Arable weed decline in Northeast Spain: Does organic farming recover functional biodiversity?

• Weed arable flora have sharply reduced from the 50s in Catalonia (Spain).
• Arable weed decline has been especially dramatic in segetal and rare species.
• Agricultural intensification affected flora interesting to birds and invertebrates.
• Organic farming recovers arable weed biodiversity in Catalonia.
• Rare and segetal weeds and weeds related to fauna increase in organic fields.

The comparison of the frequency, richness and weed cover of total species and functional groups of weeds, including those of interest for birds, pollinators and other invertebrates, and the subset of segetal and rare species from the 1950s to the present, has allowed to detect the consequences of the agricultural intensification in Catalonia (NE Spain) at regional and field scales. We analyzed field plots of conventionally managed cereal fields of the periods 1953–88 and 1996–99 while cereal fields assessed in the period 2005–07 were organic and conventionally managed. Our results indicate a remarkable reduction in weed frequency (58%), species richness (47%) and total weed cover (69%) from the 1953–1988 to 2005–2007 periods. The diminishing species richness was observed in species that are important for birds, pollinators and other invertebrates, but the most drastic decline was observed in the segetal and rare species subsets (75% and 87%, respectively). In current organic crops, the frequency, richness and total weed cover per relevé are significantly higher than in conventional crops, especially for those groups of species that are interesting for fauna and for segetal (more than twice) and rare species (4-fold). Nevertheless, the increase in arable weeds by current organic management is still insufficient to recover the highest plant biodiversity values that were observed before the widespread agricultural intensification in Catalonia.