Agricultural intensification has led to a loss of biological diversity at various spatial and temporal scales and understanding the mechanisms driving these changes would help target conservation efforts accordingly. In this study we used additive partitioning of diversity and the Jaccard index of similarity to estimate the spatial and temporal patterns of plant diversity on ditch banks under different management regimes (nature reserves and agricultural areas). We focused on a total of 118 species, including 18 indicator species of conservation interest, at 42 sites in three successive sampling periods. For all species taken together, beta diversity contributed most to total observed species diversity, but was less than expected under random distribution. Indicator species showed greater beta diversity on a spatial scale compared to all species, but much less so on a temporal scale. Importantly, the differences in indicator species composition on a spatial scale are probably due to environmental heterogeneity and dispersal limitation, indicating that management strategies should focus on both factors. Nature reserves showed higher alpha diversity within sites because of possible lower nutrient inputs and grazing intensity compared with agricultural areas, while both exhibited scale-dependent dispersal limitation.