Vegetation patterns and environmental gradients in Benin: implications for biogeography and conservation
ADOMOU Aristide Cossi (2005)
PhD, Wageningen University, the Netherlands
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.
The study was conducted in Republic of Benin, situated in the Western Africa between the latitudes 6o10’N and 12o25’N and longitudes 0o45’E and 3o55’E. The analyses involved 598 phytosociological relevés performed according to Braun-Blanquet’s approach at “relatively undisturbed sites” hosting the major plant formations. We set up a matrix of 598 relevés and 1021 plant species using the Community Analysis Packages (CAP). We also collected climatic data. We summarised data using multivariate and chorological analyses, ombrothermograms, and climatic index of Mangenot. Vegetation-environment relationships were examined using regression analyses. We proposed the first floristic-based and objective phytogeographical subdivision of Benin into ten phytogeographical districts (PDs); these were merged into three major phytochorological zones: 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. The PD 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 were discussed. We also focused on elucidating the gradients in species composition and underlying ecological factors. The overall numerical analysis of the matrix of 598 relevés and 1021 plant species resulted in twenty distinct vegetation types. Chorological and Detrended Correspondence Analyses (DCA) of these plant communities led to the recognition of four main chorological groups patterned along a climatic gradient. The four chorological groups correspond to four geographically separate phytochorological zones: the Guineo-Congolian and Sudanian regions separated by two transition zones. This was supported by regression analyses which exhibited strong correlations between the first DCA-axis and chorological categories. Water availability – expressed as a function of rainfall, length and intensity of the dry season, and air humidity – appeared as the major factor determining the vegetation gradient in Benin, and explained 80% of the variation in species composition. Rainfall, considered alone, only accounted for 30% of the variation in species composition. Thus, it is not correct to consider rainfall alone as the prime water factor underlying the species distribution in West Africa. These results demonstrate 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. Moreover, we examined the semi-deciduous forest patterns 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 Benin’s Plant Red List and conservation areas were established using the available botanical information. As many as 280 threatened plant species were recorded, of which 90% was categorised as Endangered and Critically Endangered. Among them, 19 were recognised to be of regional conservation concern while 10 figure on the IUCN Red List. The chorological analysis revealed that the Guinean species are dominant and represent 77% of the threatened species reported. The richest sites in rarest species include the semi-deciduous forest islands. Emphasis was put on the semi-deciduous forest relics as of high conservation concern, since they include c. 20% of Benin’s flora. Contrary to rainfall, water availability in terms of several climatic factors is a good predictor of the vegetation patterns in Benin. Knowledge of modern plant community patterns is vital for the palaeo-vegetation reconstruction. This study also provides baseline information for the development of ecosystem-based approach biodiversity conservation.
Keywords: Vegetation patterns, water availability, Plant Red List, conservation areas, Dahomey Gap