Mapping and predictive modelling of time-space changes of vegetation in the district of Djidja (Benin): lesson for regional planning
Arouna Osséni (2012)
PhD, University of Abomey-Calavi, Bénin
The vegetation of the district of Djidja which is under high anthropogenic pressures was studied. The aim is to assess the spatio-temporal changes of vegetation and their impacts in the regional planning. The vegetation mapping based on the remote sensing data, predictive modelling of vegetation, phytosociology and perceptions’ analysis, were the main methods used. The mapping of spatial and temporal changes of vegetation showed that the area of natural vegetation decreased from 82.27 % in 1974 to 41.50 % in 2010 while fields and fallows increased from 8.77 % to 53.17 %. Whithin this time periode, dry forests were disappeared completely by conversion of other land covers units. Residual vegetation types were riparian forests, woodlands, savannas woodlands and tree and shrub savannas which occupied respectively 1.69 %, 4.85 % and 35.50 % of that district land. The hierarchical classification of phytosociological relevés based on the presence-absence of species led to five individual plant communities. The characterization of the vegetation structure through these plants communities showed the dominance of small-diameter timber. The most represented families were the Leguminosae and Poaceae. The phanerophytes and the therophytes were the most represented, while in chorological terms, it was the sudanian species and pantropical species that were most represented. The Shannon diversity index ranged from 2.09 bits in Daniellia oliveri and Tephrosia bacteolata plants community to 3.72 bits in Pterocarpus erinaceus and Aframomum alboviolaceum plants community. The density of wood’s dbh greater than 10 cm dbh ranged from 89 individuals/ha in Daniellia oliveri and Tephrosia bacteolata plants community to 577 individuals/ha in Pterocarpus erinaceus and Aframomum alboviolaceum plants community. Then the basal area were 3.88 m2/ha in Daniellia oliveri and Tephrosia bacteolata plants community and 32.66 m2/ha in the Pterocarpus santalinoides and Phaulopsis barteri plants community. Agriculture, logging, charcoal production and at less extent, livestock and hunting were perceived by local people as determinants of vegetation degradation driven by population growth, the arrival of migrants, the deficient forest regulation and agriculture policies, land tenure, urbanization and climate change. On the assumption that current practices of natural resources would be held over time, predictive modelling of vegetation portends that the mosaic of fields and fallows lands would occupy about 60 % of the district, followed by tree and shrub savannas, which cover’s would be about 30 % in 2020. Riparian forests, woodlands and savannas woodlands would be represented by small spots with a land cover less than 3 % of that district land. A model of spatial planning and management designed to reverse the regressive trend of vegetation types has been designed for regional planning purposes.
Key words : Vegetation, spatio-temporal changes, predictive model, regional planning, Djidja, Benin