The distribution of 203 forest plant species over 234 isolated forest patches in the western part of Belgium and the most northern part of France was studied. An analysis that considered species richness in the context of the SLOSS (single large or several small reserves) debate gave no evidence of habitat subdivision reducing total plant species richness in the forests. The presence of some functional ecological plant species groups was correlated with habitat features and patch area. Habitat diversity was found to be important in explaining the presence of species groups of high conservation value, but patch age (as an indicator for habitat quality) also played a major role. Habitat diversity was not a surrogate for patch age. For most of the species groups, patch area sensu stricto is a redundant variable in explaining species richness relative to habitat diversity and patch age; area-dependent stochastic extinctions of forest plant species are of minor importance, at least at the present level of forest fragmentation. In contrast we suggest that extinction of forest plants occurred and still occurs mainly in a deterministic way. Finally, we conclude that even small forest fragments can be very important for maintaining plant species diversity, at least if they are of high habitat quality and if the forest management is appropriate. (C) 1998 Elsevier Science Ltd. All rights reserved.