Here we report on long-term secondary succession in the temperate deciduous forest Dalby Söderskog, southern Sweden. Vegetation analyses of the years 1935, 1969, 1976 and 2002 were compared as well as complete floristic surveys of the years 1925, 1935, 1970 and 2002. From 1925 to 2002, overall species richness declined continouously from 248 to 124 vascular plant species. This was mainly due to tree canopy closure excluding light-demanding species. In 2002, shade-tolerant forest species constituted the largest group for the first time. In the upper tree layer Froxinus excelsior had become the most important species in 2002, at the expense of Quercus robur and Ulmus glabro. In the herb layer, mean plot species richness decreased until 1969 but remained stable thereafter. However, the former dominant species Mercurialis perennis declined markedly between 1976 and 2002. After release from a historical regime of livestock grazing and irregular cuttings, the predicted steady-state community dominated by ulmus and Mercuriolis only prevailed for a few decades before Dutch elm disease and invasion by the slug Anon lusitanicus initiated a successional change. We conclude that unexpected events can play a decisive role in forest dynamics and strongly interact with more gradual autogenic changes of forest structure and composition.