Note 1: — Nymphanthus namkadingensis is also similar to N. chantaranothaii distributed in Thailand (Pornpongrungrueng et al. 2019), and N. glaucescens distributed in India to southern China and western Malesia (Chantaranothai 2007; Li & Gilbert 2008) in having elliptic laminae 1–3 cm long and red to purple sepals, but clearly distinguished from them by its larger stipules (3–9 × 1–2 mm in N. namkadingensis vs. 1.4–2 × 0.1–0.4 mm in N. chantaranothaii and 1.3–4 × 1.5–2 mm in N. glaucescens), more lateral nerves on laminae (8–12 pairs vs. 4–7 pairs and 4–6 pairs, respectively), and sepals with non-fimbriate apexes and slightly undulate, almost entire margins (vs. apexes and margins fimbriate in both species) (Table 1).

Note 2: — Bouman et al. (2022) divided the genus into two sections: section Nymphanthus and section Scepasma (Blume 1826: 582) R.W.Bouman (in Bouman et al. 2022: 23). Nymphanthus namkadingensis is placed in the former because of its pedicellate staminate and pistillate flowers, 3-locular ovaries, and bifid stigmas (in contrast the section Scepasma shows sessile or subsessile flowers, [4–]5–8-locular ovaries, and usually entire stigmas). The phylogenetic position of the new species is yet to be clarified, and thus further molecular studies will be needed to implement the phylogenetic relationship within the genus and its evolutionary history.