A comparative survey of weed vegetation in organic and conventional dryland winter cereal fields was performed in central Catalonia (NE Spain) in order to assess the effects of agricultural intensification on the diversity, structure and composition of weed communities. A total of 36 cereal fields were surveyed in nine agricultural sites, where a pair of one long-established organic and one conventional farms were selected. Weed surveys were carried out before harvest in 2003 and 2004, taking into account the spatial pattern. Organic practices produced an increase in weed cover, species richness and Hill's first order diversity (but not in equitability), as well as a shift in weed vegetation composition, which favoured potentially rare arable, broad-leaved, insect-pollinated and legume weeds. Weed diversity was concentrated in the crop edges, especially in the weed communities of conventional cereal fields, which were found to be more spatially heterogeneous than the organic ones.
