Weed community diversity in conservation agriculture: Post-adoption changes

Conservation agriculture has been identified as one of the farming systems likely to deliver sustainable agriculture but its effects over time on the diversity and composition of weed communities are poorly documented. Using a network of 100 winter wheat fields selected to encompass a gradient of years in conservation agriculture from 1 to 20 years in the Bourgogne-Franche-Comte? region (France), we analyzed the changes that occurred in the diversity of weed communities over several years, both in a-and I3-diversity using a taxonomic (species level) and functional (10 response traits) approach. Based on three weed surveys (before the last herbicide use, before harvest, before sowing the following crop), we identified weeds able to maintain and/or to produce propagules. All the observed weed communities were rich (average species richness of 23.9 species), diverse (average Shannon diversity of 2.15) and equitably composed of low-density species. The results showed an increase in species richness, total weed abundance and a-functional diversity but no change in species diversity and species evenness over time. Heterogeneity and average values of I3-taxonomic and I3-functional diversity between communities were high in the early years following the adoption of conservation agriculture. Heterogeneity and average I3-taxonomic and I3-functional diversity decreased over time, leading to a homogenization of weed community assemblages. Despite major changes in cultural practices related to conservation agriculture, homogenization of weed community was not immediate and did not concern all the traits studied.