Introduction

The genus Voacanga Thouars (Apocynaceae) occurs in the Old World tropics and includes trees and shrubs of rainforest areas. It belongs to tribe Tabernaemontaneae G.Don subtribe Tabernaemontaninae A.DC. (Endress et al. 2018) and is closely related to Tabernaemontana L. They have in common a dichotomous branching pattern, petioles connate at the base into a short ochrea, corolla lobes contorted to the left, absence of a corona, anthers tailed at the base and included (or with only the very tips exserted), fleshy fruits with two carpels and arillate seeds. The two genera differ by their corolla lobes, which are inflexed in bud in Tabernaemontana vs not inflexed in Voacanga; the pistil head which is free from the anthers vs coherent with the anthers and, therefore, shed with the corolla; and the calyx which is persistent vs caducous.

The genus as a whole was last revised by Leeuwenberg (1985), who recognised twelve species, with seven occurring in Africa (one of them also in Madagascar) and five in Asia. However, some of the African species were very broadly delimited in this revision. Five new species have recently been described and a further three resurrected from synonymy (Jongkind & Lachenaud 2022; Jongkind 2022). Taking into account the new species treated in this paper, Voacanga now includes 21 species, of which 16 are African. This number is likely to increase further, since some taxonomic issues remain among the Central African taxa (Jongkind 2022).

The new species, treated here, was initially discovered in south-western Cameroon, and recognised as new by Cheek et al. (2004: 245), but was not described at that time, since only fruiting material was available. In 2010, while conducting an ethnobotanical field survey in Príncipe Island, M. C. Madureira made another fruiting collection near Picos Mencorne (Madureira & Equipa Proj. Pagué III 87). Several later expeditions to the same locality resulted in additional specimens, but only in 2020 was a specimen with open flowers finally collected (D. Dias & M. Sebastião 200). It proved identical with earlier flowering collections from Bioko (Equatorial Guinea) and south-west Cameroon, which had been misidentified as either Voacanga africana Stapf or V. psilocalyx Pierre ex Stapf. The new species is here described as V. madureirae Paiva, O.Lachenaud & Covelo, and compared with its relatives. An assessment of its conservation status is also presented.

Materials and Methods

This paper is based on a study of herbarium collections in BM, BR, BRLU, COI, EA, K, LBV, MA, P, PRP, STPH, WAG and YA (herbarium acronyms according to Thiers 2023, continuously updated, except PRP – Herbarium of Principe, São Tomé and Príncipe). The species description is based on dried material, flowers preserved in spirit, field photographs, and collectors’ notes. Specimens seen by the authors are denoted with a ‘!’. The following abbreviations are used: diam. – diameter; fl. – flowering; fl.b – flower buds; fr. – fruiting; imm. fr. – immature fruits; and st. – vegetative.

The conservation status of part of the species was assessed following the IUCN Red List Categories and Criteria and guidelines (IUCN 2012, 2019). The extent of occurrence (EOO) and area of occupancy (AOO) were calculated using GeoCAT (Bachman et al. 2011) with a cell size of 2 × 2 km. The number of ‘locations’ (as defined by IUCN 2012) was calculated with regard to the main threats, such that a single location may encompass several occurrences.

Taxonomic Treatment

Voacanga madureirae Paiva, O.Lachenaud & Covelo sp. nov. Type: São Tomé and Príncipe, Príncipe, D. Dias & M. Sebastião 200 (holotype COI! [sheets, COI00100961 & COI00100962 + spirit collection, COI00100963]; isotypes BRLU!, MO!, PRP, STPH).

http://www.ipni.org//urn:lsid:ipni.org:names:77336451-1

Voacanga sp 1. auct.: Cheek & Leeuwenberg in Cheek et al. (2004: 245); Beentje & Harvey in Harvey et al. (2004: 84); Cheek et al. in Harvey et al. (2010: 110)

Shrub or small tree, 3 – 15 m high, up to 20 cm in diam., with abundant white latex in all parts; bark brown. Branches pale brown to greyish, inconspicuously lenticellate; branchlets terete, glabrous, 2 – 4 mm thick. Leaves opposite, those of a pair unequal or subequal, petiolate; petiole 0.5 – 3.8 cm long, glabrous, those of a pair connate into a short ocrea, with several colleters; leaf blade papyraceous, elliptic to slightly obovate, 2 – 3.5 × as long as wide, 9 – 28 × 3.2 – 11 cm, acute to acuminate at the apex, cuneate at the base, margin entire, glabrous on both sides; midrib flat or slightly depressed above, prominent below; secondary veins 6 – 16 pairs, 5 – 20 mm from each other, slightly prominent above and prominent below; tertiary veins invisible or very laxly reticulate. Inflorescences terminal (often becoming lateral due to sympodial growth of the twigs) and paired, umbellate to shortly cymose, 4.5 – 17 cm long, 3.5 – 8 cm in diam., 7 – 33-flowered, glabrous; peduncle 4.5 – 13 cm long; ramifications 0 – 1.3 cm long; bracts soon deciduous, narrowly elliptic, 5 × 1.5 – 2 mm, rounded at apex, glabrous. Flowers 5-merous; pedicel 8 – 18 mm long, glabrous. Calyx pale green to yellowish-white (paler inside), 9 – 15 mm long, 12 – 21 mm in diam., glabrous, deciduous in fruiting stage; tube campanulate, 1.5 – 4 (– 7) × 3 – 5 (– 8) mm, with a continuous row of thick colleters inside at the very apex or up to 1 (– 2) mm below; lobes elliptic to obovate, 6 – 12 × 2.5 – 9 mm, rounded at the apex, erect and completely enclosing the corolla in bud, patent to recurved at anthesis. Corolla white or cream, smaller than the calyx, 10 – 16 mm in diam., tube 6 – 9 × 3 – 5 mm, 5-angled, constricted at the throat and a little below mid-height, only slightly twisted, glabrous outside, inside with a dense band of short appressed hairs 1 – 2 mm wide below the insertion of the stamens; lobes contorted to the left in bud, obovate, 4 – 8 × 1.7 – 6 mm, rounded at the apex, glabrous, patent at anthesis. Stamens included, or with the very tip exserted, attached on the corolla tube towards mid-height; anthers subsessile, narrowly triangular, 3 – 4.5 × 1.2 – 3 mm, acute at the apex, sagittate at the base, glabrous, coherent and forming a cone around the pistil head, slightly twisted with the corolla. Pistil included, shorter than the stamens, 5 – 6 mm long; carpels two, connected at the base, 1.5 mm long, 1 – 2 mm in diam., rounded, glabrous; disk annular, c. 0.8 mm long, glabrous; style not split at the base, 4 – 5 mm long, glabrous, more or less gradually thickened at the apex; clavuncula capitate, 1 – 1.5 × 1 – 1.2 mm, with a ring 1.7 – 2 mm in diam.; ovules numerous in each carpel. Fruit green, or green spotted yellow, drying black, ellipsoid and more or less heart-shaped, the apex usually slightly bilobed, 1.8 – 5 × 2.5 – 7 cm, smooth, glabrous, on a thickened accrescent pedicel 2 – 2.5 cm long, tardily dehiscent along a single terminal suture. Seeds brown, obliquely ellipsoid, 8 – 11 × 4 – 5 mm, strongly furrowed and minutely tuberculate. Figs 1, 2.

Fig. 1.
figure 1

Voacanga madureirae. A flowering branch; B flower; C open calyx with colleters; D section of corolla with two stamens. From D. Dias & M. Sebastião 200. drawn by paula silva.

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Fig. 2.
figure 2

Voacanga madureirae. A flowers, Q. Luke 13274; B fruit, M. C. Madureira & Equipa Proj. Pagué III 87; C fruit cut in two halves, showing the inside, M. C. Madureira & M. Sebastião 3 MCM. photos: a quentin luke, b – c maria do céu madureira.

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recognition. Voacanga madureirae is easily recognised by its syncarpous fruits, its almost straight (not strongly twisted) corolla tube, and its calyx distinctly exceeding the corolla and bearing inside a dense row of colleters in the upper half of the tube. The first two characters are also found in V. chalotiana Pierre ex Stapf, which, however, has a smaller calyx, bearing a row of colleters near the base and much exceeded by the corolla. All other Central African species of Voacanga have apocarpous fruits, strongly twisted corolla tubes and a corolla usually exceeding the calyx, or equalling it in V. diplochlamys K.Schum., which, however, has puberulous stems and inflorescence while these parts are glabrous in V. madureirae.

distribution. This species occurs in the western highlands of Cameroon (Mt Cameroon, Rumpi Hills, Bakossi Mts, Mt Kupe, Kagwene Hills, Tofala Hills (Lebialem Highlands), Bali Ngemba) and on the islands of Bioko (Equatorial Guinea) and Príncipe (São Tomé and Príncipe); it may conceivably occur in south-eastern Nigeria, but has not been collected there so far. Map 1.

Map 1.
figure 3

Distribution of Voacanga madureirae (blue dots).

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specimens examined. cameroon. South-West Region: Mt Cameroon, path from Likombe towards grassland and top of Mt Cameroon, 1060 m, 24 Feb. 1995, fl., Cable et al. 1356 (BR!, K!, MO, P, SCA, WAG!, YA); trail N of Likombe village, 850 m, 21 Feb. 1995, fl.b., Groves 114 (K!); Bwassa, 13 March 1992, fl., Tchouto Mbatchou 42 (K!); Bwassa, 13 March 1992, fl., Nkeng 59 (K!, SCA, YA); above small Koto village, 700 m, 6 March 1985, fl., D. W. Thomas 4462 (BR!, K!, WAG! YA!). Mt Kupe, around Nyasoso, at the base of Mt Kupe, 856 m, 6 Feb. 1984, fl., D. W. Thomas 3046 (BR!, K!, LBV!, WAG! YA!); ibid. above Kupe Village, 1400 m, 17 July 1996, fr., Etuge 2861 (K!, YA); ibid. 950 m, 10 July 1996, fr., Etuge 2711 (K!, MO, P, SCA, WAG, YA); Mbulle village, 1100 m, 26 June 1996, fr., Cable 3365 (K!, YA); Bakossi Mts, Kodmin, 1300 m, 19 Nov. 1998, fr., Gosline 179 (K!, YA); ibid., 1350 m, 19 Jan. 1998, fr., Gosline 69 (K!, YA!); Rumpi Hills, near Dikome Balue, 1500 m, March 1984, fl., D. W. Thomas 3294 (BR!, K!, WAG! YA!); Kagwene Hills, Abibi path, 1650 m, 25 May 2009, fr., Ashworth 249 (K!, P!, WAG!, YA); Tofala Hills (Lebialem Highlands), 1700 m, 17 April 2004, fr., Tchiengué 1939 (K!, MO, SCA, WAG, YA); Northwest Region: Bali Ngemba Forest Reserve, above Mantum village, 1310 m, 10 April 2002, fr., Zapfack 2002 (K!, SCA, YA); ibid. 1400 m, 20 April 2002, fr., Pollard 1045 (K!, YA); ibid. 1310 m, 11 April 2002, fr., Onana 1992 (K!, YA!); ibid. 1500 m, 5 Oct. 2001, fr., Onana 1837 (K!, SCA!, YA!). equatorial guinea. Bioko Norte: carretera del Pico Basilé, 1190 m, 5 July 1986, fr., Fernandez Casas 10194 (MA [image!], WAG!). Bioko Sur: Belebú Balachá, camino de Ureca, 1310 m, 28 Feb. 1989, fl., Fernández Casas 12181 (BM!, BR!, K!, MA [image!], P!, WAG!); C. Ureca/Fondo Caldera, 930 m, 13 March 2007, fr., Galán & Barberá 4685 (BR!, MA); Badja N Trail, 1040 m, 20 Jan. 2009, fl., Q. Luke 13100 (EA!, K!, MA [image!]); Hormiga Camp, 800 m, 1 Feb. 2009, fl., Q. Luke 13274 (K!). são tomé and príncipe. Príncipe: Base do Morro do Leste, 01.88905°N; 007.40551°E, 467 m, 22 Jan. 2021, fl., D. Dias & M. Sebastião 200 (holotype COI!; isotypes BRLU!, MO!, PRP, STPH); entre Morro de Leste e no caminho para Picos Mencorne, Príncipe, 10 Aug. 2010, fr., M. C. Madureira & Equipa Proj. Pagué III 87 (PP III) (COI! [COI00100964]); subida para os Picos Mencorne, Príncipe, 27 Jan. 2011, fl. (fallen), M. C. Madureira & M. Sebastião 22 MCM (2011) (COI! [COI00103113]); entre Morro de Leste e no caminho para Picos Mencorne, Príncipe, 4 Aug. 2011, fr., M. C. Madureira & M. Sebastião 3 MCM (Salk I.) (COI! [COI00103114]); Assentada antes do Pico Príncipe, 1°35'21"N 7°22'50"E, 642 m, 16 Feb. 2018, st., Príncipe Transects 118 (BRLU!, COI! [COI00103198], PRP, STPH).

habitat. Mid-altitude forest, sometimes secondary, or rarely in farmland (Groves 124) at elevations of 467 – 642 m in Príncipe, 800 – 1310 m in Bioko, and 700 – 1700 m in Cameroon.

conservation status. Voacanga madureirae occurs in mid-altitude forest (467 – 1500 m) in western Cameroon and on the Gulf of Guinea islands of Bioko (Equatorial Guinea) and Príncipe (São Tomé and Príncipe). It is known from 26 specimens representing 18 occurrences. None of these is considered extirpated since forest cover is still present in the collection areas. Its EOO is calculated as 22,511 km2, above the threshold for “Vulnerable” status under Criterion B1. Based on a 2 × 2 km cell size, its AOO is estimated as 72 km2, within the range for “Endangered” status under Criterion B2. The 18 occurrences represent 10 – 11 sub-populations. The 11 occurrences in Cameroon are all threatened by deforestation for small-scale agriculture (which is important in the region, especially in the lower part of the species’ altitudinal range) with the possible exception of one occurrence that is protected within the Tofala Hill Wildlife Sanctuary. This occurrence represents one location in the sense of IUCN (2019). One occurrence is in Mount Cameroon National Park, but very close to its limits and consequently at risk from illegal deforestation; it represents a second location. One occurrence in the Rumpi Hills Forest Reserve and another in the Bali Ngemba Forest Reserve, both of which have only weak levels of protection (essentially none at the moment, due to political troubles in the region), represent two additional locations. The remaining occurrences are not protected, one in the Kagwene Hills represents a fifth location, two on the southern flank of Mt Cameroon (outside the national park) a sixth, two on Mt Kupe a seventh and two in the Kodmin area an eighth location. Four of the occurrences in Bioko are in the same protected area (the Reserva Cientifica de la Caldeira de Luba) and represent a ninth location; the last one, in the north of the island, is unprotected and is a tenth location. The two occurrences in Príncipe are in a protected area (the Zona Ecologica de Príncipe) and seem relatively safe for the moment, they represent an eleventh location. The species is thus known from eleven locations with regard to the most serious plausible threat, small-scale agriculture, and, because of that, a past, present and future continuing decline in habitat extent and quality is inferred. The number of locations is just above the threshold for Vulnerable status (IUCN 2012) under criterion B2ab(iii) and considering that some of them are likely to disappear in the near future, the species is provisionally assessed as Near-Threatened [NT].

phenology. Flowers have been collected from January to March; fruits in January, March, April, July, August and October.

local names. Cata d’obo (Príncipe), name also used for Tabernaemontana spp.

notes. Voacanga madureirae is a rather distinctive species, at least when fertile. In flower characters (especially the large size of the calyx in relation to the corolla) it resembles V. diplochlamys, which is partly sympatric in Cameroon (but which usually occurs at lower altitudes). In fruiting characters it resembles V. chalotiana, which has a widely separate range, south of the Equator. Differences with these species are summarised in the diagnosis and in Table 1 above. Although Leeuwenberg (1985) included V. diplochlamys in the synonymy of V. bracteata Stapf, the two species are clearly different (Jongkind & Lachenaud 2022).

Table 1. Differences between Voacanga madureirae and similar species.
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As noted in the introduction, flowering collections of Voacanga madureirae have also been confused with V. africana and V. psilocalyx. The earliest collection of the new species, D. W. Thomas 3046, was cited under V. africana by Leeuwenberg (1985: 76) in his revision of the genus, and subsequently by Cheek et al. (2004: 245), while Cable & Cheek (1998) cited other specimens under both V. africana (Groves 124, Nkeng 59) and V. psilocalyx (Cable 1356, Tchouto 42). These two species are easily separated from V. madureirae by their corolla distinctly exceeding the calyx (greatly so in V. africana), their strongly twisted corolla tube, their calyx tube with ± scattered colleters inside (never forming a dense row near the apex) and their fruits with separate carpels (Table 1).

While at least five species of Voacanga occur in south-western Cameroon: V. africana, V. diplochlamys, V. lanceolata (Stapf) Jongkind, V. madureirae and V. psilocalyx; only two are known from the Gulf of Guinea islands: V. africana and V. madureirae. Both species occur in Bioko, only V. madureirae in Príncipe and only V. africana in São Tomé. The São Tomé population of V. africana, originally described as V. lemosii Philipson, has very large fruits and could possibly represent a separate taxon, but the material seen is too fragmentary for a definite conclusion on the matter.

The seeds of Voacanga madureirae are reportedly eaten by giant rats (Etuge 4698).