Crop presence, but not genetic diversity, impacts on the rare arable plant <italic>Valerianella rimosa</italic>.

<bold><italic>Background</italic></bold>: Intensive farming affects farmland biodiversity, and some arable plants in particular. Increasing crop genetic diversity can increase crop productivity or resilience and could also benefit rare arable plants. <bold><italic>Aims</italic></bold>: We examined whether barley presence, sowing density and genetic diversity impacted the rare plant <italic>Valerianella rimosa</italic> and explored possible underlying mechanisms. <bold><italic>Methods</italic></bold>: In a field study near Dundee, Scotland, we sowed plots of five single barley genotypes, and all five genotypes combined, at three densities; we also had barley-free plots. <italic>Valerianella</italic> seeds were sown into half of all plots. Measured responses included early-season cover and harvest biomass of barley and common weeds, abiotic parameters (soil moisture, light) and establishment, biomass and seed production by <italic>V. rimosa</italic>. <bold><italic>Results</italic></bold>: Barley presence promoted <italic>V. rimosa</italic> establishment early in the growing season, but without barley density or genetic diversity effects. By harvest, the impact of barley presence on <italic>V. rimosa</italic> abundance was lost; there were no effects on <italic>Valerianella</italic> seed production. Barley negatively impacted common weeds, but <italic>V. rimosa</italic> did not benefit from any indirect facilitation by barley, being bigger without barley. <bold><italic>Conclusions</italic></bold>: Early beneficial effects of barley on <italic>V. rimosa</italic> abundance appear offset by late-season competition. However, limited impacts of barley on <italic>V. rimosa</italic> reproductive success, and negative impacts on common weeds, indicate crops might play a role in conservation management of rare arable plants by creating space in the weed community. [ABSTRACT FROM AUTHOR]