Plantations of single tree species which are usually set up to reafforest cleared lands in the tropics, have often been criticized for being associated with a low level of diversity in the ecosystems. However, our study of understorey plant species in various plantations of single exotic or indigenous species has shown that more plant species grew within stands near a natural forest than within grasslands in Sakaerat, northeastern Thailand. These forests contribute to the acceleration of the secondary succession from grassland, which before reafforestation was achieved by recurrent fires. In the natural regeneration of understorey tree species in planted forests, large tree species in the natural forest seem to be the major common seed source among the stands studied. The small leaf biomass in these planted forests generated enough shade to prevent the invasion of 2 fire-adapted competitive grasses and to provide a suitable light environment for seedling establishment at an early stage of forest development. These factors facilitated the establishment of other species within the plantations of single tree species. Faster-growing exotic species accumulated larger amounts of understorey biomass than indigenous species. The most suitable species was Acacia mangium. The association of forests planted with single tree species with a high species richness in the ecosystems may enable to achieve a high biological diversity at the landscape level, which could bring various benefits to the forests of Sakaerat.