Pleiocarpa robusta O. Lachenaud, 2022

Jongkind, Carel C. H. & Lachenaud, Olivier, 2022, Novelties in African Apocynaceae, Candollea 77 (1), pp. 17-51 : 39-40

Pleiocarpa robusta O. Lachenaud , sp. nov. ( Fig. 9E , 13 ).

Holotypus: GABON. Ogooué-Lolo: right bank of Ogooué R. near Lastoursville, c. 0°50'S 12°41'E, 21.IX.1970, fl. & imm. fr., Breteler 6558 ( BR [ BR0000014550201 ]!; iso-: LBV!, WAG [ WAG.1620439, WAG.1620438, WAG.1620437]! + spirit!) .

Pleiocarpa robusta O. Lachenaud resembles P. mutica Benth. in its uniovulate carpels, otherwise not found in the genus, and in corolla characters, but has secondary leaf veins prominent and strongly curved (vs. very weak and almost straight) and fruits covered with small domed warts, while those of P. mutica are smooth in vivo and minutely wrinkled when dried. It also resembles P. rostrata Benth. , especially in vegetative characters, but the latter species has 3 – 4(– 6) ovules per carpel and larger fruiting mericarps, 20–50 × 15–25 mm as opposed to (9–)15–18 × 7–11 mm in P. robusta .

Shrubs 1 – 2 m tall, branched, with white latex. Twigs glabrous, 1 –2.5 mm thick, blackish, ± angled and longitudinally striate when dried, somewhat lenticellate. Leaves glabrous; blade elliptic, 12.5–25 × 3.8–11.5 cm, acute at base, abruptly caudate for 1.5– 3 cm at apex, coriaceous, slightly shiny, drying olive green to greyish above, pale grey-green to olive-brown below; midrib prominent in centre and sulcate laterally on upper side, prominent on lower side; secondary veins 6–14 pairs, well-marked and conspicuous, moderately ascending, bullate above and strongly prominent below, anastomosing <1 mm from leaf margin; tertiary veins forming a transversely stretched network, rather dense and prominent above in dry state, laxer and less conspicuous below; petiole 0.7–1.7 cm long, wrinkled when dried, upper side canaliculate with margins almost touching each other. Inflorescences terminal or axillary, glomerulate, sessile, 4–10-flowered; bracts minute, ovate, 1–1.8 × 0.8–1.2 mm, rounded at apex, glabrous. Flowers 5-merous, fragrant; pedicel 0–1 mm, glabrous. Calyces glabrous, divided almost to base, lobes imbricate, elliptic to ovate, 1.5–1.8 × 1.3 mm, rounded or obtuse at apex, without colleters inside. Corollas white; tube 15–16 × c. 1 mm (at midheight), slightly swollen at base (1.5 mm broad) and at apex (1.3 mm broad) which is ± pentagonal, glabrous outside, papillose inside around stamens and sparsely villose for c. 6 mm below their insertion; lobes narrowly elliptic, 5–6 × 2–3 mm, laterally auriculate at base, rounded at apex, overlapping and contorted to left, glabrous on both sides. Anthers included, sessile, inserted c. 2 mm below mouth of tube with their apex almost reaching throat, narrowly ovate, c. 1.6 × 0.8 mm, acute or obtuse at apex, glabrous. Pistils included, reaching just under base of stamens; ovaries glabrous, with 5 carpels c. 1 × 0.4 mm, uniovulate; styles c. 12 mm long, glabrous; stigma swollen, ovoid, c. 1 mm. Fruits dark green when young, orange when mature, with 5 free carpels, these obovoid to ellipsoid, (9–)15–18 × 7–11 mm, acute at apex, glabrous, surface densely covered with domed warts, sometimes with irregular larger warts. Seeds obovoid, one per carpel, c. 11 × 6.5 mm.

Distribution, ecology and phenology. – This species is endemic to Gabon, where it is restricted to the middle Ogooué valley between Ndjolé and east of Lastoursville; it appears to be rare in the west of its range, but relatively frequent in the east ( Fig. 6 ). It occurs in lowland evergreen forests on drained soils, at elevations of 250– 500 m. Flowers have been collected in January, July and September-October; immature fruits from September to January, mature from March to May.

Conservation status. – Pleiocarpa robusta is endemic to Gabon and occurs in lowland evergreen forests; it is known from 16 collections representing 16 localities. All occurrences are considered still extant, due to the recent dates of collection and the important forest cover still present in the areas. Based on a 2 × 2 km cell size, its Area of Occupancy (AOO) is estimated as 64 km ², which falls under the threshold for “Endangered” status under Criterion B2. Its Extent of Occurrence (EOO) is calculated as 9,032 km ², which falls under the threshold for “Vulnerable” status under Criterion B1. The 16 occurrences represent 4–7 subpopulations. None of them are included within protected areas, but the species is likely to be present in Lopé and Ivindo National Parks, since it has been collected not far from their borders. Most of the occurrences (15) are located within logging concessions, one occurrence near Ndjolé is not included in a concession, but was collected near an access road. All the 16 sites are threatened by logging, which suggests a decline of the extent and quality of habitat of this species. Two of them, which are in the same logging concession near Booué, are additionally threatened by a road building project, which is expected to cause a decline in the extent and quality of habitat and number of individuals; they represent a single location (sensu IUCN). The occurrence near Ndjolé represents another location. Three occurrences are included into three different logging concessions, and therefore represent three different locations. Ten occurrences are located within a single logging concession, east of Lastoursville; one is in a conservation zone within the concession and represents one location, the other are in exploited areas and represent a second location. Consequently, the 16 occurrences represent seven locations, with regard to the most serious plausible threat (logging). We infer a past, present and future continuing decline of the extent and quality of the habitat of this species and number of individuals. Pleiocarpa robusta therefore qualifies for “Vulnerable” [VU B1ab(iii,v)+2ab(iii,v)].

Notes. – This species has previously been mistaken for Pleiocarpa mutica , from which it differs by the characters mentioned in the diagnosis like the small domed warts on the fruit carpels ( Fig. 9E, F ); it is also generally more robust in all its parts (hence the specific name) but the dimensions overlap widely. The maximum size of the leaves of P. mutica (29.5 × 11.4 cm), as well as those of the corolla tube (22 mm long), lobes (13 × 4.3 mm) and style (16 mm long) mentioned in the description by OMINO (1996: 142), presumably refer to another taxon, since we found these dimensions to be consistently smaller in P. mutica s.str. (see Table 5). From the three Gabonese collections cited under P. mutica by OMINO (1996: 146), two are P. robusta (Breteler 6558 and 13338) while the third is P. brevistyla Omino (Breteler & van Raalte 5635). Three other specimens, McPherson 17968, Nguema Miyono 1879 & Wieringa 856, two of them collected after the revision, are now representing P. mutica for Gabon. Pleiocarpa mutica occurs from Sierra Leone to Gabon and appears to reach its southern limit in the Monts de Cristal in north-western Gabon; its range is thus separate from that of P. robusta , which occurs in central and east-central Gabon.

The identity of a single fruiting collection from south Cameroon, J.J.F.E. de Wilde 8150A, is uncertain for the moment since it resembles Pleiocarpa robusta in fruit ornamentation but P. mutica in leaf characters; complementary material is needed to solve its status.

In addition to Pleiocarpa mutica , the new species may be confused with P. rostrata and Hunteria hexaloba that are both very similar in vegetative characters, and with Pleiocarpa picralimoides (Pichon) Omino ; differences between all these taxa are summarised in Table 5. It must be noted that P. rostrata , as currently delimited, is exceedingly variable in the size of its flowers and fruits, and in the shape and ornamentation of the latter; it may quite probably consist of more than one taxon. The main range of P. rostrata is in Nigeria and Cameroon; a single collection from Gabon, Le Testu 8871, cited by OMINO (1996), appears to be correctly identified, though quite distant geographically from the rest and occurring within the range of P. robusta (from which it differs by its pluriovulate carpels and larger corolla).

Additional specimens examined. – GABON. Moyen-Ogooué: 15 km NE of Ndjolé , 0°07'S 10°49'E, 31.I.1991, fl. bud, de Wilde & Sosef 10323 ( LBV, WAG) . Ogooué-Ivindo: 3 km SE de Booué , 0°08'S 11°57'E, 13.VI.1987, fl., Wilks 1574 ( WAG) ; Forêt des Abeilles , autour du camp 1, concession forestière CFAD, 0°08’S 11°58’E, 21.II.2021, fr., Paradis et al. 302 ( MO) . Ogooué-Lolo: c. 30 km NE of Lastoursville, forest exploitation on right bank of Ogooué River , 12.IV.1990, fr., Breteler et al. 9898 ( WAG) ; c. 30 km E of Lastoursville, road to l’Ogooué , 7.V.1992, fr., Breteler et al. 11358 ( WAG + spirit); c . 14 km NE of Bambidie , 17.X.1994, fl. bud, Breteler et al. 13338 ( WAG) ; Bambidie , c . 30 km E of Lastoursville , 0°44'S 12°58'E, 9.XI.1999, imm. fr., Breteler 15421 ( BR, LBV, WAG) ; Forêt des Abeilles , Station de la Makandé, 0°40'39"S 11°54'35"E, 1.XI.1996, imm. fr., Doucet 448 ( BR) ; E of Lastoursville , near Bambidie, C.E.B. chantier, 0°46'S 13°03'E, 21.IX.1996, fl. buds, McPherson 16660 ( BR, LBV, WAG) ; E of Lastoursville , near Bambidie, C.E.B. chantier, 0°46'S 13°00'E, 23.IX.1996, fl., McPherson 16669 ( LBV) ; Bambidi, axe Lastoursville- Ndangui , 0°45"S 12°58"E, 21.V.2003, fr., Nziengui et al. 518 ( LBV) ; environs du village Vouboué, Forêt des Abeilles , 29.VIII.1983, fl., Sita 5177 ( LBV) ; Concession CEB, Réserve de Milolé , 0°36'52"S 12°47'17"E, 15.III.2017, fr., Texier et al. 913 ( BRLU, MO) ; 33 km ENE of Lastoursville , 6 km on CEB exploitation road B, 0°42'S 13°0'E, 14.XI.1994, imm. fr., Wieringa et al. 3175 ( WAG) ; c. 30 km ENE of Lastoursville, 15 km on forestry road from Bambidie to Akieni, 0°39'S 12°56'E, 29.I.2008, imm. fr., Wieringa et al. 6262 ( BR, LBV, MO, WAG) .

Figures

Fig. 9. – Landolphia persooniana O. Lachenaud & Jongkind: A, B. Leaves and inflorescence. Landolphia subrepanda (K. Schum.) Pichon: C. Inflorescence. Landolphia verrucosa Jongkind & O. Lachenaud: D. Fruiting branch from above.Pleiocarpa robusta O. Lachenaud: E. Surface of a dried monocarp covered with domed warts. Pleiocarpa mutica Benth.: F. Surface of a dried monocarp.

Fig. 13. – Pleiocarpa robusta O. Lachenaud. A. Flowering branch; B. Calyx and ovary (two sepals removed); C. Flower bud (longitudinal section); D. Corolla; E. Flowering carpel (cross-section); F. Infructescence; G. Fruiting carpel; H. Fruiting carpel (part of pericarp removed) showing

Fig. 6. – Distribution map of Hunteria maasiorum Jongkind & E. Bidault (open circle) and Pleiocarpa robusta O. Lachenaud (closed circles). The open circle includes all cited collections of H. maasiorum.