The East Usambara mountains are globally important for biodiversity conservation. The work reported here aimed to clarify whether plantations of Maesopsis eminii, an introduced pioneer tree, are a source of alien tree invasion or a temporary element in a protected conservation forest ecosystem in these mountains. The natural regeneration dynamics of M. eminii and indigenous primary tree species were assessed in M. eminii plantations which were up to 35 years old, using clustered systematic sampling. A total of 103 and 95 woody species were observed in the sapling and lower canopy layers, respectively. Both layers showed an abundance of Cephalosphaera usambarensis, a local species also used in plantations. M. eminii was virtually absent in the sapling layer, and its frequency decreased in the lower canopy with time. Abundant spontaneous regeneration of native primary forest species occurred under M. eminii. The sapling density of Allanblackia stuhlmannii, Beilschmiedia kweo and Greenwayodendron suaveolens increased, and that of Newtonia buchananii decreased, with time. In the lower canopy, the density of G. suaveolens, Parinari excelsa and Strombosia scheffleri increased in relation to plantation age. Therefore, no active, risky eradication of Maesopsis is needed in the conservation area, but a priority management task should be to avoid canopy gaps.