The plantations of Acacia auriculiformis determine a progressive loss of floristic identity. The phytodiversity of the Acacia auriculiformis plantations of the classified Ouèdo forest and fallow land has been evaluated in southern Benin. For this, the Sorensen similarity index, the Shannon diversity index and Piélou's equitability are calculated. Phytogeographic types and biological types are determined. Data from field surveys are mainly processed with the Excel 2016 software. The results obtained following the establishment of 40 plots of 100 m2 in the different ages of plantations and fallows reveal a floristic richness of 76 species belonging to 36 families. The most common species are from the Rubiaceae family. Species richness varies between 5 and 16 species/survey. The similarity index of Sorensen (S <50%) shows a floristic independence between the phytodiversity of fallows and that of plantations. On the other hand, within plantations, species are floristically dependent (S> 50%). The Shannon diversity index and Piélou's fairness vary respectively between 0.87 and 1.92 bits and between 0.28 and 0.55. The Simpson indices between 0.57 and 0.73 indicate very low diversity. Variation in floristic composition increases with age of planting because of maintenance (thinning). Phytogeographically, the ground cover is controlled by introduced species with weighted spectra between 85 and 98%. As for life forms in plantations, the ground cover is dominated by mesophantophyte species with weighted spectra between 87 and 98.99%. Sustainable management of forest plantations should be considered to ensure conservation and efficient protection of undergrowth resources.
