Rapport scientifique du projet 375

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Rapport scientifique du projet 375: Endémisme et conservation des Orchidaceae et des Rubiaceae en Afrique centrale atlantique

Rapport scientifique de du Projet 375
Titre du projet: Endémisme et conservation des Orchidaceae et des Rubiaceae en Afrique centrale atlantique
Deux années de mise en œuvre du projet 375 nous a permis d’obtenir une masse critique de données qui commencent à être publiées. Quatre publications sont parues dans des journaux scientifiques à comité de lecture. Une communication orale et cinq posters ont été présentés au cours des rencontres scientifiques. Deux étudiants de doctorat ont commencé leurs travaux de recherches et les résultats sont promoteurs. Un étudiant de niveau Master II a défendu son mémoire à l’Université Libre de Bruxelles.
1. Publications scientifiques dans des revues à comité de lecture
1.1 Droissart V., Sonké B., Simo M. & Stévart T., 2009. New Orchid Records from West Central Africa. Edinburgh Journal of Botany 66 (1): 115-132. Abstract: Several papers dealing with Orchidaceae from Atlantic Central Africa were recently published. However, orchid distribution in this area remains poorly known as indicated by the results presented here. During the last decade, we conducted an intensive fieldwork program in Cameroon and Equatorial Guinea. These activities allowed us to collect 22 taxa that are new national records. Ten taxa are new records from Cameroon, eight from Equatorial Guinea and four from both countries. One of these taxa is also a new record from Nigeria. Fifteen taxa are endemic to Lower Guinea area and six were only known from the type locality.
1.2 Droissart V., Simo M., Sonké B., Cawoy V. & Stévart T., 2009. Le genre Stolzia (Orchidaceae) en Afrique centrale avec deux nouveaux taxons. Adansonia 31 (1): 25-40. Résumé: Les taxons du genre Stolzia Schltr. (Orchidaceae) présents en Afrique centrale ont été révisés. Neuf taxons ont été identifiés. Deux nouveaux taxons collectés au Cameroun et en Guinée Équatoriale (Rio Muni et Bioko) sont décrits et illustrés. Stolzia repens (Rolfe) Summerh var. cleistogama Stévart, Droissart & Simo diffère des deux autres variétés déjà décrites par son sépale dorsal soudé aux sépales latéraux sur plus d’un tiers de sa longueur, ainsi que par ses pièces florales plus petites et non réfléchies. Stolzia grandiflora P.J.Cribb subsp. lejolyana Stévart, Droissart & Simo est proche de la sous-espèce type, endémique d’Ethiopie, mais présente une inflorescence biflore plus longue et des fleurs plus petites. Il se rapproche également des différentes variétés de Stolzia repens (Rolfe) Summerh., mais s’en distingue par son inflorescence biflore et un pédoncule plus long et plus fin. Stolzia cupuligera (Kraenzl.) Summerh. et S. peperomioides (Kraenzl.) Summerh. sont néotypifiés. Une nouvelle combinaison est proposée, Stolzia peperomioides (Kraenzl.) Summerh. subsp. thomensis (Stévart & P.J.Cribb) Stévart, Droissart & Simo. Plusieurs nouvelles signalisations sont données. Une clef d’identification et un tableau synoptique des taxons présents en Afrique centrale sont proposés.
1.3 Sonké B., Simo M. & Dessein S., 2009. Synopsis of the genus Mitriostigma (Rubiaceae) with a new monocaulous species from Southern Cameroon. Nordic Journal of Botany 27: 305-312. Abstract: Mitriostigma monocaulis, a new Rubiaceae species from South Cameroon is described and illustrated. The novelty is easily separated from the other four Mitriostigma species by its monocaulous growth form, the other species being shrubs, subshrubs, or occasionally trees. Another typical character for the species is its supra-axillary inflorescences paired at the nodes. The novelty is related to M. barteri, from which it further differs in the somewhat larger leaves with a more pronounced acumen and a higher number of secondary veins. A first conservation status for the species is given. A synopsis of the genus Mitriostigma with a taxonomic key is also provided.
1.4 Simo M., Droissart V., Sonké B. & Stévart T., The orchid flora of the Mbam Minkom Hills (Yaoundé, Cameroon) (in press). Belgian Journal of Botany 2: 2009. Abstract: Despite its tremendous biodiversity that 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. Our inventories have shown that 75 orchid taxa within 27 genera are found in the Mbam Minkom Hills. Sixty one (81%) are epiphytic, eleven (15%) are terrestrial and three (4%) are lithophytic. Six of them, two Stolzia, one Bulbophyllum, one Diaphananthe, one Polystachya and one Rhipidoglossum are new taxa. Genera with the most number of taxa are 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 are endemic or near endemic to the Guineo-Congolian regional centre of endemism, of which 14 (19%) are endemic to the Lower Guinean Domain. Moreover, Bulbophyllum teretifolium, B. sp. nov., Cheirostylis divina var. ochyrae, Polystachya sp. nov., Stolzia grandiflora P.J.Cribb subsp. lejolyana Stévart, Droissart & Simo and Stolzia repens (Rolfe) Summerh. var. cleistogama Stévart, Droissart & Simo are endemic to Cameroon.

According to IUCN criteria, fifty taxa (67%) are least concerned (LC), four (5%) are vulnerable (VU) and one (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).

2 Posters et communication orale

2.1 Droissart V., Sonké B., Dahdouh-Guebas F., Lejoly J. & Stévart T. 2009. A new phytogeographical map of Atlantic Central Africa based on orchids distribution: 73 In Dahdouh-Guebas F. (ed.) Proceedings of the Symposium on African Botany in Brussels. Université Libre de Bruxelles – ULB / Vrije Universiteit Brussel – VUB, Brussels, Belgium. 120 pp. Abstract: Atlantic Central Africa (ACA) covers the Lower Guinean Domain endemism area, the four islands of the Gulf of Guinea and the Cameroonian part of the afromontane archipelago. Different centres of endemism have been identified in this area and are usually considered to be related to Pleistocene forest refugia. However, the origin of this endemism, the localization of the centres and the methods used to identify them are still subject to debate. This phytogeographical study focuses on Orchidaceae and analyses the distribution of the taxa endemic to ACA. We use an original dataset resulting from important field sampling efforts in Central Africa and from identification of specimens deposited in the main European herbaria (BR, BRLU, K, P, WAG) and YA. Our findings were that 203 specific and infraspecific taxa are endemic to ACA. Despite the potential long-distance dispersal abilities of the Orchidaceae, their distribution coincides with several centres of endemism previously identified on the basis of other taxonomic groups (Begonia, Caesalpinioideae, Rubiaceae). This allowed us to propose a new phytogeographical map of ACA, but we question the use of dispersal ability as a proxy for forest refugia, and more generally as a proxy for chorological markers. The results also highlight the role of the environmental heterogeneity of mountainous areas to explain patterns of endemism in Central Africa.
2.2 Simo M., Sonké B., Droissart V., Hardy O. J. & Stévart T. 2009. The Angraecum genus (Orchidaceae) in tropical Africa: revision and biogeography: 107. In Dahdouh-Guebas F. (ed.) Proceedings of the Symposium on African Botany in Brussels. Université Libre de Bruxelles – ULB / Vrije Universiteit Brussel – VUB, Brussels, Belgium. 120 pp. Abstract: With 206 species, Angraecum Bory is one of the most diverse genera of African orchid. Nineteen sections were recognized in this genus based on morphological characters, of which sections Dolabrifolia (Pfitz.) Garay and Pectinaria (Benth) Schltr. are distributed in Tropical Africa and in Madagascar. The extensive and ongoing botanical survey undertaken by ULB in collaboration with African partners in West Central Africa has led to the discovery of three new Angraecum belonging these two sections. Moreover, the geographic disjunction (West Central Africa and Madagascar) between members of Sect. Pectinaria raises some doubt about the monophyletic origin of this section.

Therefore an updated revision of these sections was needed. Combining molecular phylogeny and population genetic studies based on new DNA samples that we have collected, we will provide new insight for the classification of members of the two Sections. Preliminary results are presented here.

2.3 Taedoumg H., Randriamboamvonjy-Ranarivelo T., Hamon P., De Block P. & Sonké B., 2009. Craterispermum, un genre énigmatique de la famille des Rubiaceae: 109. In Dahdouh-Guebas F. (ed.) Proceedings of the Symposium on African Botany in Brussels. Université Libre de Bruxelles – ULB / Vrije Universiteit Brussel – VUB, Brussels, Belgium. 120 pp. Résumé Craterispermum Benth. (sous-famille Rubioideae) est un genre strictement tropical, distribué en Afrique, à Madagascar et aux Seychelles. Il compte environ vingt espèces et se distingue par ses inflorescences supra-axillaires souvent très compactes. Le matériel sec présente une teinte jaune caractéristique due à l’accumulation de l’aluminium. Le genre est taxonomiquement mal connu et sa position au sein des Rubiaceae n’est pas claire (tribu monogénérique des Craterispermeae ou sous-tribu des Morindeae). Son statut unique dans la famille, s’exprime aussi par son tégument (exotesta) atypique.

Des observations préliminaires nous indiquent qu’à coté des caractères floraux usuels, les caractères végétatifs (nervation, coriacité et couleur des feuilles, forme, pubescence des stipules…) seront d’une importance capitale dans la délimitation des espèces.

La révision suivra les règles classiques de la taxonomie en Herbier. La phylogénie, le calcul les paramètres génétiques classiques, la comparaison entre les génomes au niveau des populations et une étude chromosomique détaillée nous.permettrons d’énoncer une hypothèse sur l’évolution de ce genre. Pour étudier l’accumulation de l’aluminium, nous emploierons diverses méthodes semi-quantitatives («aluminon et Chrome Azurol-S Solution test ).

En raison de la relative rareté de Craterispermum, les travaux de terrain sont d’une grande importance et s’envisagent au Cameroun, au Gabon, à Madagascar

2.4 Simo M., Sonké B., Droissart V., Hardy O., Cachapa B. & Stévart T. Taxonomic revision of the Angraecum distichum and A. bancoense complex based on molecular and morphological data. Poster présenté au Young Botanist’s Day Forum, 24-25 novembre 2009, ULB, Bruxelles, Belgique. Abstract: This study aims to make a taxonomic revision and phylogeny of Sections Dolabrifolia and Pectinaria of the genus Angraecum. Nineteen sections are recognized and 225 species are currently accepted. The Section Dolabrifolia, with four described species, is characterized by very short leaves, laterally compressed and densely imbricate. Central Africa appears to be the center of endemism and diversity for this Section. A. distichum, described in 1836, is the type species of the Section while A. bancoense was published in 1980. These two species have the same habit and morphological variation in leaf-shape. They are sometimes found growing in sympatry. Recent phylogenetic analyses using three chloroplastic molecular markers (matK, rps16 and trnC-petN) did not display any genetic differentiation between the two taxa.

The main objective of this study is to define a clear delimitation of these two species through examination of all specimens and using tnrL-F, another chloroplastic molecular marker to heighten genetic differentiation between these species.

Presently, precise delimitation of these two species relies only on flower features (lip shape, flower size). Detailed examination of their specimens in several herbariums revealed that almost 90% of collections identified as A. distichum belong to A. bancoense. These results showed that A. bancoense seems to be more frequent and widespread than previously supposed. This species, known from Ivory Coast, Benin, Nigeria, Cameroon and Gabon, is newly recorded from Guinea, Liberia, Equatorial Guinea and Democratic Republic of Congo. A. distichum is also widespread (from Sierra Leone eastwards to D.R. of Congo, Uganda and Angola) but appears rare in its habitat. Therefore, the species might be threatened. In the future, a detailed examination of all their specimens will be carried on in order to make new distribution maps and to revise their conservation status.
2.5 Taedoumg H., De Block P., Hamon P. & Sonké B. Three new species of Craterispermum, a poorly known Rubiaceae genus. Poster présenté au Young Botanist’s Day Forum, 24-25 novembre 2009, ULB, Bruxelles, Belgique. Abstract: The genus Craterispermum Benth. (subfamily Rubioideae) is distributed in tropical Africa, Madacascar and the Seychelles. It is easily recognized by the often very compact axillary inflorescences and the typical yellow colour of the dried plants, which is the result of aluminium accumulation. As typical in Rubioideae , Craterispermum has heterostylous flowers. The genus is badly known taxonomically. No recent monograph of Craterispermum exists and therefore the identification of herbarium specimens to species level is difficult. The taxonomic position of Craterispermum also remains unclear: It is usually placed in the monogeneric tribe Craterispermeae close to Morindeae. Testament to this isolated position, Craterispermum possesses a number of highly derived characters, such as its unique seedcoat, consisting of isolated exotesta cells with ring-like thickenings.

According to the world checklist of Rubiaceae, sixteen species of Craterispermum are known. However, our ongoing taxonomic revision of the genus has resulted in the identification of eight novelties in West Central Africa. These new species are from the Lower Guinea Domain (Cameroon, Equatorial Guinea, Gabon, Nigeria), which seems to be the centre of diversity of the genus. Two other new species occur in the Congolian Domain (Democratic Republic of Congo). The following characters are most useful for discriminating taxa: leaf characters such as higher order nervation, colour and leaf texture, length of acumen; inflorescence characters such as length of peduncle, number of flowers, length .of pedicel; fruit characters such as shape and size of fruit.

We are currently describing three new species from Cameroon and Gabon. They are known from an important number of specimens, but their description is not easy. Indeed, flowers and fruits are very rare on herbarium specimens. Flowers are very short-lived and not often collected, fruits fall off easily during drying and pressing because of the compact inflorescences. Often, only one flower morph is available. The distinguishing characters of the two new species from Cameroon are: small leaves with secondary nerves ± closely parallel, mostly uniflorous and not branched inflorescences for the first one; several-flowered and inflorescences with 2 or 3 branches for the second one. The new species from Gabon is unique within the genus because of the strong dimorphism between the brevistylous and longistylous flowers and inflorescences.
2.6 Sonké B., Droissart V. & Stévart T. Apport des inventaires des Orchidaceae et des Rubiaceae dans l'identification des Zones d'Importance pour la COnservation (ZICO) au Cameroun: 31 In Dahdouh-Guebas F. (ed.) Proceedings of the Symposium on African Botany in Brussels. Université Libre de Bruxelles – ULB / Vrije Universiteit Brussel – VUB, Brussels, Belgium. 120 pp. Résumé: Malgré leur grande diversité, les écosystèmes forestiers tropicaux d’Afrique centrale comptent parmi les moins bien décrits du globe. Des incertitudes demeurent sur le nombre et la distribution des espèces qu’ils abritent ainsi que sur l’origine de leur diversité. L’histoire de ces forêts a vraisemblablement joué un rôle majeur pour expliquer les patrons de biodiversité actuellement observés. En effet, nombreux sont ceux qui soutiennent que les phases arides du Quaternaire, synchrones des périodes glaciaires des régions tempérées, ont été l’un des éléments importants expliquant la distribution actuelle des espèces. Ainsi, les forêts humides se seraient confinées dans des endroits de tailles plus petites appelés refuges, disparaissant des régions devenues trop sèches. L’identification des zones de refuges forestiers est donc fondamentale pour déterminer les zones d’importance pour la conservation car les refuges constitueraient une source de diversité inter et intra spécifique. La localisation et l’étendue de ces refuges forestiers font l’objet de nombreuses discussions entre les scientifiques.

Le Cameroun possède une des plus riches biodiversités d’Afrique tropicale, plus de 8000 espèces végétales y sont représentés. Les Rubiaceae et les Orchidaceae représentent ensemble 10 à 15 % de la flore de ce pays. Certaines parmi elles sont à distribution restreinte ou sont connues seulement par un petit nombre de récoltes, voire uniquement par le spécimen type. Les activités anthropiques qui ont cours dans les zones de récolte de ces espèces font craindre le pire pour leur survie. En effet, si aucune action n’est entreprise pour la protection de ces espèces, elles risquent de disparaître définitivement. La protection durable de ces espèces rares ou endémiques passe par la définition d'une politique cohérente de gestion des écosystèmes. La stratégie de gestion doit prendre en compte les zones de forte diversité spécifique qui constituent un bon indicateur des zones à conserver.

En 1986, le Professeur Lejoly fait sa première mission de terrain au Cameroun. Il inaugure par la même occasion la collaboration entre le Laboratoire de Botanique systématique et de Phytosociologie (ULB) et l’Université de Yaoundé I. Très vite, le principal partenaire local de l’ULB, le Prof. Sonké se spécialise dans l’étude des Rubiaceae et devient l’interlocuteur naturel du Jardin Botanique National de Belgique au Cameroun. En près de 20 ans, sept Docteurs en Sciences auront été formés à l’ULB sous la direction du Professeur Lejoly et dans le cadre de cette collaboration. A la faveur du congrès de l’Aetfat à Bruxelles en 2000, les premiers contacts entre le Prof. Sonké et le Dr Stévart marquent le début d’une longue collaboration portant sur l’étude conjointe des Rubiaceae et des Orchidaceae. Ainsi, en 2001 débutent les premiers inventaires d’Orchidaceae dans la Réserve du Dja. Une première ombrière est construite à Somalomo, rejointe en 2004, par une seconde ombrière à l’Université de Yaoundé I. Ces inventaires sont réalisés initialement avec le soutien des projets ECOFAC (CE) et DIVEAC (CIUF). Ils recevront par la suite un soutien financier du Fonds Léopold III, du Fonds Cassel, du FNRS et du National Geographic. En 2002, en hommage au Prof. Lejoly, Tricalysia lejolyana Sonké & Cheek, est publié. Il est suivi du Polystachya lejolyana Stévart, Bertiera lejolyana Nguembou & Sonké et de Stolzia grandiflora subsp. lejolyana Stévart, Droissart & Simo (Fig. 1).

En plus de la Réserve du Dja, nos équipes ont visité 13 localités situées au centre, au sud-ouest et au sud du Cameroun. Ces inventaires menées par 9 chercheurs et étudiants belges et camerounais ont permis d’enrichir de plus de 10,000 spécimens d’herbiers les collections de YA, BRLU, BR, MO, K, WAG. Durant ces missions, près de 3000 échantillons d’Orchidaceae vivants ont été mis en culture dans l’ombrière de Yaoundé, aboutissant ainsi à la création d’une des collections d’Orchidaceae les plus riches d’Afrique. Cette méthode a ainsi permis la récolte de plus de 2000 échantillons d’herbiers fertiles d’Orchidaceae, permettant au Cameroun de devenir l’une des régions d’Afrique les mieux connues pour la flore en Orchidaceae (Fig. 2). L’examen des spécimens disponibles d’Orchidaceae et de Rubiaceae (nouvellement récoltés ou existants) a par ailleurs permis d’identifier de nombreux spécimens dont certains étaient soit de nouvelles espèces (Tableau 1) soit des signalisations nouvelles pour le Cameroun. Ces travaux ont permis de faire plus d’une trentaine de publications scientifiques conjointes. Une étude comparative des patrons de distribution des Orchidaceae et des Rubiaceae au Cameroun utilisant une méthode permettant de corriger les biais liés à la variation de l’effort d’échantillonnage a mis en évidence un gradient de continentalité de diversité. Pour les deux familles, les parties côtières sont plus riches en taxons endémiques. Les patrons de distribution observés pour les Orchidaceae et des Rubiaceae permet de conclure que le sud Cameroun, bien qu’encore mal étudié, présente une richesse spécifique et taux d’endémisme élevés, et mérite ainsi d’être conservé au même titre que le sud-ouest.

Depuis plus d'une décennie, d'intenses activités d'inventaire des Rubiaceae et des Orchidaceae menées par l’ULB et l’Université de Yaoundé I ont donc cours au Cameroun. Ils ont permis d'accroître fortement les connaissances sur la distribution de ces deux familles. Les Orchidaceae et les Rubiaceae comptent parmi les familles les plus diversifiées, notamment en forêt tropicale humide, et elles présentent un large éventail de formes biologiques et de modes de dispersion. Ces caractéristiques rendent particulièrement intéressant la comparaison de leurs patrons de biodiversité respectifs. Il est aujourd'hui admis que les centres de diversité générique, les modèles d'endémisme et les disjonctions des aires observées chez certaines Rubiaceae de forêt de basse altitude corroborent la théorie des refuges forestiers. De même, les Orchidaceae et surtout celles qui sont épiphytes peuvent être utilisées pour identifier les zones d’endémisme. Leur distribution montre qu'elles peuvent être utilisées comme familles modèles dans l'identification des zones à forte diversité et donc importantes pour la conservation.

Nous nous proposons maintenant de faire progresser nos connaissances sur la localisation et l’origine des zones de forte richesse spécifique et d’endémisme au sein des forêts d’Afrique centrale atlantique en général et au Cameroun en particulier par l’étude et la comparaison des patrons de biodiversité inter-spécifiques chez les Rubiaceae et les Orchidaceae, ainsi que les patrons de diversité génétique au sein de quelques genres et espèces choisies parmi ces familles.

3 Publications en préparation

3.1 Droissart V., Sonké B., Hardy O. J., Simo M., Taedoumg H., Nguembou K. C. & Stévart T. Do Orchidaceae and Rubiaceae show similar endemism patterns in Cameroon? Journal of Biogeography.

3.2 Simo M., Sonké B., Droissart V., Cachapa B., Hardy O. & Stévart T. Synopsis of the Section Dolabrifolia in the genus Angraecum. Systematic Botany.

3.3 Simo M., Sonké B., Droissart V. & Stévart T. The Angraecum of Central Africa, Orchids.

3.4 Sonké B., Taedoumg H. & Robbrecht E. A reconsideration of the Lower Guinean species of Sericanthe (Rubiaceae, Coffeeae), Botanical Journal of Linnean Society.

3.5 Sonké B., Lachenaud O. & Steven D. Synopsis of the Lower Guinean species of Cuviera (Rubiaceae). Systematic Botany.

3.6 Taedoumg H., Sonké B., Hamon P. & De Block P. Craterispermum gabonicum (Rubiaceae) a new species from Gabon. Nordic Journal of Botany.

3.7 Taedoumg H., De Block P., Hamon P. & Sonké B. Two new Craterispermum species from Cameroon. Blumea.

3.8 Taedoumg H., Vanhecke L., Sonké B., Hamon P. & De Block P. Phenetic analysis of the Craterispermum laurinum-schweinfurthii-cerinanthum complex (Rubiaceae). Taxon.
4. Communications et posters pour l’AETFAT 2010

4.1 The new database on Orchidaceae-Rubiaceae endemic to Atlantic Central Africa: (1) use for IUCN conservation assessments.

Authors: Sonké B., Droissart V. and Stévart T. (Talk)

Abstract: Africa Central Atlantic (ACA) area includes low-Guinean domain, fragmented Afromontane zone Cameroon and the islands of the Gulf of Guinea. This part of Africa has a unique biodiversity. Many rare plant species are endemic to this zone and among them the Orchidaceae and Rubiaceae. Some among them are endemic to the ACA and sometimes with a very narrow distribution. The intense human activities that are under way are fears the disappearance of these species. There is an urgent need for conservation measures based on scientific criteria are taken for protecting these species which are very important to understand the history of forests in the Congo Basin. Today, the most appropriate approach to define consistent policies of conservation is to first assess the threats to species using the IUCN criteria. If in some parts of Africa (Southern, Eastern) this method is often used to make assessments of species status, it is not the case in Central Africa. In recent years we have built a database on Orchidaceae and Rubiaceae endemic to the ACA. This database is a compilation of data collected by our research teams and identifications of herbarium specimens available in major herbaria. The assessment focuses for a start on genra that have recently been reviewed and therefore does not have any taxonomic problems. The exploitation of this database allowed us to assess the conservation status of species endemic to Central Africa and reassess other species with new data available. The results confirm that the status of a species can evolve over time.

4.2 A new specimen’s database for Atlantic Central Africa (2): using Orchidaceae and Rubiaceae to explore endemism patterns in the sub-region.

Authors: Droissart V., Sonké B., Hardy O., Couteron P. and Stévart T. (Talk)

Abstract: Orchidaceae and Rubiaceae are among the world’s five most diverse plant families and differ strongly in their life form and dispersal abilities. Rubiaceae are usually understorey shrubs and have limited dispersal capability while Orchidaceae are mostly epiphytic and, because of their tiny seeds, are potentially able to disperse over long distances by wind. The high number of species, the important endemism level and the local endemism encountered for these two families suggest that they could be used as a proxy for the identification of general endemism patterns in Atlantic Central Africa (ACA).

The aim of this study is to characterize and compare geographic patterns of diversity by querying our Orchidaceae and Rubiaceae database (based on xxx herbarium specimens) database for taxa endemic to ACA. To correct for heterogeneous sampling effort, we used rarefaction principles to compute unbiased indices of species diversity and floristic similarity.

Our results show that sub-sampling methods are an efficient way to identify and correct potential biases observed in raw species richness due to heterogeneous sampling intensity. In Cameroon, we confirm that the western (Atlantic) part of the evergreen forests is richer in endemic species than the more inland forests. However, the southern part of these Atlantic forests are as diverse as the more studied northern part, and should also be considered as another hotspot of plant diversity in ACA. Similar endemism patterns (at least in the lowland rainforest) were found for both families, although the Orchidaceae are expected to have better long distance dispersal abilities. The dispersal ability of Orchidaceae seems thus limited by different factors like the need of specific mycorhizal associations for seed germination or the relationship between phorophyte/epiphytes.
4.3 Taxonomy, phylogeny and phylogeography of continental Angraecoid orchids: a case study with the Section Dolabrifolia of the genus Angraecum.

Authors: Simo M., Sonké B., Droissart V., Hardy O., Cachapa B. & Stévart T. (Talk).

Abstract: This study is a part of the Ph.D. project of the first author which aims to elucidate the taxonomy, the phylogenic position and the distribution of two Sections of the genus Angraecum: Sect. Pectinaria and Sect. Dolabrifolia. This latter is characterized by very short, laterally compressed and densely imbricate leaves and has his center of endemism and diversity in Atlantic Central Africa (ACA). Four species are currently described in the Section Dolabrifolia. However, recent fieldworks in ACA allow the discovery of two additional taxa.

Main objectives of the present study are (1) to make clear delimitation of each member of the Section Dolabrifolia through examination of all specimens available in different herbaria, the review of the literature, and provide an accurate identification key, (2) to use four plastid regions (trnC-petN, trnL-F, matK and rps16) to produce phylogenetic trees of the targeted species and (3) to make the taxonomical revision of the section, including the description of the two new taxa.

Preliminary results based on examination of specimens in the Sect. Dolabrifolia in BR, BRLU, P and WAG revealed that almost 90% of collections identified as A. distichum belong to A. bancoense. Molecular results derived from representative sampling of taxa from two Angraecum sections show high levels of resolution and strong bootstrap support in preliminary analyses. Sections Dolabrifolia and Pectinaria appeared closely related and form a monophyletic clade among Angraecoid orchids. A first phylogeographical observation of Angraecum bancoense has been made, showing a haplotypes’ repartition which is not opposed to Pleistocene forest refuges hypothesis, however sampling was not enough to make any firm conclusions. A detailed examination of all specimens of members of Section Dolabrifolia is still ongoing and will allow to make new distribution maps and to assess their conservation status.
4.4 Phenetic analysis of the Craterispermum laurinum-schweinfurthii-cerinanthum complex (Rubiaceae)

Authors: Taedoumg H., Vanhecke L., Sonké B., Hamon P. & De Block P. (Talk).

Abstract: Craterispermum Benth. is an Afro-Madagascan genus comprising ca. 35 species, half of which remain undescribed. This genus of shrubs and small trees is characterized by pedunculate, often very compact, inflorescences that are paired at the nodes, small heterostylous white flowers, a bilocular ovary with one pendulous ovule per locule and a small drupaceous fruit with a single bowl-shaped seed. Craterispermum species accumulate aluminium in vegetative tissue, which results in the typical yellow or pale green colour of the dried leaves. Because of the above-mentioned characters, Craterispermum is easily recognized at genus level. Identification at species level is difficult because taxa often look similar.

During our ongoing revision of Craterispermum, we become aware of the considerable nomenclatural and taxonomic confusion involving C. laurinum (Poir.) Benth., C. schweinfurthii Hiern and C. cerinanthum Hiern. These three species have partly overlapping distribution areas, and, while typical specimens are readily identified, many intermediate specimens cannot easily be placed. We conducted a phenetic study to determine whether this complex is best treated as one polymorphic species or as three distinct species. In a preliminary analysis 56 morphological characters were examined for their usefulness in separating the three taxa. In a second stage, those 27 variables explaining the most phenetic variation were studied on 150 specimens, representing the full morphological and geographical variation of the three species. Principal component and canonical variance analyses were used to analyse this data set.

Our results show that C. laurinum, C. cerinanthum and C. schweinfurthii are distinct species and that phenetic overlap between the taxa is low. The best distinguishing characters are leaf (e.g., venation, leaf texture, length of acumen) and inflorescence characters (e.g., length of peduncle and pedicels, number of flowers). The colour of the dried leaves, often used to identify C. laurinum, is not a reliable character.

4.5 New records for the flora of Cameroon
Authors: Sonké B., Lachenaud O., Simo M., Taedoumg H., Droissart V., Lemaire B., Stévart T. & Dessein S. (Poster).

Abstract: The Republic of Cameroon covers an area of ca. 475,000 km2 and extends from about 1°45’N to 13°N and between 8°25’E to 16°20’E. This central-west African country has an exceptionally rich flora of ca. 8,000 vascular plant species of which nearly 10% are endemic. Cameroon’s remarkable floristic richness reflects its biogeographic diversity comprising four main regions: (i) the lowland forest region, almost entirely covered in Guineo-Congolian rainforest; (ii) Mount Cameroon and the western highlands, an area of largely volcanic upland with a SW to NE alignment extending from Mount Cameroon on the coast at ca. 4°N inland to the Mandara mountains; (iii) the Adamaoua Plateau, a large massif that divides the country in two from W to E; and (iv) northern Cameroon, a region extending from the Bénoué plain to Lake Chad, with altitudes not exceeding 300 m.

In recent years, intensive botanical exploration activities are underway in Cameroon to expand our knowledge on the flora. They have uncovered many new species to science and new records for the flora. As part of a long scientific collaboration between Cameroonian and Belgian researchers, two expeditions were held in Cameroon in the months of April-May and October 2009. This contribution gives preliminary results arising from these surveys. Two new taxa of Balsaminaceae for science were collected and eleven species were found new to the flora of Cameroon including four Orchidaceae, four Rubiaceae one Annonaceae, one Liliaceae and one Asclepiadaceae.

6 Do Orchidaceae and Rubiaceae show similar endemism pattern in Cameroon?

Authors: Droissart V., Sonké B., (Hardy O., Couteron P.) and Stévart T. Poster.
7 The endemic flora of Sao Tomé and Principe (Gulf of Guinea).

Authors: Stévart T., Oliveira F. and Figueiredo E. (Talk)

5 – Thèses, mémoires
5.1 Mémoire de Master II

Bruno Cachapa Bailarote. Phylogénie et phylogéographie au sein du genre Angraecum en Afrique centrale. Mémoire défendu en septembre 2009. Promoteurs: Tariq Stévart (Missouri Botanical Gardens) & Olivier Hardy (Université Libre de Bruxelles).

Abstract: Few studies have been so far conducted by molecular taxonomists on Angraecoids (Orchidaceae, Vandae). Some studies focusing on Indian Ocean Angraecoids showed that the actual taxonomy, based on morphological criteria, does not correspond to genetic data. In this work, we wanted to: (1) test the monophyly of Continental Africa’s Angraecum; (2) make a synthesis of the distribution of species from the Dolabrifolia and Pectinaria sections and test the monophyly of each; (3) test the recent taxonomic cleavage of Angraecum bancoense and Angraecum distichum with molecular markers and; (4) see if the haplotypes’ distribution pattern of Angraecum bancoense is related to Pleistocene forest refuges. In order to do it, we sequenced three chloroplastic DNA regions (matK, rps16, trnC-petN) of Continental African Angraecoid species, mainly from the Angraecum genus. 98 DNA samples were extracted from 19 species. On these 98 samples, 78 where sequenced for at least one molecular marker. These sequences, along with others that were available from other works, have allowed us to establish a phylogeny from Angraecoids and Angraecum Bory genus. We have been able to confirm the polyphyly of Continental African Angraecum, of which some species should be placed in other genera. The Dolabrifolia section is monophyletic, however we still have a doubt about the Pectinaria section as we can see species of another section included in the Pectinaria clade on some phylogenetic trees. We have not been able to observe a clear distinction between Angraecum bancoense and Angraecum distichum based on molecular data. Groups obtained were mixes of species. A first phylogeographical observation of Angraecum bancoense has been made, showing a haplotypes’ repartition which is not opposed to Pleistocene forest refuges hypothesis. Distribution maps of African species from Dolabrifolia and Pectinaria sections are also provided.
5.2 Thèses de doctorat en cours

Murielle Simo. Révision systématique et analyse biogéographique de deux sections du genre Angraecum (Orchidaceae) en Afrique tropicale. Promoteurs: Bonaventure Sonké (Université de Yaoundé I) & Tariq Stévart (Missouri Botancal Garden).

Ce travail repose sur l’hypothèse selon laquelle la théorie des refuges forestiers explique une grande partie de l’endémisme et de la diversité en Afrique centrale atlantique. La spéciation des espèces étudiées s’expliquerait dès lors par cette théorie et illustrerait les différents scénarios évolutifs observés en Afrique centrale atlantique, et en particulier les modes de spéciations allopatriques et sympatriques liés à la présence de refuges forestiers et à leur dynamique.

Hermann Taedoumg. Révision systématique du genre Craterispermum Benth. en Afrique continentale. Promoteurs: Bonaventure Sonké, Petra De Block (Jardin Botanique National de Belgique) & Perla Hamon (IRD Montpellier).

Le présent projet de thèse se propose de faire la révision des espèces continentales du Craterispermum, de clarifier les relations qui existent entre les différentes espèces du genre (Afrique continentale et Madagascar) et énoncer des hypothèses sur l’évolution et la filiation des taxons au sein de ce genre, étudier l’écologie, la biologie, et la biogéographie de Craterispermum, en mettant un accent particulier sur l’accumulation de l’aluminium.

Novembre 2009

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