Latitudinal distribution, co-occurring tree species and structural diversity of the threatened palm Borassus aethiopum (Arecaceae) in Benin, West Africa

Abstract

Background and aims – Borassus aethiopum Mart. is a wild palm species with high subsistence importance in West Africa. Extensive agriculture and overharvesting of its stem and fruits for multiple uses have caused a decrease of its natural populations in its native range. For conservation purposes, the distribution, abundance and structural diversity of the species were investigated across ten phytodistricts in three biogeographical zones in Benin. Two hypotheses were tested (i) tree floristic composition of B. aethiopum natural habitat changes with phytodistricts and (ii) structural diversity of B. aethiopum changes with phytodistricts, both as potential adaptation strategies to changing ecological conditions. Methods – Geographical coordinates of the species occurrence were recorded. Abundance was assessed in 852 one-ha plots. Structural diversity was studied using structural indices on data from ecological inventories and neighbourhood survey in 70 one-ha plots. Key results – The two hypotheses proved true. B. aethiopum was found in all phytodistricts but with strong variations in abundance. Overall, floristic composition of its natural habitats showed dissimilarities among phytodistricts. Three main vegetation types sheltered B. aethiopum : mixed grass and shrub savannas, savanna woodlands and woodlands, all of which were found in gallery forest landscapes. The density of B. aethiopum was lower in grass savannas but larger, shorter and distant individuals were found there than in savanna woodlands and woodlands. In the latter vegetation types, its density was high with thin, tall and closely spaced individuals. B. aethiopum tolerates mingling with several other tree species but increased mingling tends to lead to positive differentiation in diameter and height. Conclusions – Borassus aethiopum is a sun-demanding species and establishes successful populations in various ecological conditions. It could be mixed with other tree species in tree plantations and modern agroforestry systems as long as water requirements are met. However, it would be preferable that the species is associated with shade tolerant or medium sun-demanding species.