Introduction

In the course of revising the species of Violaceae of Africa, mainly in preparation for the account of the Violaceae for the “Flore du Cameroun”, the first author has, with collaborators, published 17 new species to science for this group (Achoundong & Onana 1998; Achoundong & Bos 1999; Achoundong & Bos 2001; Achoundong 2003; Achoundong & Cheek 2003; Achoundong & Cheek 2005; Achoundong & Bakker 2006; Achoundong et al. 2021). The new species published here have previously been referred to under provisional names (Achoundong 1996; Achoundong 1997; Amiet & Achoundong 1996; Achoundong 2000; Bakker et al. 2006). In this paper, these two provisional species names are formally published to validate these names.

The most recent studies of the phylogeny and classification of African Rinorea Aubl. are set out by Wahlert (2010), Wahlert & Ballard (2012) and van Velzen et al. (2015). However, the classification of Brandt (1914) has still not been formally replaced.

The genus Rinorea is pantropical, with 210 species currently accepted by Plants of the World Online (POWO, continuously updated, accessed April 2022). Africa is the most species-diverse continent for Rinorea with 110 – 150 species (van Velzen et al. 2015). Rinorea species are forest understorey shrubs or small trees. Morphologically, in continental Africa, they are characterised by having alternate, simple leaves, often with petioles of different lengths on the same stem and a usually long, curving apical bud (in the Neotropics and Madagascar, some species e.g. Rinorea sect. Pubiflora Wahlert & H.E.Ballard have opposite leaves). The flowers are often green, dull yellow, or shades of white and are usually markedly zygomorphic (rarely appearing more or less actinomorphic). There are three sets of petals in Rinorea: an anterior petal (also known as the lower or ventral petal), two lateral petals and two posterior petals. These are likely homologous to the three sets of petals in other strongly zygomorphic genera of Violaceae, such as Viola L.(Wahlert 2010).

The anterior petal is larger than the other petals, and often modified, with taxonomically important, often diagnostic characters. The androecium has a staminal tube which is also zygomorphic: the anterior (lower or ventral) side is longer and entire, while on the dorsal side of many of the African species, the staminal tube is generally shorter and incised with a V-shaped cleft. The fruits are typical of the family: hard, dry tricoccal, septicidal capsules with parietal placentation.

Rinorea are ecologically important and diverse in African forests, often with several sympatric species in one forest. For example, ten species were recorded in a few square kilometres of the Mefou Proposed National Park near Yaoundé (Achoundong in Cheek et al. 2011). The most species diverse area is in the littoral plain in the polygon bounded by Edea-Kribi, Bella and Bipindi. Many species are range-restricted, found in such small areas that they are at risk of extinction from forest clearance. Rinorea dewitii Achound., R. fausteana Achound., R. simoneae Achound., and R. thomasii Achound., are all assessed as threatened in the Red Data Book of the Flowering Plants of Cameroon (Onana & Cheek 2011) and all but the first can be found on the IUCN Red List (iucnredlist.org) e.g., Rinorea thomasii (Darbyshire & Cheek 2004a; Cheek 2017; Darbyshire & Cheek 2004b). In neighbouring Gabon, the recently published Rinorea calcicola Velzen & Wieringa is also range-restricted and of conservation concern (van Velzen & Wieringa 2014). Cameroon has the highest species-diversity for the genus in tropical Africa with 53 species listed (Onana 2011), followed by Gabon, with 46 species (Sosef et al. 2006). However, the superficial similarity between species has made identification difficult for taxonomists, e.g., 194 specimens of Rinorea are listed as unidentified to species for Gabon in Sosef et al. (2006).

African Rinorea species are of great interest to entomologists, being important larval food plants of the butterfly genus Cymothoe (the gliders). Twenty-eight species of Cymothoe are known to feed on Rinorea, of which 18 are strictly monophagous, six are oligophagous and three feed on up to six species of Rinorea (Amiet 1997; Amiet 2000; Amiet & Achoundong 1996).

Materials & Methods

Fieldwork by the first author was mainly carried out in Cameroon from 1987 – 1996 in connection with his doctoral studies of Rinorea (Achoundong 1997). Specimens were collected using the patrol method (e,g. Cheek & Cable 1997). All specimens cited have been seen. Herbarium citations follow Index Herbariorum (Thiers continuously updated). Specimens were studied online, on loan from or at BR, K, P, WAG and YA principally by the first author. We also searched JSTOR Global Plants (https://plants.jstor.org/ accessed April 2021) for additional materials. Taxonomic authorities follow the International Plant Names Index (IPNI 2021), and nomenclature follows Turland et al. (2018). The conservation assessment was made using Bachman et al. (2011) following the categories and criteria of IUCN (2012). Herbarium material was examined with a Leica Wild M8 dissecting binocular microscope fitted with an eyepiece graticule. Measurements were made from rehydrated material. The terms and format of the description follow the conventions of Beentje & Cheek (2003) and Achoundong et al. (2021). Post-facto georeferences for specimens without coordinates were obtained from Google Earth. (https://www.google.com/intl/en_uk/earth/versions/).

Results: Taxonomy

Rinorea spongiocarpa Achound. sp. nov., the first of the two species described below is placed in Rinorea [unranked] Brachypetalae because it is closely similar to R. gabunensis Engl. and R. leiophylla M.Brandt which are placed in this group on molecular grounds (Wahlert & Ballard 2012; van Velzen et al. 2015) and because it fits the description of this group: alternate leaves, six ovules per ovary, cymose inflorescence, anthers sessile on the border of the staminal tube.

Rinorea dimakoensis Achound. sp. nov. the second species described, is placed in Rinorea [unranked] Ilicifolieae on molecular grounds (Wahlert & Ballard 2012; van Velzen et al. 2015) and because it fits the description of the group: alternate leaves, six ovules per ovary; cymose inflorescences, sepals clearly ribbed fanwise; staminal tube sinuate between insertion of anthers, tube without a free margin or a lobed margin subtending anthers. Further data are given on Ilicifoliae in Wahlert et al. (2020).

1. Rinorea spongiocarpa Achound. sp. nov. Type. Cameroon, South Region, Ebolowa, “Hill facing village of N’Kolandom, in primary forest on slope, Alt. c. 700 m, 2.48N, 11.10E”, fl. 20 Feb. 1975, J. J. F. E. de Wilde 7985 (holotype WAG; isotypes BR, K000593339, P, YA).

http://www.ipni.org/urn:lsid:ipni.org:names:77308122-1

Rinorea spongiocarpa ined. Achoundong (1996: 536 – 544; 1997: 193 – 198); Amiet & Achoundong (1996: 465); Onana (2011: 151).

Tree or shrub 2 – 6 (– 12) m high; stems glabrous. Leaf-blades leathery to thickly papery, glossy, dark green above, pale yellow-green beneath, elliptic, ovate to narrowly oblong, 14 – 30 × 11 – 6 cm, apex acuminate, base cuneate to attenuate, lateral nerves 9 – 12, on each side of the midrib, tertiary nerves subscalariform, leaf margin crenate, glabrous; petiole 5 – 7 cm long. Inflorescence a terminal panicle up to 7 cm long, lateral ramifications constituted by few-flowered cymes of 5 – 7 flowers each. Bracts triangular, 1.5 × 2 mm, median nerve raised, conspicuous. Flowers yellow or yellow-white, zygomorphic, 4 – 5 (– 6) × 3 – 4 (– 5) mm. Sepals purple, unequal, triangular to elliptic 2 × 3 mm, apex rounded. Petals yellow, unequal, anterior (lower) petal oblong 6 × 2 mm, spreading, not or barely revolute at maturity, lateral and posterior petals smaller, 3 – 4 mm long, distal half strongly revolute. Androecium zygomorphic, 3.5 – 5 mm long, staminal tube 1 – 2.5 mm long, staminal filaments short, extending from the tube rim, anthers subsessile, thecae 2 mm long, outer surface puberulent, connective appendage 2 mm long, red, decurrent deeply on the anther thecae, thecae appendage entire, not bifid. Gynoecium up to 5.3 mm long. Ovary subglobose, 1.5 × 1 mm, glabrous, style straight, enlarged at the base, 3.5 mm long. Fruit subglobose-ovoid, 30 × 20 mm surface smooth, lacking ribs, fruit wall c. 3 mm thick, six-seeded. Seeds white, tetrahedral, 9 – 10 (– 11) × 9 – 10 × 5 (– 6) mm, drying pale brown, glossy. Fig. 1.

Fig. 1
figure 1

Rinorea spongiocarpa. A habit, flowering stem; B portion of inflorescence with two open flowers and one flower in bud; C flower, note the spreading anterior petal; D anterior petal; E lateral and posterior petals; F sepals; G androecium, side view; H fruit, mature. From J. J. F. E. de Wilde 7985 (holotype, WAG). drawn by j. m. (hans) de vries.

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recognition. Similar to Rinorea parviflora Chipp in that the fruits are subglobose (in most species the fruits are 3-angular), having a thick mesocarp (in almost all other members of the genus the mesocarp is thin), differing in that the abaxial surface of the leaf-blade lacks glands, the staminal tube of the flower is not free (the stamens arise from it), and that the inflorescence branches are many-flowered (vs glandular, staminal tube free, and 1 – 2-flowered respectively).

distribution. Cameroon. The species is restricted to the area of the Dja Forest (from Mbalamayo to Abong Mbang) in East Region, extending westwards to Ngovayang and Ebolowa-Lolodorf in Littoral Region. (Map 1).

Map 1
figure 2

Global distributions of Rinorea spongiocarpa and R. dimakoensis.

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specimens studied. cameroon. East Region, Alat Makay, Dja National Park, fl. 23 Feb. 1987, Achoundong 1411 (P, YA); Timbe II, near Abong-Mbang, fl. 20 Jan. 1990, Achoundong 1562 (P, YA); Dja National Park., fl. 25 April 1993, Lejoly & Sonké 154 (BR, YA); Dja National Park, Mekas, fl. 10 Jan. 1995, Sonké 1385 (BR, YA); Mekas, fr. 26 May 1995, fr, Sonké 1548 (BR, YA); South Region. Ebolowa, Hill facing village of N’Kolandom hill, in primary forest on slope, fl. 20 Feb. 1975, J. J. F. E. de Wilde 7985 (holotype WAG; isotypes BR, K000593339, P, YA); Nkoladom village, S of Ebolowa, fr. 10 Sept. 1989, Achoundong 1495, (K000593338, WAG, YA); ibid., fl. fr. Sept. 1992, Achoundong 1951 (YA); ibid., Achoundong 1952 (YA); ibid., Achoundong 1953 (YA); Bongolo I, 30 km on Ebolowa-Lolodorf road, fl., 14 Sept. 1989, Achoundong 1501, (K, YA); ibid., fl., fr., 14 Sept. 1989, Achoundong 1501 (K000593337, P, YA); Avobengon village, 24 km S of Djoum, 12°55'E, 2°40'E, fl., 22 Dec. 1990, Achoundong 1598 (YA); ibid., fl., 22 Dec. 1990, Achoundong 1632 (YA); ibid., fl. 22 Dec. 1990, Achoundong 1700 (P, YA); ibid., fl. 10 April 1991, Achoundong 1760 (K000593336, WAG, YA); ibid., imm.fr. Achoundong 1802 (K000593335, YA); Medjap, near Djoum, st. 20 May 1990, Achoundong 1681 (P, YA); Mill Hill, Lolodorf, Sept. 1992, Achoundong 1974 (YA); Bibondi near Ngovayang, NW of Lolodorf, fl., 4 March 1993, Achoundong 2017 (YA); ibid., Achoundong 2024 (YA); Mezese, fr. 24 May 1993, Achoundong 2066, (YA); Mezese, fr. 16 Sept. 2004, Achoundong 2335 (YA, WAG); Ebienemeyong, fl., Jan. 1993, Achoundong 2338 (YA).

habitat. Rinorea spongiocarpa is widespread in dense lowland evergreen forest from Lolodorf and Ngovayang at the edge of the littoral plain through the Dja forest of the Cameroon Congolese forest zone (in the sense of Letouzey 1985), which is characterised by Gilbertiodendron dewevrei and Sterculia subviolacea. It occurs at an altitudinal range of 400 – 700 m. Rinorea spongiocarpa does not occur in the littoral plain itself and is completely absent from the semi-deciduous forest of the South Cameroon Plateau.

conservation status. Rinorea spongiocarpa is only known from the border of Littoral Region at Ngovayang and Lolodorf to the Dja Forest area of adjoining East Region (Map 1). On the basis of the specimen records cited above, we calculate the total extent of occurrence of Rinorea spongiocarpa as 25,295 km2. However, within this fairly large area, there are currently only nine scattered locations known and the global area of occupation is calculated as only 44 km2 using the IUCN-preferred cell size of 4 km2. Surveys for plant conservation management in forest areas north, south and west of the range of distribution of this species have resulted in many thousands of specimens being collected and identified, but failed to find any additional specimens of Rinorea spongiocarpa (Cheek 1992; Cable & Cheek 1998; Cheek et al. 2000; Maisels et al. 2000; Chapman & Chapman 2001; Tchoutou 2004; Harvey et al. 2004; Cheek et al. 2004; Cheek et al. 2010; Harvey et al. 2010; Cheek et al. 2011). Although there are still poorly sampled forest locations with intact natural habitat in Cameroon, it is likely that the known range of Rinorea spongiocarpa is close to the actual. Only one location for the species occurs in a designated protected area: the Dja forest. The species appears absent from the largest protected area in South Region, the Campo-Ma’an National Park. Despite being formally unprotected, forest at several of the known locations appears mainly intact (Google Earth imagery viewed 29 Oct. 2021), with forest entirely or largely removed only in two or three locations, apparently due to small-holder agricultural operations near roads. Rinorea spongiocarpa is therefore here assessed, on the basis of the ten or less locations, small known area of occupation and threats described above, as Vulnerable, VU B2ab(iii). Another example of a range-restricted endemic species in the same range and habitat is Kupeantha spathulata (A.P.Davis & Sonké) Cheek (Cheek et al. 2018a), which is also assessed as Vulnerable (Rokni et al. 2017).

etymology. The name of this species was coined by Amiet who inspired and supervised the doctoral research of the first author on Rinorea. “spongiocarpa” derives from the appearance of the fruit, which is not 3-angled as in most species of the genus, but subspherical, appearing inflated and spongy (even though it is hard).

notes. In herbarium collections, specimens belonging to this species appear similar to those of two other species: Rinorea gabunensis and R. leiophylla with which R. spongiocarpa has long been confused. The species have separate ranges. Rinorea gabunensis and R. leiophylla occur exclusively in the littoral plain whereas R. spongicarpa is restricted to forest inland in the Cameroon “congolese forest” zone. The three species are distinguished as follows:

  • 1. Small shrub of less than 5 m high; partial-inflorescences (secondary branches) of the inflorescence uniparous; fruit ribbed………………………...……….........................................................................................Rinorea gabunensis

  • 1. Small tree up to 12 m high; partial-inflorescences (secondary branches) of the inflorescence biparous; fruit smooth (not ribbed)………………………….…………………………….………….....………………….…………2

  • 2. Leaves glandular on abaxial surface; staminal tube with free margin.................………………….Rinorea leiophylla

  • 2. Leaves not glandular on abaxial surface; staminal tube margin not free………………………Rinorea spongiocarpa

2. Rinorea dimakoensis Achound. sp. nov. Type. Cameroon, East Region, Dimako, rive droit de la riv. M Bonda, fl. fr. 18 Jan. 1960, Letouzey 2663 (holotype P00123533; isotypes P00123534; P00123535; P00123536; P00123537; P0012358; P00123539; YA).

http://www.ipni.org/urn:lsid:ipni.org:names:77308123-1

Rinorea dimakoensis Achound. ined. Achoundong (1996: 544); (1997: 156); Amiet & Achoundong (1996: 466); Bakker et al. (2006); Onana (2011); van Velzen et al. (2015); Wahlert (2010); Wahlert & Ballard (2012).

Tree or shrub up to 12 m tall; stems glabrous. Leaves thickly coriaceous, dark green above, blades elliptic, ovate to narrowly oblong, 10 – 30 × 6 – 18 cm, apex acuminate, base rounded, obtuse or attenuate, lateral nerves 8 – 10 on each side of the midrib, leaf margin acutely serrate, glabrous; petiole 3 – 6 cm long. Inflorescence terminal or subterminal panicle 8 – 12 × 2 – 7 cm, lateral branches with 2 – 5-flowered cymes; bracts ovate 1.5 – 2 × 1 – 2 mm, rounded at the summit, median nerve conspicuous. Flower yellow, zygomorphic, 4 – 5 (– 6) × 3 – 4 (– 5) mm. Sepals unequal, triangular to elliptic orbicular, 2 × 3 mm, apex rounded or emarginate. Petals yellow, unequal, oblong, 6 – 7 × 2 – 4.2 mm, lateral and dorsal petals smaller, anterior (lower) petal bigger, straight, not revolute at maturity. Androecium zygomorphic, 3.5 – 5 mm long. Staminal tube 0.5 mm long, tube margin not free, anthers sessile on the rim of the staminal tube; staminal thecae 2 mm long, connective subelliptic, c. 2 mm long, apex rounded, thecae base decurrent slightly, thecal appendage entire (not bifid). Gynoecium 3 mm long. Ovary glabrous, subglobose, 1 mm long, style 2 mm long. Fruit ovoid, 3.5 × 3 cm, longitudinally 3-ribbed, 6-seeded. Seeds tetrahedral, 8 × 5 mm. Fig. 2.

Fig. 2
figure 3

Rinorea dimakoensis. A habit, fruiting stem; B portion of infructescence axis; C two flowers, side view; D flower, side view; E androecium, side view. A, C – E from Achoundong 3033 (WAG, YA), B from Letouzey 2663 (holotype P). drawn by j. m. (hans) de vries.

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recognition. Rinorea dimakoensis Achound. is similar to R. ilicifolia Kuntze, in the shape and size of the leaves which are leathery and robustly toothed at the margin, however in R. dimakoensis the leaves are wider ( 6 – 18 cm vs 3 – 9 cm) and the spines are shorter (<0.5 mm long vs 1 – 2 mm long), the pedicels, flowers and fruits are longer and/or larger, reaching 4.5 mm, 7 × 4 mm, and 3.5 cm long respectively (vs 1 mm, 3.5 – 4 × 3 – 4 mm, and 1.2 – 1.6 cm long and the sepals have only the midrib raised and partly rib-like, not with multiple raised longitudinal ribs.

distribution. Cameroon & Gabon. The species is globally endemic to a narrow area on the South Cameroon Plateau of East Region, extending from Dimako in the south to the Lom-Pangar in the north and crossing into Gabon (Map 1), however it is suspected that the range may be larger.

specimens studied. cameroon. East Region. South Dimako, MBonda R., fl. fr. 10 Aug. 1987, Achoundong 1878 (P, YA); Confluence du Lom et du Pangar, fl. date unknown, Achoundong 3033 (WAG, YA); S of Dimako, MBonda R., fl. fr. 18 Jan. 1960, Letouzey 2663 (holotype P00123533; isotypes P00123534; P00123535; P00123536; P00123537; P0012358; P00123539; YA); Ndemb II, 55 km along Ndemba road, st. 17 Nov. 1955, Nana 343 (P, YA); 60 km on Bertoua Road, Essengue ll path, st. 18 Feb. 1956, Nana 489 (YA). gabon. c. 30 km NE of Lastoursville, forest exploitation on right bank of Ogooue R., fl. 11 April 1990, Breteler 9868 (BR barcode BR00000565803; MA sheet No. 841394); Rabi, Shell oil concession, c. 300 m SSW of Rabi 46, fl. fr. 23 Nov. 1989, W. J. J. E. de Wilde 9675 (BR sheet number 2034648; LBV; WAG sheet number 1870759).

habitat. Rinorea dimakoensis is globally restricted to forest at up to 720 m alt. The distribution range of this species falls within the transition from forest to grassland in Cameroon, with forest mainly along drainage lines interdigitating with grassland on better-drained areas. The species is completely absent from semi-deciduous forest in the adjoining Centre Region, e.g. in the Yaoundé and Bafia areas. It is also completely absent from Cameroon coastal (littoral) evergreen forest, but extends far into Gabon.

conservation status. Five locations are mapped for Rinorea dimakoensis (Map 1). However, three additional locations are thought to exist but require confirmation and documentation. Moreover, the species has been lost at its former northernmost location which has now been inundated (viewed on Google Earth 29 Oct. 2021) by the reservoir behind the Lom-Pangar hydro-electric dam, which was completed in 2017. It is to be hoped that searching in surviving suitable habitat in the area might discover additional individuals but this is not certain. Forest at the remaining locations in Cameroon appears to have been shrinking and has been degraded over recent years, probably due to urbanisation, and the demand for fuel in neighbouring Dimako and Bertoua, towns along the transnational highway that links Douala, and Bangui, the major artery for the Central African Republic. The two locations in Gabon seem more secure and forest is under much less pressure. On the basis of the specimen records cited above and the likely but so far unsubstantiated locations referred to above, we estimate the total extent of occurrence of R. dimakoensis as 119,962 km2 (including the Lom-Pangar site) and the global area of occupation is calculated as only 20 km2 using the IUCN-preferred cell size of 4 km2. There are currently only five extant locations known but another three suspected. It is possible that further additional locations will be found (especially within Gabon) within the extent of occurrence, and that this itself might be extended since this area of Cameroon is less well surveyed than that of the main forest zone (see references cited under R. spongiocarpa). We here assess R. dimakoensis on the basis of the data given above, as Vulnerable, VU B2ab(iii). Another example of a range-restricted endemic species with a similar range is Allophylus bertoua Cheek (Cheek & Haba 2016).

etymology. The species is named after the Dimako locality in East Region, where the first fertile specimen representing this species was collected in 1960 by Letouzey.

notes. Rinorea dimakoensis appears both closely similar and related to R. ilicifolia. The Gabonese specimens cited had been identified as R. ilicifolia. From field observations it appears that R. dimakoensis differs in the shape and size of the habit, the structure of the leaf margin, the architecture of the inflorescence and the habitat.

This close similarity probably constitutes one of the reasons why this species has not been recognised previously. Since discriminating characters are mainly located in mature flowers, it was not easy for botanists formerly to separate this species. Rinorea dimakoensis is also similar to R. keayi Brenan. In fact, the type specimen of this species, collected by Letouzey, was first identified as Rinorea aff. keayi. However, in R. dimakoensis leaf margins are so not spiny as those of this species and the lamina is not glandular beneath.

Discussion

Documented global extinctions of plant species are increasing (Humphreys et al. 2019) and recent estimates suggest that as many as two fifths of the world’s plant species are now threatened with extinction (Nic Lughadha et al. 2020). Cameroon has the highest documented number of plant species extinctions of any country in tropical Africa (Humphreys et al. 2019). The endemic Cameroon species Oxygyne triandra Schltr. and Afrothismia pachyantha Schltr. are among those now known to be globally extinct (Cheek & Williams 1999; Cheek et al. 2018b; Cheek et al. 2019) and recently two species of Pseudohydrosme (Moxon-Holt & Cheek 2021; Cheek et al. 2021a) have been shown to be extinct in adjoining Gabon. In some cases, species appear to have become extinct even before they are known to science, such as Vepris bali Cheek (Cheek et al. 2018c) and Monanthotaxis bali (Cheek et al. 2022a), both also in Cameroon. Even areas known to be of high conservation value have been slated for development, threatening the species they contain with extinction, e.g. the Ebo forest in Cameroon (Lovell 2020).

About 2000 plant species new to science are published each year, with Cameroon contributing more than any other tropical African country in 2019 (Cheek et al. 2020). New species to science from Cameroon are being published steadily (Alvarez-Aguirre et al. 2021; Cheek & Onana 2021; Cheek et al. 2017, 2021b, 2021c, 2022b), even new genera to science (Litt & Cheek 2002; Cheek et al. 2003, 2018a). Only when such species as Rinorea spongiocarpa and R. dimakoensis (this paper) are formally known to science are they fully visible and only then can extinction risk assessments be accepted by IUCN allowing the possibility of measures being taken to protect them (Cheek et al. 2020).

Efforts are now being made to delimit the highest priority areas in Cameroon for plant conservation as Tropical Important Plant Areas (TIPAs) using the revised IPA criteria set out in Darbyshire et al. (2017). This is expected to help avoid the global extinction of additional endemic species such as the Rinorea species published in this paper which it is intended will be included in the future TIPAs.