Plant species diversity in semi-natural grasslands. Effects of scale, landscape structure and habitat history

Sammanfattning: The thesis examines the impact of present and past landscape structure, habitat history and management on different aspects of present-day plant species diversity in fragmented semi-natural grasslands in Sweden and in Estonia. Species diversity was significantly associated with present and/or past grassland connectivity in the majority of the studies in the thesis. However, relationships between diversity and habitat connectivity differed between the studies – depending on the the group of studied species and on the scale of study. Present grassland connectivity was positively associated with fine-scale (0.25–1 m2), rather than large-scale (>0.1 ha), species diversity – suggesting that small-scale diversity may be more sensitive to habitat fragmentation than large-scale diversity. High landscape diversity in the surroundings of the grassland sites had a positive effect on species richness, emphasizing the importance of considering landscape context when examining the influence of landscape-scale processes on plant communities. Within-habitat environmental heterogeneity had a positive influence on both large-scale and fine-scale species diversity. Grassland history (age; previous land-use) had a significant impact on present-day species diversity. Large-scale species richness increased with grassland age (30 to >280 years of grassland continuity). While fine-scale species richness was independent of habitat age, the proportion of grassland specialist species on a fine scale was higher in old grasslands and in grasslands on previously forested land than in previously arable sites. Diversity was positively associated with Late Iron Age human population density – indicating that human land-use legacies that influence present-day plant species diversity may reach back more than 800 years. Grassland management by grazing or mowing is, by definition, essential for the persistence of semi-natural grasslands. Fine-scale species diversity decreased with decreasing grazing intensity in the studied grasslands. The results also suggest that simple descriptors (distance from settlements; human population density) related to human activities may provide good indicators for management processes that influence levels of small-scale plant species diversity. Comparison of the responses of fine-scale species evenness and richness to differences in landscape structure and history showed that evenness was associated mainly with historical factors, while richness was also influenced by contemporary landscape structure. Patterns of species co-occurrence were associated with different factors on different spatial scales. The results of the thesis emphasize the importance of (a) examining different aspects of species diversity on multiple spatial scales (b) including information on historical habitat distribution, when studying the community responses to habitat fragmentation and when making conservation decisions for fragmented habitats.