Belg. J. Bot. 142 (2) : 111-123 (2009) © 2009 Royal Botanical Society of Belgium THE ORCHID FLORA OF THE MBAM MINKOM HILLS (YAOUNDÉ, CAMEROON) Murielle SIMO1, Vincent DROISSART2,5, Bonaventure SONKÉ1,2 and Tariq STÉVART2,3,4,* 1 Laboratoire de Botanique systématique et d’Écologie, Département des Sciences Biologiques, École Normale Supérieure, Université de Yaoundé I, B.P. 047, Yaoundé, Cameroun 2 Herbarium et Bibliothèque africaine, Université Libre de Bruxelles - ULB, 50 Av. F. Roosevelt, CP 169, 1050 Bruxelles, Belgique 3 Missouri Botanical Garden, Africa & Madagascar Department, P.O. Box 299, 63166–0299, St Louis, Missouri, USA 4 National Botanic Garden of Belgium, Domein van Bouchout, B-1860 Meise, Belgium 5 Present address: Institut de Recherche pour le Développement (IRD°, UMR AMAP, Botanique et Bioinformatique de l’Architecture des Plantes, Bd de la Lironde, TA A51/PS2, 34398 Montpellier cedex 5, France (* Author for correspondence; e-mail: tariq.stevart@mobot.org) Received 27 August 2008; accepted 10 July 2009. ABSTRACT. — Despite its tremendous biodiversity, which results from a strong elevation gradient and high habitat diversity, the flora of the Mbam Minkom Hills (Cameroon) is poorly documented. Moreover, these hills have recently become an urgent and major challenge for conservationists because their proximity to the city of Yaoundé has considerably increased human pressure on this area considered as the last main block of primary submontane forest around this city. As a consequence, the main objectives of this paper are to provide the first orchid account of the Mbam Minkom Hills, to document the ecology and the distribution of these species and to highlight the importance of this family for the conservation of this threatened ecosystem. A total of 75 orchid taxa within 27 genera were found in the Mbam Minkom Hills. Sixty-one (81%) were epiphytic, 11 (15%) were terrestrial and 3 (4%) were lithophytic. Six of them (8%), one Bulbophyllum, one Diaphananthe, one Polystachya, one Rhipidoglossum and two Stolzia are new taxa. Genera with the highest number of taxa were Polystachya (19 taxa) and Bulbophyllum (12 taxa). Polystachya carnosa is newly recorded for Cameroon. Lowland forest (51 taxa) and inselbergs (29 taxa) are the two habitats with the highest species richness. Species flower mainly between March and May and between July and September. Thirty-four (45%) taxa were endemic or near endemic to the Guineo-Congolian regional centre of endemism, of which 14 (19%) were endemic to the Lower Guinean Domain. Moreover, Bulbophyllum teretifolium, Bulbophyllum sp. nov., Cheirostylis divina var. ochyrae, Polystachya sp. nov. and Stolzia repens var. cleistogama are endemic to Cameroon. According to IUCN criteria, 50 taxa (67%) are least concerned (LC), 4 (5%) are vulnerable (VU) and 1 (1%) is near threatened (NT). Three taxa (4%), Cheirostylis divina var. ochyrae, Diaphananthe bueae and Polystachya letouzeyana are endangered (EN). Seventeen taxa (23%) are not evaluated (NE). The orchid flora of Cameroon is far from being completely known, as shown by the six new taxa and the new national record found during this study. KEY WORDS. — Orchidaceae, conservation status, epiphytes, inselberg, Lower Guinea Domain, submontane forest. 112 BELGIAN JOURNAL OF BOTANY 142 INTRODUCTION With a number of species estimated to 25 000 (GOVAERTS et al. 2007), orchids are one of the largest families of flowering plants. Most species are found in the tropics, the New World tropics being the most diverse (PRIDGEON 1992). Because of their reputed beauty and important specific richness in the tropics, orchids are a major component of conservation policies. Orchids are threatened by habitat loss, particularly because many species are epiphytic, or by the avoidance of eutrophicated soils for the terrestrial species (PILLON & CHASE 2007). In addition to the epiphytic habit, endemism leads to high vulnerability or extinction through habitat loss, as the destruction of habitat in one area results in the loss of the species (WHITMORE & SAYER 1992, PRIMACK 1993). It seems likely that such extinctions have already been widespread, although the species may not always have been documented (WHITMORE & SAYER 1992). In Cameroonian tropical rainforest, most orchids are epiphytes (ZAPFACK & ENGWALD 2008) and are mainly found in the canopy of lowland and submontane forests, or sometimes on the rock of inselbergs (facultative epiphytes). The ‘Flore du Cameroun’ lists 360 orchid species in Cameroon (SZLACHETKO & OLSZEWSKI 1998, 2001a,b) but the orchid flora is still far from being completely known. These authors also listed many species that had not yet been recorded in this country but collected in adjacent countries. In order to fill this gap, several publications on the flora of Cameroon including orchids have recently been published (CRIBB 1998, CRIBB et al. 2000, STÉVART 2003, CRIBB & POLLARD 2004, POLLARD et al. 2004, DROISSART et al. 2006, DROISSART et al. 2009a). However, none of these papers dealt with the orchid flora of the Mbam Minkom Hills. The only publications mentioning orchid records from the Mbam Minkom Hills (SZLACHETKO & OLSZEWSKI 1998, 2001a,b, DROISSART & STÉVART 2004) list nine taxa identified to species (six), subspecies (one) or varieties (two). In Cameroon, ecosystems that harbour most of the rainforest biodiversity are a source of great concern and their conservation has become an issue of increasing priority because of the ongoing effect of human activities (TCHOUTO 2004). The Mbam Minkom Hills ecosystem is probably one of the most endangered of Cameroon, and is the last block of primary submontane forest around Yaoundé. Its tremendous biodiversity (SONKÉ et al. 2006), the lack of conservation status, and the important human pressure due to the proximity of the city of Yaoundé (DROISSART & STÉVART 2004, SONKÉ & STOFFELEN 2004) make it one of the main challenges for conservationists in Cameroon. The objectives of this study are, therefore, to provide the first specific and extensive account of orchids occurring in the Mbam Minkom Hills, to document the ecology and the distribution of these species and to highlight the importance of the hills for orchid conservation in Cameroon. MATERIAL AND METHODS STUDY SITE This study was conducted in the Mbam Minkom Hills, which lie 20 km NW of Yaoundé (3°52’ – 4° N and 11°20’ – 11°27’ E) and cover an area of approximately 100 km2 (Fig. 1). The area possesses the highest elevation of the Centre Province of Cameroon with an altitudinal range of 600 to 1295 m. Primary vegetation is lowland and submontane forest. Several inselbergs are also present in the area. Orchids were collected in summit forest as well as on both east-facing and west-facing slopes of the Mbam Minkom Hills. Because they are exposed to the monsoon winds, the west-facing slopes receive more rain (KUETE 1977). This climatic contrast results in pronounced differences in the physiognomy and floristic composition of the forests covering both sides. The west side is characterized by an uneven relief and steep slopes, and is covered by lowland forest, interspersed with cultures and fallow land. The east versant is more degraded than the west versant and is characterized by its gentle slopes and lowland forest. The summit forests are characterized by a striking physiognomic and floristic homogeneity. The vegetation of the Mbam Minkom Hills has previously been investigated by LETOUZEY (1968), KUETE (1977), ACHOUNDONG (1985, 1996) and NOUMI (1998). Studies on the Rubiaceae family carried out by SONKÉ & STOFFELEN (2004), SONKÉ et al. (2006) and NGUEMBOU (2006) revealed the presence of many species endemic to Cameroon. ORCHIDS OF THE MBAM MINKOM HILLS (CAMEROON) 113 Fig. 1. Topographic map of studied area and localisation of different hills (triangle) surrounding Yaoundé (Cameroon). Collections sites are showed by stars. Floristic data were collected during 44 days in 51 sites. FLORISTIC DATA Floristic data used in this study were gathered from recent field inventories (244 fertile specimens), literature (nine specimens) and examination of previous herbarium collections (eight specimens from YA). For each species, we determined: the habitat, the elevation range, the phenology based on field and cultivation observations, the IUCN Red List Categories (2001) taken from the literature (CRIBB et al. 2000, CRIBB & POLLARD 2004, POLLARD et al. 2004, DROISSART et al. 2006) and the phytogeographical status supplied by Kew Monocot Checklist (GOVAERTS et al. 2007) using White’s chorological classification (WHITE 1979, 1983). Field data were collected from March 2004 to March 2007. During our fieldwork, sterile living plants were collected and put in culture in a shadehouse built in Yaoundé in order to obtain accurate identification based on flowered specimens. For this study, identification was thus improved by ex-situ cultivation of 574 living specimens, which yielded the 244 fertile specimens. For some fertile specimens, in addition to herbarium specimens, we collected silica-dried material for molecular studies. RESULTS FLORA The flora of the Mbam Minkom Hills includes 75 taxa of orchids distributed in 27 genera, of which 61 (81%) are epiphytic, 11 (15%) are terrestrial and 3 (4%) are lithophytic (Table 1). Stolzia grandiflora P.J.Cribb subsp. lejolyana Stévart, Droissart & Simo and Stolzia repens (Rolfe) Summerh. var. cleistogama Stévart, Droissart & Simo are abready published (DROISSART et al. 2009b). Bulbophyllum sp. nov., Diaphananthe sp. nov., Polystachya sp. nov. and Rhipidoglossum sp. nov. will be published elsewhere. The most represented genera are Polystachya Hook. (19 taxa), and Bulbophyllum Thouars (12 taxa). Polystachya carnosa P.J.Cribb & Podz. is a new national record for Cameroon while Bulbophyllum teretifolium Schltr., Bulbophyllum sp. Species 1 2 3 4 5 6 7 22 23 24 25 26 27 Reference specimen LF Conservation status DSS 373 E NE DSS 815 E LC *** DSS 387 E LC *** DSS 581 E LC *** Simo 4 E LC *** Droissart & Simo 214 E VU **** DSS 394 E LC *** Simo 3 Droissart 83 Droissart & Simo 211 SDS 2434 Droissart 3 DSS 880 DSS 447 DSND 185 Droissart 1 Droissart 66 DSND 124 DSS 644 Droissart 12 DSS 990 E E T E E E E E L E E E E E NE LC *** LC *** LC *** LC *** NE LC *** LC *** LC *** LC *** NE LC *** LC *** LC *** Droissart 5 DSS 389 DSS 1025 Liv. spec DSS 412 Dang 752 E E E E E T VU **** NE LC *** LC ***** LC *** EN **** LowFor Insel SubFor Chorology X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X Wide GCR Sub end GCR Sub end Wide Linking GCR-ZAM GCR-LG GCR-LG GCR- LGUG Linking GCR-SOM GCR Sub end GCR- LGUG GCR Sub end GCR-LG GCR Sub end Wide Wide Wide GCR Sub end Linking GCR-SOM GCR GCR GCR-LG Wide Linking GCR-ZAM Wide Linking GCR-SOM GCR-LG BELGIAN JOURNAL OF BOTANY 142 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Aerangis collum-cygni Summerh. Ancistrochilus rothschildianus O’Brien Ancistrorhynchus capitatus (Lindl.) Summerh. A. metteniae (Kraenzl.) Summerh. Angraecum distichum Lindl. A. egertonii Rendle A. eichlerianum Kraenzl. var. curvicalcaratum Szlach. & Olszewski Bolusiella batesii (Rolfe) Schltr. B. talbotii (Rendle) Summerh. Brachycorythis macrantha (Lindl.) Summerh. Bulbophyllum bidenticulatum J.J.Verm. ssp. bidenticulatum B. calyptratum Kraenzl. var. calyptratum B. sp. nov.** B. falcatum (Lindl.) Rchb.f. var. falcatum B. fuscum Lindl. var. melinostachyum (Schltr.) J.J.Verm. B. lupulinum Lindl. B. oreonastes Rchb.f. B. oxychilum Schltr. B. pumilum (Sw.) Lindl. B. resupinatum Ridl. var. filiforme (Kraenzl.) J.J.Verm. B. sandersonii (Hook.f.) Rchb.f. ssp. stenopetalum (Kraenzl.) J.J.Verm. B. teretifolium Schltr. Calyptrochilum christyanum (Rchb.f.) Summerh. C. emarginatum (Afzel. ex Sw.) Schltr. Chamaeangis odoratissima (Rchb.f.) Schltr. C. vesicata (Lindl.) Schltr. Cheirostylis divina (Guinea) Summerh. var. ochyrae Szlach. & Olszewski 114 Table 1. List of 75 Orchids present in the Mbam Minkom Hills with reference specimens. Species 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 D. pellucida (Lindl.) Schltr. D. sp. nov. ** Epipogium roseum (D.Don) Lindl. Eulophia euglossa (Rchb.f.) Rchb.f. ex Bateman E. odontoglossa Rchb.f. Graphorkis lurida (Sw.) Kuntze Habenaria procera (Afzel. ex Sw.) Lindl. Liparis nervosa (Thunb.) Lindl. ssp. nervosa L. tridens Kraenzl. Oeceoclades maculata (Lindl.) Lindl. O. saundersiana (Rchb.f.) Garay & P.Taylor Plectrelminthus caudatus (Lindl.) Summerh. var. caudatus Polystachya adansoniae Rchb.f. var. adansoniae P. affinis Lindl. P. albescens Ridl. ssp. albescens P. albescens Ridl. ssp. polyphylla (Summerh.) Stévart P. calluniflora Kraenzl.var. calluniflora P. caloglossa Rchb.f. P. carnosa P.J.Cribb & Podz.* P. cultriformis (Thouars) Lindl. ex Spreng. P. elegans Rchb.f. P. fusiformis (Thouars) Lindl. P. sp. nov.** P. golungensis Rchb.f. P. letouzeyana Szlach. & Olszewski LowFor Insel SubFor Chorology X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X Wide — Pal GCR- LGUG GCR Sub end Wide GCR Sub end GCR Sub end GCR-LG Wide — Pan Linking GCR-SOM Wide Wide Wide Wide — Pan Wide Wide — Pan Wide GCR Wide Wide GCR GCR-LG GCR Sub end GCR Sub end GCR-LG Wide — Pal GCR-LG Wide — Pal GCR-LG Wide GCR-LG 115 Corymborkis corymbis Thouars Cribbia confusa P.J.Cribb Cyrtorchis chailluana (Hook.f.) Schltr. Diaphananthe bidens (Afzel. ex Sw.) Schltr. D. bueae (Schltr.) Schltr. LF Conservation status SDS 2431 T LC *** Droissart 15 E LC *** DSND 309 E LC *** Droissart 13 E LC *** DSS 424 E EN A1c+2c ****** DSND 111 E LC *** DSS 370 E NE Satabié 795 T LC *** Droissart 44 T NE Letouzey 11641 T LC *** DSS 894 E LC *** Droissart & Simo 210 T LC *** Droissart & Simo 220 T LC *** Droissart 73 E LC *** DSND 55 T LC *** Dang 672 T NE Liv. spec. E LC *** DSS 423 E LC *** Simo 5 E LC *** Simo 2 E NT *** Droissart 55 L VU **** Droissart & Simo 209 E LC *** DSS 381 E LC *** DSS 363 E NE DSS 395 E LC *** DSS 409 E VU **** Simo 1 E LC *** DSS 814 E NE DSND 257 E LC *** Droissart 18 E EN **** ORCHIDS OF THE MBAM MINKOM HILLS (CAMEROON) 28 29 30 31 32 Reference specimen 116 Species 70 71 72 73 74 75 P. modesta Rchb.f. P. odorata Lindl. var. odorata P. paniculata (Sw.) Rolfe P. polychaete Kraenzl. P. tenuissima Kraenzl. P. tessellata Lindl. Rangaeris muscicola (Rchb.f.) Summerh. R. rhipsalisocia (Rchb.f.) Summerh. Rhipidoglossum curvatum (Rolfe) Garay R. rutilum (Rchb.f.) Schltr. R. sp. nov.** Stolzia grandiflora P.J.Cribb subsp. lejolyana Stévart, Droissart & Simo. S. repens (Rolfe) Summerh. var. cleistogama Stévart, Droissart & Simo. Tridactyle anthomaniaca (Rchb.f.) Summerh. var. anthomaniaca T. gentilii (De Wild.) Schltr. T. lisowskii (Szlach.) Szlach. & Olszewski T. tridentata (Harv.) Schltr. Zeuxine elongata Rolfe LF Conservation status DSND 83 E NE Droissart 61 E LC *** DSS 386 E LC *** DSS 840 E LC *** Droissart & Simo 218 E LC *** DSS 410 E LC *** DSS 746 E LC *** Droissart 17 E LC *** DSS 805 E NE Droissart 74 E NE DSND 90 E NE Droissart 45 E NE LowFor Insel SubFor Chorology X X Wide Wide Linking GCR-SOM Wide Wide Wide Wide Linking GCR-ZAM GCR- LGUG Wide GCR-LG GCR-LG X GCR-LG X X X X X X X X X X Droissart 68 E NE DSS 812 E LC *** X Wide Droissart 63 DSS 433 Liv. spec. SDS 2415 E L E T NE NE LC *** LC X X X Wide GCR-CLG Wide Wide X X Notes. LF, life form: E = epiphyte, L = lithophytic, T = terrestrial; conservation status according to IUCN Red List Categories: EN = Endangered, LC = Least concerned, NE = Not evalued, NT = Near threatened, VU = Vulnerable; habitat: LowFor = Lowland forests, Insel = Inselbergs, SubFor = Submontane forests; chorology: GCR = Guineo Congolian wide, GCR Sub end = Guineo Congolian subendemic, GCR-LG = Guineo Congolian endemic to Lower Guinea, GCRLGUG = Guineo Congolian linking species (Lower Guinea – Upper Guinea), GCR-CLG = Guineo Congolian linking species (Lower Guinea – Congolian), Linking GCR-ZAM = Regional linking species Zambezian Region / GCR, Linking GCR-SOM = Regional linking species Somalia Masai Region / GCR, Wide = Tropical Africa wides, Wide — Pan = Pantropical wides, Wide – Pal = Paleotropical wides), * = new record, ** = new taxa, *** = from CRIBB & POLLARD 2004, **** = from DROISSART et al. 2006, ***** = from POLLARD et al. 2004, ****** = CRIBB et al. 2000, DSS = Droissart, Stévart & Simo, SDS = Stévart, Droissart & Simo, DSND = Droissart, Stévart, Nguembou & Djuikouo, Living specimen not flowered yet = Liv. spec. BELGIAN JOURNAL OF BOTANY 142 58 59 60 61 62 63 64 65 66 67 68 69 Reference specimen ORCHIDS OF THE MBAM MINKOM HILLS (CAMEROON) nov., Cheirostylis divina var. ochyrae Szlach. & Olszewski, Polystachya sp. nov. and Stolzia repens var. cleistogama are endemic to Cameroon. ECOLOGY AND DISTRIBUTION Species richness was highest in lowland forests (51 taxa) and inselbergs (29 taxa). Thirty-nine 117 taxa were found only in lowland forests while 15 taxa were restricted to inselbergs (Fig. 2). Thirteen taxa were collected in submontane forests, of which 6 were only encountered there (Table 1). Most orchids collected in the field and in the shadehouse flower from March to May and from July to September. Few species flower between December and February (Fig. 3). Fig. 2. Specific richness in the Mbam Minkom Hills and in the three main habitats (lowland forest, inselbergs and submontane forest) encountered in this area. Restricted species are those only collected in the under mentioned habitat. Ground species = terrestrials and the lithophytes, Epi = epiphytes. Fig. 3. Flowering of orchids and mean monthly precipitations of the Mbam Minkom Hills. Data from the meteorological station of the military airport of Yaoundé (1997-2006). 118 BELGIAN JOURNAL OF BOTANY 142 According to White’s chorological classification (WHITE 1979, 1983), 34 taxa are Guineo-Congolian endemic / subendemic, 8 are regional linking species (species present in two regional centres of endemism) and 33 are present in at least three regional centres (wide species, Table 2, Fig. 4). Five taxa are present in the Guineo-Congolian regional centre and the Somalia-Masai regional centre and 3 in the Guineo-Congolian regional centre and the Zambezian regional centre. Inside the Guineo-Congolian regional centre, 11 taxa (19%) are endemic to the Lower Guinean Domain. Four taxa are restricted to the Upper Guinean and Lower Guinean Domains and 1 is restricted to the Congolian Domain and the Lower Guinean Domain. CONSERVATION According to the IUCN Red List Categories (IUCN 2001), 50 (67%) of the taxa recorded from the Mbam Minkom Hills are least concerned, 1 (1%) is near threatened and 17 (23%) are not evaluated (Fig. 5). Seven taxa (9%) are threatened: Cheirostylis divina (Guinea) Summerh. var. ochyrae, Diaphananthe bueae (Schltr.) Schltr. and Polystachya letouzeyana Szlach. & Olszewski are endangered while Angraecum egertonii Rendle, Bulbophyllum teretifolium, Polystachya albescens Ridl. subsp. polyphylla (Summerh.) Stévart and Polystachya elegans Rchb.f. are vulnerable (Table 1). Table 2. Distribution of the 75 taxa recorded in the Mbam Minkom Hills (abbreviation given in Table 1). Chorological elements 1 Guineo Congolian endemic / subendemic 1a Guineo Congolian wide 1b Two Domains (linking species) 1b1 LGUG 1b2 LGC 1c Endemic to LG 1d Sub/near endemic or Marginal Intruder 2 Regional linking species 2a Somalia Masai Region / GCR 2b Zambezian Region / GCR 3 Wides 3a Africa wides 3b Paleotropical 3c Pantropical DISCUSSION FLORA With 75 taxa reported over 100 km2, the orchid flora of the Mbam Minkom Hills is one of the richest of Central Africa (Fig. 6; Table 3). Other sites that are above the curve are Bali Nguemba forest reserve, Príncipe Island, Mount Cameroon area, Mount Kupe area, Gabon and Cameroon. Moreover, it is very likely that the specific richness of this area is still underestimated as several plants in culture have not flow- Number and % of taxa 34 (45%) 4 (5%) 8 (11%) 4 (5%) 1 (1%) 14 (19%) 11 (15%) 8 (11%) 5 (7%) 3 (4%) 33 (44%) 27 (36%) 3 (4%) 3 (4%) ered yet and, therefore, cannot be accurately identified. Orchid specific richness of Mbam Minkom might be similar to that of Mount Oku and the Bali Ngemba Forest Reserve (Table 1), two other Cameroonian sites. This quite surprising richness and the important number of new taxa (8%) found in the Mbam Minkom Hills can be explained by the method used in this study. Contrary to other studies that only relied upon collection of flowering plants in the field, our study used the shadehouse method, which has successfully been applied by different authors to ORCHIDS OF THE MBAM MINKOM HILLS (CAMEROON) 119 Fig. 4. Distribution of orchid flora of the Mbam Minkom Hills (GCR = Guineo Congolian wide; Wide = widespread, Linking = Regional linking species, GCR = Guineo Congolian wide, GCR sub end = Guineo Congolian subendemic, GCR-LGC = Guineo Congolian – Congolian/Lower Guinea linking species, GCR-LG= Guineo Congolian endemic to Lower Guinea, GCR-LGUG = Guineo Congolian linking species – Upper Guinea/Lower Guinea). The phytogeographical status is supplied by Kew Monocot Checklist (GOVAERTS et al. 2007) using White’s chorological classification (WHITE 1979, 1983). study orchid diversity in tropical Africa (SAN1969, 1971, PEREZ-VERA 2003, STÉVART 2003, DROISSART et al. 2006, DROISSART 2009). FORD ECOLOGY AND DISTRIBUTION Most orchids from the Mbam Minkom Hills are widespread species or Guineo-Congolian species. Of these Guineo-Congolian species, 19% are endemic to the Lower Guinea Domain. This figure differs strongly from the Gamba Complex in South Gabon where STÉVART & DROISSART (2006) found only 7% of Lower Guinea endemics for a similar number of species. The Gamba complex is situated at a low elevation, more to the South and is close to the sea. It is covered mainly by lowland forest, savannah and swampy vegetation. Moreover, the inselbergs and the submontane forests of the Mbam Minkom Hills possess seven of the 14 species endemic to Lower Guinea Domain. These habitats are less frequent in the Gamba complex, even if the area is much larger. The highest specific richness in the Mbam Minkom Hills was encountered in lowland forests (51 taxa). This habitat also harbours most restricted species (76%). However, in the Mbam Minkom Hills, most inselbergs are surrounded by submontane forests. Inselbergs usually shelter the epiphytic flora of the surrounding vegetation (STÉVART 2003). Therefore, we should consider Fig. 5. Number of taxa according to conservation status using IUCN Red List Categories (number of taxa and percentage of the total orchid flora). Conservation categories used in this paper come from existing publications (CRIBB et al. 2000, CRIBB & POLLARD 2004, POLLARD et al. 2004, DROISSART et al. 2006). 120 BELGIAN JOURNAL OF BOTANY 142 Fig. 6. Relation between log specific richness (number of taxa) and log area expressed in km in 15 countries and sites. Values for area and species richness are summarized in Table 3. Table 3. Area (km2), specific richness (SR), and orchid species density (species/km2) of 15 countries and sites in West Central Africa. Data from STÉVART (2003), Kew checklists (CRIBB 1998, CRIBB et al. 2000, CRIBB & POLLARD 2004, POLLARD et al. 2004) and SIMO (2003). Countries/Sites Bali Nguemba Forest Reserve Annobon Mbam Minkom Hills Principe Banyang Mbo Wildlife Sanctuary Sao Tomé Mount Oku Bioko Mount Kupe Mount Cameroon Cristal Mountains Dja Fauna Reserve Rio Muni Gabon Cameroon Area (km2) 8 17 100 136 665 854 1550 2000 2390 2700 4100 5260 26000 257700 475000 the flora of both habitats as a unique flora that possesses 36 taxa, 66% of which are restricted to those habitats. One of the functions of the shadehouse is to study long-term orchid phenology in order to complete field observations, especially for species with fugacious flowers. The flowering duration is SR 85 19 75 71 93 104 85 98 183 150 126 111 182 400 489 Density (species/km2) 10.6250 1.1176 0.7500 0.5221 0.1398 0.1218 0.0548 0.0490 0.0766 0.0556 0.0307 0.0211 0.0070 0.0016 0.0010 indeed very variable between species, from a few days to more than one month. Maximal flowering periods for the species of our study site are observed from March to May and from July to September. The peak flowering periods correspond to the small rainy season, the small dry season and the beginning of the long rainy season. ORCHIDS OF THE MBAM MINKOM HILLS (CAMEROON) CONSERVATION Major threats for orchid populations are habitat destruction and overcollection (HEYWOOD & STUART 1992, WHITMORE & SAYER 1992). In the Mbam Minkom Hills, orchids are threatened by loss and fragmentation of their habitat. According to BULAFU et al. (2007), the most important and vulnerable habitats are moist forest habitats. Although the exact relationship between habitat destruction and orchid species extinction is not known (HEYWOOD & STUART 1992, WHITMORE & SAYER 1992), it has been shown that epiphytes are vulnerable to extinction through habitat destruction (TURNER et al. 1994). Most orchids are light-demanding epiphytes that grow on the upper part of the canopy of largest and oldest trees. Local human populations of the Mbam Minkom Hills are very scarce, but are dependent on forest resources to meet their basic needs. As a consequence, the recent increase of local human population in the Mbam Minkom Hills has an obvious ecological impact on the forest ecosystem. These activities include agriculture, logging and hunting. Forest habitat in the Mbam Minkom Hills is believed to be threatened with clearance for agriculture. Farmers are involved in the destruction of a considerable portion of the lowland forests for the establishment of cocoa and plantain plantations. Timber exploitation by private owners is also an important economic activity, especially during the dry season. Trees are logged every year, particularly Triplochiton scleroxylon K. Schum., which harbours many orchid species situated at the base and in the middle parts of the canopy (JOHANSSON 1974). Another conservation problem for orchids lies in their dependence on particular species of pollinators for their reproduction. If the habitat becomes too small or fragmented to support the pollinator, the orchid species will not regenerate and eventually die out (HEYWOOD & STUART 1992), even though there may be enough habitat left for the plants to grow in. The Mbam Minkom Hills are a site of concern for orchid conservation in Cameroon because it harbours some of the rarest or most endangered orchids of the Lower Guinea endemism area. Even some Rubiaceae are known only from this 121 site (SONKÉ et al. 2006). The study site, despite its proximity to Yaoundé, also harbours two emblematic and highly threatened animal species: the gorilla and the grey-necked picathartes. However, the area is under important anthropic pressure and primary forest and its associated biodiversity might well disappear in a near future in the absence of an active conservation strategy. CONCLUSION The orchid flora of Cameroon is far from being completely known, as shown by the six new taxa and the new national record found during this study. The orchid flora of the Mbam Minkom Hills is one of the richest of Central Africa. Moreover, knowledge of the flora will increase since some living plants collected in that remain to be identified because they have not flowered yet. New sampling efforts using climbing material should be made in the area to complete our preliminary work. Although the diversity of its flora and fauna has been highlighted, a conservation plan is still needed. Ex situ orchid conservation in the Yaoundé shadehouse is possible, but an in situ conservation strategy would be preferable considering the submontane ecology of most threatened species and the difficulty to maintain these species in culture in Yaoundé. ACKNOWLEDGEMENTS We would like to thank the National Herbarium of Cameroon staff for allowing access to the herbarium collection, and late Dr. Dzikouk, Program Officer of Cameroon Biodiversity Conservation Society for logistic help and accommodation offered during fieldwork activities. Fieldwork in Cameroon was funded by the Communauté Française de Belgique, the Fonds David et Alice Van Buuren and the Fonds Leopold III pour l’Exploration et la Conservation de la Nature. We are also grateful to the Royal Belgian Institute of Natural Sciences and the Belgian National Point to the Global Taxonomy Initiative for financial support offered in 2007 to the first author. 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