Weed species diversity and community composition in organic and conventional cropping of spring cereals

Interest in organic farming is growing rapidly in Europe. The resulting expansion of the organic farming area has been expected to enhance the biodiversity of agricultural habitats. Organic cropping practices can be hypothesized to support a higher number of weed species than conventional cropping and also to favor herbicide-susceptible and less-nitrophilous species. The diversity and species composition of weed communities were compared in spring cereal fields cultivated by organic, conventional cereal and conventional dairy cropping methods in southern Finland between 1997 and 1999. Samples were taken before (May-June) and after (July-August) the treatment of conventional fields with herbicides. The number of species was used as a measure of species diversity. The total and mean numbers of species between cropping practices were compared by applying rarefaction and ANCOVA, respectively. In organically cultivated, fields the adjusted mean number of species per field exceeded that in conventionally cultivated fields by about two species in both May-June and July-August. The total number of species was higher in organically cultivated fields only in July-August. The species composition of organically cultivated fields was closer to that of conventional dairy than to that of conventional cereal fields. Species susceptible to herbicides (e.g. Chenopodium album) were abundant in organically cultivated fields in both sampling periods whereas the relative abundance of some of them declined in conventional fields from May-June to July-August. In conventional fields, the relative abundances of Viola arvensis, Stellaria media and Elymus repens increased from May-June to July-August. Only weak support was found for the hypothesis that organic cropping favors less-nitrophilous weed species. It is concluded that organic cropping promotes weed species diversity at an early phase of cropping history but that a change in species composition would require a longer period of organic cropping.