Tetrastigma fruticosum Soulad., Phonep. & Tagane, sp. nov. (Figs 1, 2)

TYPE: — LAOS. Khammouane Province, Khounkham District, The Rock Viewpoint, Phou Pha Marn, in semi-shaded limestone karst, 18.177560°N, 104.48400°E, 455 m elev., 7 Sep. 2023, young fr., Tagane et al. Z534 (holotype: FOF [FOF0005255!]; isotypes KAG [KAG186966!, KAG187205!], VNM!).

Tetrastigma fruticosum is distinct from all other species in the genus by its shrubby habit to 6 m tall, absence of tendril, pedately 5-foliolate leaves with leaflets shallowly emarginate at apex, and 1-seeded oblong-ellipsoid fruits. Among the species of Tetrastigma in the Indochina Peninsula, T. fruticosum is most similar to T. calcicola Kochaiphat & Trias-Blasi in Kochaiphat et al. (2016: 4) known from an open limestone mountain in northern Thailand, in leaf shape and size, but clearly distinguished by the above diagnostic characters.

Plants shrub or tree-like shrub, 2.5–6 m tall, up to 10 cm d.b.h. Stems terete, bark grayish brown, corky. Young twigs terete, glabrous, lenticellate, yellowish green in vivo, blackish brown in sicco, old twigs grayish brown in sicco, with ridges; tendrils absent. Stipules very broadly ovate, 1–1.2 × 2.8–3 mm, glabrous. Buds ovate, apex acute. Leaves compound, pedately 5-foliolate, rarely 4, 6 or 7-foliolate; petiole 4.1–7.7 cm long, glabrous, base slightly pulvinate; leaflets thinly coriaceous; terminal leaflet petiolule 0.9–1.7 cm long, glabrous, terminal leaflet blade elliptic-obovate, oblong-obovate, 4.2–6.2 × 1.4–2 cm, apex slightly emarginate, margin crenate, with (2–)3–5 teeth on upper 2/3, base acute to cuneate, petiolules of lateral leaflets complex 0.8–2.1 cm long, glabrous, lateral leaflet blades elliptic, elliptic-obovate, obovate, 1.6–5.5 × 0.9–2.1 cm, apex and margin as for terminal leaflet, base cuneate, asymmetrical; midrib sunken adaxially, prominent abaxially, secondary veins 4–6 pairs, prominent abaxially, tertiary veins inconspicuous adaxially, reticulate, slightly prominent abaxially. Inflorescences and flowers not seen. Infructescence compound cyme, up to 5.2 cm long, 6.5 cm in diam., peduncle 0.9–3.2 cm long, blackish brown, glabrous, lenticellate. Berries oblong-ellipsoid, 0.9–1.1 cm long, 0.5–0.7 cm in diam., surface smooth, yellowish green, 1-seeded. Seeds oblong-ellipsoid, 7–8 × 4.5–5 mm, yellowish brown to reddish brown, testa slightly transversely rugulose when young, smooth when mature, adaxial surface with a Y-shaped furrow, abaxial surface with an oblong-elliptic chalaza, endosperm Mshaped in cross-section.

Additional specimen examined:— LAOS. Khammouane Province: Khounkham District, the Rock Viewpoint Phou Pha Marn, 18.177497°N, 104.483794°E, 446 m elev., 2 July 2022, Souladeth et al. PPM15 (FOF); same locality, 18.17689°N, 104.48409°E, 405 m elev., 19 March 2024, with fruits, Tagane et al. Z1202 (FOF, KAG [KAG187601, KAG187602], VNM); Gnommalat District, Pha Katai trail, 17.5529°N, 105.1641°E, 173 m elev., 5 September 2024, sterile, Tagane et al. Z1636 (FOF, KAG [KAG188301], VNM); Hinboun District, walking trail on limestone hills nearby Khoun Kongleng, 17.64508°N, 104.81160°E, 203 m elev., 6 September 2024, sterile, Tagane et al. Z1703 (FOF, KAG [KAG188366], VNM).

Distribution:— Laos (Khammouane Province).

Habitat:— Open to semi-shaded limestone karst, 170–460 m elev.

Phenology:— Fruiting from July to March.

Etymology:— The specific epithet fruticosum refers to the shrubby habit of the new species.

Vernacular name:— Ton Khao Poun (ຕົ້ນເຂົາປູນ; Lao), suggested here. Ton in Lao refers to a shrubby habit, Khao Poun refers to common name for Tetrastigma in general.

GenBank accession no.: — PP920557 (ITS; sequenced from Tagane et al. Z534).

Preliminary conservation assessment:— Endangered (EN). Currently Tetrastigma fruticosum is known only from three localities in Khammoaune Province, central Laos, where the above specimens collected. We observed about 30 to 50 mature individuals in each location. The Area of Occupancy (AOO) is 12 km 2 and the Extent of Occupancy (EOO) is calculated as 958.1 km 2 using GeoCAT (Bachman et al. 2011). The species is considered to be endemic to the limestone karst in this area, which is widely seen and mostly located within Phou Hin Phoun National Protected Area. As the topography is very steep and sharp (Fig. 2A), the habitat of T. fruticosum is likely stable, but more or less negative impact may be suffered, such as limestone mining, dam construction and construction for tourist attraction. Given this situation, it is assessed as Endangered (EN) according to IUCN criteria B1a, b(iii), 2a, b(iii) (IUCN 2024).

Note: — Among the Tetrastigma species in Laos, T. fruticosum may be similar to T. pachyphyllum (Hemsley 1886: 135) Chun (1940: 235) in its leaf shape and size, but differs in its shrubby habit (vs. woody climber in T. pachyphyllum) and smaller and oblong-ellipsoid berries 0.9–1.1 × 0.5–0.7 cm (vs. globose, (1–) 1.4–1.8 cm in diam.).

Limestone areas are known to have unique ecosystems and plant diversity due to their alkaline soils and complex geographic features (Clements et al. 2006, Maneeanakekul & Sookchaloem 2012). Most species of Tetrastigma are dominant climbers in rainforests and subtropical forests and only a few species inhabit lowland shrublands or rocky mountain cliffs (Jackes 1989, Ren & Wen 2007). The shrubby habit of T. fruticosum (Fig. 2A, B) is considered to be an adaptation of the severe environment of open and rocky limestone karst of the Phou Pha Marn. This might represent a good example characterizing the endemism of this area. Indeed, several endemic plant species were reported from this limestone and its surrounding areas (Newman 2008, Haevermans et al. 2020, Phonepaseuth et al. 2021, Vongthavone et al. 2024). In addition to these plants, we collected several taxa in various groups including Apocynaceae, Araliaceae, Gesneriaceae, Malvaceae, Melastomataceae, and Phyllanthaceae, which did not match any previously known species. Further botanical inventories and subsequent taxonomic studies are needed to elucidate the overall flora of the limestone area in central Laos.