Plant species diversity patterns in a Swedish rural landscape : effects of the past and consequences for the future

Abstract: Plant species richness in rural landscapes is threatened as grassland management ceases or changes, and as many remaining semi-natural grasslands are becoming increasingly fragmented. This thesis is a study of plant species diversity in a rural landscape in south-eastern Sweden, and its relationship to management history and landscape pattern. Past and present landscapes were analysed using old cadastral maps, aerial photographs, a soil/bedrock map and a digital elevation model for the spatial land-cover distribution and a pollen stratigraphy as point data. Species richness was analysed in 146 sample-sites representing 14 different habitats. Patterns of plant species richness and the occurrence of 52 individual species were analysed with respect to topography, top- and subsoil and land use history. Further, a tentative model was developed to analyse the effects of grazing regimes and landscape patterns on species persistence is presented. Human activities for more than 4000 years in the area are evident from archaeological and palynological analyses. Grazed grasslands have been present throughout this time. All semi-natural grasslands with a long continuity of management (8% of the total area) are located on shallow soils, less than 50 cm depth. These long-continuity grasslands, mainly dry open semi-natural grasslands and midfield islets, together with road verges, were the most species-rich habitats, and also contained the highest number of habitat specialists. Half the number of species (49% of 361) were found to be restricted to one or two habitats only. Habitats on dry substrates had the highest alpha diversity, while beta diversity was highest in moist to wet habitats. Plant species richness and some of the species occurrences could be associated with different topographic and edaphic properties. Management history was also associated with high species richness, and with the occurrence of 17 species. Open grasslands encroached by trees and shrubs show a decline in species number. In semi-open grasslands 57% of the species present in open grasslands persisted. Deciduous and wet deciduous forests were further affected by species decline. However, wet deciduous forests still appear to have a potential for restoration of moist to mesic grassland habitats. Modelling different grassland regimes and landscape patterns indicated the existence of a threshold where grassland species are strongly reduced: when grasslands have decreased in area to between 10 and 30% of the total area. Continuous but low-intensity grazing is more positive to grassland species than discontinuous but highly intensive grazing. This effect is particularly strong when frequency and/or intensity of grazing drop below 20%. The pattern of fragmented grasslands was also found to be important for the persistence of grassland species. This thesis gives support to the suggestion that small remnant habitats are important for plant species richness and that past history is one of the most important factors to influence current plant species richness. Further, a modelling approach is useful to understand some of the effects of grazing management and landscape pattern on species diversity at present and for conservation issues for the future.