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Question The intensification of crop cultivation in much of Europe since the mid-20th century has greatly increased crop yields but caused dramatic biodiversity losses in arable fields. We investigated the extent of these losses at the level of plant community types. Location Ten areas in central Germany with different soil/climate conditions and various arable plant communities. Methods We compiled historical surveys of arable fields in the 1950s/early 1960s before the onset of pervasive agricultural industrialization, and in 2009 revisited 392 arable fields. Historical and recent data were compared with supervised manual classification, detrended correspondence analysis ( DCA) and ANOVA. Results Ten out of 16 plant communities at association rank observed in fields in the 1950s/1960s were not recorded again. The proportion of relevés assignable at association level decreased from 75% to 5%, while the proportion of relevés assignable only at higher syntaxon level or not assignable at all had increased from 2% to 75%. The impoverishment of vegetation was slightly less pronounced at field margins, where around one quarter of the recent relevés could be assigned to associations. Present arable plant communities in the region are species-poor and consist chiefly of common, often herbicide-tolerant, generalist species, with no clear preference for cereal vs root crops, autumn- vs spring-sown crops or base-rich vs base-poor soils. Conclusion Our new approach using phytosociological syntaxa and a semi-permanent plot design enabled us to quantify biodiversity losses at the community type level. The currently used set of phytosociological associations is inadequate to represent present-day arable plant assemblages. The concept of residual plant communities provides a useful methodological supplement. [ABSTRACT FROM AUTHOR]

Question How do different weed management systems based on glyphosate-resistant ( GR) crops affect weed community structure and composition? Location A total of 156 agricultural fields across six US states. Methods Fields were selected and categorized into three weed management systems based on a 3-yr history: (1) a single continuous GR crop; (2) a rotation of two GR crops; and (3) a GR crop rotated with a non- GR crop. Weed species population density was surveyed in 20 0.5-m2 plots per field four times each season from 2006 to 2010. Weed population density, species richness and diversity were analysed using repeated measures mixed models to test the effect of year, geographic location (based on plant hardiness zone) and weed management system (as described above). Weed community composition was analysed using NMDS ordination. The importance of geographic location, planted crop and weed management system were analysed using analysis of similarity and permutational ANOVA based on the Bray- Curtis similarity among sites calculated from the maximum weed species population density per site per year. Results A total of 329 weed species were identified across the six states and all time periods. Weed communities were most strongly correlated with geographic location and secondarily to the crop planted. Weed management system affected similarity among weed communities through an interaction with site location but not every year. The weed management systems of crop rotation and GR trait rotation generally reduced weed population density and species diversity, but the effect of crop rotation varied by geographic location. Conclusions The overall effects of weed management systems in fields including the GR trait on arable weed communities reinforce the need for locally adapted weed management programmes to steward the GR crop trait to manage for high weed diversity while reducing crop-weed competition and maximizing yield. [ABSTRACT FROM AUTHOR]