Rinorea spongiocarpa and Rinorea dimakoensis (Violaceae), new threatened species of forest trees from East and South Regions of Cameroon and Gabon

Two tree species are described as new to science: Rinorea spongiocarpa Achound. sp. nov. (placed in Rinorea [unranked] Brachypetalae) and R. dimakoensis Achound. sp. nov. (placed in Rinorea [unranked] Ilicifolieae). Both species occur in Cameroon, south of the Sanaga river, the first from South and East Regions, occurring in evergreen forest from Ngovayang and Lolodorf across the southern plateau to the Dja forest. The second extends also to Gabon, occurring in Cameroon in the northern part of East Region in semi-deciduous forest towards the interface with woodland habitats in the Dimako-Bertoua area. The two species are mapped and illustrated, and their affinities and conservation status according to the 2012 IUCN categories and criteria are discussed. Both R. spongiocarpa and R. dimakoensis are threatened with extinction due to habitat destruction, both are assessed as Vulnerable.


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 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 Vshaped 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;Darbyshire & Cheek 2004b). In neighbouring Gabon, the recently published Rinorea calcicola Velzen & Wieringa is also rangerestricted and of conservation concern (van Velzen & Wieringa 2014). Cameroon has the highest speciesdiversity 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.
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 km 2 . 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 km 2 using the IUCN-preferred cell size of 4 km 2 . 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 smallholder 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 rangerestricted 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 3angled 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: 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.
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. 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 km 2 (including the Lom-Pangar site) and the global area of occupation is calculated as only 20 km 2 using the IUCNpreferred cell size of 4 km 2 . 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 . 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. 2017Cheek et al. , 2021bCheek et al. , 2021cCheek et al. , 2022b, even new genera to science (Litt & Cheek 2002;Cheek et al. , 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.

Acknowledgements
The first author thanks IRAD-National Herbarium of Cameroon (YA) for support in his retirement that has enabled him to continue and finalise his taxonomic research on Violaceae of Cameroon for the Flore Du Cameroun. The Nederlandese Organisatie voor Wetenschappelyk Onderzoek (NOW), Institut de Recherche pour le Developement (IRD) and Royal Botanic Gardens, Kew have supported the cost of visits by the first author to the European herbaria of BR, BM, K, P and WAG. The first author is particularly grateful to Jos van der Maesen, Frans Breteler and Eric Chenin at these institutions. He also thanks colleagues at Wageningen for retrieving and transmitting the excel-lent figures that illustrate this paper, drawn by J. M. (Hans) de Vries. Xander van der Burgt is thanked for making the map. This paper was completed as part of the Cameroon TIPAs (Tropical Important Plant Areas) project at RBG, Kew, which is supported by players of People's Postcode Lottery. Formal Red Listing of these two tree species, once they are formally published, will be supported by the John S. Cohen Foundation.

Declarations
Conflicts of interest. The authors declare no conflicts of interest.
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