Ecological Assessment of Riparian Forests in Benin
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Ecological assessment of riparian forests in Benin: Phytodiversity, phytosociology, and spatial distribution of tree species Armand Kuyéma NATTA Promotor: Prof. Dr. Ir. L.J.G. van der Maesen Hoogleraar in de Plantentaxonomie, Wageningen Universiteit, Nederland Prof. Dr. Ir. B. Sinsin Professor of Ecology, Department of Environment Management, Faculty of Agronomic Sciences, University of Abomey-Calavi, Benin. Co- promotor: Dr. A. Akoègninou Department of Vegetal Biology, Faculty of Science and Technique, University of Abomey-Calavi, Benin. Promotiecommissie: Prof. Dr. Ir. A. de Gier (ITC – Enschede, Nederland) Prof. Dr. F.J.J.M. Bongers (Wageningen Universiteit, Nederland) Prof. Dr. J. Lejoly (Free University of Brussels, België) Prof. Dr. S. Porembski (University of Rostock, Germany) Ecological assessment of riparian forests in Benin: Phytodiversity, phytosociology, and spatial distribution of tree species Armand Kuyéma NATTA Proefschrift ter verkrijging van de graad van doctor op gezag van de rector magnificus van Wageningen Universiteit Prof. Dr.Ir. L. Speelman in het openbaar te verdedigen op woensdag 26 november 2003 des namiddags te vier uur in de Aula NATTA Armand Kuyéma (2003). Ecological assessment of riparian forests in Benin: Phytodiversity, phytosociology, and spatial distribution of tree species Ph.D. Thesis Wageningen University, with summaries in English, French and Dutch. ISBN 90-5808-954-1 Key words: Riparian forests, flora, diversity, endangered species, structure, plant community, ordination, classification, rivers, streams, spatial distribution, ecological factors, sampling designs, Benin, West Africa. To My mother, Gwendoline, Kévin. CONTENTS Chapter 1 General introduction 1 Chapter 2 Study area 7 Chapter 3 Riparian forests, a unique but endangered ecosystem in Benin 15 Chapter 4 Assessment of riparian forest fragments plant diversity in West African savanna regions: an overview from Benin 27 Chapter 5 Structure and ecological spectra of riparian forests in Benin 41 Chapter 6 A phytosociological study of riparian forests in Benin (West Africa) 53 Chapter 7 Spatial distribution and ecological factors determining the occurrence of Pentadesma butyracea Sabine (Clusiaceae) in Benin (West Africa) 73 Chapter 8 Forest structural parameters and floristic composition spatial variation and modelling across rivers in Benin 83 Chapter 9 Ouémé and Comoé: forest-savanna border relationships in two riparian ecosystems in West Africa 105 Chapter 10 Assessing the density of Khaya species through Simple Random Sampling, Stratified Sampling and Systematic Sampling 117 Chapter 11 General discussion 129 Chapter 12 General conclusion 143 Summary 149 Résumés 155 Samenvatting 161 References 167 Riparian forests plant species list 193 Acknowledgements 211 Curriculum vitae 213 List of publications 215 Chapter 1 GENERAL INTRODUCTION A.K. Natta Chapter 1: General Introduction Chapter 1 GENERAL INTRODUCTION 1.1. RESEARCH BACKGROUND The loss and fragmentation of tropical forest is the single greatest threat to the world’s biological diversity (Whitmore 1990, Huston 1994). In 1992, the Convention on Biological Diversity highlighted that measures must be implemented for the conservation of natural ecosystems, especially for tropical forests, which are famous for being the most species rich ecosystems on earth. Although many species have been described, very little is known about their ecology (Sayer & Wegge 1992). In Africa the most species-rich forests, at least for woody plants, are in the wetter areas of West Central Africa (from the base of Mt Cameroon South East into Gabon), and in the North East of Madagascar (Gentry 1992). Unfortunately this diversity is characterised by a deforestation rate of 0.7 %, more than twice the world average of 0.3 % (FAO 2000). In West and Central Africa, the rain forest block is interrupted between South East Ghana and South West Nigeria. Biogeographers call this discontinuity the Dahomey-Gap. Innumerable animals and plants are common to Central Africa and upper Guinea, showing that the two forests blocks have been connected in the past. However, this does not mean that the forests are identical. The two rain forests blocks are characterised by special endemic species to confirm that they have not only been isolated but are intrinsically different (Kingdon 1990). On the other hand, the West-African countries located in the Dahomey-Gap, such as Benin, are characterised by the absence of tropical rain forests and a low percentage of dense semi-deciduous forests. In consequence the need to conserve the biological diversity of remaining fragmented forests, represented by sacred forests, few protected dense semi- deciduous upland forests and riparian forests, in this dry wedge cannot be underestimated. All over the world, and particularly in tropical savanna, the natural vegetation associated with waterways and mostly represented by riparian forests (or RFs) is credited to be among the most species-rich ecosystems (Meave & Kellman 1994, PGRN/IUCN 1994, Roggeri 1995, Nilsson et al. 1997). Riparian forests are important areas for global biodiversity (Sala et al. 2000), because they protect key resources for mankind, such as water sources and quality, and stream environment (Vought et al. 1994, Lowrance et al. 1997, Montgomery 1997, Trimble 1999), and harbour a diversified flora and physical structure (Gibbs & Leitão 1978, Gregory et al. 1991, Bersier & Meyer 1994, Kellman et al. 1994, Meave & Kellman 1994, Woinarski et al. 2000, Kokou et al. 2002). Ecologists have also examined the value of riparian forests as habitats for many animals, and recognised them as a priority area for conservation of terrestrial mammals (Doyle 1990, Darveau et al. 1998, de Lima & Gascon 1999, Darveau et al. 2001), as well as birdlife (Stauffer & Best 1980, Gates & Giffen 1991, McGarigal & McComb 1992, Larue et al. 1995, Murray & Stauffer 1995, Darveau et al. 1995, Whitaker & Montevecchi 1997, Saab 1999, Woinarski et al. 2000). As tropical forests become more fragmented due to deforestation, riparian forests play a crucial role in providing habitat corridors between forest patches to increase landscape connectivity (Forman & Godron 1986, Forman 1997, Machtans et al. 1996). In proportion to their area within a watershed, they perform more ecological and productive functions than do adjacent uplands (NRC 2002). The vital ecological, hydrological and biogeochemical vital functions of these forests bordering waterways are now largely acknowledged. In recognition of these roles, they have been increasingly protected by policies and legislation (DNRE 1996, de Lima & Gascon 1999, NRC 2002, Natta et al. 2002). Unfortunately, they are under severe 1 Chapter 1: General Introduction threat worldwide (Sparovek et al. 2002), and current management strategies, particularly in the tropics, seem to have limited effects. A large body of work has demonstrated that riparian forests play a critical role regulating interactions between terrestrial and aquatic components of temperate zones landscapes (Gregory et al. 1991, Gilliam 1994, Naiman & Décamps 1997), however there have been relatively few studies dedicated to them in the tropics (Bowden et al. 1992, McDowell et al. 1992, McClain et al. 1994, Chestnut & McDowell 2000, Groffman et al. 2001) in general, and Benin in particular. Flora and fauna reserves represent 11% of Benin surface area, and the flora is estimated to have 3,000 species (PFB 1997), which is relatively poor in comparison with neighbouring humid countries. The natural wooded vegetation represented by savanna and open forests occupy 72.2 % of the country, but dense semi-deciduous forests and riparian forests cover only 3.42 % of the country (CENATEL 2001). During the preparation of Benin’s Environmental Management Plan, it was noted that the lack of scientific knowledge of the country’s environment constituted a major obstacle to the realisation of the plan and measurement of the effectiveness of the proposed actions. In 1994, following the spirit of the Rio conference, the sustainable development agreement between Benin and the Netherlands emphasised on the knowledge and valorisation of the biological diversity. In 1995 this agreement has retained, as emergency programs, scientific surveys of plant species and the compilation of Benin’s flora. The flora is known to be a major component of a region’s or a country’s biodiversity. Scientific research on species diversity is of great importance in providing accurate information and can enhance the management and sustainable use of phytogenetic resources, especially for endangered ecosystems (PFB 1997), such as riparian forests, which are among the most important forest ecosystems in the majority of degraded woodlands and mosaic of savanna landscapes (Natta 2000). In Benin the rapid changes in land use have led to the progressive destruction and fragmentation of riparian forests, which provide fertile soil for cultivation, give an opportunity for irrigation, and shelter a wide range of valuable and scarce plants and animals. Therefore, they are systematically targeted for illegal selective tree cutting, hunting and conversion to agriculture. Also, when the riparian forests degrade, forest species become sparse, leaving the vegetation open for the savanna species to invade. Their rich biological resources, especially plant species, are disappearing before they can be inventoried and assessed (Natta 2000). We see that riparian forests can be classified among the endangered ecosystems