The impact of forest age (indicating the number of years of continuous forest cover), historical forest management and soil conditions were studied on the forest plant species richness and composition of 168 forest fragments in the Moervaart valley. The Moervaart valley is a large alluvial plain in the north of Flanders which originated from the natural drainage of a lake at approximately 11 000 BC and where gyttja (lime) is locally present. Historical- ecological research indicated that the area was managed as grassland (both pasture and meadow) during medieval times, while forest was absent. Grassland was gradually afforested from the 18th century onwards. Analysis of the floristic composition indicated that forest age was the dominant factor explaining species richness. Older forests contained both more acidophilous and basophilous species than younger forests, suggesting a large internal variation in soil acidity caused by the presence of a pattern of ridges. However, this could not be confirmed by soil pH measurements, as analysis was performed on a mixture of soil samples for each fragment. Dispersal characteristics of the inventoried woodland species confirmed that the Moervaart valley has not been permanently covered by forest. Most of the woodland species in the oldest forests, appeared to be relatively fast colonizing anemochores and ornithochores, while slow colonizing barochores, myrmecochores and plants with vegetative reproduction were almost absent.