Restoration of oak forest: Effects of former arable land use on soil chemistry and herb layer vegetation

Stands of pedunculate oak (Quercus robur) planted 50-80 years ago on two types of land (previously forested land and former arable fields) were compared regarding vegetation and soil. Former arable soils were characterized by a higher pH, higher nitrate concentration and higher soil density, but had lower organic matter content and lower ammonium concentration in the topsoil (0-5 cm). These differences, however, decreased with soil depth. Phosphorus concentration was consistently higher in former fields throughout the soil profile (0-45 cm). Nitrogen mineralization, determined by in situ incubation, showed a strong seasonal pattern with peak values in spring. Non-metric multidimensional scaling ordination revealed marked compositional differences in the vegetation between the two land-use categories, and also compositional turnover along gradients in soil pH and nitrogen availability. Differences in soil pH between land-use categories occurred in a range critical for the establishment of many typical forest herb layer species. Plant indicator species were identified for the two land-use categories. The results showed that acid-sensitive forest herbs may benefit from the higher pH soils in new woodlands, in contrast to ancient forest soils with little buffer capacity towards natural and anthropogenic acidification. In conclusion, former arable use has long-lasting effects on soil properties and vegetation composition in broadleaved forests. New woodlands on former fields can thus offer relatively persistent new habitats for acid-sensitive species that have suffered from reduction in habitat area during historic periods of deforestation and cultivation.