Weed communities are more diverse, but not more abundant, in dense and complex bocage landscapes

Bocage landscapes are characterized by a network of hedgerows that delimits arable fields. Such landscapes provide many ecosystem services, including biodiversity conservation, but their effects on weed communities remain largely unknown. Bocage landscapes could affect weed communities through two main processes: plant spillover from hedgerows and increased environmental heterogeneity in arable fields. These bocage effects are also likely to vary between farming systems (conventional vs. organic) due to differences in management practices. We sampled weed communities more than 20 m from field margins in 74 arable fields (37 per farming system). Fields were located along two independent landscape gradients of total length of hedgerows (with or without a shrub layer) and organic farming cover, in Brittany (France). We analysed the effect of 'bocage' (i.e. the density and complexity of hedgerow networks) and farming systems at field and landscape scales on species and functional diversity of weed communities. Further, we used fidelity to non-crop habitats and Ellenberg indicator values to assess the 'plant spillover' and 'environmental heterogeneity' hypotheses, respectively. Weed communities were more diverse and more abundant in organic farming systems. In addition, weed communities were more diverse, but not more abundant, in denser and more complex bocage landscapes. 'Bocage' increased species diversity of weeds, but also community-weighted variance of specific leaf area, plant height and seed mass. Positive effects of 'bocage' on weed diversity were driven by increased environmental heterogeneity rather than spillover of transient species from hedgerows. 'Bocage' effects were independent of farming systems at field and landscape scales. Synthesis and applications. Maintaining diverse weed communities is key to agroecological weed management and biodiversity conservation in agricultural landscapes. Farmers are often concerned that hedgerows harbour competitive plants spreading into field edges, thereby increasing weed pressure. However, our study shows that dense and complex bocage landscapes promote weed diversity in field cores, most likely by increasing environmental heterogeneity. Thus, bocage landscapes could actually enhance ecosystem services provided by weed communities and reduce weed-crop competition.