Two new species of Diospyros (Ebenaceae) from Central Africa

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INTRODUCTION
The area of Central Africa known as Lower Guinea (White 1979) extends from Nigeria to the mouth of the Congo River, and is centred on Gabon and southern Cameroon.The great floristic richness of this region was stressed by White (1979) and is becoming more and more obvious with the progress of botanical exploration and taxonomy.New species from this region, particularly from Cameroon and Gabon, are described every year, often from recent collections made in previously poorly-known or unexplored areas (e.g.Sonké et al. 2009, Janssens et al. 2010, Ntore et al. 2009, Lachenaud & Breteler 2011, Lachenaud & Séné 2012, Fischer & Lachenaud 2013, Lachenaud et al. 2013, Droissart et al. 2014, van Velzen & Wieringa 2014, Couvreur et al. 2015, Lachenaud & Zemagho 2015).
One of the genera particularly well-represented in Lower Guinea is Diospyros L. (Ebenaceae).This genus, here considered in the broad sense (including Royena L.) includes 736 species worldwide, 104 of which occur in continental Africa (Govaerts 2017).It has a mostly tropical distribution, although a few species occur in temperate regions.The taxonomy and distribution of African Diospyros were studied in detail by White (Letouzey & White 1970a, 1970b, White 1978, 1980, 1987, 1988) who identified Lower Guinea as an important center of diversity and endemism for the genus.Since his revision, five new species from this area have been described: one from Gabon, D. rabiensis Breteler (Breteler 1994); one from the Republic of the Congo, D. whitei Dows.-Lem.& Pannell (Dowsett-Lemaire & Pannell 1996); and three from Cameroon, D. kupensis Gosline (Gosline & Cheek 1998), D. korupensis Gosline and D. onanae Gosline (Gosline 2009).Two further species have been resurrected: D. simulans F.White (Senterre 2005), previously synonymised with D. cinnabarina (Gürke) F. White, and D. rubicunda Gürke (Jongkind 2007), previously regarded as conspecific with D. barteri Hiern.
Between 2012 and 2016, three of the authors (O.L., G.D. and T.S.) took part in several collecting expeditions in western Gabon, particularly around Lambaréné and in the upper Ngounié valley.Both areas were previously somewhat neglected by botanists, as shown by the map of collection Pl.Ecol. Evol. 150 (2), 2017 density in Sosef et al. (2006: 12, fig. 5).The field work there resulted in many interesting findings, including two species of Diospyros, which were identified as new after detailed comparison with available material of the genus from Central Africa.These two species are here described and illustrated, and compared with their most similar relatives; an assessment of their conservation status is also provided.One of the new species, Diospyros cleistantha O.Lachenaud & G.E.Schatz is sparsely distributed in Cameroon, Equatorial Guinea and Gabon, while the other, D. subargentea O.Lachenaud, Dauby & G.E.Schatz, is a very local Gabonese endemic, only known from the Lambaréné region.Both species exhibit leaf dimorphism, which is relatively common in the genus: the leaves of the lateral branches are normal, while those of the main stem are modified, being much smaller and relatively broader in proportion; these modified leaves are only visible near the apex of the axis.

MATERIAL AND METHODS
This paper is based on a study of the material from BR, BRLU, LBV, MO, P and WAG (abbreviations according to Thiers 2017).The descriptions are based on field observations, herbarium specimens, and spirit material.All cited specimens were seen by the authors, unless indicated by 'n.v.'The following abbreviations are used: fl.(flowering), fr.(fruiting), ster.(sterile).Phytogeographical considerations follow White (1979).
A preliminary extinction risk assessment was made using the IUCN Red List Categories and Criteria (IUCN 2012).Georeferenced specimen data were imported into Geocat (Bachman et al. 2011) to estimate the area of occupancy (AOO) and extent of occurrence (EOO).The AOO cell size was set at 2 km × 2 km, as recommended by IUCN (2016).Each locality was regarded as a separate subpopulation.The number of 'locations' (as defined by IUCN 2012) was calculated with regard to the known threats, such that a single 'location' may encompass more than one subpopulation.

Diospyros cleistantha
Diospyros sp. 3 sensu White (1978: 353).Diagnosis -Diospyros cleistantha closely resembles D. itu rensis and D. zenkeri in the male flowers, which are 3-merous, with the anthers included, and the corolla glabrous or nearly so, with an inflated tube and minute lobes.It differs from these two species mainly by the leaves, which have the lateral veins almost reaching the margin (not forming conspicuous loops far from the margin), the tertiary veins dense and more or less parallel (not coarsely reticulate), and a dense indumentum on both sides when young, which persists below on the midrib (vs.no indumentum at all, even on young leaves); it also has a densely pubescent calyx (vs.glabrous or very sparsely pubescent).-Type: Gabon, Collines au sud-est du lac Ezanga, 1°10′04.3″S10°20′23.2″E, fl., 23 Nov. 2013, Lachenaud et al. 1577 (holo-: BR; iso-: BRLU, LBV, MO, P, WAG).Tree, 7-10 m tall (probably becoming taller), up to 21.5 cm in diameter; trunk blackish or dark grey, straight, without buttresses; slash with orange-brown outer layer and cream-white inner layer; branches horizontal, arranged in pseudowhorls.Twigs when very young with an indumentum of long ± crispate buff hairs, these very soon caducous; adult twigs minutely puberulous, with dense, short, ± patent hairs < 0.1 mm long.Leaves of lateral branches with petiole 0.4-0.7 cm long, pubescent with same indumentum as the twigs; lamina ovate to elliptic, (6.7-)7.5-15.8× (2.5-)3.7-8.1 cm, base rounded or obtuse (rarely cuneate), apex obtusely acuminate for 0.5-1 cm, papyraceous to slightly coriaceous, green on both sides, the lower side slightly paler, when very young entirely covered with a dense indumentum of floccose buff hairs, soon glabrescent, in the adult stage glabrous except for the shortly hairy midrib on the lower surface; midrib impressed or rarely slightly prominent above, prominently raised and brownish below; secondary veins 5-8 on either side, slightly impressed above, raised below, markedly ascending and semicraspedodromous (almost reaching the margin); tertiary veins dense, darker and conspicuous on the lower surface, the main elements parallel, the finer elements reticulate and less prominent; leaf glands few, small and dispersed across the lamina; basal glands absent; leaves of main stem (only present near apex) much smaller, suborbicular, apex obtuse or faintly acute.Male flowers 3-merous, sessile, solitary or in fascicles of 2-4 on older branches below the leaves, sometimes also in the axils of the lower leaves; bracts very small, c. 1.3 × 1 mm, broadly ovate, densely covered with appressed hairs.Calyx brown, cupular, 1.3-3 mm long, densely covered with short, appressed hairs, with 3 very short, broadly triangular lobes (these sometimes hardly distinct).Corolla white or pinkish white, very thick and waxy, ovoid to ellipsoid and almost closed, 8 × 6 mm when fresh, 6-7 × 3.5-4 mm when dry, with 3 very short triangular lobes of c. 1 mm long, entirely glabrous or with short appressed hairs near the margin of the lobes.Stamens 9, included in the tube, subsessile; anthers 6 × 1.2 mm, almost linear, acuminate, glabrous.Female flowers and fruits unknown.Fig. 1.Distribution -Lower Guinea subcentre of endemism.Dios pyros cleistantha has a scattered distribution in southwest Cameroon, Equatorial Guinea (Rio Muni) and west-central Gabon, south to the upper Ngounié valley (fig.2).It might be expected in the Republic of the Congo, since it occurs in Gabon not far from the border.As shown by the small number of collections, it is overall a rare species, although it was  based on male flowers in the Flore du Gabon (Letouzey & White 1970a) is used on material of D. cleistantha, it indeed leads to couplet 28 (D. iturensis and D. zenkeri), and the key based on vegetative characters also leads to D. ituren sis.However, D. cleistantha differs from these two species by the characters mentioned in the diagnosis; in addition, the leaf midrib of D. iturensis and D. zenkeri is always prominent above, while that of D. cleistantha is usually impressed.
In the vegetative state, Diospyros cleistantha might be confused with D. rubicunda Gürke and D. soyauxii Gürke & K.Schum., whose leaves have a rather similar shape and venation pattern.However, in these two species the lower surface of the leaves is persistently pubescent and distinctly glaucous when fresh (the glaucous tinge is not always obvious in dry material), whereas D. rubicunda also differs in its very short petioles.White (1980) placed both D. rubicunda and D. soyauxii in D. sect.Noltia, which differs from D. sect.Calvitiella by the deeply lobed male calyx and the narrowly conoidal, usually pubescent male corolla.Etymology -The species name refers to the nearly closed flowers.Conservation status -Diospyros cleistantha is given a preliminary Red List status of Endangered [EN].Its extent of occurrence (EOO) is estimated to be 39,944 km 2 , and its area of occupancy (AOO) to be 24 km 2 ; the latter value falls within the limits for the Endangered status under criterion B2.The species occurs in Cameroon, Gabon and Rio Muni (Equatorial Guinea), in lowland wet evergreen forest, on periodically flooded soils or more rarely on well-drained soils.The forests where it is found are subject to degradation due to the long history of timber exploitation (more than one century), even though D. cleistantha is not a commercially exploited species.In addition the gallery forest of the Dola River in Gabon, where the species is particularly frequent and where two of the subpopulations occur, will presumably be affected by the development of industrial palm plantations in the surrounding savanna.We do not expect a continuous decline of the EOO, AOO and number of mature individuals, but the extent and quality of the habitat are likely to decline.Diospyros cleistantha is known from seven specimens, representing six subpopulations; these represent five different locations in the sense of IUCN (2016), which meets the threshold for Endangered under criterion B, subcriterion a.Since the conditions for applying subcriteria a and b under criterion B are met, D. cleistantha is assigned a preliminary status of EN B2ab(iii).Notes -The earliest collection of this species, Leeuwenberg 9730, was already recognised by White (1978: 353) as an imperfectly known species related to Diospyros iturensis.
Only three collections with flowers are known, all male: Lachenaud et al. 1577 and 2558 from Gabon, and Leeuwen berg 9730 from Cameroon.They differ slightly in the indumentum of the corolla: entirely glabrous in the Gabonese specimens vs. with some hairs near the margin of the lobes in the Cameroonian material.Otherwise, they are so similar that we have no hesitation in referring them to the same species.

Diospyros subargentea
Small tree or shrub, up to 8 m tall and 12 cm in diameter; trunk blackish, straight, without buttresses; slash with orange-brown outer layer and cream-white inner layer; branches horizontal and arranged in pseudowhorls, usually in groups of 5. Twigs densely covered with short appressed greyish hairs.Leaves of lateral branches with petiole 0.2-0.5 cm long, with same indumentum as the twigs; lamina ovate to elliptic, 4-11.2 × 1.4-4.7 cm, base rounded or obtuse (rarely cuneate), apex gradually and rather long acuminate with a c. 1 mm long apical mucro, papyraceous, densely covered with silvery hairs on both sides when very young, in the adult stage strongly discolorous, the upper surface green and glabrous, the lower surface silvery-white covered with very dense, strongly appressed sericeous hairs c. 0.15 mm long; midrib slightly impressed above, prominently raised below; secondary veins 3-6 on either side, slightly impressed above, raised below, markedly ascending (except sometimes the lower ones) and brochidodromous; tertiary veins ± parallel and rather lax, conspicuous to hardly distinct on the lower surface; leaf glands few, small and dispersed across the lamina; basal glands absent; leaves of main stem (only present near apex) much smaller, suborbicular, 1.5-2 × 1.5-1.7 cm, apex obtuse or faintly acute.Male flowers unknown.Female flowers only known in bud, 4-merous, borne in small axillary cymes of 3-5 (usually producing a single fruit) on young leafy twigs; bracts small, lanceolate, c. 2 mm long, with appressed greyish hairs; pedicels 1-2 mm, not accrescent in fruit, with appressed greyish hairs.Calyx entirely covered with dense appressed hairs, tube 0.5-1 mm long, lobes triangular, 1-1.2 mm long.Corolla in bud narrowly conical, 4 × 1.3 mm when dry, covered with dense appressed hairs outside.Ovary cylindrical, c. 2 mm long, covered with dense appressed hairs and ending in a glabrous style c. 1 mm long.Fruits green (not fully mature), narrowly cylindrical, 3-3.6 × 0.9-1.4cm, apex acute, circular in cross section, smooth, sparsely pubescent, eventually glabrescent, 4-locular; calyx persistent at the base of the fruit, with lobes only slightly accrescent, up to 2 mm long.Fig. 3.
When the key based on vegetative characters in the Flore du Gabon (Letouzey & White 1970a) is used on material of this species, it leads to couplet 25, which includes a group of species with discolorous leaves (Diospyros fragrans Gürke, D. cinnabarina (Gürke) F.White, D. gracilescens Gürke, D. sanzaminika A.Chev., D. soyauxii and D. simu lans F.White).All these species have the lower surface of the leaves with much sparser indumentum than in D. subar gentea.The key separates D. fragrans from the rest based on the obtuse to rounded leaf base, a character we found to be not entirely reliable, since in both D. soyauxii and D. subar gentea the leaf bases vary from rounded to cuneate.Etymology -The species name refers to the silvery underside of the leaves.Conservation status -Diospyros subargentea is given a preliminary Red List status of Vulnerable [VU].Its extent of occurrence (EOO) is estimated to be 3,975 km 2 , and its area of occupancy (AOO) to be 32 km 2 ; these values both fall within the limits for the Endangered status under criteria B1 and B2, respectively.The species is endemic to westcentral Gabon, and grows in lowland, wet, evergreen forest, where it can be locally dominant in the undergrowth.It is found only in unprotected areas.The forests where it occurs are subject to degradation due to the long history of logging (more than a century), even though D. subargentea is not commercially exploited.Some of the localities are under possible threat from mining (i.e.Mabounié) and conversion to palm plantations, a situation that might quickly impact the habitat of D. subargentea.We predict that these threats will continue in the future and will lead to a continuous decline of the EOO, AOO, extent and quality of the habitat, number of subpopulations and number of mature individuals.Diospyros subargentea is known from eleven specimens, representing eight subpopulations; these represent eight different locations in the sense of IUCN (2014), which meets the threshold for Vulnerable under criterion B, subcriterion a.The species is thus assigned a preliminary status of VU B1ab(i,ii,iii,iv,v) +2ab(i,ii,iii,iv,v).