Crop diversification affects weed communities and densities in organic spring cereal fields in northern Europe

Within organic crop production, weed interference remains one of the main constraints on productivity, yet diverse weed vegetation supports the ecosystem services of arable fields. The use of crop diversification could alter weed communities and increase diversity, preventing the dominance of single detrimental weed species. This study investigated the type of crop diversification implemented in organic cropping systems in northern Europe and whether this affects the arable weed vegetation. A weed survey was conducted in 207 organic spring-sown cereal fields in five northern European regions (Denmark, Finland, Germany, Latvia and Sweden). Weed density data were combined with management and crop diversification data collected on the farms. The data set was analysed using linear mixed models and multivariate ordinations. The 197 weed species recorded were representative for the type of organic arable farming conducted in the regions. The management and crop diversification variables studied differed considerably among the regions. Management, such as the type of spring-sown cereal and crop mixture during monitoring, the preceding crop and weed harrowing, primarily influenced weed densities, while weed diversity increased with the duration of organic management and use of winter cover crops. The weed species communities were altered by the crop diversification applied. This study reflects the impact of regionally varied crop diversity and reinforces the importance of regionally adapted crop and weed management.