Summary1. Agricultural intensification, at field and landscape scales, has caused a decrease in weed richness and changes in species composition. In order to prevent this loss of diversity and develop efficient management regimes, it is important to understand how both aspects of intensification affect plant diversity and the way in which they interact. 2. This study assessed plant diversity at the centre, edges and boundaries of 29 organic and 29 conventional cereal fields distributed in 15 agrarian localities of the NE Iberian Peninsula. We focused on the composition of plant assemblages and on the specific richness at the field level, which was calculated for the whole set of species and for that of characteristic arable weeds. The percentage of arable land together with human settlements was used as a surrogate for landscape complexity and the amount of nitrogen inputs for land-use intensity. 3. Our results show that both scales of agricultural intensification have a similar negative effect on the total plant species and characteristic arable weed richness, and they also affect plant assemblages. Furthermore, we found no strong interaction between landscape and land-use intensity for explaining total plant richness. 4. The relative importance of farming intensity and landscape varies depending on the location within the field, which can be attributed to differences in the agricultural impact and limited seed dispersal from adjacent habitats. Management is the main factor in explaining differences among field centres, whereas changes at boundaries are mainly due to landscape characteristics, and at edges both factors are relevant. 5. Synthesis and applications. To preserve agricultural plant diversity it is equally important to prevent agricultural intensification at field and landscape scales. Policies enhancing low-intensity management techniques, such as organic farming, are the main way to promote diversity inside the fields and will be equally beneficial in simple and complex landscapes for total plant species richness. To maintain diversity within agricultural areas, it is also important to reduce farming intensity at the edges, which would favour characteristic arable weeds and margins' overall plant diversity.

International audienceThis report shows that weed harrowing in organic cereal fields is an efficient alternative to herbicides since weed harrowing does not reduce yields compared to weed-free plots. Arable weeds provide resources and habitat to many organisms. However, weeds are the most important constraint to crop production. Indeed, the potential crop losses of the eight major crops due to weed–crop competition amount to about 30 %. New ways of food production are needed due to the current severe biodiversity decline, about 1,000 times higher than the natural rate of species loss, and the growing food demands. Herbicides are highly efficient at reducing crop losses due to weed–crop competition, but at the expense of declining biodiversity. Studies have shown a poor efficiency of weed harrows in terms of weed reduction in organic farming systems. Here, we evaluated the feasibility of weed harrows in organic fields to reduce weeds to a threshold that does not limit crop production, while maintaining a rich flora. The results were compared to results obtained using herbicides in conventionally managed fields. Eleven organic and conventional cereal field pairs in Catalonia, Spain, were evaluated for one season in 2006–2007. Three different weed control treatments were applied: weed-free plots; weed-controlled plots, using herbicide in conventional fields and weed harrowing in organic ones and non-weeded plots. Crop yield and the abundance, richness and composition of the weed flora, which was dominated by ryegrass and poppies, were evaluated. Our results show that weed harrowing prevents weeds from being a limiting factor of crop productivity in organic cereal fields, since weed-controlled plots did not reduce yields compared to weed-free plots. A similar trend was observed in herbicide-controlled plots. However, herbicides diminished weed species richness in approximately 47 % and changed the species composition whereas harrowing allowed the maintenance of high levels of weed diversity in the organic fields

José‐María L & Sans FX (2011). Weed seedbanks in arable fields: effects of management practices and surrounding landscape. Weed Research51, 631–640. Summary Weed seedbanks are a reserve of weed diversity and can contribute to the prediction of future weed problems in arable fields. Managing seedbanks should therefore help in optimising biodiversity and controlling weed infestations. This study assessed the effects of management system (organic vs. conventional) and landscape complexity on seedbank size and species richness at the edges and centres of Mediterranean dryland cereal fields and examines the relationship between specific management practices and seedbanks. Field edges and organic fields had more species‐rich, denser seedbanks than field centres and conventional fields, and landscape complexity had a limited effect on arable seedbanks. Accordingly, the promotion of low‐intensity farming practices regardless of landscape complexity, especially at field edges, would be an effective measure for conservation purposes in Mediterranean agroecosystems. Nevertheless, the high seed density of organic seedbanks reveals the need for more effective seedbank management. The analysis of the effects of specific management practices highlights the importance of cleaning crop seeds properly to reduce seedbank size and using complex rotations, especially as this tends to conserve species richness while reducing seed abundance.

Field boundaries are expected to support the maintenance of biodiversity in agroecosystems, since they provide the habitat for a range of plant species. However, plant diversity in field boundaries has decreased substantially in recent decades. This pattern is generally linked with the intensification of agricultural land use at field and landscape level. Therefore, we aimed to test the effect of farming management (field and boundary management), boundary structure (width and habitat assemblage considering the Mediterranean grassland element), and landscape heterogeneity on plant species richness of field boundaries. Plants were recorded along 30 field boundaries next to organic fields and 30 next to conventional fields located in 15 agrarian localities of NE Iberian Peninsula along a gradient of landscape complexity. A total of 517 plant species were identified in the 60 field boundaries. We recorded 162 species (31%) catalogued as rare, very rare or extremely rare in the flora of the Catalan Countries. Our results showed the importance of landscape heterogeneity, field management and habitat assemblage, since they were found to be the most influential variables for plant species richness; whereas boundary width and boundary management were seen to contribute less to explaining plant diversity. Accordingly, agri-environmental schemes should be designed to promote organic farming and maintain the structure of the landscape mosaic in order to benefit plant diversity in field boundaries in the Mediterranean region.

Questions: Do diversity components (a, b and c) differ across contrasting regions? What is the relative contribution of organic and conventional farming practices to the structuring of arable weed species diversity and different functional groups (legumes, grasses and broad-leaves)? To what extent do arable weed communities differ between regions and farming systems (organic vs conventional)?Location: Twenty-six farms in total in northeast Spain (Catalonia) and north Germany (Lower Saxony). Methods:We examined the weed flora in paired organic and conventional farms at each locality and assessed diversity components by additive partitioning of species richness (a, b and c).Results: The weed species composition differed greatly between the two regions. Only 18 of 135 arable weed species were shared. The a-, b-and c-diversity of all functional groups was significantly higher under organic farming practices for both regions, indicating an increasing homogenization of local communities by agricultural intensification within each region. b-diversity contributed most to the total observed species richness in both regions (69.8% and 35.8% on organic and conventional farms, respectively, in Catalonia; and 62.4% and 53.0% on organic and conventional farms, respectively, in Lower Saxony). These results demonstrate the great importance of environmental heterogeneity and of farm-specific differences in agricultural practices for the richness of arable weed species.Conclusions: Regardless of the substantial differences in arable weed community composition across regions, our study emphasizes the great importance of organic farming for arable weed species richness. Understanding the patterns and causes of the dissimilarity of local communities appears to be a key factor for species conservation and the development of effective European-wide agrienvironmental schemes at landscape and regional scales. This approach is in contrast to current nature conservation practices that are restricted primarily to local (field-and farm-scale) implementation.