Secondary succession in old-fields in the Transylvanian Lowland (Romania).

The main trends in spontaneous regeneration were studied in old-fields in the Transylvanian Lowland (Câmpia Transilvaniei) over a period of 40 years using the chronosequence method. Succession proceeds to grassland, because the establishment of woody vegetation is hindered by grazing and mowing of the old- fields and by the scarcity of woodlands in the vicinity. Community properties and population-level changes were recorded at different stages of succession and compared with semi-natural grassland in the surrounding landscape. Due to favourable soil conditions and temperate climate, vegetation cover develops quickly after the fields are abandoned. Annuals dominated only in the first year. After two years the fast growing clonal grass, Elymus repens, became dominant. After approximately 12 years, Elymus was replaced by Festuca rupicola, which is more resistant to stress and disturbance. In the later stages of succession various species, some typical of surrounding grassland, attained high cover values. A steady increase in species diversity, measured by the Shannon index, and richness was recorded at both the field (1.0–2.5 ha) and plot (4 × 4 m) scales. Species richness increased rapidly in early and middle stages and stabilized after the 14th year. Specific features of the succession in the old-fields in the Transylvanian Lowland can be attributed to the continued grazing and mowing of the fields after they are abandoned. This increases species richness because it arrests succession at a stage when species diversity is high. The management directs regeneration towards secondary grassland rather than species poor woodland.