Four new threatened species of Rinorea (Violaceae), treelets from the forests of Cameroon

Four species of Rinorea are described as new to science; all four species are endemic to evergreen rain forest in Cameroon. Rinorea villiersii Achound. and R. amietii Achound are placed in Rinorea [unranked] Crassiflorae M.Brandt, while, R. dewildei Achound. and R. faurei Achound. fall in Rinorea [unranked] Dentatae Engl. The first species appears to be endemic to the Solé Forest Reserve northeast of Yabassi in Littoral Region. The second and the third species are found mainly in Littoral and South Regions, R. amietii extending to South West Region and R. dewildei extending to Central Region. The fourth species, R. faurei is an endemic of the Santchou Forest Reserve at the foot of Dschang Plateau in West Region. The taxonomic affinities of the four species are discussed, illustrations and a distribution map are provided, and their conservation status is assessed. All four species are threatened with extinction according to the 2012 IUCN categories and criteria.


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 various collaborators, already published 13 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). Other publications have used provisional names which are now used for the new species published here Achoundong 1997;Amiet & Achoundong 1996;Achoundong 2000;Bakker et al. 2006). In this paper, four such 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 first two new species described below, R. villiersii Achound. and R. amietii Achound. fall within Rinorea [unranked] Crassiflorae M.Brandt, while the second two species, R. dewildei Achound. and R. faurei Achound. fall in Rinorea [unranked] Dentatae Engl.
The genus Rinorea is pantropical, with 206 species currently accepted by Plants of the World Online, (continuously updated), accessed April 2021). Africa is the most species-diverse continent for Rinorea with 110 -150 species (van Velzen et al. 2015). Rinorea species are mainly 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 an 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 zygomorphic. 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: 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). 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., R. thomasii (Darbyshire & Cheek 2004a;Cheek 2017;Darbyshire & Cheek 2004b). In neighbouring Gabon, the recently published R. 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). Twentyeight 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 & Achoundong 1996).

Materials & Methods
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. 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 continuously updated), and nomenclature follows Turland et al. (2018). The conservation assessments were 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 Achoundong & Cheek (2005). Post-facto georeferences for specimens without coordinates were obtained from Google Earth.
RECOGNITION. In its habit, its ferruginous leaves and its very short apical stem bud, Rinorea amietii is morphologically (but not phylogenetically) close to R. subsessilis M.Brandt, but differs in the leaves being pubescent and non-glandular below (vs glabrous and glandular), the petal midrib pubescent (not glabrous), and in additional features in the key below. The two species are separated by the following characters:  Many of the species are narrow endemics and so are vulnerable to extinction from forest clearance. Littoral forest habitat is highly threatened due to its ability to host industrial agriculture and because it is a target for logging activity. Many industrial farms are already located there, cultivating crops such as banana (Musa spp.), palm oil (Elaeis guineensis), and rubber (Hevea brasiliensis). Due to its proximity to the coastal ports and the density of roads, littoral forest habitat is in demand for clearance for the expansion of these industries. Currently eight locations are recorded for Rinorea amietii based on the 23 specimens studied, following extensive surveys in Cameroon detailed above under R. villiersii). The extent of occurrence is estimated as 9,545 km 2 , and the area of occupation as 60 km 2 . Given the threats above, we assess R. amietii as Vulnerable VU B1+2ab(iii). ETYMOLOGY. The specific epithet commemorates Professor Jean-Louis Amiet, lecturer at University of Yaoundé. He studied the relationship between butterflies and plants, especially that between Cymothoe (the gliders with about 75 species in Africa) and the genus Rinorea (Amiet 1997;Amiet 2000), and inspired the first author in his life's taxonomic research on the genus Rinorea in Africa (Amiet & Achoundong 1996). He is commemorated by several other species such as Afrothismia amietii Cheek (Thismiaceae, Cheek 2007). Amiet, an entomologist, came to understand in the 1980s that the Cymothoe butterflies he studied were often monophagous, the larvae of some species eating only one species of Rinorea (see Introduction). However, at that point, these different species of Rinorea were not recognised by taxonomists because of the superficial similarity of many other species. To address this difficulty he recruited the first author! NOTES. Recent data from the chloroplast marker trnL-F and the nuclear ribosomal ITS, indicate that Rinorea amietii is related to R. longisepala Engl., R. ledermannii Engl. and R. umbricola Engl. (Robin van Velzen pers. comm. to Achoundong, June 2021) supporting placement in Rinorea [unranked] Crassiflorae. 2 mm long, expanded at the summit. Fruit white, ovoid, tuberculate, 1.5 (-2) × 1 (-1.5) cm. Seeds tetrahedral, 0.5 × 0.5 cm. Fig. 3. RECOGNITION. In Rinorea dewildei the lamina is distinctive in that 3 -4 of the lateral nerves are more prominent than the others. Rinorea cerasifolia M.Brandt shows the same character. Rinorea dewildei has petioles 0.5 -1.5 cm long, sepals twice as long as broad (vs to 5 cm long, sepals as long as broad in R. cerasifolia). HABITAT. Rinorea dewildei occurs in littoral lowland evergreen rainforest. The altitude ranges from 0 to 700 m. This species appears common in many areas of Cameroonian littoral forest but despite this has surprisingly few collections. CONSERVATION STATUS. According to Tchouto, Rinorea dewildei is common in the Campo-Ma'an National Park. Consequently, the species is well protected within the park. However, at locations outside the park, such as at Makak, Edéa and Eséka, the habitat of this species is being cleared for agriculture and urbanisation. Currently, after extensive surveys in Cameroon (see under Rinorea villiersii), six locations are recorded for R. dewildei (see specimens above), the extent of occurrence is estimated as 12,430 km 2 , and the area of occupation as 36 km 2 . Given the threats above, we assess R. dewildei as VU B1+2ab(iii). ETYMOLOGY. The specific epithet commemorates Dr W. J. J. O. de Wilde. He and his wife, Brigitte de Wilde-Duyfjes collected the type specimen. Together, from their base at N'Kolbisson in Yaoundé in the 1960s, they collected numerous excellent botanical herbarium specimens in large sets through many parts of Cameroon, including the littoral areas where Rinorea dewildei still grows. At this time their home institution, Herbarium Vadense (WAG), then at Wageningen, Netherlands, was highly active in conducting botanical surveys in poorly studied but speciesrich areas of tropical Africa, such as Cameroon and Gabon. Sixty years later, we are still naming species new to science resulting from their efforts, such as this. NOTES. One anomalous specimen from Gabon (Moyen-Ogooué, Camp Mboumi base, river bank, fr., fl. 20 Aug. 1999, Issembe 191 (K, LBV, WAG), shares many similarities with Rinorea dewildei, and was initially identified as this species, but is here excluded. It may represent a further undescribed new species to science. Rinorea faurei ined. Achoundong (1996: 544), Achoundong (1997: 255); Onana (2011: 150).

RECOGNITION. In its habit and its thyrsoid inflorescence
Rinorea faurei is close to two other Rinorea species of the forest understory: R. sinuata Chipp and R. thomasii, differing from the first in having the flowers dispersed in an elongated, lax cyme (vs a dome-shaped, compact cyme), differing from the second in the leaves elliptic, ovate or obovate and long-acuminate (vs oblong and shortly acuminate): HABITAT. Rinorea faurei occurs in understory of evergreen rain forest in the Santchou Plain at the foot of the Dschang Plateau. The estimated elevation above sea level is 770 m. The area of forest is more than 7000 ha. Santchou Forest Reserve is also the home of R. dentata Kuntze, R. oblongifolia C.Marquand, R. yaundensis Engl. and R. batesii Chipp (Achoundong pers. obs. 1991(Achoundong pers. obs. -1992. CONSERVATION STATUS. Santchou Forest Reserve has official protection status. However, lack of concrete action on the ground means a strong risk of the reserve being invaded by farmers since the Santchou Plain is intensively cultivated. Currently, after extensive surveys in Cameroon (see under Rinorea villiersii above), only a single location in the sense of IUCN (2012) is recorded for R. faurei (see specimens above), the extent of occurrence and the area of occupation is estimated as 70 km 2 . Given the threats above, we assess R. faurei as Critically Endangered, CR B1+2ab(iii). ETYMOLOGY. The specific epithet honours Jean-Jacques Faure, former Head of the Forestry High School in Cameroon. His contribution to the protec-tion of the environment of Central Africa has been important.

Discussion
More collections of Rinorea in Cameroon are needed. The more widespread species such as R. dewildei (see note there) have surprisingly few collections, even though the first author has noted they can be quite common within their range. It appears that many collectors avoid targeting the genus in their botanical inventory work, perhaps doubting that one species differs from another and seeking to avoid duplicating collections of the same taxon. The first author, in contrast, targeted the genus in his field surveys, accounting for his collections being the majority of those cited in this paper. This demonstrates how under-collected these Cameroon littoral forests are generally and confirms that more herbarium specimens should be collected to determine which species are present in these forests before they are lost.
About 2000 species of vascular plant have been described as new to science each year for the last decade or more. Until species are known to science, they cannot be assessed for their conservation status and the possibility of protecting them is reduced (Cheek et al. 2020). To maximise the survival prospects of range-restricted species there is an urgent need not only to document them formally in compliance with the requirements of the relevant nomenclatural code (Turland et al. 2018), but also to formally assess the species for their extinction risk, applying the criteria of a recognised system, of which the IUCN Red List of Threatened Species is the most widely accepted ). Despite rapid increases over recent years in numbers of plant species represented by assessments on the Red List, the vast majority of plant species still lack such assessments .
Increasing representation of plants on the Red List is imperative but documenting threatened species in the level of detail required for inclusion on the Red List is resource intensive (Juffe-Bignoli et al. 2016) and securing the required peer-review for completed assessments to be accepted for the Red List can be slow and problematic . Alternative approaches to evaluating extinction risk, including automated or semi-automated methods involving digitally available information and artificial intelligence, have been applied to plants in recent years (e.g., Darrah et al. 2017;Nic Lughadha et al. 2019;Zizka et al. 2020), often with demonstrable success in correctly predicting the threat status of species previously assessed for the Red List. However, caution is needed in considering such automated assessment approaches for adoption as conservation tools, not only for a variety of technical reasons ) but also because they lack the power that an IUCN Red List assessment has to help change the fate of a species.
As a global standard, the IUCN Red List not only provides stable and evidence-based methods for biodiversity scientists to assess and document the extinction risk of individual species, thus facilitating comparison of the extinction risk of taxonomic groups and areas worldwide (e.g., Nic Lughadha et al. 2020; Mair et al. 2021), it also supports the safeguarding and sustainability frameworks used by businesses and their major lenders (Bennun et al. 2018;Juffe-Bignoli et al. 2016). For example, clients of the International Finance Corporation (World Bank Group) are required to use the Red List to inform project risks and to refrain from activities leading to a net reduction in populations of species assessed on the Red List as Endangered (EN) or Critically Endangered (CR), over a reasonable timescale. Species whose likely extinction has been avoided or reduced as a result of these rules include National governments and leaders also recognise the importance of species assessed as threatened by the Red List, as demonstrated recently in Cameroon when, in part due to the high number of plant species on the Red List (Lovell 2020), a logging concession was revoked for the Ebo forest (Kew Science News 2020).
Thus while the Conservation Status statements included in the species treatments in the present paper suffice to highlight the threatened status of these species to the taxonomic botany community, completion and review of full assessments must follow so that they can be published on the IUCN Red List. Their assessments on the Red List will facilitate their inclusion in larger scale studies, including future global extinction risk estimates and conservation prioritisation exercises, but, most importantly, it will enable not only scientists, but also local communities, NGOs and national authorities to promote and take action to safeguard them.
Documented 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 . In Cameroon, Oxygyne triandra Schltr. and Afrothismia pachyantha Schltr. of South West Region, Cameroon are now known to be globally extinct (Cheek & Williams 1999;Cheek et al. 2018a;Cheek et al. 2019). In some cases, Cameroon species appear to have become extinct even before they are known to science, such as Vepris bali Cheek (Cheek et al. 2018b).
Most of the 815 Cameroonian species in the Red Data Book for the plants of Cameroon are threatened with extinction due to habitat clearance or degradation, especially of forest for small-holder and plantation agriculture following logging (Onana & Cheek 2011). 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 intended to help avoid the global extinction of additional endemic species such as the Rinorea species published in this paper which will be included in the future proposed 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. He also thanks colleagues at Wageningen for retrieving and transmitting the excellent figures that illustrate this paper, mainly drawn by J. M. (Hans) de Vries, with valuable contributions by Jean Michel Onana. He thanks Robin van Velzen for data on the molecular phylogenetic placement of taxa. This paper was completed as part of the Cameroon TIPAs (Tropical Important Plant Areas) project at RBG, Kew, which is supported by Players of Peoples Postcode Lottery. Formal Red Listing of these four species, once they are formally published, will be supported by the John S. Cohen Foundation.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/ licenses/by/4.0/.