Drivers of plant community completeness differ at regional and landscape scales

Although many grassland species may exist within agricultural landscapes, some are still absent from individual grassland patches. Understanding which processes limit plant biodiversity within these landscapes is important, and may be better understood if we consider local diversity relative to its species pool size (community completeness). Here we test how habitat availability, dispersal limitation, grassland age, and local environmental conditions (productivity, fine scale heterogeneity) affect the completeness of grassland plant communities at two spatial scales in an agricultural region in Southeast Estonia, Northern Europe. We additionally test how species dispersal and persistence traits (seed production, potential dispersal distance, and clonal ability) depend on the landscape and local environmental factors.We collected vascular plant data from 31 grassland patches and used species co-occurrences to estimate which species from the surrounding region were suitable, but absent for each grassland site (dark diversity). We partitioned dark diversity for each site into species that were found within the surrounding 10 ha area (landscape scale) or species found only at other visited grassland sites (regional scale), and used these estimates to calculate completeness for each site at the landscape and regional scale. Environmental data (various soil characteristics, light availability, plant biomass) was collected to characterize productivity and fine scale heterogeneity of the sites.We found that community completeness was limited by seed production ability, dispersal distances, and grassland availability at the regional scale, whereas local environmental conditions (productivity) limited completeness at the landscape scale. Species dispersing long distances were more often present in areas historically bordered by more grasslands, whereas potential dispersal limitation was reduced in older grasslands with heterogeneous and sparse vegetation.We conclude that dispersal limitation, habitat availability, and local environmental conditions all determine completeness of grassland patches; however, their relative importance is scale dependent. Nevertheless, the presence of good dispersers in isolated grasslands is likely a disappearing legacy of historical landscapes. To support high taxonomic and functional diversity in agricultural landscapes, it is therefore crucial to increase the area and connectivity of grassland patches and maintain their moderate management by mowing or grazing.