Reintroduction of rare arable plants by seed transfer. What are the optimal sowing rates?

During the past decades, agro‐biodiversity has markedly declined and some species are close to extinction in large parts of Europe. Reintroduction of rare arable plant species in suitable habitats could counteract this negative trend. The study investigates optimal sowing rates of three endangered species (Legousia speculum‐veneris (L.) Chaix, Consolida regalis Gray, and Lithospermum arvense L.), in terms of establishment success, seed production, and crop yield losses. A field experiment with partial additive design was performed in an organically managed winter rye stand with study species added in ten sowing rates of 5–10,000 seeds m(−2). They were sown as a single species or as a three‐species mixture (pure vs. mixed sowing) and with vs. without removal of spontaneous weeds. Winter rye was sown at a fixed rate of 350 grains m(−2). Performance of the study species was assessed as plant establishment and seed production. Crop response was determined as grain yield. Plant numbers and seed production were significantly affected by the sowing rate, but not by sowing type (pure vs. mixed sowing of the three study species), and weed removal. All rare arable plant species established and reproduced at sowing rates >25 seeds m(−2), with best performance of L. speculum‐veneris. Negative density effects occurred to some extent for plant establishment and more markedly for seed production. The impact of the three study species on crop yield followed sigmoidal functions. Depending on the species, a yield loss of 10% occurred at >100 seeds m(−2). Synthesis and applications: The study shows that reintroduction of rare arable plants by seed transfer is a suitable method to establish them on extensively managed fields, for example, in organic farms with low nutrient level and without mechanical weed control. Sowing rates of 100 seeds m(−2) for C. regalis and L. arvense, and 50 seeds m(−2) for L. speculum‐veneris are recommended, to achieve successful establishment with negligible crop yield losses.