Study of spatial variability of herbaceous biomass, phenology and structure of vegetation in the upper Oueme catchment in sudanian zone of Benin
Ismaïla TOKO IMOROU (2008)
PhD, University of Abomey-Calavi
Factors determining spatial variability of herbaceous biomass, phenology and structure of vegetation were investigated in the upper Oueme catchment. This study were carried out in five sub catchments inside of which fifty three harvesting biomass plots of 10 m x 10 m were established according to transect method. Phytosociological relevés were done in two hundred plots of 30 m x 30 m to determine plant communities occuring in such sub catchments. Community typology and ordination were derived from a Detrended Canonical Analysis (DCA). They were distinguished eight plant communities. Topography, soil properties and human activities impact were the main factors which explain the establishment of those plant communities. 467 species belonging to 287 genera and 86 families were recorded. Leguminoseae (87 species) and Poaceae (81 species) were the most represented families. Phanerophytes (37 per cent) followed by therophytes (32 per cent) were the dominant life forms. Sudano-Zambezian (19 per cent), Sudanian (16 per cent) and Pantropical (16 per cent) species were the most abundant in the community. Species richness varied from 12 species on inselberg to 47 species in middle and upland per 900 m2. Shannon index, Pielou evenness and tree basal area varied respectively from 0.68 bits, 0.47 and 0.37 m2/ha on inselberg to 3.92 bits, 0.89 and 28.28 m2/ha in riparian forests. Tree density varied from 10 stems/ha on inselberg to 810 stems/ha in middle and upland. Herbaceous biovolum varied from 1319 m3/ha in fallows to 16180 m3/ha in depression. Stem diameter distribution showed a decreasing curve in natural vegetation type while in fallows, the big stems diameter were conserved. The biomass varied from 0.40 t DM/ha (June) to 8.43 t DM/ha (November). The herbaceous biomass was significantly different between topography levels, soil properties, plant life forms, plant community and sub catchments. This herbaceous biomass was significantly correlated with soil depth, drainage intensity, biovolums, tree density (dbh ≥ 10 cm) and tree canopy cover. The herbaceous plants dynamics showed several phases of trend. The peak of biomass varied according to the topographic levels and the sub catchment. Thus, the peak of the herbaceous production was reached in October in Aguimo and Térou-Wannou sub catchments then in November in Soomi-Igbèrè and Ouémé-Bétérou sub catchments.
Key words: Herbaceous biomass, phytosociology, upper Oueme catchment, Benin