Notes on Tricalysia elmar sp. nov. (Rubiaceae, Coffeeae), and cloud forest of the Cameroon Highlands

Background and aims – This paper reports a further discovery in the context of a long-term botanical survey in the Cross River-Sanaga interval of west-central Africa, focussing on species discovery and conservation. Methods – Normal practices of herbarium taxonomy have been applied to study the material collected. The relevant collections are stored in the Herbarium of the Royal Botanic Gardens, Kew, London and at the Institute of Research in Agronomic Development – National Herbarium of Cameroon. Key results – Species new to science recently discovered from the cloud (submontane) forests of the Cameroon Highlands in Cameroon are reviewed. Most of these species are rare, highly localised, and threatened by habitat destruction. These discoveries increase the justification for improved conservation management of surviving habitat. Tricalysia elmar Cheek (Coffeeae-Rubiaceae) is described as an additional cloud forest species new to science. Its taxonomic position is discussed, and it is compared with similar species of the genus. Restricted so far to four locations, Mt Kupe, Bali Ngemba, Lebialem and Nta Ali, its conservation status is assessed as Endangered (EN B1+2ab(iii)) according to the 2012 criteria of IUCN.


INTRODUCTION
The new species of Tricalysia A.Rich. reported in this paper was discovered as a result of the long-term survey of plants in Cameroon to support improved conservation management led by botanists from Royal Botanic Gardens, Kew and the IRAD (Institute for Research in Agronomic Development)-National Herbarium of Cameroon, Yaoundé. This study has focussed on the Cross-Sanaga interval (Cheek et al. 2001) which contains the area with the highest species diversity per degree square in tropical Africa (Barthlott et al. 1996), including several endemic genera. Medusandraceae sensu stricto (Medusandra Brenan) was until recently considered endemic to the Cross-Sanaga interval (Heywood 2007) but data now shows that Medusandra, with Soyauxia Oliv., are confamilial with neotropical Peridiscaceae (Soltis et al. 2007;Breteler et al. 2015).
The herbarium specimens collected in these surveys formed the primary data for the series of Conservation Checklists that began at Mt Cameroon (Cheek et al. 1996), with the Plants of Mt Cameroon (Cable & Cheek 1998) and continued with Mt Oku and the Ijim Ridge , Bali-Ngemba (Harvey et al. 2004), the Mt Kupe, the Bakossi Mts and Mwanenguba , Dom  and Lebialem ). So far, over 100 new species and several new genera have been discovered and published as a result of these surveys, many from the cloud forest altitudinal band of 800-2000 m altitude, and two new national parks have resulted, the Mt Cameroon National Park, and The Bakossi Mts National Park, the first to be specifically for plant conservation (rather than for animals) in Cameroon. The species described in this paper was first noted as "a new species, apparently endemic to Mt Kupe and Bali Ngemba" in Cheek et al. (2004: 394) and in Harvey et al. (2004: 124), and was subsequently found at Lebialem (Harvey et al. 2010: 142) and was given the provisional name of "Tricalysia sp. B aff. ferorum". Here it is formally named as Tricalysia elmar Cheek, allowing IUCN to accept a conservation assessment for the species, and for it then to be incorporated into conservation prioritisation initiatives.

The genus Tricalysia
The new species was readily placed in Tricalysia of tribe Coffeeae (Rubiaceae) by the combination of shortly sheathing and cylindrical interpetiolar stipules with long awns, contracted axillary inflorescences with a series of 3(-4) cupular calyculi subtending the flowers, calyx teeth and tube reduced but conspicuous, corolla tubes about as long as lobes, anthers largely exserted (sometimes included in Sect. Ephedranthera), inserted at the mouth of the corolla tube which lacks exserted hairs.
The genus Tricalysia currently contains about 78 described species, which are mainly evergreen hermaphroditic shrubs or small trees of lowland evergreen forest in tropical and southern Africa and Madagascar. However, several species are subshrubs of drier habitats in southern Africa, and at least one species is deciduous. Formerly, the name Tricalysia was extended to Asian species now included in Diplospora DC. and Discospermum Dalzell (Ali & Robbrecht 1991). Those Asian species are distinct from Tricalysia s.s. (in Africa) by their unisexual tetramerous flowers. Tricalysia is now restricted to Africa and Madagascar. Sericanthe Robbr., an African genus segregated from Tricalysia by Robbrecht (1978), is distinguished by seeds with a scar-like area from apex to ventral face, basifixed anthers, often possessing bacterial nodules in the leaves and a sericeous indumentum on the outer corolla surface (Bridson & Verdcourt 2003). The molecular phylogenetic analysis of Tricalysia by Tosh et al. (2009) showed that the former Tricalysia subgenus Empogona had a sister relationship with Diplospora. Accordingly, the African genus Empogona Hook.f. was resurrected. Its species are distinguished from Tricalysia in the strict sense by black fruits, flag-like anther connectives, hairs projecting from the corolla throat and free, often alternate distal bracts. The phylogeny of the group was refined by Arriola et al. (2018) who included Asian representatives in the sampling and postulated that Tricalysia sensu stricto is sister to a clade (Sericanthe (Diplospora, Empogona)).
A key to the 12 genera in tribe Coffeeae can be found in Cheek et al. (2018a). This tribe has its highest generic diversity in Cameroon with nine of the genera present.
Most African Tricalysia species are well studied as a result of Robbrecht's revisions (Robbrecht 1979(Robbrecht , 1982(Robbrecht , 1983(Robbrecht , 1987. Within Tricalysia as currently defined, Robbrecht (1979,1982,1983,1987) recognised five sections in Africa: Sect. Ephedrantha Robbr., Sect. Probletostemon (K.Schum.) Robbr., Sect. Tricalysia, Sect. Rosea (Klotzsch) Robbr. and an unnamed Madagascan section. However, only eleven species of the 78 accepted Tricalysia were sampled in the phylogenetic study by Tosh et al. (2009), and of these, only six were African, so further, more intensively sampled, phylogenetic studies are called for if the current sectional classification is to be tested. Since the monographic studies of Robbrecht, seven new taxa have been published from the Flora Zambesiaca region by Bridson (in Bridson & Verdcourt 2003) and nine new taxa published for the Madagascan section now named Sect. Androgyne Robbr. (Ranarivelo-Randriambovanjy et al. 2007). Apart from these, only two other new species have been published, both from Cameroon: Tricalysia lejolyana Sonké & Cheek (Sonké et al. 2002a) and T. achoundongiana Robbr., Sonké, & Kenfack (Sonké et al. 2002b). However, new species are likely to be discovered in Gabon, where 118 Tricalysia specimens are recorded as being unidentified to species versus 219 identified (Sosef et al. 2005: 373-375), and in Cameroon, where six unidentified, possibly undescribed species are recorded from the Bakossi area in Cheek et al. (2004). Many species of the genus are geographically localised, rare and threatened. For example, twelve species of Tricalysia were assessed as threatened in Cameroon (Onana & Cheek 2011), mainly because they are known from a single or few locations, and some are threatened by logging followed by agriculture such as palm oil plantations, e.g. Tricalysia lejolyana (Sonké et al. 2002a).
Tricalysia elmar is distinguished by having fruits with a highly conspicuous, large and dome-like accrescent disc. This feature is otherwise only known in the genus in T. ferorum Robbr., placed in Sect. Probletostemon due to its free distal bracts, yet placed there with reservations: "of isolated position, does not fully match with the enumerated characters, but is accomodated here" (Robbrecht 1983: 299). However, Tricalysia elmar has distal bracts united in a cupular calyx, and due to this character, together with the entire calyx (separating it from Sect. Rosea, which further, is restricted to eastern Africa), stalked and fully exserted stamens with distinct filaments (separating it from Sect. Ephedranthera), it merits placement in Sect. Tricalysia. In the key to the species of this section (Robbrecht 1987: 67-71), Tricalysia elmar keys out to couplet 44, due to the calyx lobes exceeding the tube in length, the 6-merous flowers, glabrous styles and anthers, pubescent ovary and corolla lobes 8-11 mm long. However its characters fit neither of the two choices offered in that couplet, which lead to T. niamniamensis Hiern and, in couplet 45, to T. bagshawei S.Moore (except subsp. malaissei Robbr.) and T. oligoneura K.Schum. Tricalysia elmar differs from the first species in lacking corolla lobes with entirely pubescent abaxial surfaces (instead they are only sparsely hairy at the base and apex of the lobes). It differs from the last two species in lacking hairs at the inner base of the corolla tube (they are present only at the throat), and lacking pubescence only on the half of the corolla lobes not covered in bud (hairs present are spread equally on both sides of the lobe). Table 1 uses diagnostic characters separating Tricalysia elmar from these species, drawing on data from Robbrecht (1983Robbrecht ( , 1987.

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 Coffeeae e.g. Cheek et al. (2018a). Terminology for specialised structures, e.g. for colleters, and domatia, generally follows Robbrecht (1987Robbrecht ( , 1988. 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). The map was produced using simplemappr software (https://www. simplemappr.net/). Herbarium codes follow Index Herbariorum (Thiers continuously updated).

RESULTS
The new species, Tricalysia elmar, shares uniquely within the genus an accrescent floral disc with T. ferorum of Sect. Probletostemon. The disc in both species conspicuously occupies the apex of the fruit. In other species of the genus the disc is not accrescent and is usually invisible in fruit, where it is concealed by the calyx. However, the taxonomic placement of the new species is postulated as being not with Sect. Probletostemon but with species of Sect. Tricalysia, since it has the following traits, that characterise that group (Robbrecht 1987) yet which are unknown in Sect. Probletostemon (Robbrecht 1983). 1) distal bracts united in a cupular calyculus; 2) calyx entire, not spathaceous or split; 3) anthers completely exserted from the corolla mouth, with distinct filaments.
Using the dichotomous key of Robbrecht (1987), Tricalysia elmar keys out to the vicinity of three species (see introduction). Its affinities are therefore likely to be with these, although molecular phylogenetic studies are needed to confirm this hypothesis. The taxa can be separated using the characters shown in table 1.

The accrescent disc in Rubiaceae
The floral disc in Rubiaceae e.g. in Tricalysia, is considered to secrete nectar for the attraction of pollinators (Robbrecht 1987: 53). Therefore, it is not unexpected that as the ovary develops into the fruit, it is generally invisible or inconspicuous, concealed by the calyx, having no obvious function at that stage. However, several species of Rubiaceae in a range of genera and tribes are distinguished from their congeners at the fruiting stage by the presence of accrescent discs which are highly conspicuous to the naked eye, occupying much of the apex of the fruit. Elsewhere in the Coffeeae this feature can be seen in Coffea bakossii Cheek & Bridson where it helps to distinguish it from the similar C. liberica Hiern ). In the Vanguerieae, massive accrescent discs are an important character for recognising Keetia susu from other West African species of the genus. In all these examples, those species with accrescent discs are distinguished by having larger fruits than their congeners suggesting that disc accrescence is linked to the development of larger fruit size which can be conjectured to have selective dispersal advantage in some scenarios, where animal dispersers might favour larger fruits.

Cloud, or submontane forest in the Cameroon Highlands
A fault, running nearly NE-SW between two major African plates, is the origin of the Cameroon Highlands that lie in a band 50-100 km wide along that fault and which were formed over four separate periods of mountain building beginning in the Tertiary, although a Cretaceous origin has also been suggested. The highest point, at 4095 m, is Mt Cameroon, an active hawai`ian type volcano, but the highlands begin in the bight of Biafra 40 km to the SW with the mountain-island of Bioko ( ) and the Kilum Ridge, the second highest peak in Cameroon. The highlands branch northwestwards into Nigeria in two places, as the Obudu plateau, and from the Bamenda Highlands, the Mambilla Plateau. Continuing northeastwards to Tchabal Mbabo, the highlands turn eastwards as the Adamaoua Plateau, extending into the west of the Central African Republic (Courade 1974;Cable & Cheek 1998;Cheek et al. 2004). Moisture laden southwesterly monsoon winds from the Atlantic result in rainfall of 3-4 m p.a. on the more coastal highlands, decreasing steadily inland. The wettest spot in Africa, Cape Debundscha, at the foot of Mt Cameroon, receives 10-15 m p.a., and rainfall there is almost continuous throughout the year. Rainfall in most of the highlands occurs mainly from May to November (Courade 1974;Cable & Cheek 1998). The mountains disrupt the normal bimodal pattern seen in West Africa, resulting in one long, instead of two shorter rainy seasons per annum. Submontane or cloud forest, is generally recognised as occurring in the 800-2000 m alt. band and is characterised by the presence of black, humic soils and the presence of abundant epiphytic, pendulous mosses on woody plants . Cloud forest once extended from the coastal highlands inland to the Bamenda Highlands at least, but its clearance has been near total in the Bamileke Highlands (now intensively cultivated for Coffea arabica L.) and agriculture is also extensive in the Bamenda Highlands where human population is also dense. Total primary forest loss for Cameroon is reported as 47.7% and loss per year from 2001-2018 at 52,272 ha per annum (Mongabay, https:// rainforests.mongabay.com/deforestation/archive/Cameroon. htm downloaded on 18 Jul. 2019). However, forest loss has not been evenly spread throughout the country, and has been most extensive in high altitude areas above the malaria zone with relatively fertile soils. In the Bamenda Highlands the sole surviving patch of any size is the Bali Ngemba forest reserve (c. 8 km 2 ), which is not managed for conservation purposes but for timber extraction. Further south some cloud forest remains in places along the steep west-facing escarpment that links the plateaux with the lowlands, such as at Lebialem, but this is steadily also being cleared for agriculture (Tchiengue in Harvey et al. 2010). The largest surviving blocks of submontane forest in Cameroon are thought to be those in the Bakossi Mts and Mt Kupe where extensive tracts still blanket the mountains, although even here clearance upslope for small-holder agriculture continues ). The cloud forest vegetation and species composition at several of these locations have been characterised in the series of conservation checklists referred to above, and additionally that for Mt Cameroon by Tchouto et al. (1999). However, despite this, some areas, such as the Bakossi Mts, remain incompletely sampled, and the Rumpi Hills and Lebialem Highland areas are even more poorly known to science, while some highland areas remain practically unsampled. There is no doubt that many tens of species remain to be discovered and published. Among other cloud forest plant species discovered in cloud forest in the Cameroon highlands in the last 20 years, are (grouped by life-form, in alphabetical order by genus): Herbs  (Mackinder & Cheek 2003), Rhaptopetalum geophylax Cheek & Gosline , and Ternstroemia cameroonensis Cheek (Cheek et al. 2017).
These cases illustrate the scale at which new discoveries are being made in the Highlands of western Cameroon which already contains the most species-diverse degree squares documented in tropical Africa; and which includes several Pleistocene refuge areas (Cheek et al. 2001).
Most of the species listed above are threatened with extinction, since they are narrow endemics with small ranges, restricted to mainly submontane (cloud) forest patches which are steadily being cleared (Onana & Cheek 2011).
Pl. Ecol. Evol. 153 (1), 2020 The number of flowering plant species described worldwide as new to science each year regularly exceeds 2000, adding to the estimated 369 000 already known (Nic Lughadha et al. 2016), although the number of flowering plant species known to science is disputed (Nic Lughadha et al. 2017). Only 7.2% have been assessed and included on the Red List using the IUCN (2012) standard (Bachman et al. 2019), but this number rises to 21-26% when additional evidence-based assessments are considered, and 30-44% of these assess the species as threatened (Bachman et al. 2018). Newly discovered species, such as that reported in this paper, are likely to be threatened, since widespread species tend to have been already discovered. There are notable exceptions to this rule (e.g. Vepris occidentalis Cheek (Cheek et al. 2019) a species widespread in West Africa from Guinea to Ghana). Generally, it is the more localised, rarer species that remain undiscovered. This makes it all the more urgent to find, document and protect such species before they become extinct, as is Oxygyne triandra Schltr. (Cheek et al. 2018b), or possibly extinct, in the case of another Cameroon Highland cloud forest tree, Vepris bali Cheek (Cheek et al. 2018c). Most of the 815 Cameroonian species Red Listed in the "Red Data Book, Plants of Cameroon" are threatened with extinction due to habitat clearance, mainly 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 Tricalysia elmar.