Disepalum rawagambut Randi, D.C.Thomas & Wijedasa, 2022

Randi, Agusti, Wijedasa, Lahiru S. & Thomas, Daniel C., 2022, Disepalum rawagambut (Annonaceae), a new tree species from peat swamp forest of Sumatra, Indonesia, Phytotaxa 530 (1), pp. 121-126 : 121-125

Disepalum rawagambut Randi, D.C.Thomas & Wijedasa , sp. nov.

( Figs 1–2 ).

Type:— INDONESIA. South Sumatera Province: Musi Banyuasin Regency, Bayung Lencir District , Muara Medak Village , conservation area of PT Tri Pupajaya forest production concession, 10 m. elev., 1°46’14.52”S, 104°12’6.82”E, 30 Nov 2020, Randi GB-035 (holotype: BO; isotypes: WAN, FIPIA, SING) .

Disepalum rawagambut has flowers with two sepals and four relatively short, spatulate corolla lobes that are thickened in the distal part with blunt apices similar to those of D. anomalum (Borneo) and D. longipes (Malay Peninsula, Sumatra). The new species can be differentiated by a combination of characters including its habit, large sepals, smaller corolla lobes and larger fruit (larger torus, longer carpophores, and larger monocarps; Table 1 ).

Medium-sized tree to 25 m tall and to 28 cm DBH; monopodial, lateral branches almost horizontal; stilt roots often present on older trees. Bark rough, irregularly cracked to scaly and peeling off, dull to dark brown with grey-white patches; inner bark pale brown to chocolate; sapwood cream to pinkish; bark at the sapling stage smooth or shallowly cracked and often with conspicuous horizontal lines. Leafless twigs cylindrical with conspicuous and persistent petiole scars; leafy portion of the twigs 2–5 mm thick with a conspicuous raised edge adaxially, glabrous, green to shiny brown with age. Leaves simple, alternate, coriaceous, elliptic or sometimes lanceolate or oblanceolate, 3.0–12.0 × 1.7–6.0 cm; base cuneate to shortly decurrent; margin entire, slightly thickened at the base and minutely recurved; apex bend downwards, acute to shortly acuminate, acumen 2–8 mm long, apex blunt; adaxial surface green, glabrous, abaxial surface pale green, puberulent and appressed pubescent on the midrib, glabrescent; midrib sunken adaxially, raised abaxially; secondary veins slender, 7–10 pairs, brochidodromous, visible adaxially and indistinct abaxially when fresh; tertiary venation widely reticulate, slightly raised and visible on adaxial surface when fresh, slightly sunken and indistinct abaxially. Petiole 4–9 mm long, channeled adaxially, glabrous, green to pink or pale brown when fresh. Flowers solitary or rarely in a 2-flowered inflorescence, borne terminally, or sometimes leaf-opposed with the opposing leaf reduced in size; 2-flowered inflorescence a fan-shaped cyme (rhipidium), peduncle and internodes ca. 1–2 mm long, bracts 12–23 × 4–12 mm, oblanceolate to narrowly oblanceolate, with scattered simple hairs on both surfaces, caducous; pedicel 4.1–7.5 × 1.2–2.5 mm, with scattered, appressed hairs, pink to pale brown when fresh, rarely green. Sepals 2, broadly obovate or broadly elliptic, 10–16 × 8–14 mm, boat-shaped or flat when young and becoming boat-shaped with age, coriaceous, apex obtuse, adaxial surface glabrous or rarely puberulous, light green and sometimes pinkish to dull brown in the basal part, abaxial surface with scattered appressed hairs, green to crimson. Petals connate, corolla tube 5–11 mm long, obconical, slightly thickened at the base, adnate to torus, dark red to blackish at the base, distally greenish, distal half of tube pubescent with appressed hairs abaxially; corolla lobes 4, greenish yellow to brownish yellow, spatulate, wider and thicker at apex, strongly incurved, 2.0–4.0 × 0.8–1.2 mm at the widest point, apex rounded, appressed pubescent on both surfaces; stamens numerous, 1.7–2.2 mm long, connective extending beyond the pollen sacs at the apex, apex nearly flat to rounded, puberulent; carpels numerous (to 170), ovules (1–)2 per carpel, ovaries 0.4–0.9 mm long, surface glabrous or puberulent with appressed hairs and with long-erect hairs at the base, style and stigma oblong, 0.8–1.2 mm long, setulose. Pedicel in fruiting stage 4.5–10.0 cm long, 3.5–8.0 mm thick, woody, glabrous, light to pale brown. Sepals persistent in fruit, yellow and then drying pale to dark brown. Torus in fruiting stage 2.0– 3.5 cm in diameter; carpophores (10–) 40–86 mm long, 2–6 mm thick, thickest at the apex, dark red when fresh. Monocarps (22–)56–142, ovoid to ellipsoid, 10–21 × 6–15 mm, base rounded, apex rounded or apiculate, the surface often foveate, glabrous; immature monocarps red, turning shiny black when ripe; pericarp 1–2 mm wide, juicy, deep purple to blackish when fresh. Seeds 2, rarely 1, flattened-ellipsoid, 6.2–11.0 × 4.0–9.0 mm, light to dark brown with clearly visible grooves on the surface when fresh.

Distribution:— So far only known from the type locality. However, a wider distribution seems likely given the wide distribution of suitable peat swamp forest habitat along the east coast of Sumatra.

Ecology and phenology:— Intact peat swamp forest with soil conditions that are always wet to flooded. Growing scattered in relatively undisturbed forest, D. rawagambut is uncommon at the type location. Only two mature individuals were found within a two-hectare plot, and no seedlings were observed in the vicinity of the mature trees. Although rare, some seedlings were observed in regenerated peat swamp forest not far from the intact forest. Flowering and fruiting March–April and October–November. The fruits were observed to be eaten by hornbills, and fallen fruits are eaten by terrestrial mammals like wild boars and mouse deer.

Etymology:— The species epithet refers to the specific habitat of the new species in Bahasa Indonesia: rawa gambut (peat swamp).

Vernacular name: — Pisang-pisang (Palembang Malay); this vernacular name is also applied to other Annonaceae species.

Provisional conservation status assessment:— Critically endangered: CR C2a(i) ( IUCN 2019). Disepalum rawagambut grows in relatively undisturbed lowland peat swamp forest, which is protected by mandatory regulations for private forestry concessions. More extensive collecting in the wider area may reveal otherwise, but for now we must assume a small populations size and a restricted distribution in Sumatra. Threats from active illegal logging operations and forest fires in the area surrounding the type location were observed.

Uses:— Local use for machete handles and firewood.

Notes:— Morphologically, D. rawagambut is most similar to D. longipes , but apart from morphological differences ( Table 1 ), this species prefers forests at 600–1200 m in Sumatra, whereas D. rawagambut is a lowland peat swamp species.

Figures

FIGURE 1. Disepalum rawagambut. A. Trunk and stilt roots on old tree. B. Twig with flowers and infructescence. C. Close-up of twig. D. Adaxial surface (upper) and abaxial surface (below) of leaves.All photos by A. Randi from the holotype (Randi GB-035).

FIGURE 2. Disepalum rawagambut. A. Flower with pedicel (side view). B. Flower (top view). C. Flower (side view). D. Infructescence. E. Carpophore and monocarp. F. Immature monocarp (longitudinal section). G. Mature monocarp (longitudinal section) and seeds. All photos by A. Randi from the holotype (Randi GB-035).