Taxonomy of Atlantic Central African orchids 5. A new species of Angraecum sect. Conchoglossum (Orchidaceae, Angraecinae) from Gabon and Cameroon

Abstract Recent field inventories and taxonomic research in Central Africa have resulted in the discovery of many new orchid species. Five specimens of an apparently new Angraecum species were collected in Gabon and Cameroon. They stand out for their hanging habit and short zig-zag stem. Morphology of leaves and habit is somewhat comparable to Angraecum cultriforme and Angraecum stolzii, two species from East Africa. Flowers of the novelty share the general morphology of Angraecum pyriforme from which the new species is distinguished by being smaller and with a different lip-spur ratio. Here we show that these five specimens represent a new species, described here as Angraecum lanceolatum. The distinguishing traits include thin lanceolate leaves, convolute distally, with a rhombic lip shape. Dichotomous key to four Central African species of sect. Conchoglossum and a table of the diagnostic characters of the seven related Continental African Angraecum taxa are included here. A preliminary assessment of the conservation status of Angraecum lanceolatum is provided, using the IUCN Red List Categories and Criteria.


Introduction
According to the latest count of WCSP (Govaerts et al. 2015), the genus Angraecum Bory comprises 223 species. With 173 species recorded in the Malagasy region (Govaerts et al. 2015), Madagascar and the Mascarenes are considered as the centre of diversity of Angraecum. Nevertheless, Central Africa also shows a high orchid diversity and endemism rate (Stévart 2003, Droissart 2009) where many new species remain to be described. A cultivation system established in São Tomé, Gabon, Equatorial Guinea and Cameroon by Stévart (2003) and his collaborators has allowed collection of thousands of fl owering specimens. Th is has enabled taxonomic revisions of several orchid genera (Verlynde et al. 2013) and the description of more than 25 new orchid taxa (e.g. Droissart et al. 2014, D'Haijere et al. 2015, many of which still remain to be published.
A revision of Angraecum species belonging to sections Afrangraecum Summerh. and Conchoglossum Schltr. was conducted by the fi rst author in 2015. A careful examination of specimens from main herbaria has confi rmed the status of fi ve new species, of which one is described here.
Th e fi rst collection of the new species originates from Mont Seni in the Monts de Cristal National Park in Gabon (IUCN Category II National Park). Th is specimen was collected by Nguema Miyono (N. Miyono 2037) in 2001 and deposited in BRLU and LBV (abbreviations after Th iers continuously updated). Unfortunately, the material was sterile and identifi ed as Angraecum angustipetalum Rendle. A few years later, during fi eldwork in the Ivindo National Park in Gabon, a living plant of the same species was collected by Diosdado Nguema. Th e specimen was sent to the garden of M. Biteau (Jardi-Gab, Libreville) who cultivated it in his shade-house under number BTO23. Since then, the plant has produced three fl owering specimens (D. Nguema s.n., JBB 244 and JBB 263) after which it died. Following examination of the three fl owering specimens and the living plant (BTO23), Stévart considered it as a potentially new species. Finally, fi eldwork conducted in Cameroon by Droissart in February 2015 enabled another collection of that new species (Droissart et al. 1874). Th e specimen was collected in the Campo-Ma'an National Park (South Region of Cameroon) and cultivated in Yaoundé shade-house under number Y 5652 NY where it fl owered in June 2015. Comparison of these fi ve specimens with the type material of related Angraecum species confi rmed that these specimens represent a new species, described here as Angraecum lanceolatum.
Th is paper is the fi fth in a series of publications (Stévart et al. 2010, D'Haijere et al. 2015) based on recent intensive fi eldwork and focusing on collections-based taxonomic revisions of Orchidaceae in Atlantic Central Africa.

Material and methods
Th is study was conducted under the framework of the fi rst author's Master's thesis. A revision including 109 specimens from all Angraecum species belonging to sections Afrangraecum and Conchoglossum was undertaken. Collections of BR, BRLU, K, WAG, MA, MO, P and YA were examined and did not reveal any additional specimens of the new species. Description of the new species is based on fi ve spirit preserved specimens originating from Gabon and Cameroon. Th e terminology used for description followed Systematics Association Committee for Descriptive Biological Terminology (1962a, 1962b, Botanical Latin (Stearn 1992) and Th e Kew Plant Glossary (Beentje 2010). Two living specimens of the new species were collected by teams of the Missouri Botanical Garden (MBG) and the Institut de Recherche pour le Développement (IRD) during fi eldwork in Gabon and Cameroon. Sterile material collected in the fi eld was grown in the shade-houses until obtaining fl owered specimens preserved as spirit collections. Colour and habit characteristics given are based on the fi eld data and high resolution photographs. Additional photographs, measurements and morphological study of spirit material were carried out using an optic microscope Zeiss STEMI SV11.
A preliminary risk of extinction assessment was made using the IUCN Red List Categories and Criteria (IUCN 2001(IUCN , 2014. Georeferenced specimen data were imported into GIS to calculate area of occupancy (AOO) and extent of occurrence (EOO). Th e cell size for AOO was set 2 × 2 km as recommended by IUCN (2014). Each locality was regarded as a separate subpopulation. Th e number of 'locations' (as defi ned by IUCN 2014) was calculated with regard to the kind of threats, such that a single 'location' may encompass more than one adjacent population. Diagnosis. Angraecum lanceolatum is close to A. stolzii Schltr. (1915) but diff ers from it by shorter narrowly ovate leaves that convolute in the distal half, by a shorter zig-zag stem and by a rhombic lip shape. Th e species also resembles A. cultriforme Summerh. (1958) but diff ers from it by smaller fl ower size and slightly curved spur. Angraecum lanceolatum is also close to A. pyriforme Summerh. (1936) in the shape of fl oral parts, but diff ers from it by previously stated vegetative characters, hanging habit, singlefl owered infl orescence and smaller fl ower. Description. Small epiphytic herbaceous plant. Stem hanging, slightly zig-zag in form, unbranched, up to 8.5 cm long. Leaves alternate, spaced, narrowly ovate to lanceolate, sometimes slightly falcate and always convolute in the distal part, margins entire; distinct midvein forming slight channel, accompanied with 2 or 3 nerves on each side merging into one throughout; small stomata spots visible in young leaves; leaf apex unequally bilobed, acuminate, with the larger lobe 1.5-2.8 mm long and the smaller 0.3-1.2 mm long, leaf blade 2.3-4.1 × 0.6-0.9 cm; leaf internode about 5-6 mm long. Infl orescence single fl owered, eventually two-fl owered; peduncle elongated 13-23 mm long, opposite to the leaf at the node. Bracts acute, 2 mm long. Flowers white, opening diameter about 12.5 mm. Ovary and pedicel not resupinate, 8 mm long. Dorsal sepal 6.2-8.5 × 3 mm, elliptic, acute, thick, with entire margins. Lateral sepals 6-7 × 2-2.2 mm, elliptic, acute, thick, with entire margins. Petals 5-6.5 × 2-2.2 mm, obliquely elliptic, acute, entire margins, similar in shape to lateral sepals. Lip 5-6 × 4.5-5 mm, concave, rhombic when fl attened, widest between fi rst third and the half, acute; spur 16-19.5 mm, cylindric, slender, straight, somewhat elliptically infl ated and greenish at the apex. Column 1.5 × 2 mm. Pollinia 2, pyriforme. Fruit capsule, 18-24 × 3.5-5 mm. Distribution and habitat. Endemic to the Lower Guinea Domain (Cameroon and Gabon, Fig. 3). Th e specimen collected in Cameroon was found at 850 m elevation in submontane forest with Gilbertiodendron unijugum (Pellegr.) J. Léonard (Fabaceae). Th e plant was epiphyte at about 1.5 m from the ground, on the trunk of a shrub with a diameter less than 10 cm.

Angraecum lanceolatum
Phenology. Flowering occurs in June and September.

Conservation. IUCN Red List category: Least Concern [LC]
. Th e extent of occurrence (EOO) of Angraecum lanceolatum is estimated to be over 23,884 km 2 , exceeding the 20,000 km 2 upper limit for Vulnerable status under the criterion B1, whereas its area of occupancy (AOO) is estimated to be 12 km 2 (which falls within the limits for Endangered status under the criterion B2). Th e species is now known from three subpopulations in Gabon and Cameroon. Th ese three subpopulations represent three diff erent locations (sensu IUCN 2014), less than fi ve locations, which is the up- per limit for Endangered status under the subcriterion 'a' of criterion B2. Angraecum lanceolatum has only been collected in protected areas (Monts de Cristal and Ivindo National Parks in Gabon and Campo-Ma'an National Park in Cameroon). None of these protected areas is under threat and they appear well managed. Angraecum lanceolatum is thus not threatened. Th e available information suggests that the number of subpopulations and mature individuals, as well as its EOO and AOO, will not decrease noticeably in 10 years or 3 generations the future. Application of the IUCN criteria therefore indicates that it cannot be regarded as Endangered despite the fact that its AOO is limited. Angraecum lanceolatum is therefore assigned a preliminary status of LC.
Etymology. Th e specifi c epithet of the new species owes to the particular leaf shape. Even though there are several interpretations of "lanceolate" shape according to diff erent authors (Linnaeus, Lindley), we relied on the current depiction from Beentje (2010) that describes it as narrowly ovate and tapering to a point at the apex.

Notes
Th e diagnostic characters of species from Central African region that belong to the section Conchoglossum, as well as one morphologically related species of the section Afrangraecum are summarized (Table 1). Morphometric results of the mentioned Master's thesis and molecular data on Angraecum (Stévart unpublished) confi rmed the status of the new species. Vegetative morphology and habit of the new species resemble Angraecum stolzii Schltr. in having single fl owered infl orescence and sometimes slightly falcate leaves. Large and small leaf apex lobes are not prominent as in A. stolzii, in which the larger lobe reaches at least 10 mm, comparing to a maximum of 2.5 mm in A. lanceolatum. Spur is similar in shape but in the new species it is at least three times longer than the lip, while the fl ower in A. stolzii has approximately equal spur and lip lengths. Floral morphology, particularly the lip shape of new species is similar to A. pyriforme Summerh. from the sect. Afrangraecum. Nevertheless, ratio between lip and spur lengths is close to 1:3 in the new species compared to 1:2 in A. pyriforme. Additionally, the new species has a distinctive habit.
Th e novelty is a representative of Conchoglossum section according to Stewart et al. (2006, see also Summerhayes 1958, Angraecoides sensu Garay 1973), due to its continental distribution and its white single fl owered infl orescence.