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QUESTIONS: How does small linear grassland elements (paddy levees and road verges) contribute to plant diversity patterns in sites with modern or traditional management? Which factors determine small‐scale plant species richness of small linear grassland elements in a Japanese agricultural landscape? LOCATION: Shiga Prefecture, Japan. METHODS: Aerial photos from 1963, 1985, 1995 and 2008 were used to investigate landscape changes and to measure landscape structures within a typical agricultural landscape (~25 km²). Plant diversity was investigated in ten paddy levees and ten road verges in each of four traditional (barely any land cover alterations after 1963) and five modern (almost completely changed after 1985) sites. α‐diversity (mean number of species per plot) and γ‐diversity (total number of species) for all species, specialist and invasive species, respectively, were analysed in relation to site history types and landscape structures. Plot species richness was analysed in relation to site history type, patch age and width, proximity to forest and litter removal. RESULTS: In total, 204 plant species were found, 55 species unique to traditional sites and 13 unique to modern sites. α‐ and γ‐diversity of paddy levees and road verges was significantly higher in traditional sites. Number of specialist species on paddy levees and number of invasive species in road verges was also higher in traditional sites. Age was the strongest predictor of plot species richness on levees; older levees supported higher species richness than younger ones. Litter removal was the strongest predictor (positive effect) of plot species richness in road verges. CONCLUSIONS: These results suggest that the colonization credit of new levees in modern sites is decreasing quite rapidly as species quickly establish, approaching almost the same richness values as in traditional sites. However, long continuity of traditional management is still one of the most important contributors to plant diversity and crucial for many species. But small grassland elements can still be of great significance for preserving and enhancing plant diversity in otherwise depleted landscapes, as long as they are managed and connected throughout the landscape.
