Distylodon sonkeanum Droissart, Stevart & P.J.Cribb, 2014

Droissart, Vincent, Cribb, Phillip J., Simo-Droissart, Murielle & Stevart, Tariq, 2014, Taxonomy of Atlantic Central African orchids 2. A second species of the rare genus Distylodon (Orchidaceae, Angraecinae) collected in Cameroon, PhytoKeys 36, pp. 27-34 : 27-30

Distylodon sonkeanum Droissart, Stevart & P.J.Cribb sp. nov. Figs 1 , 2

Diagnosis.

Distylodon sonkeanum Droissart, Stévart & P.J.Cribb, is close to Distylodon comptum Summerh. but differs from it in having several-flowered inflorescences, longer leaves and spur, and shorter pedicel and ovary.

Type.

Cameroon. Bifa (piste sur la route Kribi-Ebolowa), à 5 km au SE de Zingui, le long d’une piste de chasseurs entre les rivières Nyété et Nyamefoo, 02 °41.308'N, 010°16.406'E, 5 July 2007, Droissart, Stévart & Simo M. 585 (holotype: BRLU!; isotype: YA!).

Description.

Dwarf epiphytic herb. Stem short, up to 8 mm long, 2-2.5 mm in diameter, leafy, unbranched. Roots more than 120 mm long, 1-1.8 mm in diameter, numerous, distributed at the base of the plant, unbranched. Leaves few (up to 5), 25-37 × 6-11 mm, obliquely narrowly elliptic, slightly coriaceous, with entire margins, with an indistinct and irregular reticulate venation; leaf apex obliquely subacute, unequally bilobed, with distance between the two lobes less than 1 mm. Inflorescences up to 41 mm long, 2- to 5-flowered, almost horizontal to pendant, unbranched, emerging at the base of the stem, with peduncle 13 mm long; floral internode about 6-7 mm, one flower per node; rachis terete. Bracts up to 1.2 mm long, tubular. Flowers 17-25 × 6-8 mm, not opening widely, green, the larger one situated at the apex of the inflorescence. Ovary and pedicel not twisted, 5-7 mm long. Dorsal sepal 4-7 × 1.6-1.9 mm, linear lanceolate, acute, slightly to markedly reflexed, with entire margins. Lateral sepals 5.5-9 × 1.8-2.0 mm, obliquely linear-lanceolate, acute, slightly to markedly reflexed, with entire margins. Petals 3.0-4.8 × 1.1-1.3 mm, linear lanceolate, acuminate, slightly curved forward, with entire margins. Lip 4.5-6.0 × 2.0-3.5 mm, slightly to markedly trilobed in the basal half; side lobe obliquely triangular, subacute to acute, 0.8-1.8 mm long; mid lobe much longer, 3-4 mm long, linear-subulate, acuminate, fleshy, somewhat curved forward; spur 13.5-18.0 mm long, 0.8-1.1 mm in diameter, cylindrical, straight, slightly inflated in the apical part in larger flowers, apex rounded. Column 1-1.2 × 1.0 mm, almost cylindrical with truncate apex. Rostellum 0.3 mm long, consists of two erect, subulate teeth or fangs. Anthercap 1.0 × 1.0 mm, deltoid. Pollinia two, spherical. Viscidia two, with two stipites 0.7-0.8 mm long, independent of each other, flattened, bifurcate.

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 ). The 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’. The 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. The 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 fieldwork is required to ascertain its conservation status more objectively. Considering the general habitat of Distylodon 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 km2), and using the Criteria B of the IUCN Red List Categories and Criteria, the species was assessed as CR B2ab(iii).

Etymology.

The 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 first to highlight the high biodiversity of this area and he greatly facilitated our work in Cameroon.

Figures

Figure 1. Illustration of Distylodon sonkeanum (Droissart, Stevart & Simo M. 585): A habitus B flower, diagonal view C flower, side view D dorsal sepal E lateral sepal F petal G lip H column with pollinium, sideview I column with anthercap, frontal J pollinium with stipe K stipe.

Figure 2. Photographs of living specimen of Distylodon sonkeanum: A front view of the flower B side view of the flower C inflorescence.

Figure 3. Distribution of Distylodon sonkeanum and Distylodon comptum in tropical Africa.