Taxonomy of Atlantic Central African orchids 2. A second species of the rare genus Distylodon (Orchidaceae, Angraecinae) collected in Cameroon

Abstract While conducting field inventories in South Cameroon, we collected two specimens of a new species that we considered to belong to the genus Angraecopsis. Afterwards, a careful examination of specimens housed at main herbaria, along with the nomenclatural types, allows us to place it in Distylodon, a monotypic genus previously known from East Africa. Distylodon sonkeanum Droissart, Stévart & P.J.Cribb, sp. nov. was collected in the lowland coastal forest of Atlantic Central Africa. It is known from a single locality in the surroundings of the Campo-Ma’an National Park. The species differs from D. comptum, by its several-flowered inflorescences, longer leaves and spur, and shorter pedicel and ovary. The species appears to be rare and is assessed as Critically Endangered [CR B2ab(iii)] according to IUCN Red List Categories and Criteria. New field investigations are required to attempt to find it in the low-elevation parts of the Campo-Ma’an National Park in Cameroon.


Introduction
During fi eldwork conducted in April 2007 by the fi rst and the third authors (Droissart 2009) in the surroundings of the Campo Ma'an National Park, near the village of Bifa (South Region, Cameroon), two living specimens of an unknown orchid were collected without fl owers and were thus cultivated in an orchid shadehouse at Yaoundé. Th ese specimens fl owered in July 2007 and were initially placed in Angraecopsis through the trilobate shape of the lip. Th en, a detailed examination and comparison with material at the Orchid Herbarium of the Royal Botanic Gardens, Kew, and with literature (e.g. Summerhayes 1933, 1951, Szlachetko and Olszewski 2001, allowed us to identify these specimens as belonging to a new species of the monotypic genus Distylodon Summerh . Complementary surveys in the same area and examination of closely related material preserved at BR, BRLU, K, P, WAG and YA (herbaria acronyms according to Th iers continuously updated) did not reveal any additional specimens of this novelty. Seven years after our discovery in Cameroon, we describe the new species collected from a single locality, resulting in the present paper. Th is paper represents the second in a series of publications based on recent intensive fi eld work (Stévart 2003, Droissart 2009) and focusing on collections-based taxonomic revisions of Orchidaceae in Atlantic Central Africa.

Distylodon sonkeanum
Distribution and habitat. Distylodon sonkeanum is endemic to the coastal part of the Lower Guinea Domain (White 1979) of the Guineo-Congolian Region. It is known from a single locality in South Region of Cameroon (Fig. 3). Th e only population known so far was found in the lowland evergreen forest at 100 m elevation, growing epiphytically on a fallen branch.
Conservation. IUCN red list category: Critically Endangered, [CR B2ab(iii)]. Distylodon sonkeanum is only known from one very restricted subpopulation in Cameroon which represents one 'location'. Th e main threats known to the species in the southern Cameroon are deforestation for logging and oil palm ( Elaeis guineensis Jacq.) and rubber ( Hevea brasiliensis (Willd. Ex A. Juss.) Müll.Arg.) plantations, resulting from a strong international demand. Th e ongoing loss of the forest leads us to expect a continuous decline of its habitat in the only known locality, and therefore of its extent of occurrence and area of occupancy. Moreover, this location is accessible to local residents and through their practices of shifting agriculture and small-scale timber exploitation; they are gradually transforming this area into secondary forest.
Distylodon sonkeanum appears to be rare but, due to its inconspicuous habit, further fi eldwork is required to ascertain its conservation status more objectively. Considering the general habitat of D. sonkeanum , it is to be hoped that more specimens and additional sites will be found in the low-elevation parts of the adjacent Campo Ma'an National Park. Based on our current knowledge of the ecology and the distribution of the species (one location and AOO less than 10 km 2 ), and using the Criteria B of the IUCN Red List Categories and Criteria, the species was assessed as CR B2ab(iii).
Etymology. Th e species epithet ' sonkeanum ' refers to the Professor Bonaventure Sonké of the University of Yaoundé I, who is an internationally recognized taxonomist, specialized in the Rubiaceae's family. He has collected extensively in the Bipindi-Akom II area, and adjacent Ngovayang massif and Campo-Ma'an National Park. He was one of the fi rst to highlight the high biodiversity of this area and he greatly facilitated our work in Cameroon.

Discussion
Th e genus Distylodon was established about 50 years ago by Summerhayes (1966). Before our discovery, it was represented only by Distylodon comptum Summerh., known from one specimen collected in Uganda in 1944 ( Eggeling 5381 , holotype K!). As suggested by its etymology, this genus is mainly characterized by the shape of the rostellum which is bilobated, the two lobes standing up parallel to one another forming two narrow almost subulate acute teeth or fangs ( Fig. 1 H, I).
Distylodon sonkeanum is easily distinguished from the only taxon described in the genus so far, D. comptum . Th e new species produces plurifl orous infl orescences, longer leaves and spur, and shorter pedicel and ovary. As pointed out by Summerhayes (1966), the genus appears to be closely allied to Angraecopsis Kraenzl. Both genera present short stemmed plants with distichous, conduplicate leaves with unequally lobed tips; their infl orescences are short bearing small, spurred and pale yellow to green colored fl owers. Nevertheless, the genus Distylodon can be distinguished by its sepals with similar shape and size, and the characteristic shape of its rostellum. Further molecular evidences are required to test the monophyly of Distylodon . Unfortunately, no material suitable for DNA studies has been collected so far and consequently the phylogenetic placement of the species remains to determine.
Th e distance between the localities of Distylodon sonkeanum and D. comptum is more than 2,000 km (Fig. 3). Th e Albertine Rift is well known for the concentration of many narrow endemics, being a hotspot of plant diversity in East Africa (Brooks et al. 2004). Th e territories surrounding the Campo-Ma'an National Park also harbor many orchids that are endemic to Atlantic Central Africa (Droissart 2009), and our discovery stresses the need of further explorations and plant protection in this area. Th e large gap between the two taxa, covering the Congolian sub-centre of endemism (White 1979), remains largely unsampled and future botanical explorations may reveal that the geographic disjunction between the two species is not as large as we may believe today.