Dalby Söderskog revisited: long-term vegetation changes in a south Swedish deciduous forest

In this study, we report on 67 years of secondary succession in the temperate deciduous forest Dalby Söderskog, southern Sweden. In 2002 vegetation analyses were conducted in 74 systematically distributed plots and the results compared with similar investigations during the years 1935, 1969 and 1976. Additionally, a floristic survey of the forest was made and compared with inventories of the years 1925, 1935 and 1970. From 1970 to 2002, overall species richness of the forest continued to decline, but at a lower rate than before. In 2002, for the first time, shade-tolerant forest species constituted the largest group. In the upper tree layer Fraxinus excelsior became the most important species, at the expense of Quercus robur and Ulmus glabra. In the herb layer, no changes in total or mean species richness were exhibited at plot scale between 1976 and 2002. In contrast to the lack of quantitative changes in species richness, we observed major changes in species composition at the plot level. In particular, the former dominant species Mercurialis perennis had declined markedly. After release from a historical regime of livestock grazing and irregular cuttings, the predicted steady-state community dominated by Ulmus and Mercurialis only prevailed for a few decades before unexpected external events, in this particular case Dutch elm disease and invasion by the slug Arion lusitanicus, initiated a successional change. We conclude that unexpected diseases or pests or rare climatic extremes can play a decisive role in forest dynamics and strongly interact with more gradual autogenic changes of forest structure and composition.