Vegetation patterns and environmental gradients in Benin

Key words: West Africa, Benin, vegetation patterns, floristic areas, phytogeography, chorology, floristic gradients, climatic factors, water availability, Dahomey Gap, threatened plants, biodiversity, conservation.

Understanding plant species distribution patterns and the underlying factors is a crucial step for the conservation and management of plant communities and ecosystems. The geographical distribution of plant species is not entirely random. Clear patterns exist and can be detected by careful analysis of the distribution patterns of a large number of species. Following this chorological approach suggested by White (1983), we explored the vegetation patterns in Benin using modern statistical tools and discussed the biogeographical and ecological implications. Chapter 1 presents the background, the objectives and the study area.

In Chapter 2, we proposed a phytogeographical subdivision of Benin based on multivariate and chorological analyses. Two-Way Indicator Species Analysis (TWINSPAN) and Detrended Correspondence Analysis (DCA) performed well on our data and suggest an objective subdivision of Benin into ten phytogeographical districts (PDs). Chorological analysis proves to be a complementary approach to the characterisation of phyto-sociologically-defined districts and allows their merging into three major phytochorological zones, namely the Guineo-Congolian and Sudanian regions, linked by the Guineo/Sudanian transition zone. The biogeographical patterns observed were interpreted in terms of both ecological factors (climate, soil, geology and land form) and historical factors (past climatic changes). The biogeographical particularity of the PD of Pobè was elucidated by the exclusive presence of the Guineo-Congolian endemic genera such as Amphimas, Anthonotha, Distemonanthus, Hymenostegia, Anthrocaryon, Coelocaryon, and Discoglypremna. The only Guineo-Congolian endemic family found in Benin is Octoknemataceae with Octoknema borealis (now in Olacaceae), which appears to be restricted to the PD of Ouémé valley. The PDs of Bassila and the Atacora Chain within the Sudanian Region turn out to be ecologically and biogeographically outstanding floristic areas. The 8th Guineo-Congolian endemic genus reported in Benin was Aubrevillea with A. kerstingii, which is restricted to the PD of Bassila. The Sudanian endemic genus Haematostaphis with H. barteri and the Benin's endemic species Thunbergia atacoriensis were found to be confined to the Atacora Chain. The proposed floristic areas reflect the climatic zonation and express a great similarity to the previous physiognomically-based subdivisions. The relationships between our floristic areas and the regional chorological units as proposed by White (1983) were discussed.

In Chapter 3, we classified and described the major vegetation types based on species composition. It focused on elucidating the gradients in species composition and underlying ecological factors. Two-Way Indicator Species Analysis (TWINSPAN) performed on a matrix of 598 relevés and 1021 plant species result in twenty distinct vegetation types. Chorological and Detrended Correspondence Analyses (DCA) of these plant communities lead to the recognition of four main chorological groups patterned along a climatic gradient. The four chorological groups correspond geographically to four distinct phytochorological zones: the Guineo-Congolian and Sudanian regions separated by two transition zones. This was supported by the strong correlation between the first DCA-axis and chorological categories. The results show that water availability - expressed as a function of rainfall, length and intensity of the dry season, and air humidity - is the major factor determining the vegetation gradient in Benin, and explains 80% of the variation in species composition. Rainfall, considered alone, only accounts for 30% of the variation in species composition along the vegetation gradient. Thus, it is not correct to consider rainfall alone as the prime moisture factor underlying the species distribution in West Africa. This demonstrates the predictive value of vegetation type with regard to chorological categories and climate. Phytosociology and chorology turn out to be two complementary approaches to phytogeography.

Chapter 4 focused on the patterns in semi-deciduous forest and their biogeographical significance. The six semi-deciduous forest types described and their floristic composition (both at species and family levels) are comparable to those recognised in other parts of West Africa where the semi-deciduous forest is more continuous. The ecologically dominant families include Sterculiaceae, Ulmaceae (now Celtidaceae), Leguminosae, Moraceae, Ebenaceae, Rubiaceae and Euphorbiaceae. The Drypetes aframensis-Nesogordonia papaverifera community appeared to be the vegetation type in which the Celtidaceae, Sterculiaceae, Euphorbiaceae, and Violaceae are best represented. The record of many Upper Guinean endemic species in the semi-deciduous forest islands provided strong evidence for past floristic connections of the Dahomey Gap with the West African rain forest zones. The restriction of the Khaya grandifoliola-Aubrevillea kerstingii community to the centre-northern section was interpreted as resulted from past floristic connections between this area and the Upper Guinean forest block. The overall species composition of the semi-deciduous forest islands situated in southern Benin is in good agreement with the pollen assemblages of the humid period of early-mid Holocene (8500-4000 years BP), a period during which the Dahomey Gap was shown to be covered by a semi-deciduous forest. The Terminalia superba-Piptadeniastrum africanum community, which corresponds to the West African moist semi-deciduous forest, was assumed to dominate the vegetation of the gap during this period. Its subsequent retreat to forest stream banks was interpreted as an ecological response to the general dry climate of the late Holocene, which induced the fragmentation of West African rain forest. The results show that knowledge of modern plant community structure, species composition and distribution is vital for the in-depth interpretation of pollen data in the process of palaeo-vegetation reconstruction.

In Chapter 5, plant species and areas of conservation priority were identified using the available botanical information. As many as 280 threatened plant species were recorded, of which 90% was categorised as Endangered and Critically Endangered. This indicates that the risk of extinction facing these species is high. Among them, 19 were recognised to be of regional conservation concern while 10 were of globally conservation concern and are on the IUCN Red List. The chorological analysis revealed that the Guinean species are dominant and represent 77% of the threatened species reported. This is in agreement with the high representation of threatened species in the closed forest remnants. The richest sites in rarest species (i.e. species confined to one site) include the semi-deciduous forest islands such as Pobè (17 range-restricted species), Dangbo (11 species), Ewè (9 species), and Ahozon (8 species). Emphasis was put on the semi-deciduous forest relics as of high conservation concern, since they include c. 20% of Benin's flora. Other areas of conservation priority include the swamp forest of Lokoli, the mangrove of Ouidah, the inselbergs of the phytogeographical district of Zou, and the Atacora Chain which harbours the two Benin's endemic plant species. Furthermore, most of these sites proved to harbour threatened plant communities. This study also provides basic botanical information for the improvement of the existing reserve network.

Chapter 6 presented a general discussion and conclusions on issues tackled in this thesis. The distribution of Rinorea (a bio-indicator genus) provided further evidence for the biogeographical significance of the semi-deciduous forest islands. We discussed the implications of our research outputs for biodiversity conservation in Benin.