Weeds are not always evil: crop-weed relationships are context-dependent and cannot fully explain the positive effects of intercropping on yield

Implementing sustainable weed control strategies is a major challenge in agriculture. Intercropping offers a potential solution to control weed pressure by reducing the niche space available for weeds; however, available research on the relationship between crop diversity and weed pressure, and its consequences on crop yield is not fully conclusive yet. In this study, we performed an extensive intercropping experiment using eight crop species and 40 different species mixtures to examine how crop diversity affects weed communities and how the subsequent changes in weeds influence crop yield. Field experiments were carried out in Switzerland and in Spain, which differ drastically in terms of climate and soil, and included monocultures, 2- and 4-species mixtures, and a control treatment without crops. Weed communities were assessed in terms of biomass, species richness and evenness, and community composition. Results indicate that intercropping per se does not reduce weed performance or diversity, but crop species composition does. In particular, the presence of cereals in crop mixtures significantly reduced weed biomass and diversity. Despite the lack of crop diversity effect on weeds, crop yield increased with crop species richness, particularly in Switzerland. Moreover, in Switzerland, where soil resources were abundant, increasing crop yield correlated with increased weed suppression and reduced weed diversity. In Spain, where water and nutrients were limited, crop yield was not related to weed biomass or diversity. Synthesis and applications: We demonstrate that in our study, increased crop yield in mixtures was not due to increased weed suppression in diverse crop communities, but must be the result of other ecological processes. We also show that crop-weed relationships vary across environmental conditions, which emphasizes the need for a better understanding of weed communities' assembly mechanisms, as well as locally adapted weed management strategies.