Notes on the threatened lowland forests of Mt Cameroon and their endemics including Drypetes burnleyae sp. nov., with a key to species of Drypetes sect. Stipulares (Putranjivaceae)

This paper reports a further discovery of a new endemic threatened species to science in the context of botanical surveys in the lowland coastal forests of Mount Cameroon in the Cross River-Sanaga interval of west-central Africa. These studies now focus on species discovery and conservation through the Tropical Important Plant Areas programme. New species to science continue to be discovered from Mt Cameroon. Most of these species are rare, highly localised, and threatened by habitat destruction, increasing the justification for improved conservation management of surviving habitat. Drypetes burnleyae is placed in and keyed out in Drypetes sect. Stipulares, a group mostly confined to Lower Guinea, here adjusted to accommodate nine species. The conservation status of Drypetes burnleyae is assessed as Endangered (EN B1+2ab(iii)) according to the 2012 criteria of the International Union for the Conservation of Nature. An updated overview of the plant endemics of Mt Cameroon is presented.


Introduction
The new species described in this paper as Drypetes burnleyae was brought to light as a result of botanical surveys to aid conservation management at and around Mount Cameroon (Cheek & Hepper 1994;Cheek et al. 1996), long considered a Centre of Plant Diversity (Cheek et al. 1994). The species is a small tree or shrub, a near endemic of the coastal forests at the foot of Mt Cameroon. In this paper we provide the evidence that this species is new to science, formally describe it, place it within the current classification of the genus, and review the ecology, endemics, conservation importance and threats to the largely unprotected coastal forest of Mt Cameroon in which it occurs.

Putranjivaceae and Drypetes Vahl
The family Putranjivaceae, formerly included in Euphorbiaceae, is considered to have up to three genera, Putranjiva Wall. (four species, restricted to Asia from India to New Guinea), Sibangea Oliv. (three species in tropical Africa) and the most species-diverse genus, Drypetes Vahl which is pantropical. Putranjiva and Sibangea are sometimes subsumed into Drypetes. Drypetes is considered to have about 200 species, with only c. 20 in the Americas, the remainder in Africa (c. 70 spp.) and Asia and Australasia (c. 100 spp. (Hoffmann 2007)).
Plants of the World Online (Plants of the World Online, continuously updated) gives 96 accepted names in Drypetes for 'Africa' but 15 of these are endemic either to Madagascar, Mascarenes, Comores or Seychelles, with two in Saõ Tomé, so about 79 taxa are accepted for continental Africa. Only five species occur in South Africa (e.g. D. natalensis (Harv.) Hutch.) while 25 species were accepted by Keay (1958) for Flora of West Tropical Africa. Cameroon with 31 species (if Sibangea is included) is the most speciesdiverse African country for Drypetes (Onana 2011) followed by Gabon with 26 species (Sosef et al. 2005).
In continental Africa, Drypetes species are mainly confined to the understorey of evergreen, usually lowland, forest. They are indicators of good quality (species-diverse, undisturbed forest), in the same way as are species of Cola (Cheek pers. obs. 1984(Cheek pers. obs. -2012Cheek 2002). Both genera are slow-growing shrubs and trees, not pioneers, and do not regenerate well after major forest disturbance. High species diversity in these genera at a site indicates forest of high conservation value in tropical Africa. Several species in Cameroon have small global ranges and have been assessed as threatened, e.g., Drypetes magnistipula Hutch. (EN), D. preussii Hutch. (VU), D. tessmanniana Pax & K.Hoffm. (VU) (respectively Cheek 2004a, Cheek 2004bCheek & Cable 2000) and once revisionary work is completed, the number of threatened species of Drypetes is expected to rise (Cheek in Onana & Cheek 2011: 144).
Only three new species of Drypetes have been described in Africa in the 21 st century. These are D. moliwensis Cheek & Radcl.-Sm. (Cheek et al. 2000a), D. bakembei D.J. Harris & Wortley (2006) and D. morocarpa J.Léonard ex D.J. Harris & Quintanar (Quintanar et al. 2020). This low number is partly because of the difficulty caused by the destruction of many German type specimens of Drypetes from Central Africa at B in 1943. Loss of this reference material especially hampers the elucidation of new Drypetes material from Central Africa because the former German colony of Kamerun (now the Republic of Cameroon) was the source of many if not most of these type specimens. It is also here that the genus is most species-diverse, and where discoveries of novelties are therefore still most likely. Currently, newly collected specimens of Drypetes often remain unidentified to species. In the Gabon checklist, while 146 specimens are identified to species, a further 147 remained unidentified (Sosef et al. 2005). Burkill (1994: 54 -60) reports on the uses of 19 species of Drypetes in the West African region. The wood of many species is reported as hard and durable, resistant to termites and is valued for constructing homes and mortars for pounding food. The fruits of several species are reported as edible and sometimes sold on markets. The majority of the uses reported are for treating a large variety of ailments, from toothache, wounds, internal and external parasites, fevers, rheumatism, boils and to relieve pain. Some species are used as fish or rat poisons. Johnson et al. (2009) report on the presence of "mustard oil" volatiles in Drypetes.

Material and Methods
The methodology for the surveys in which this species was discovered is recorded in Cheek & Cable (1997). Nomenclatural changes were made according to the Code (Turland et al. 2018). Names of species and authors follow IPNI (continuously updated). Herbarium material was examined with a Leica Wild M8 dissecting binocular microscope fitted with an eyepiece graticule measuring in units of 0.025 mm at maximum magnification. The drawing was made with the same equipment with a Leica 308700 camera lucida attachment. Specimens were inspected from the following herbaria: BM, K, P, WAG, YA. The format of the description follows those in other papers describing new species in Drypetes e.g. Cheek et al. (2000a). All specimens cited have been seen unless indicated "n.v.". The conservation assessment follows the IUCN (2012) categories and criteria. GeoCAT was used to calculate red list metrics (Bachman et al. 2011). Herbarium codes follow Index Herbariorum (Thiers, continuously updated).

Results
The specimens on which the new species, Drypetes burnleyae, is based had been provisionally identified as "Drypetes ?principum" in preparation for publication of the Conservation Checklist for Mount Cameroon (Cable & Cheek 1998). But, by oversight, they were not included in that work. Both species have large leaf-blades of similar size and shape and which dry dark brown on the lower surface and bear densely brown hairy fruits of about equal size from the leafy branches. However, the two species can be easily separated using the information in Table 1.
Drypetes sect. Stipulares Pax & K.Hoffm. A remarkable feature of Drypetes burnleyae are the large leathery persistent stipules. In most species of the genus the stipules are vestigial, triangular <1 mm long and early caducous. Persistent stipules are only found in a minority of the species of the genus, most of which occur in West-Central Africa and which in the latest classification of the genus (Pax & Hoffmann 1922) were grouped in sect. Stipulares. This section is characterised by medium to large persistent stipules 5 -60 mm long, stamens 4 -13 encircling a central disc which lacks a rudimentary ovary. In the female flowers the ovary is 2 (-3)-locular, lacks sculpture and is placed on a spreading disc. The stigmas are sessile or subsessile, dilated and undivided (Pax & Hoffmann 1922). Drypetes burnleyae fits this description. A key to the species with persistent stipules is presented below. In this, our new species keys out in a couplet with D. similis (Oliv.) Hutch. This species was previously segregated with D. arborescens Hutch. as the genus Sibangea but was united with Drypetes by Hutchinson (1912) who also grouped these persistent-stipuled species together as his species 1 -7. Pax & Hoffmann(1922) followed Hutchinson in sinking Sibangea, but placed Drypetes similis in sect. Hemicyclia (Wight & Arn.) Pax & K.Hoffm. on account of the unilocular ovary, despite the large, persistent stipules. We have added to this group Drypetes dinklagei (Pax) Hutch. since this species also has conspicuous persistent stipules which seem to have been overlooked by Pax & Hoffmann (1922). It is possible that this group is a natural monophyletic unit. Apart from the traits listed by Pax & Hoffmann (1922), these species are all geographically coherent, occurring from eastern Nigeria through Cameroon and Gabon, with some species ranging beyond, e.g. D. mildbraedii (Pax) Hutch. in Democratic Republic of the Congo (DRC). These species also share leaves that completely or mainly lack, the marginal teeth that often characterise the genus, and have leaves that dry brown on the lower surface rather than grey-green. The last feature also occurs in some species outside the group, such as in D. principum. Molecular phylogenetic research is needed to test the monophyly of this group.

Mt Cameroon, its endemic plants and its coastal forests
At 4095 m high, Mt Cameroon, locally known as Fako, is by far the highest mountain, and the only active volcano, in continental West-Central Africa. One has to travel about 1300 km eastwards to the Virunga Mts of eastern DRC before encountering mountains of comparable height, or active volcanoes. Fako erupts roughly every 20 years and the last lava flow dates from May 2000. Rainfall varies dramatically from one part of the mountain to another, most falling in the 8 -9 month wet season between April to November. There are no months with less than 50 mm precipitation at Cape Debundscha, on the SW, seaward side, which receives 10 -15 m p.a., but at Tiko to the E there are two months with <50 mm, and rainfall declines to 2 -3 m p.a. while at the summit and in the rainshadow to the E, it is 1 -2 m per year (Courade 1974).
The volcanic massif is about 45 km along its longest axis, SSW -NNE, and 28 km across at its narrowest (Cable & Cheek 1998). Together with the foothill areas and lower slopes that surround it, it has the highest documented species diversity for vascular plants in tropical Africa, with 2435 species, of which 49 have been considered strict endemics and of which 116 were reported as threatened (Cable & Cheek 1998 (Cable & Cheek 1998).
The Mount Cameroon National Park was created in 2009, to a large degree because of its unique botanical importance. The mountain is a tourist attraction, particularly for those who wish to walk to the summit from the former German colonial capital of Buea on the eastern slope of the massif. Forest elephants and the endemic Mt Cameroon Francolin (Pternistus camerunensis) are also attractions, although rarely seen.
Arguably the most spectacular botanical discovery at Mount Cameroon, from its northern foothills, the Southern Bakundu Reserve, was that of Medusandra richardsiana Brenan, basis of a new family, the Medusandraceae (Brenan 1952), to which was later added the genus Soyauxia Oliv. Medusandraceae sensu stricto (Medusandra Brenan) was until recently considered endemic to the Cross-Sanaga interval (Heywood 2007). Subsequently, however, both Soyauxia and later Medusandra were shown to be confamilal with the rare and little known South American family, Peridiscaceae (Soltis et al. 2007;Breteler et al. 2015). The Cross-Sanaga interval (Cheek et al. 2001), comprising largely of South West Region, Cameroon, contains the area with the highest species diversity per degree square in tropical Africa (Barthlott et al. 1996;Cheek et al. 2006), and many of these species are narrow endemics, and a large number are at Mt Cameroon.
The emphasis of the UK government Mt Cameroon Plant Genetic Resources Project, later known as the Mount Cameroon Project was to focus on conservation management of the forest plant diversity, especially the highly threatened lowland forest vegetation. Following rehabilitation of the Limbe Botanic Garden, its base, the project conducted surveys in the eastern foothills formerly known as Mabeta-Moliwe (Cheek 1992) now known as Bimbia-Bonadikombe, and then of the Etinde area (Thomas & Cheek 1992), and in the coastal forests between Idenau and Limbe, with the adjoining coastal eastern foothill forest of Onge (late 1993) and finally Mokoko forest (early 1994). Following identifications, a checklist for Mt Cameroon and its foothills was published (Cable & Cheek 1998). The specimens of Drypetes burnleyae cited in this paper were collected during these surveys.
Several Mount Cameroon endemics occur above 2000 m alt. both at the forest-grassland ecotone or in the montane grassland above that begins the summit area. These include Silene biafrae Hook.f. (Hooker 1864), Myosotis cameroonensis Cheek & R.Becker (Cheek & Becker 2004) and Luzula mannii (Buchenau) Kirschner & Cheek subsp. mannii (Kirschner & Cheek 2000). Yet some species among the nine previously listed as endemic to this summit area (Cable & Cheek 1998) are now known to have a wider distribution. Both Genyorchis macrantha Summerh. and also, Isoglossa nervosa C.B.Clarke have subsequently been found on another peak in the Cameroon Highlands, Mt Oku (Darbyshire et al. 2011), which at 3095 m alt., is also a centre of plant diversity of conservation importance (Maisels et al. 2000;Cheek et al. 2000b). Yet a greater number of the Mt Cameroon endemics are found in the submontane or cloud forest that extends from c. 800 -2000 m altitude (Tchouto et al. 1999;Thomas & Cheek 1992). Eleven are listed in Cable & Cheek (1998), several of which were only formally named and published subsequently. These include Oxygyne duncanii Cheek (Cheek et al. 2018a), Impatiens etindensis Cheek & Eb.Fisch (Cheek & Fischer 1999), Impatiens frithii Cheek (Cheek & Csiba 2002) and orchids such as Angraecopsis cryptantha P. J.Cribb (Cribb 1996). However, most of the Mt Cameroon endemics and the most threatened species overall, are found in the evergreen lowland forest that survives in the foothills and in coastal areas below 800 m altitude.
Twenty-nine species of lowland endemics are listed by Cable & Cheek (1998: xxxii) of which 17 were newly discovered and still awaited publication at that time. Subsequently, some of these supposed endemic species were either synonymised, e.g., Trichoscypha camerunensis Engl. or found to have wider distributions, e.g., Trichoscypha bijuga Engl. (Breteler 2004). Some of those subsequently published as new to science and thought to be endemic, such as Salacia nigra Cheek (Gosline et al. 2014) and Belonophora ongensis S.E. Dawson & Cheek, were later found to extend beyond Mt Cameroon to Littoral Region (Cheek et al. 2018b;Cheek & Dawson 2000). This was also the case with Stelechantha arcuata S. E.Dawson (2002) now Pauridiantha arcuata (S.E. Dawson) Smedmark & B.Bremer (2011) which extends to Mt Kupe and the Bakossi Mts (Cheek et al. 2004), Psychotria elephantina O. Lachenaud & Cheek (Cheek & Lachenaud 2013) extending to Korup (Lachenaud 2019), and also, Ancistrocladus grandiflorus , subsequently found to extend to the Rumpi Hills.
However, other endemics recorded for Mt Cameroon in Cable & Cheek (1998), under working names that were formalised later, remain strictly endemic to the coastal lowland forests such as Cola cecidifolia Cheek (2002), Psychotria bimbiensis Bridson & Cheek (Cheek & Bridson 2002) and Drypetes moliwensis Cheek & Radcl.-Sm. (Cheek et al. 2000a). Reexamination of specimens collected in the surveys of the 1990s has uncovered further endemic new species, not listed as such in Cable & Cheek (1998), such as Dracaena mokoko Mwachala & Cheek (2012) and Octoknema mokoko Gosline & Malécot (2012). Aframomum tchoutoui D.J.Harris & Wortley is an example of one of several near-endemics discovered in the same way, in this case extending about 50 km west and 100 km N of Mt Cameroon (Harris & Wortley 2018). New surveys at Mt Cameroon in the 21 st century have discovered other highly threatened species entirely overlooked in the 1990s, such as Kupea martinetugei Cheek (Cheek et al. 2003), Psychotria asterogramma O. Lachenaud (2019), and a further new endemic, Afrothismia foertheriana, T.Franke, Sainge & Agerer (Franke et al. 2004).
The rarest of the rare endemic Mt Cameroon plant species were considered those nine species listed as possibly extinct globally in Cable & Cheek (1998) since they had not been seen for over 60 years (now 80 years), not being found in the surveys of the 1990s. Eight of these species were lowland forest endemics. In one case, Coleus dissitiflorus Gürke (treated as Plectranthus dissitiflorus (Gürke) J.K.Morton in Cable & Cheek (1998)), a collection was discovered from the 1970s giving hope that it survives today. But the remainder have not been seen, leading to fears that they are indeed extinct. In one case, Oxygyne triandra Schltr., a series of concerted efforts were made to rediscover it over several years, without success, leading to the conclusion that it is indeed now globally extinct (Cheek & Williams 1999;Cheek et al. 2018a). This is also the case with a further mycotroph, Afrothismia pachyantha Schltr., that had been collected with the Oxygyne. This Afrothismia was thought to have been locally extinct at Mt Cameroon but was mistakenly considered to have been rediscovered on Mt Kupe (Cheek 2004c), until the Mt Kupe population was found to be a separate species rendering Afrothismia pachyantha restricted to the lowland forest of Mt Cameroon and also globally extinct .
The habitat of all these coastal lowland forest species falls outside the Mount Cameroon National Park which is restricted to higher altitudes. Lowland forest and its species are undoubtedly the most threatened at Mt Cameroon. Clearance of coastal forest had begun in the eastern foothills area of Mabeta by the early 19 th century to produce food for slaves being held at Bimbia by the Isuwu. Later the Christian settlement of Victoria (now Limbe) was founded in 1858 at Ambas Bay by Alfred Saker of the London Missionary Society. More extensive forest clearance began with the advent of the German colony of Kamerun (now Cameroon) in 1884. The fertile volcanic soils and abundant rainfall merited extensive lowland forest clearance to support plantation crops for export to Europe, initially of bananas, later of rubber and oil palm. Oil palm plantation now extends in a belt around the seaward southern base of Mount Cameroon from Cameroon's largest port, Douala, but does not yet occupy the more infertile western foothill areas. Today, the lowland forest areas that remain are threatened by clearance for timber, followed by subsistence agriculture and plantation expansion. Limbe is now a city, and a port, with an oil refinery and a population of about 118,000 persons (Cameroon data portal 2015).
These threats make it a priority to name Mt Cameroon coastal forest endemic species such as Drypetes burnleyae, since until this is done, conservation assessments are not accepted by IUCN (IUCN 2012), and as a result, proposals for conservation measures are less likely to be enacted.
The  (Cheek et al. 2020), and a high proportion of these are likely to be threatened, since more widespread species tend already to have been discovered, although there are exceptions (Cheek & Etuge 2009). This makes it all the more urgent to find, document and protect undescribed species before they become globally extinct, as have the eight species discussed above from Mt Cameroon.
Efforts are now being made to delimit the highest priority areas in Cameroon for plant conservation through the Tropical Important Plant Areas (TIPAs): Cameroon programme (Cheek, continuously updated) which uses the revised Important Plant Areas (IPA) criteria set out in Darbyshire et al. (2017). The coastal forests of Mt Cameroon, the habitat for Drypetes burnleyae will be among the TIPAs so designated.
Postcode Lottery. Janis Shillito typed the manuscript. George Gosline and two anonymous reviewers gave advice on an earlier version of the manuscript. Fieldwork funding in the 1990s leading to the discovery and collection of most of the specimens cited in this paper was received from the former Overseas Development Administration (ODA) of the UK government (now incorporated in the Foreign, Commonwealth and Development Office) through the former Mount Cameroon Project at Limbe Botanic Garden, which was co-managed by ODA with the Forest Department of the Cameroon Government. The fieldwork during which the type collection was collected was supported by the Earthwatch Institute. The authors thank Dr Jean Michel Onana of Yaoundé and an anonymous reviewer for constructive comments on an earlier version of this manuscript.
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