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The conversion of natural forests to monoculture tree plantations (MTPs) has been known to reduce above-ground tree diversity. However, information is lacking on the impact of MTPs on below-ground tree species diversity. This study evaluated below- and above-ground tree species diversity in a Strict Nature Reserve (SNR), a monoculture plantation of an indigenous tree species – Nauclea diderrichii (Nd), and four MTPs of exotic species – Gmelina arborea (Ga), Tectona grandis (Tg), Pinus caribaea (Pc) and Theobroma cacao (Tc) within Omo Biosphere Reserve in South-western Nigeria. Below-ground tree species was ascertained by evaluating the soil seed banks from the natural forest and the monoculture plantations using the seedling emergence method while tree species enumeration aboveground was done using the quadrat method. Below- and above-ground tree species diversity was higher in the SNR than in the MTPs and higher in the MTP of indigenous species (Nd) than in those of the exotic species. Below-ground tree species diversity was zero in both the SNR and MTPs below 0-5 cm soil depth. Similarity in tree species composition was also higher between the SNR and the indigenous species (Nd) plantation. The negative impact of natural forest conversion to MTPs on below- and above-ground tree species diversity was found to be lower in the indigenous species (Nd) plantation than in the exotic species (Ga, Tg, Pc & Tc) plantations. Consequently, plantation forestry may consider native species for biodiversity restoration.
Keywords: Forest conversion, species origin, regeneration potential, tree diversity