Rhaptopetalum rabiense Kenfack & Nguema, 2019

Kenfack, David & Nguema, Diosdado Ekomo, 2019, A new species of Rhaptopetalum (Lecythidaceae) from south-western Gabon, PhytoKeys 128, pp. 39-46 : 41-44

Rhaptopetalum rabiense Kenfack & Nguema sp. nov. Figs 1 , 2

Type.

GABON. Ogooue Maritime , 25-ha Rabi Forest plot, 1°55'37.57"S, 9°52'50.66"E, 23 m alt., 27 Aug 2014 (fl), Nguema et al. 2825a (holotype: LBV, isotypes: BR, MO, US, K, P)

Diagnosis.

Rhaptopetalum rabiense is similar to R. belingense by its dome-shaped ovary, its entire calyx margin, its apex placentation and smooth fruits, but differs by its pubescent (vs. glabrous) young branches, its longer pedicel (5-7 mm vs. 3 mm) articulated 1 mm below the calyx (vs. articulated directly below the calyx), and its uni-ovulated loci (vs. multi-ovulated) (Table 1 ).

Description.

Tree 4-6 m tall, bole cylindrical, to 20 cm diameter at 1.3 m aboveground; bark pale brown, slash fibrous, pink in the outer part, yellowish in the inner part; young branches slender, rusty brown, densely puberulous, conspicuously lenticellate. Leaves distichous; petiole 2-4(6) × 2 mm, densely puberulous; lamina obovate to elliptic, coriaceous, abundantly punctate, 8-18(21) × (2.5)5-9 cm, acuminate to broadly acute at apex, slightly unequal and rounded at base; margin entire; midrib conspicuous and flattened above, prominent and minutely puberulous beneath; secondary veins 8-11 pairs, plane above, prominent beneath, arching and joined towards the margin of the lamina. Inflorescence ramiflorous, axillary and supra-axillary, fasciculate, with 1-8 flowers. Bracts ovate, about 1 mm long. Flower bud ovoid to globose-oblong, rounded at apex, (2.5)3-4 mm long, pink to red directly above the calyx rim, light pink to whitish towards the apex. Pedicel in flower buds (2.5)3-5 mm, but generally 5-7 mm long in opened flowers, yellow-green, minutely puberulous to glabrescent, articulate directly beneath calyx. Calyx cupuliform, yellow-green, glabrescent to pu berulous, the margin entire, 2-2.5 mm in diameter on the rim, receptacle about 1 mm long. Pseudocorolla fleshy, splitting into (3)4 lobes 2.5-4 mm long oblong to ovate lobes. Stamens 35(37), the filament light pink to whitish, about 0.3 mm, the poricidal anthers bright yellow, slightly arched towards the centre of the flower, 2-2.5 mm long; ovary superior, globose, about 1 mm high and 1.5 mm diameter, 3-4-locular, each locule with 1 or 2 axile pendulous ovules. Style 3-4.5 mm long. Fruit a globose capsule, green, smooth, 15-20 mm diam., fruiting pedicel 5-7 mm long, seeds 8-12 × 5-8 mm.

Phenology.

Flowering August-October, Fruiting September-December.

Geographic distribution.

Rhaptopetalum rabiense is only known from the type locality, the Rabi forest (Figure 3 ).

Habitat.

Rhaptopetalum rabiense grows in old growth forest, on both terra firme dry and wet depressions, with elevation 20-50 m.

Additional specimens examined.

GABON. Ogooue Maritime : Rabi, 25-ha permanent plot, 24 m elev., 1°55'28.1"S, 9°52'48.26"E, 21 August 2013 (st), Nguema et al. 1743; Rabi, 25-ha permanent plot, 50 m elev., 1°54'51.36"S, 9°52'41.56"E, 28 October 2013 (st), Nguema et al. 1922; Rabi, 25-ha permanent plot, 50 m elev., 1°55'27.09"S, 9°52'41.56"E, 5 November 2013 (st), Nguema et al. 2057; Rabi, 25-ha permanent plot, 32 m elev., 1°55'37.57"S, 9°52'50.66"E, 2 September 2014 (fl), Nguema et al. 2825B; Rabi, 25-ha permanent plot, 32 m elev., 1°55'37.57"S, 9°52'50.66"E, 2 September 2014 (fr), Nguema et al. 2825C; Rabi, 25-ha permanent plot, 32 m elev., 1°55'30.22"S, 9°52'42.18"E, 23 September 2014 (fl), Nguema et al. 2832; Rabi, 25-ha permanent plot, 32 m elev., 1°55'30.35"S, 9°52'41.79"E, 2 October 2014 (fl), Nguema et al. 2833; Rabi, 25-ha permanent plot, 61 m elev., 1°55'36.44"S, 9°52'46.61"E, 2 December 2014 (fr), Nguema et al. 2926 .

Preliminary conservation status.

The conservation status of Rhaptopetalum rabiense was evaluated using the IUCN Red List Categories and Criteria Version3.1 ( IUCN 2012). The extent of occurrence (EOO) and the area of occupancy (AOO), estimated using the web Geospatial Conservation Assessment Tool or GeoCAT ( Bachman et al. 2011) and the auto-value cell size length of 2 km, were 0.214 km2 and 8.00 km2, respectively. These two values meet the criteria B1 (EOO <100 km2) and B2 (AOO <10 km2) for Critically Endangered, following the IUCN Red List Categories and Criteria Version 3.1 ( IUCN 2012). The species is not under legal protection. The type locality is an oil and gas production field and, at the same time, a logging concession. These activities are likely to result in its population reduction and/or in a fragmentation of its range. R. rabiense in the studied plot has 299 individuals with dbh ≥ 1cm (12 individuals per hectare) and seems to be regenerating, based on diameter size class distribution. It is known only from its type locality and has not been recorded in the adjacent national parks of Loango National Park on the west and Moukalaba Doudou National Park on the east. We therefore assess R. rabiense with the preliminary IUCN Red List status of Critically Endangered CR B12ab(iii).

Figures

Figure 1. Rhaptopetalum rabiense A flowering twig B close-up of the lower surface showing the indumentum and the punctate lamina C detail of inflorescence and stem showing lenticels D flower bud E opened flower F flower with pseudocorolla and stamens removed showing superior ovary G longitudinal section of F showing pendulous ovules H fruiting branch.

Figure 2. Rhaptopetalum rabiense A flowering branch B lateral view of the flower C flower view from above showing the poricidal anthers and the gynoecium D fruiting branch. Photographs by Diosdado Nguema.

Figure 3. Type locations of Rhaptopetalum rabiense and the closely related species R. belingense in Gabon.