Impact of mid-successional dominant species on the diversity and progress of succession in regenerating temperate grasslands.
Questions: (i) Which species dominate mid-successional old-fields in Hungary? How does the identity of these species relate to local (patch-scale) diversity and to the progress of succession? (ii) Which species have the strongest negative impact on diversity in spontaneous old-field succession and what generalizations are possible about traits of these species? (iii) Are these species dominant or subordinate components in mature target communities? (iv) Do native or alien species have stronger effects on the diversity and progress of succession? Location: Abandoned agricultural fields (abandoned croplands, orchards and vineyards) at various locations throughout Hungary. Methods: Vegetation patterns on 112 old-fields, in 25 sites varying in soil and climatic conditions, topography, landscape contexts and land-use histories were sampled. Most old-fields had appropriate seed sources in the immediate vicinity, i.e. natural or semi-natural grasslands (meadows steppes, closed and open sand steppes) as source and target habitats. Age of abandoned fields ranged from 1 to 69 yr, but most sites were between 15 and 60 yr. The cover of vascular plant species (%) was estimated in 2×2-m plots. Relationships between diversity, progress of succession (similarity to target communities) and identity of dominants were tested. Results: A small portion of successional dominants (eight species) had strong negative impacts on diversity. These species belonged to Poaceae, Asteraceae and Fabaceae. Most of these species were wind-pollinated, and capable of lateral vegetative spread. Dominant species varied in size and had, on average, a low requirement for N but a high requirement for light. With one exception, Solidago gigantea, they were native to the Hungarian flora. Significant differences were found among the impact of successional dominants when dominant species were grouped according to their original role (dominants or subordinates) in natural communities. The overall effect of species identity was also significant. Bothriochloa ischaemum was identified as the species with the strongest negative effect on species diversity. Conclusions: Our results suggest that mid-successional dominant species differ in their impact on diversity and progress of succession. Mid-successional plots dominated by alien species, or by native species that were originally subordinate in natural communities, regenerate less successfully and may temporarily arrest succession. Therefore, early colonization of native dominants should be enhanced with restoration measures.